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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 06-23-2019, 10:30 AM
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1985 Corvette
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Default 1991 power gone and rough idle once up to operating temp

Hey guys,

I'm sure there have been plenty of these types of issues discussed before but wanted to run this by the group before I go digging in the archives. Also digging in the archives for these issues sometimes is like going through Google to self-diagnose a physical condition: it's always the worst thing returned....in this case I came across faulty ignition module. 1991 Z that is not throwing trouble codes. I just replaced the fuel pumps and fuel filter. The injectors were replaced by a previous owner and they are the stainless brand. The coils I could not identify but they are blue and have "made in USA" on each one, so not sure what brand they are. The plug wires were also replaced by a previous owner. I replaced the spark plugs with AC Delco brand sometime last year. I pulled these for a compression test and they all looked to be in good order.

The car starts up with no issues and idles at 725 - 750 rpm (digital read fluctuates between 29,30 and 31). If I drive the car a bit, and come to a stop at a redlight, the rpm readout goes to 850 - 875. No issues feel noticeable at this point other than this quirk on the readout. The real issue happens once the car is up to operating temperature. Today I took the car out for a drive and once the coolant was up to temp I wound it out to about 6500 rpm in 2nd. She was happy cruising along 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 readout fluctuate between 33 -35, but prior to this when it shows a higher reading earlier in the drive, there's no shaking and loping. 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. Also at this point, if I have it in say 2nd gear and let it run to about 4K rpm and then let off the gas, I can hear the 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. Has anyone experienced this similar situation? I don't think this is a vacuum leak because the funky idle and lope should be there at initial startup. I want to get some feedback on this from you guys because a similar thread I read about this had a few posters saying it sounded like the ignition module was going south....and that's fun.

EDIT: I have not had backfire issue since replacing the pumps but the that was a thing if taking the car up to 4K rpm and letting off the gas to coast down.

Last edited by 1985 Corvette; 06-23-2019 at 10:38 AM.
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Old 06-23-2019, 11:17 AM
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Your best bet at diagnostics is if you can find someone in the area with a live time scanner so you can see what reading are when the issues occur, could be as simple as a bad senor. You can find them used on e-bay and they are well worth the investment. I bought an OTC Genisys EVO 5.0 many years ago on e-bay.

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Old 06-23-2019, 12:19 PM
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Originally Posted by FASTAZU View Post
Your best bet at diagnostics is if you can find someone in the area with a live time scanner so you can see what reading are when the issues occur, could be as simple as a bad senor. You can find them used on e-bay and they are well worth the investment. I bought an OTC Genisys EVO 5.0 many years ago on e-bay.
Thanks for the information and call, Steve! So Iíll add for anyone else reading this thread that this recent condition became a thing AFTER I did a plenum pull to replace the breather box gasket and injector housing gaskets. I am aware of a leaky passenger side actuator that I will be replacing on the July 4th weekend.

Steve mentioned it could be a pin inside one of the plugs that plugs into the underside of the plenum that isnít fully seated, so Iíll be doing a thorough check of the plugs. Iíll also add when the loping and reduced power condition occurs I can see where the coolant temp has passed the middle mark on the gauge. Itís not a whole lot but itís noticeable. The voltage needle also bobbles around 12 - 15 volts while the car is at a stop and just idling.
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Old 06-23-2019, 02:34 PM
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When you engine gets warm, if you have original injectors, the heat is it is like causing one or more of the injectors to short, resulting in your rough idle. Another tell tale sign of bad injectors is if you can hear the dual mass flywheel rattling at idle. Very likely time to replace.

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Old 06-23-2019, 02:47 PM
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Originally Posted by ZR WON View Post
When you engine gets warm, if you have original injectors, the heat is it is like causing one or more of the injectors to short, resulting in your rough idle. Another tell tale sign of bad injectors is if you can hear the dual mass flywheel rattling at idle. Very likely time to replace.

Brian A.
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The injectors appear to be the Delphi stainless body set that you can find on John's FIC website. I'm owner number 5 and it was owner number 3 who replaced the original fuel injectors, along with the coils, plug wires and starter. As you said, I can hear the dual mass flywheel loudly clunking and rattling when it's in full lope and shaking the car.

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Old 06-23-2019, 03:17 PM
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The heat is telling you something. An electrical circuit is opening (or a weak connection is causing a separation due to expansion) when the engine gets to a certain temperature. The only way to rule out injectors is to measure resistance when the car is cold and again when it's hot and misfiring. There is a way to check the resistance without removing the plenum, I think it's in the owners manual but can't be 100% certain on that (and don't have it handy). If not in the owners manual, Marc Haibeck has the process on his site. You will need to get the car REALLY hot before the 'warm test' as it could cool to the point that it won't show low resistance during the testing process. Regarding new injectors not failing, I had a six month old set of John's injectors produce similar symptoms to what you are outlining. He replaced them (great customer service btw) with an even better set than I ordered (stainless) and knock on wood, no problems since.

