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1991 power gone and rough idle once up to operating temp

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1991 power gone and rough idle once up to operating temp

 
Old 07-02-2019, 01:17 PM
  #21  
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Yes 02 sensor....could it be it failed again ? Hmm...?.
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Old 07-02-2019, 01:21 PM
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Originally Posted by 1985 Corvette View Post
......
The car starts up with no issues and idles at 725 - 750 rpm.....
once the coolant was up to temp I wound it out to about 6500 rpm in 2nd.
.... happy cruising in 3rd after that for about 5 minutes.
I got to a red light and like clockwork the engine sounded like a cammed SBC and the car was shaking.
The loping and shaking is accompanied by reduced power ......exhaust note sounds different on acceleration.... shudders and struggles to accelerate if full throttle is applied.....
2nd gear ....to about 4K rpm.... let off the gas.... occasional backfire or pop.

So if I park the car and it cools down, and then I restart the car, it's back to normal 750 rpm idle.
Injector problem not likely as engine runs good in open loop startup & at closed loop temp
No DTC's stored or visible on display panel, so not likely a sensor monitored condition
The most common problem that does not throw a DTC is usually spark or fuel related. Fuel pressure issues aren't usually intermittent. That leaves spark. I have seen similar symptoms from coil issues.

A weak coil issue can frequently be discovered with a 6th gear, low rpm (1000 or so) acceleration test. Depress the throttle. The engine should accelerate smoothly. If it does not accelerate smoothly (bucks like a cammed engine at too low engine speed) then coils/wires should be checked. High rpm disguises weak spark.

For a no cost or disassembly check, I recommend performing a spark test after symptoms appear and while the engine is hot. At idle, good spark will be blue & throw an arc about 3/4" to ground. Weak spark will be orange. A bad coil does not always display bad spark on both paired cylinders, depending on where the problem is with the coil. An internal winding problem will affect both paired cylinders. A terminal connection problem can affect only a single cylinder. The terminal problem can be corrosion at the coil and/or the plug wire.
Get a used plug, remove the plug wire from a single cylinder only, put the used plug in the wire & lay it on the engine, have a helper start the engine & while holding the plug wire boot, gradually move the spark plug away from the engine(ground) until you can observe the spark. Best done in a low light condition(shade) rather than bright sunlight. Repeat for each cylinder. You won't need to be reminded but once to turn off the engine before repeating the test for the next cylinder.

After the spark test, if you have identified a weak spark condition, removing the plenum is necessary to replace the coil or the plug wires. Ohm each individual plug wire to verify condition in case wires are suspect.

A bit of information regarding coils. There are a few different aftermarket coil out there. The ZR-1 LT5 coils are unique in spite of their appearance. The correct ACDelco coil for the LT5 is D545. The other ACDelco coil used on other GM models is D555. Do not use the D555 or any coil that shows it is correct for both the D545 & D555.
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Old 07-02-2019, 02:04 PM
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Originally Posted by A26B View Post
Injector problem not likely as engine runs good in open loop startup & at closed loop temp
No DTC's stored or visible on display panel, so not likely a sensor monitored condition
The most common problem that does not throw a DTC is usually spark or fuel related. Fuel pressure issues aren't usually intermittent. That leaves spark. I have seen similar symptoms from coil issues.

A weak coil issue can frequently be discovered with a 6th gear, low rpm (1000 or so) acceleration test. Depress the throttle. The engine should accelerate smoothly. If it does not accelerate smoothly (bucks like a cammed engine at too low engine speed) then coils/wires should be checked. High rpm disguises weak spark.

For a no cost or disassembly check, I recommend performing a spark test after symptoms appear and while the engine is hot. At idle, good spark will be blue & throw an arc about 3/4" to ground. Weak spark will be orange. A bad coil does not always display bad spark on both paired cylinders, depending on where the problem is with the coil. An internal winding problem will affect both paired cylinders. A terminal connection problem can affect only a single cylinder. The terminal problem can be corrosion at the coil and/or the plug wire.
Get a used plug, remove the plug wire from a single cylinder only, put the used plug in the wire & lay it on the engine, have a helper start the engine & while holding the plug wire boot, gradually move the spark plug away from the engine(ground) until you can observe the spark. Best done in a low light condition(shade) rather than bright sunlight. Repeat for each cylinder. You won't need to be reminded but once to turn off the engine before repeating the test for the next cylinder.

