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Cant solve rough Idle, 90 L98 Corvette

posted Mar 31, 2016 10:05:14 by Eugene Tuorto
I have a 90 L98 Corvette with a rough idle, I am not a mechanic by trade and do most my own work, IAC counts drop to 0 when warm, engine vacuum is at 16hg which is normal for the aftermarket cam that is installed. Things I have done so far is check fuel pressure, leak down test, injectors are about 6 months old, I have checked the ignition system and cant find anything wrong and no codes, The original motor had the same rough idle. I replaced most the sensors with the new build IAC valve, TPS, MAP, and 02 sensor with all new pig tails. The car itself has 190K on it and I believe it is a vacuum leak but cant find any. Thanks For any help.
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16 replies
Noah said Apr 01, 2016 01:48:47
IAC counts at 0 i think support the possibility of a vacuum leak.
Just out of curiosity, did you try checking for vacuum leaks after the engine had been warmed up? That's when you said the counts drop, so it seems that would be when the leak is greatest.
Maybe the old ScannerDanner spray it with water test would turn up some results.
Just squirt some water around the intake mating areas, injector O-rings, vacuum lines etc. and listen for the water to be sucked in and the idle to change.

I gotta ask, The rough idle isn't from the performance cam?
New distributor, cap, rotor, wires and plugs during the engine swap or everything from the old engine?
Massachusetts, USA
Eugene Tuorto said Apr 01, 2016 09:46:10
Its a mild cam,,cap, rotor, wires, p/u coil and module are new. The old stock motor (18hg vacuum) had the same idle problem. I also have a new throttle body, I tried the minimum idle procedure but was not able to set idle low anuff without motor stalling. I have idle with IAC valve hooked up at around 700rpm and it fluctuates up and down about 50 rpm (650 to 750 rpm) I still need to finalize the tune but I am not going to go any further until I solve the idle problem. Other than the idle the motor runs real strong. I have tried everything I could think of including the water trick trying to find a vacuum leak all the 25 year old vacuum lines have been replaced. I will play around with it some more this weekend and if I have no luck I will start looking for recommendations for a shop in my area that does diagnostic work.
[Last edited Apr 01, 2016 17:02:20]
Eugene Tuorto said Apr 01, 2016 17:25:52
Well I had time to work on it some more, I still can't find a vacuum leak but I was able to get IAC counts to hold at around 13 counts in neutral and 17 counts when in drive. RPM's was 675 to 725 in neutral and 575 to 625 in gear. In gear it feels like it wants to stall and is very rough.
Chris said Apr 02, 2016 01:00:56
GM's speed density systems aren't the greatest back then. The tune port injection had there fair share of problems. Honestly I think I'd start looking at the TPS and MAP sensors to see if they are holding steady voltage at idle. Those 2 sensors influence idle quality the most. I'd also look at the O2's at idle and fuel trims/ block learn and see where they are sitting.

Have you looked at the distributor? I've seen carbon tracking in these cause the spark to scatter. Also seen the shaft wear the bushings causing the rotor to slop around and mess with the pickup in the base of it.
[Last edited Apr 02, 2016 01:02:06]
Chris said Apr 02, 2016 01:11:07
Have you checked out the EGR valve? I slightly stuck open one will cause rough idle problems
Eugene Tuorto said Apr 02, 2016 11:56:57
I deleted the EGR via block off plates and disabled in the tune, tps is steady, MAP at 700 rpm is 1.74V/43.9hg and 1.89v/45.78, I used a distributor from a low mile vette that had no side to side play in the shaft and was shimmed to remove vertical play , cap and rotor is new but i will recheck. here's a link to a short data log I did at idle in and out of gear. thanks everyone for your imput. If you look at the data keep in mind that the tune has not been finalized. when the motor was stock it had the same idle problem.
[Last edited Apr 02, 2016 12:38:23]
PaulDanner said Apr 02, 2016 20:11:07
What is your block learn and integrator numbers at idle (engine hot and O2 hot of course)
I believe this is a single wire O2, so hold it at 2500 for a few minutes before giving us that data.
Also it wouldn't hurt to know the same numbers (BL and INT) at 2500
Eugene Tuorto said Apr 02, 2016 23:58:13
INT seems to be between 125 and 130 at 700 rpm, BLM is at 124 to 122 at 700 rpm, I will try and get some data at 2500 rpm tomorrow. it is single wire O2 unheated sensor obd1.
Eugene Tuorto said Apr 03, 2016 16:33:15
At 2500 INT is 124 to 128, BLM is steady at 138. O2 is fluctuating rapidly from about 120 to 800.
Eugene Tuorto said Apr 09, 2016 09:39:34
well I can't think of anything else to check I was wondering if my rough idle could be caused by a problem with the engine wiring harness and if so what to check. if no one has anymore suggestions then I would like any recommendations for a diagnostic tech in the Ashland,PA 17921 area. thank you
Eugene Tuorto said Apr 15, 2016 23:18:47
Well I brought the car in for new tire and alignment, I also asked the tech to look at my idle and he hooked a smoke machine and found no vacuum leaks, he also checked the ignition system and he said everything looks real good except the rough idle. when i had the car back home i reead the rpm off the number 1 spark plug via timing light and it did not fluctuate more than 20 rpm.
I am throwing this out there to see if anyone ever had a problem with an out of balance torq converter this is on of the few things that the old motor has in common with the new motor. thanks for any imput.
Noah said Apr 16, 2016 01:07:32
Did you change the engine mounts? I don't know man, hope you get it figured out.
Massachusetts, USA
AndyMacFadyen said Apr 16, 2016 12:16:43
By rough running do you mean the rpm is continuously rising and falling (what old school guys call "hunting") or random shaking.?

I dont know these engines but if it is hunting although you seem to have excluded vacuuum leaks I would look to air getting into the manifold perhaps by the throttle butterfly idle setting. My thinking is that IAC will have better smoother control if it is not completely closed at normal idle
[Last edited Apr 16, 2016 12:17:20]
"Rust never sleeps"
Chris said Apr 16, 2016 14:50:10
Torque converters are balanced at the factory. I have seen cases where changing the holes they bolt into helps a small vibration. Not so much an idle issue. Also seen a harmonic balancer for a 305 installed on s 350. <<<that one F'ed me hard. Out of options and finally checked #'s on it and low and behold. Can you give us another snapshot of the O2 and block learn. Hold it at 2500 for a few to warm up the O2 then let it idle and record the idle portion of it. That way we can see what idle numbers are with the O2 hot.
Eugene Tuorto said Apr 16, 2016 22:41:00
Engine mounts are new, I will try and make a video of the motor shake, to me O2 looks normal and the shake is the same weather its in closed loop or not. The crank is an eagle 3.75 inch stroke and is balanced to use a balencer for 400 sb witch it has and runs very smooth above idle. I did notice that it ran a bit smoother when I switched back to the original GM map sensor. I played around with that today I'm not sure if my tuner can adjust for that if I can get my hands on the adapter for changing the tune on the fly I might give it a try myself.
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