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Old 06-23-2019, 03:59 PM
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Originally Posted by ZR WON View Post
The heat is telling you something. An electrical circuit is opening (or a weak connection is causing a separation due to expansion) when the engine gets to a certain temperature. The only way to rule out injectors is to measure resistance when the car is cold and again when it's hot and misfiring. There is a way to check the resistance without removing the plenum, I think it's in the owners manual but can't be 100% certain on that (and don't have it handy). If not in the owners manual, Marc Haibeck has the process on his site. You will need to get the car REALLY hot before the 'warm test' as it could cool to the point that it won't show low resistance during the testing process. Regarding new injectors not failing, I had a six month old set of John's injectors produce similar symptoms to what you are outlining. He replaced them (great customer service btw) with an even better set than I ordered (stainless) and knock on wood, no problems since.

Brian A.
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Thank you for the information, Brian. I will look over Marcís site tonight for the info. You arenít wrong about new parts not failing. Paul Workmanís ďDONíT ASSUME NEW PARTS ARE OKAYĒ posts repeats in my head while running through the mental checklist of what could be the issue.

And I hate to be that guy that starts throwing parts at it, but if Iím gonna be back in valley again, I might spring for a new set of coils and plug wires. I see Ecklers has got the coils and the plug wires. If someone has a better recommendation on where to source the parts, say at a higher quality assurance, let me know.
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Old 06-25-2019, 11:46 AM
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Is it throwing any codes? (Paperclip method) What’s the fuel pressure when hot?
H
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Old 06-25-2019, 06:09 PM
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Originally Posted by ccmano View Post
Is it throwing any codes? (Paperclip method) Whatís the fuel pressure when hot?
H
It may be waiting to go into closed loup to act up.
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Old 06-25-2019, 08:05 PM
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Agree with previous post....Do the paperclip test....I had a similar problem..First 15 mins the car was fine...After that, hesitation and power loss.....Swapped out sensor on passenger side under car near headers..
Problem solved..
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Old 06-25-2019, 10:11 PM
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Hey guys. I havenít messed with it too much. I double checked the codes but itís just flashing code 12. I read Marcís info on testing the injectors with plenum in place. Very straight forward but Iím having a moment with it: he mentions removing INJ 1 fuse and using the terminal contact closest to front of car from as an access point. The voltmeter gets grounded and then probing the pins in connector A will provide a readout of resistance of ohms.

But Iím not ďgroundingĒ to the fuse contact so I got a little lost on how probing the fuse contact came into play. Sorry if itís obvious itís been a long day. Iíve got a voltmeter but itís been a while, I guess time to get back into it.

On the off chance it is the injectors, does anyone have experience with these?

https://www.southbayfuelinjectors.co...fuel-injectors
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Old 06-25-2019, 10:24 PM
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On the off chance it is the injectors, does anyone have experience with these?

https://www.southbayfuelinjectors.co...fuel-injectors

I would not recommend trying to cut corners on quality of the parts you use. Why take a chance on having to do the job twice?
Don't really think your issue is with injectors from your description. Work out a plan to troubleshoot in an orderly manner by referring to your FSM. Don't throw parts at it hoping to get it fixed. The last part you decide to replace will be the culprit!!!
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Old 06-25-2019, 11:32 PM
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Originally Posted by fxkenny View Post
On the off chance it is the injectors, does anyone have experience with these?

https://www.southbayfuelinjectors.co...fuel-injectors

I would not recommend trying to cut corners on quality of the parts you use. Why take a chance on having to do the job twice?
Don't really think your issue is with injectors from your description. Work out a plan to troubleshoot in an orderly manner by referring to your FSM. Don't throw parts at it hoping to get it fixed. The last part you decide to replace will be the culprit!!!
I agree, probably not injectors.

Just posted this over on the registry... As for the Southbay injectors, My car was running these Bosch/Ford Orange top injectors when I got it. They have stainless bodies and ran perfectly fine. Did some research and they are rated at 19lbs by Ford but Ford runs lower fuel pressure. When running at our fuel pressures they are rated at 21lbs. I did change them out but only because I’d didn’t know how old they were on my 180k mile car and I got a killer deal on a set of FIC’. I see no issue with these injectors.
H