After the spark test, if you have identified a weak spark condition, removing the plenum is necessary to replace the coil or the plug wires. Ohm each individual plug wire to verify condition in case wires are suspect.

A bit of information regarding coils. There are a few different aftermarket coil out there. The ZR-1 LT5 coils are unique in spite of their appearance. The correct ACDelco coil for the LT5 is D545. The other ACDelco coil used on other GM models is D555. Do not use the D555 or any coil that shows it is correct for both the D545 & D555.
Thank you for the detailed analysis, Jerry. And thanks for the correct part number for the coil! Do you have a recommendation for the plug wire set? It has Delco wires on it now but I wouldnít mind putting a new set on after checking the resistance. The current coils in the car now are some brand that say made in USA on them but they are blue.

I had to go back in to replace a leaky secondary actuator. I donít mind throwing in the D545 coils and new plug wires while in there.

Last edited by 1985 Corvette; 07-02-2019 at 02:07 PM.
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Old 07-02-2019, 05:53 PM
  #24  
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Where fuel pressure comes into the picture is when cold at first startup both pumps run until the coolant temp reaches 176 degrees F. If the primary pump is weak or the internal check valve is defective the engine will run poorly once the secondary pump shuts off and primary pump pressure is low. This is why I’m asking for fuel pressure readings when hot. Just because the pump was recently replaced doesn’t mean it’s good.
H

Last edited by ccmano; 07-02-2019 at 05:54 PM.
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Old 07-03-2019, 08:52 PM
  #25  
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Originally Posted by ccmano View Post
Where fuel pressure comes into the picture is when cold at first startup both pumps run until the coolant temp reaches 176 degrees F. If the primary pump is weak or the internal check valve is defective the engine will run poorly once the secondary pump shuts off and primary pump pressure is low. This is why I’m asking for fuel pressure readings when hot. Just because the pump was recently replaced doesn’t mean it’s good.
H
That crossed my mind too Hans, but once I went back & summarized the symptoms the OP listed, it ran fine even after it warmed up. Your thoughts??

Following is the abbreviated symptoms stated by the OP. Note that he drove it to 6500rpm (insert by Jerry; adding more heat, likely in open loop & secondary pump running also) in 2nd and cruised in 3rd for another 5 min. (insert by Jerry; back to closed loop & only on Primary pump)

The car starts up with no issues and idles at 725 - 750 rpm.....
once the coolant was up to temp I wound it out to about 6500 rpm in 2nd.
.... happy cruising in 3rd after that for about 5 minutes.

Last edited by A26B; 07-04-2019 at 08:47 AM.
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Old 07-03-2019, 09:00 PM
  #26  
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Originally Posted by 1985 Corvette View Post
Thank you for the detailed analysis, Jerry. And thanks for the correct part number for the coil! Do you have a recommendation for the plug wire set? It has Delco wires on it now but I wouldn’t mind putting a new set on after checking the resistance. The current coils in the car now are some brand that say made in USA on them but they are blue.

I had to go back in to replace a leaky secondary actuator. I don’t mind throwing in the D545 coils and new plug wires while in there.
I hate to see you spending more money on any parts that have yet to be proven bad. Jerrys Gaskets has both coils & plug wires.

