My 1994 Corvette (LT1/A4) runs great when cold, but once the engine reaches approximately 200-210 degrees F (oil temp), develops a rough idle and runs very rough - sputtering and backfiring (through the intake) if I give it too much throttle.

Mods include:

Throttle body coolant bypass
160* thermostat
Crane valve springs
Crane 1.6:1 roller rockers
LT4 knock module
Open air lid
Power couplers
High flow MAF housing (stock MAF sensor)
Air foil
Ford Racing matched 24# fuel injectors (“blue tops”)

Recent tune up included the replacement of the following:

Distributor cap/rotor
Spark plugs
Spark plug wires
Fuel filter
Air filter
PCV valve
(2) Front o2 sensors (did not replace rear)
Coolant temperature sensor
Ignition control module

When I pull codes, using the paper clip method, I get the following:

1. C12
4. H77
9. H72

As I mentioned, the car runs great when cold or below 200* oil temp. Any warmer and it’s like a switch going off - the car runs awful.

Any ideas would be greatly appreciated.

Thanks,
Greg

 

H72 is a big red flag to me... that's a potential problem with the Serial Data Link.

 

The codes don't make to much sence. I would clear them and then recheck. If you did not just change the cap and rotor, I would say that it sounds like opti problems. Start with a fuel pressure check and see if thats correct. The C12 code just indicates the computer is working normal, like a selftest.

On a 94, module 1 on CCM code 41 is serial data loss,
You have to know what code is from what module 1, 4 or 9.
There is a code 72 for mod 1 (CCM) and 4 (PCM)
72 can be LED dimming output or vehical speed sensor loss.

Am I wrong? It's not my opinion, it's out of the GM book for 94.

 

I reset all codes on Saturday and they've returned. I agree, I need to get a fuel pressure reading. When the car starts acting up, I suspect that it is running lean. I will get a fuel pressure guage and test it in the next day or two. BTW, the car has 83K miles on it if that's of any significance. Thanks

 

...from a new C4 owner. I'm not well versed in electronic cars but does your car smell lean? I had missing trouble on my Z and it smelled lean as heck & some plugs confirmed it was lean in some cylinders....turned out that I had some bad injectors...their coils were shorting.

The DTC's were not much help but when I scaned the car and ran it long enough for it to get to "closed loop" the scan showed the lean condition via the O2 sensors' feed back info to the ECM. I'm not saying that you should not do the fuel pump psi diagnostic but if your fuel pump checks out okay I'm just saying maybe check the O2's with a scan tool. Mine would run okay when it was cold but as it warmed up the worse the miss became. It would get so bad that the D/M flywheel was knocking around from the miss and if I stood behind the car I could smell it being lean.

I don't know about the 94 manual but my 90 Helm had a page which gave all the numbers of a scan of a properly running motor...using that & comparing my scan numbers to it showed the lean fuel situation....don't get me wrong. I had to bring the data to a tech I know & he explained what my data was saying & what it was pointing to.:o

Good luck
Tom

 

How u reset codes? I reset codes by pulling neg battery terminal for about 2-3 min's and then reconnect it, I had 2 occasions sofar but mine has serious too many chitload of codes and car wont start so i did didconnect neg battery terminal for 2 min and reconnect it and than iam good to go. Good luck finding the problem.

If those codes are real than stay with bogus.

 

To be honest, I don't know what a lean condition smells like. I do know that it isn't running rich, as there is no black smoke or unburned fuel smell.

I suspected a faulty o2 sensor as the most likely culprit. Since the car has 83K miles, I just went ahead and replaced both front sensors (figured it would need them soon enough anyway). At the time, I didn't realize there was another o2 sensor "down stream". I haven't replaced (or tested) that one, but from what I've read, that one doesn't effect fuel mixture (please correct me if I'm wrong). The car runs no better with the new sensors in front.

sami,
Here's how you reset the trouble codes (I copy/pasted from another post on this forum):

1. With ignition "off" ground terminal #12 of the DLC (use bent paper clip to connect pin 4 to pin 12)

2. Turn ignition "on".

3. Press trip/odo button on the DIC until 1.7 appears in the trip
monitor area of the instrument cluster.

4. Press eng/met button on the DIC & hold until "---" appears in the
speedometer area of the cluster, this will clear CCM DTC'S.


To clear PCM/ECM codes do the following:


1. Press the trip reset button on the DIC until the desired system is displayed. In this case it will be 4.0.

2. Press trip/odo button on the DIC until the desired diagnostic mode(4.7) is displayed on the trip monitor area of the cluster.