Last edited by ccmano; 06-25-2019 at 11:32 PM.
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Old 07-01-2019, 06:02 PM
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Hey guys, it's been a bit but I finally got out to do some probing with the multimeter. I work at the speed of slow on these issues, the turtle is my spirit animal. I'm not too sure I'm doing this right, as Marc's walkthrough states resistance should be 12.1 ohms +/- .1 at 70 degrees F temp. So I had my instrument set to 200 Omega, I probed the pins in connector A off the ECM with one of the probes touching the fuse contact closes to the front of the car from INJ 1 fuse cavity. Then I pulled the secondary relays and probed them in conjunction with their respective contacts on connector A. Here are my results on a nice toasty 95 degree day in the garage. The car has not been started in a few days:

Primary injector and connector ID with readout

#1 A1 - 17 ohms
#2 A7 - 17 ohms
#3 A18 - 16.9 ohms
#4 A13 - 17 ohms
#5 A3 - 16.7 ohms
#6 A8 - 16.7 ohms
#7 A2 - 16.8 ohms
#8 A12 - 16.9 ohms

Secondary Relays

#1 - A on relay 1 - 16.7 ohms
#2 - G on relay 2 - 16.8 ohms
#3 - G on relay 1 - 16.8 ohms
#4 - F on relay 1 - 16.9 ohms
#5 - B on relay 2 - 16.8 ohms
#6 - A on relay 2 - 16.8 ohms
#7 - F on relay 2 - 16.8 ohms
#8 - B on relay 1 - 16.8 ohms

I was going to take it out for a drive tomorrow to get it up to operating temp to have the condition begin and park it to probe all the connections again while hot but.....what did I do wrong on this because I'm way off from the 12.1 ohm resistance per Marc's guide. 200 is as low as it goes on ohms, and bumping up to 2K just moves the decimal place and I still get the same readouts above.

Last edited by 1985 Corvette; 07-01-2019 at 06:06 PM.
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Old 07-01-2019, 09:30 PM
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When OEM Injectors go bad the resistance drops. They go from 12 ohms down to 8 and lower. Somethings not right with your readings. Are you sure you have OEM injectors? Do you have a photo of when you had the plenum off that shows the injectors that you can post. I can identify from there.

I still don’t think the injectors are the problem from the way you described it. It almost sounds like an issue when it transfers into closed loop mode or when the secondary fuel pump shuts down after cold start.
H

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Old 07-01-2019, 11:50 PM
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Originally Posted by ccmano View Post
When OEM Injectors go bad the resistance drops. They go from 12 ohms down to 8 and lower. Somethings not right with your readings. Are you sure you have OEM injectors? Do you have a photo of when you had the plenum off that shows the injectors that you can post. I can identify from there.

I still don’t think the injectors are the problem from the way you described it. It almost sounds like an issue when it transfers into closed loop mode or when the secondary fuel pump shuts down after cold start.
H
These aren't the original injectors. 3rd owner replaced them (at least his for sale ad said so anyway) Have a look. They have a three hole discharge. I replaced the fuel pumps a few weeks ago with the EP381 pumps and a new fuel filter. I was hoping it was a fuel filter problem or pump problem, but on closer observation, it's definitely a thing after 10-15 minutes of driving and warming the car up. I'll keep checking the trouble shoot charts.




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Old 07-02-2019, 10:34 AM
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As you say, those are not original, they appear to be FIC’s. Resistance will be completely different. Considering how close all your readings are I don’t see a problem here. Again, highly unlikely this is an Injector Issue. I forgot, did you check for codes? I still want to know the fuel pressure when hot.
H
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Old 07-02-2019, 12:52 PM
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Originally Posted by ccmano View Post
As you say, those are not original, they appear to be FICís. Resistance will be completely different. Considering how close all your readings are I donít see a problem here. Again, highly unlikely this is an Injector Issue. I forgot, did you check for codes? I still want to know the fuel pressure when hot.
H
No codes. Iíll have to run it again later and check pressure after shutdown. I donít believe this is a fuel problem. No fuel in vacuum line to the regulator. On the injectors, I researched some and it appears Delphi injectors and others like them are high impedance, and can range from 12 - 17 ohms. All of mine fluctuated between 16.9 to 17. My mistake for taking Marcís OEM ohm reading as uniform for across all types of injectors.

I took it out today for a short drive and like clockwork the coolant needle got to about almost mid mark on gauge and the power cut in half. Pulled it into the driveway and shut off the car. It cooled a bit and when I started it back up the flywheel was talking to me while it loped. Rpm was 31 on the readout after it cooled a little and the usual lope and shaking body. Before that it was 35 on readout before shutting down.
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Old 07-02-2019, 01:03 PM
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Change the sensor on the passenger side....Get one long enough...Amazon has them....
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Old 07-02-2019, 01:08 PM
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Originally Posted by zrc3john View Post
Change the sensor on the passenger side....Get one long enough...Amazon has them....

Hey, John. I saw your post earlier but figured you were speaking about the O2 on that side? Iíve replaced the O2s last year.
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