Last edited by A26B; 07-04-2019 at 08:48 AM. Reason: correct grammar
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Old 07-04-2019, 01:09 PM
  #27  
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Originally Posted by A26B View Post
I hate to see you spending more money on any parts that have yet to be proven bad. Jerrys Gaskets has both coils & plug wires.
You may be onto something. I took it out for a drive today and tried your 6th gear from 1K rpm test. I used full throttle once in 6th. It had a tiny shudder while accelerating but not too bad. I tried accelerating in a few gears and it's not 100% but still brisk on acceleration. On the final leg of the drive, I took it to 5K rpm in second gear and got a nice loud pop out of the exhaust and then the power was gone. It's probably common sense in this condition that the 6th gear pull from 1K rpm would cause this but I tried it anyway.

Car bucked horribly, flywheel was clunking away and the dash flickered and dimmed. Pulled it back in the driveway and the digital readout was showing 31 for rpm, whole car loping and shaking. I sometimes get backfires or pops out of the Corsa exhaust when the engine is still cold and if I accelerate in 1st gear up to 3K rpm or so. It's always been like that since I've had it and if that's normal for an LT5 with stock manifolds and a Corsa catback, then it may not be a clue to a problem that's been present and is now full blown. All the injectors checked out cold consistent across the board for resistance.

Ccmano, I haven't forgotten about you, I gotta dig out my fuel pressure gauge and check it while it's still warmed up. I might have to cave and get to a shop with a scanner to see what's going on. Code 12 all day long. Both O2 sensors are less than a year old and are Bosch brand.

Last edited by 1985 Corvette; 07-04-2019 at 01:14 PM.
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Old 07-04-2019, 02:21 PM
  #28  
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Digital readout on cluster shows only gives H72 and H74 under section 1. Car has an aftermarket touch screen head unit, so not too concerned with those codes. Section 4 has no stored codes, all I get is " _ _ _ " and then it's done cycling through. I'll have to pick up a new pressure gauge for the fuel rail.

Last edited by 1985 Corvette; 07-04-2019 at 02:24 PM.
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Old 07-04-2019, 03:22 PM
  #29  
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Originally Posted by A26B View Post
That crossed my mind too Hans, but once I went back & summarized the symptoms the OP listed, it ran fine even after it warmed up. Your thoughts??

Following is the abbreviated symptoms stated by the OP. Note that he drove it to 6500rpm (insert by Jerry; adding more heat, likely in open loop & secondary pump running also) in 2nd and cruised in 3rd for another 5 min. (insert by Jerry; back to closed loop & only on Primary pump)

The car starts up with no issues and idles at 725 - 750 rpm.....
once the coolant was up to temp I wound it out to about 6500 rpm in 2nd.
.... happy cruising in 3rd after that for about 5 minutes.
Sorry Jerry I have to disagree on this one. The OP clearly states the problem is after warm up.

ďThe real issue happens once the car is up to operating temperature. I got to a red light and like clockwork the engine sounded like a cammed SBC and the car was shaking. The loping and shaking is accompanied by reduced power that seems to start once the car has fully warmed up. Once at this point, the engine exhaust note sounds different on acceleration and it feels like the car is only running on 8 injectors. The acceleration of the car is greatly diminished and it feels like the car shudders and struggles to accelerate if full throttle is applied once at this point.Ē

Fuel pressure issues, coils and wires, even the ECM are all possibilities. I agree that thorough testing is better than throwing parts at it. With new pumps and filter itís still possible a pump is defective, a fuel line in the tank came loose or the ďYĒ is cracked on the pump assembly. The lack of codes, as you indicate, points us to a non-monitored system. If it were O2 (closed loop) related I would expect codes. It will simply be one of those step by step elimination diagnoses.
H
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Old 07-04-2019, 03:45 PM
  #30  
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Thanks for continuing to mull it over with me, guys. To stay productive on the troubleshoot, as my current fuel pressure gauge connector end is jacked up, I checked the resistance of the injectors while the engine was still warm and acting up. And I may have found the problem...or one of them. Again, these are replacement injectors with a three hole spray pattern.