3. Press & hold the eng/met button on the DIC until "---" is
displayed in the speedometer area of the cluster, this will clear PCM codes.


To clear ABS/ASR codes do the following:

Same as above but you’re looking for “9.7” on the trip odo

 

I think I may have made a breakthrough, finally! I did two things tonight:

1. Purchase a fuel pressure gauge ($36.99 @ AutoZone).
2. Purchase an ignition coil ($19.76 I couldn’t resist, besides, I’ve thrown much more expensive parts at it up to now).

I duct taped the fuel pressure gauge to the windshield, grabbed a pen & paper (& a beer) - diagnostics time. Here’s the log of events:

9:52 key in ignition, turn key, don’t start engine - hear fuel pump make noise, fuel pressure (FP) = 44 psi
- after 2 seconds, no more fuel pump noise - pressure drops to 40 psi
9:53 turn key off/on - FP jumps to 44 psi, again drops to 40 psi (and holds)
9:54 start car - FP = 36 psi @ idle
9:58 oil temp: 84* / coolant temp: 132*
10:00 oil: 100*/ coolant: 154*
- blip the throttle results in engine stumble, heard a ping (spark knock) - thanks LT4 knock module
- FP gauge ranges from 44 psi to 36 psi when on/off the throttle
10:05 oil: 132*/ coolant: 176*
- put car in Drive (D) w\ foot on break (160* thermostat, takes forever to warm up)
- fuel pressure doesn’t change when in D
10:10 oil: 159*/ coolant: 200* - no change in FP
10:12 oil: 168*/ coolant: 208*, put car in P
10:15 oil: 186*/ coolant: 222*, FP still 36 psi @ idle
- goose the throttle and FP jumps to 44 psi
- running much smoother but still has a hesitation (should fuel pressure at idle be higher?)
10:18 oil: 204*/ coolant: 236* - electric fan turns on
10:19 put car back in D w\ foot on break in attempt to heat car to temp range where it has been running rough
10:20 oil: 208*/ coolant: 225* - fan turns off
10:22 oil: 213*/ coolant: 236* (fan turns back on)
- this is the point where the car has been developing a rough idle, car is running great (besides the hesitation)
- quickly stomping on gas peddle, car hesitates for a split second, then revs up without a hitch
- again, FP shoots up to 44 psi and settles at 36 psi (at idle)
10:24 oil:219*/ coolant: 227* - put car in gear, foot on brake, bring RPM’s up to 3,000
- FP increases with throttle, settling @ 42 psi (@ 3,000 RPM)
- let off gas, FP drops to 36 psi (very consistent)
10:29 oil: 226*/ coolant: 225* - no change, engine runs good abet with a significant hesitation
10:31 car not getting any hotter, never did develop the rough idle or backfire
- I conclude that the new ignition coil fixed the erratic idle at higher temperatures

Still, is my fuel pressure too low at idle? Is that where the hesitation is coming from?

Please share your thoughts!

Greg

 

Let me be the first again to go out on a limb and say the dirty word, OPTI. I had real similar symptoms and changed the coil also. Ran good for a few days and then back to the same old stuff. Granted lots of other things can be wrong like the ICM which has a bad rep also. I hope I’m wrong on this one, just keep the thought in mind.

Your fuel pressure readings I think are in the ball park and similar to mine. Remember these gauges are not $300 gauges. They work OK for what we need to know, I'm sure there are differences from gauge to gauge. If you were in the 20s or so that would be an issue.

 

I took the car for a long ride today and it ran like crap. Now it cuts out severly at high RPM - felt like it didn't even want to shift from 1st to 2nd, just hovered at 5,500 RPM (missing) until I let off the gas.

I read somewhere that to properly check the fuel pressure, you need to disconnect the vacuum line that goes to the fuel pressure regulator. The article said that the pressure at idle (w\ stock settings) should be between 43-45 psi. Doing the check this way, I found that my car idled at 46 psi (according to my gauge). So now I'm confident that the fuel pressure is not the issue.

I'm afraid that I am also suspecting the opti at this point. I wish that I would have changed the whole thing out in the first place, instead of just the cap & rotor. Now I will have to tear everything back apart. Why oh why doesn't this car give me a trouble code?

I'm disgusted with this car beyond belief. I'm going to Chicago for a long weekend and this POS can sit in the garage for the next week or two. I don't even want to look at it for a while.

Thanks for all advice.