Primary injector and connector ID with readout

#1 A1 - 17.8 ohms
#2 A7 - 17.7 ohms
#3 A18 - 17.8 ohms
#4 A13 - 17.8 ohms
#5 A3 - 17.8 ohms
#6 A8 - 17.8 ohms
#7 A2 - 17.8 ohms
#8 A12 - No measurement returned

Secondary Injectors probed through Relays

#1 - A on relay 1 - 17.7 ohms
#2 - G on relay 2 - 17.7 ohms
#3 - G on relay 1 - 17.7 ohms
#4 - F on relay 1 - 17.8 ohms
#5 - B on relay 2 - 17.7 ohms
#6 - A on relay 2 - 17.6 ohms
#7 - F on relay 2 - 17.5 ohms
#8 - B on relay 1 - 17.5 ohms

I'll check it when the engine is fully cooled down but checking cylinder 8's secondary injector resistance (if I read the guide right) involves probing A12 on ECM connector A with cavity B on relay #1. So I know it's not the contact inside connector A acting up because it gave me a reading with the engine cold for cylinder 8 no problem. That was the only discovery from doing this. Would one faulty injector create the level of issues I have described?
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Old 07-04-2019, 04:11 PM
  #31  
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While unusual, it’s certainly possible that individual injector electronics can fail with heat or the wiring to that injector is affected by heat. If you dropped one injector it could cause those symptoms. Double check you findings and check the connector to that injector for wire breaks, abrasions or bent pins.
H
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Old 07-05-2019, 09:55 AM
  #32  
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I decided to probe A12 for the primary again on #8 this morning before going to work. With the engine fully cooled off, it gave me a 16.9 ohm reading.

So Iím curious: once I crack open the top end to go back in there, I can probe the injector connections directly but thereís really no way to simulate a heat soaked condition with it is there? Iíll be sure to check the connector and wiring.

I still may limp it into a shop to scan it while itís acting up. Iíd like to fully verify itís the injector(s) before I buy a whole new set...
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Old 07-08-2019, 10:53 PM
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I haven't done a whole bunch on this issue outside of some more resistance measuring and parts ordering. It appears as I only have one injector that seems to be acting up, I bought a single replacement injector to replace the primary on #8. ACCEL 150121 for anyone curious. It has a small tab on it that will have to be filed down to let the injector connector plug in but no biggie. I'll probably break everything down on in a few days. Injector will be another day or so before I get it anyway. I did decide to check the coils engine at room temp through the plug wires I ended up with the following:

coil 1/6 - 19.8
coil 2/3 - 19.5
coil 4/7 - 18.0
coil 5/8 - 17.5

I'll probably check them again after a drive once it acts up again. Unless a coil that has gone south or is going south is going to be that way cold or hot. I tried Jerry's 6th gear pull from 1K rpm before the engine got up to operating temp and had no issues.

Last edited by 1985 Corvette; 07-09-2019 at 08:35 AM.
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Old 07-12-2019, 11:07 PM
  #34  
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Default Last batch of silly questions

So I haven't done anything yet, I've been meaning to take it out for one more drive because I was curious about the primary injector on #8. I mentioned "no measurement returned" for that injector previously, that's because I forgot to up the readout on the voltmeter. I had it set to the lowest reading for ohms, so while the other aftermarket injectors were reading at 17.8 ohms hot, #8 primary's ohm reading was higher than 20.. So I know this is the funky one, but I'm morbidly curious about how much higher a resistance reading on that injector that I'll see before I break everything down. And also I'd like to pick some brains on this: This can rule out it most likely being a frayed wire? I had contemplated opening up the injector loom on that side and checking the wiring throughout.

Also, out of curiosity, I still have a set of ignition coils from a 1991 I helped an owner service because he said they were probably bad and making his car run rough. I'm thinking he just had a bad plug wire (one had tons of electrical tape on it) and he wanted everything ignition related replaced. I checked these old ones and got the following on 20K setting:

5/8 - 5.88
4/7- 5.95
3/2- 5.75
1/6- 5.49

I got a new set of Delco coils in the box that read in this area. So that tells me these factory coils collecting dust since 2006 that were removed for replacements are still good. OR when one or more of the coils became saturated with heat, they malfunctioned. So I've already measured my coils through the plug wires and they check out but I am curious: Can coils act up when they get hot? Haven't tried measuring the coils through the wires when the engine is hot.

Last but not least, does anyone have a technique for removing the push nut that secures vacuum actuator arm to the secondary lever arm? I'm just getting a handful of them tomorrow from the hardware store.

Last edited by 1985 Corvette; 07-12-2019 at 11:10 PM.
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Old 07-13-2019, 09:34 AM
  #35  
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Originally Posted by 1985 Corvette View Post

Last but not least, does anyone have a technique for removing the push nut that secures vacuum actuator arm to the secondary lever arm? I'm just getting a handful of them tomorrow from the hardware store.
I just pull and twist. Having extras at least if you rune one no big deal. You may want to do a search and see if someone makes a tool similar to what is used on fuel lines to push in from the rear and release the spring tension so you can pull them off.

http://patents.google.com/patent/US7231700B2/en

http://www.ebay.com/itm/push-in-Fitt...cAAOSwc0FUotNc

Last edited by FASTAZU; 07-13-2019 at 09:48 AM.
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Old 07-14-2019, 01:58 PM
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Originally Posted by FASTAZU View Post
I just pull and twist. Having extras at least if you rune one no big deal. You may want to do a search and see if someone makes a tool similar to what is used on fuel lines to push in from the rear and release the spring tension so you can pull them off.

http://patents.google.com/patent/US7231700B2/en

http://www.ebay.com/itm/push-in-Fitt...cAAOSwc0FUotNc
Thank you for the recommendation, Steve. Iím getting pressure cooked out in the garage today! Iím now kicking around just replacing all of the injectors. I took off all the bits before removing the plenum and I noticed once I had the air filter duct and housing out of the way, I got a whiff of gas smell. Operated the throttle manually and there was an apparent thick odor of it inside the plenum.

I donít recall how it all smelled when I took all this apart to replace some valley parts last time but Iím wondering if primary #8 is not only failing once hot, but also leaking. Also, what if thatís not the only injector leaking. I know it doesnít take much to get a strong gas smell, but Murphyís law and all that. I know thereís a test to prime the rails out of the injector housings to check for leaks. It would be my luck if I had a couple or more of these leaking, and I only picked up one Accel injector to replace primary #8.

And I kind of want this to be the last time I go back in here for a while so...it might be best to replace the entire set. Eyeballing some Bosh yellow tops that are flow matched and 3 year warrantied. Some guys have spoken out against these but Iíve found many posts from here and the registry of happy customers after installing them.

Once the yellows are installed, Iíll send the current set on the car now to John at FIC to look at. Then I have a back up set on hand for the future. Just thinking/rambling out loud.
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Old 07-14-2019, 02:32 PM
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Iím over in the panhandle if you ever get this way.

Iíve been following your thread and have to admit it seems frustrating. If it were me, I would trouble shoot all systems & sensors to understand what is going on.

The plenum pull(s) are a fairly easy task & it seems as if youíre going in again. Why not pull the fuel rails out of the injector housings and space them high so you can see the tips of the injectors. Once done, pressurize the system & see if you have a leak. In my cases, electrically failed injectors ohmed well below 10 & didnít necessarily leak. Also, make sure the injectors are seated & sealed appropriately in the rails.

Any fuel in the fuel pressure regulator vacuum line?

This isnít all I would investigate but thought Iíd answer WRT fuel/injectors.

Ted
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Old 07-14-2019, 03:22 PM
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At this point ( maybe you have already) bring it to a qualified mechanic and let him troubleshoot....
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Old 07-14-2019, 04:43 PM
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Ted, is right on with the injector leak test.
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Old 07-14-2019, 05:01 PM
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Originally Posted by Demps View Post
I’m over in the panhandle if you ever get this way.

I’ve been following your thread and have to admit it seems frustrating. If it were me, I would trouble shoot all systems & sensors to understand what is going on.