Greg

 

Don't rule out the possability of clogged cats, which tend to increase backpressure as they get hotter. Doe's your engine seem to bog down when you get on the throttle after the engine is at operating temp???? This is a tell tale symptom of clogged cat(s)......

just a thought......

 

With what you changed it sounds like the opti to me as well as I changed everything you did as well except the cap. have a 94 and it was doing the same thing. (Not throwing any codes) Changed my opti and it runs like a bear now. Just changing the cap and rotor is not enough to fix in most cases. When I pulled my opti the cap was fine. The module inside has a tendency to go bad as well. Just my 2 cents

 

Yea, what does a lean condition smell like? I've never had an opti go bad so I can't comment on that.

 

are you an air traffic controller by chance?

 

LOL, sorry, I guess I should have edited that a bit before posting.

Anyway, just got back from Chicago. I'm in better spirits now and ready to tackle this sick Corvette issue (again). It would be nice to get this thing to the point where I can actually enjoy it before the summer has past by!

I ordered a remanufactured opti and will be picking it up tomorrow (Monday). Will post the results once installed. On the bright side of things, my radiator should be well flushed by the time this project is done.

Later,
Greg

 

well... a bad FPR will cause pressures to go UP not down. This makes me think the pump or fuel filter are at fault.

I don't see Opti codes here, and a bad cap/rotor won't go this way.

As we can see, the great 160* stat really keeps the engine cool...

 

I installed the new opti-spark distributor tonight. I'm getting good at working on this thing - took me just over 3 hours. I was happy to see that the old one was definately in poor condition. It had significant rust on the internals. The metal cover plate was almost completely coated in surface rust. Looking through the receipts that came with the car, I noticed that the water pump was replaced by a dealership a few years back. I assume that's where the moisture came from.

Convinced that I had finally found the problem, I (re)installed the Hpertech programming and took the car for a ride. The car ran great up to 5,000 RPM and then cut out, almost like hitting a rev-limiter. I took the car home, uninstalled the Hypertech programming and the car runs great now. It no longer cuts out at 5K RPM. I went for an hour long drive and the car had no problems. I am confident that the distributor was the problem all along.

I don't understand why it doesn't run better with the Hypertech. With the programming installed, I can feel a definate power increase at the lower RPM's - the car will light up the tires from a stop, where as without the power programming, it barely breakes the tires loose. When the power cuts out, there is no pinging. I wonder if the redline somehow got screwed up in the Hypertech programming. I plan on installing it again an playing with it some more when I get a chance.

Thanks for the replies,
Greg

 

Greg.. just an observation, but there should be a 160* thermostat with the hypertech to help it fight detonation.. whether it be audible or not.. the silent type is the destructive one usually. your temps looked severely high.. my recommendation would be to pull the shrouding and the cooling radiator and clean ebtween them and try and ungunk the radiator with some simple green and a water hose... flush the cooling system and refill with a 30/70% coolant/distilled water.. on all my vettes, the temps never exceeds 180* even on 99* days and stop and go traffic.. that high of a temop is killing power and likely also pulling timing from knock your not hearing.. I would bet if you get those temps down you would notice the hypertech programming to be more effective. you will be suprised at what you find between the AC radiator and the cooling radiator especially if you have never cleaned it! C4's are notorious for collecting road debris, bags, birds nests and cigareete wrappers in there!

Chris

 

Don't feel bad about the hyperpuke,All the engine programming does that I have found was it changes the tps to read higher voltage sooner when the gas pedel is pushed.
Example tps reads .50 at idle and roughly .460 at full throttle.The hyperpuke will make the tps read .460 at 3/4 throttle instead of at real full throttle.
Mine with the 160 stat runs between 180 and 195 ,unless I have the A/C on .With a/c about 210 sometimes higher.
You have to remove the limiter or raise the top speed to get it to perform like you want.
Their is more but you will figure it out

 

lcvette, I have a 160* thermostat installed. The high temperatures shown in the log I posted above was without the hypertech programming. With the hypertech installed, the car runs much cooler. The coolant temps stay between 175 & 200 degrees.

Today, I installed the hypertech programming again and set the redline at 6200 RPM. The first time that I floored it, the car pulled good all the way through 1st gear and shifted nicely into 2nd. Problem solved, or so I thought. Once the car warmed up a bit more (I got stuck in a small traffic jam - coolent temp climbed to 198*), the high RPM miss came back. Just before the 1-2 shift, the car fumbles for a second or so before shifting into 2nd.

I think that I might contact Hypertech and see what they say.

Greg