The plenum pull(s) are a fairly easy task & it seems as if you’re going in again. Why not pull the fuel rails out of the injector housings and space them high so you can see the tips of the injectors. Once done, pressurize the system & see if you have a leak. In my cases, electrically failed injectors ohmed well below 10 & didn’t necessarily leak. Also, make sure the injectors are seated & sealed appropriately in the rails.

Any fuel in the fuel pressure regulator vacuum line?

This isn’t all I would investigate but thought I’d answer WRT fuel/injectors.

Ted
Ted, I'd love to come check out the assortment of Zs you have. Maybe sometime in the Fall or Winter one weekend, the wife and I will road trip that way. I used to live in Mobile, Alabama and she has wanted to see it, so we could make a pit stop along the way. It's been a little frustrating but it's not my daily driver, that's why I've been so slow to work on it. That and accessing the fuel system sends a gasoline smell throughout the entire house from the garage if I get any spilled gas. There is no fuel in the vacuum fitting for the regulator. So far I've paper clipped the diagnostic terminal to get no codes on the digital readout. I've got to get back in the valley to replace a bad actuator anyway so it is what it is. The O2s are new, new fuel filter and Delco pumps (still gotta pressure check the system hot). Coils ohm checked through the wires check out. New Delco plugs. I figured if it were a sensor going bad there would be a code. Jerry's 6th gear pull at 1,000 rpm turned up nothing for a coil related issue.

My major clue is when up to operating temp, A12 pin on the ECM connector provides no ohm reading, no matter what setting I try. Once the engine has completely cooled off, it ohms out at about 17 no problem. All other injectors ohm out hot or cold together. That and my knocking flywheel and shaking car at idle. Marc didn't have full power mode active at all times on this chip, so at idle it's just running off the primary set and that's when the flywheel is talking to me. This is never a problem if the car is room temp and started, settles down into a normal idle. The passenger side bank is also same side with the failing actuator. So a barely functioning/crapped out primary injector when hot combined with a secondary airflow pathway that is not fully opened up is my diagnosis.

I'm not opposed to the leak test with the rails spaced up so I can observe if there is a leak. I may give it a try. My reservation for doing so is if I have more than one leaking injector, I have to wait on the shipment of however many more injectors are needed and I'd want it just done at that point and time. The Accels seem to be $50+ or so per injector. Amazon had an individual injector in stock for $14, brand new. Why it was that price, I do not know but they restocked it again at $50+ per injector, so not sure if price was a mistake but I lucked out, I suppose. For about $330, I can clean the slate with all 16 Bosch injectors.

This Accel injector also ohms out at 15 while the current set in the car are 17. Not sure if that would have any issues cooperating with the set but I have thought about it.



Originally Posted by zrc3john View Post
At this point ( maybe you have already) bring it to a qualified mechanic and let him troubleshoot....
If this last attempt doesn't cut it, there's a shop here in Jacksonville called Alchemy EFI. Bunch of guys on LS1tech can't say enough good things about their work on GM specific performance vehicles. Though they mainly work on newer LS/LT cars, the owner is very familiar with the older F and Y body cars from 80s/90s. They're about 25 miles from me, I may consider limping it there one weekend if I decide to tap out. Believe me, I've already considered bringing it to a shop but no one around outside of Alchemy EFI seems to be worth it. The few I called, including a few Chevy dealers gave me the, "We'll have to call you when our Vette guy is here so you can bring it in". Just to hook up a scanner and monitor the car apparently requires a "Vette guy" tech just because it's a Vette....so not the place(s) I want to bring it, if I have to.

This will be my last posting on the issue until I have it resolved. I've appreciated the input of all on this one.

And just so everyone knows: FASTAZU (Steve-the-man) reached out and offered to mail me a very expensive scan tool to hook up and monitor the vehicle. I declined this offer, while I appreciated the trust to send such a valuable tool, I did not want anything to happen to it coming to me, or with me shipping it back. Great group of guys we have with these Zs....I'll get there one day.

Last edited by 1985 Corvette; 07-14-2019 at 05:05 PM.
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