Big Jim

Hello fellow Corvette enthusiusts.

This is my first post here as I am new to the forum. Interestingly, I have a topic on HobbyTalk.com (with the same software format) titled "Ask Big Jim About Motors" and I am an expert in that field. That field being electric motors used in radio controlled cars. I deal with a different kind of "motor" than the one I have a question about and that being the one in my '87 C4. But that's the difference in terminology. You can call an engine a motor but you can't call a motor an engine, or so I'm lead to believe.

Anyway, on to my problem that I hope someone here that is familiar with diagnosing TPI engines can help me with.
The engine has just been professionally rebuilt at a Corvette shop in Santa Ana, CA but I'm not that confident in the shop's work. It has had an idle problem since the day I took it home from there although it does run well at all other rpm ranges.

When I start it cold, it will idle at about 1100 rpm and after a few minutes, will settle down to a computer controlled 600-700 rpm. At that point it starts to surge. The idle will go up and then down. Then start acting like the idle is too low, dipping below 600 rpm but catches itself and readjusts to normal and then it will start all over again. If I shut it off and start it immediately again, it will idle at 1100 rpm for 15-20 seconds before coming down.

It has a Comp Cam in it with 112 degree lobe separation with a higher lift and duration than the stock cam but it was one recommended for my application by Competition Cam. I'm not sure if the cam really likes idling at 600 rpm. But if I adjust the idle up, then it really revs when I start it up.

The plugs are new Rapid Fire #5's gapped at .045" and the plug wires are new also. The ignition timing is about 2 degrees above stock but the idle problem doesn't change even if I put the timing at the recommended 6 degrees. Other than the Comp cam, K&N air filter, and freer exhaust, it's pretty much stock. The injectors are from a very low mileage '89 C4 with less than 40K miles on them.

Also, I have replaced most of the old plastic vacuum lines. At least the ones I can see and trace with no change in the idle. I have sprayed water all around and listened for vacuum leaks the best I could but I couldn't hear anything. Connecting the vacuum line to the EGR or not makes no difference and it shows no trouble codes.

So my questions are this:
1. What could be causing my irratic idle problems?
2. How can I adjust the hot idle speed without changing the rpm during start up?
3. What would be the next logical step for more power. Larger injectors, siamese intake runners, bigger MAF sensor, larger throttle body or?

Thanks for the help guys. I do this forum stuff all the time but I'm not usually on this end of the Q&A's.

BIG JIM

1991corvettte

I would check the IAC or the Mass Air Flow sensor. Tapp the mass air flow sensor with a screw driver and see what happens.

TEXAS-HOTROD

I think 91Corvette is looking in the right direction.
On the early G.M. systems, if a sensor is sending a screwey signal, the engine will not run/idle right. If a sensor is unplugged, the computer usually compensates by ignoring that signal and relying on info from other sensors.
Try this. Start the engine and let it run up on the fast idle. When the r.p.m.s start to come down, unplug the idle air control motor when the rpms fall to about 800+ and see if the idle is stable. If it still has problems idling, plug the i.a.c. back in and kill the motor.
Unplug the mass air flow sensor, start it and see what happens. If it is still wacko, do the same with the throttle sensor.

I have modded numerous IROCs and Camaros and one sensor that I always scan is the t.p.s. I found out that by setting the t.p.s. close to 1volt (maximum allowable voltage) in the closed position seems to help low and mid r.p.m. performance. On one car (a Chevy Corsica) the customer complained about crappy gas mileage (19 mpg) that it wasn't as good as it used to be. I lowered the t.p.s. to the spec. minimum and he gained something like 4mpg. Some other car I worked on had a hard time idling and for the hell of it, I twisted the t.p.s. lower and higher a small amount and surprisingly the idle would go from crappy to good. I found a happy-medium, scanned the readings, saw thay were within specs, and locked the sensor down.

As far as opening the butterflys to gain idle rpm, that usually has bad effects. If the computer wants a 600rpm idle, that's what it will try to do. The last IROC that I worked on had such a long duration cam, it had a really choppy idle, but it idled fine. The customer claimed that it was a stock computer system only w/a chip. I doubt that your cam has anything to do w/your probs. A big cam usually causes a rich idle.

Big runners and 1K c.f.m. throttles are to help w/high rpm, but you need good heads and and a good exhaust system to really benefit. You can get by w/an adjustable fuel pressure regulator to up the pressure and gain more fuel delivery. If everything is huge (runners,pipes,heads,stroker,etc...) then larger injectors would be needed.

If all else fails have the shop run a scan on the sensors and make sure all is working like it should. Pay attention to the m.a.f. and see what voltage the t.p.s. is at. Since thay did the work, they should at least check it out for you (at no cost-and be cool about it). I would, if it was me.
---TEX---

DKOV

I am having the exact same issue but it has nothing to do with hot cams or the like because I don't have them.

Tell me, does it get worse when the AC is on?

I've gone through just about everything on this car (1990 Vette Convert.) and the idle is still lopey and rough.

In 1990, there is no MAF so despite the fact that every bit of advice I got said to look there, and after 3 days of NOT finding one, I came to the conclusion that it MUST be something else.

From what I've seen and read, this idle issue is COMMON in Vettes but nobody seems to have the answer as to WHY it happens.

I've gone through the Distributor, timing, plugs, wires, coil, ECU, IAC, TPS (which isn't adjustable either on a 90), the MAP, the Temp Sensors, everything.

However...

What I have recently discovered is that the Air Conditioning "On" Circuit controls the idle even when the AC is off. According to GM, if this circuit is not communicating with the ECM it will cause poor and lopey idle that gets especially bad with the AC is on. Which describes my situation perfectly.

Here's the problem though... nobody can seen to tell me where this circuit is or where components of it are. There's got to be more than just the "on/off" button on the dash board. There's a relay for certain. A solinoid possibly. A Switch perhaps. I just don't have the wiring diagrams or a thurough enough book to find them.

Any help here would be appreciated.

Because, If I'm right... THIS is the answer to SO MANY people's problems with the c4 Idle Issues.

Thanks,

Scott "DKOV" Kovalik
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91zr1

i have the same problem with my 88 convertiable it starts and idls at about 1500rpm for about 5 min then slows to about 900 and bounces back and forth to 1500 and back down to 900 i had had a hunch it was the fuel injectors so i ran cleaner through and then replaced the injecters and the problem still remains. It never did that before i am starting to wonder if it is the chip i took it to a shop and had a performance chip put in it and i am not sure if thats what did it but i doubt it it only started happing after i had that done.i am used to the computer position in my 91 zr1 and i am unable to find the computer to check and see that the chip was actually put in if anyone can tell me where the computer modual is located it would be a huge help. thank you

asp eater

Justardnck

I am no expert but I was snooping around my 92 because I wanted to know why I had only 1 functional compartment behind the seats and when I removed the 7mm screw I opened the compartment I found what looked like the ECU and a pump of some sort... possibly the ABS pump.
Hope this helps

04Vetter

You can try removing the IAC and cleaning it with spray carb cleaner. This is quite often a idle problem fault. A cheap fix too!

miverson332

Sounds to me like your chip was programmed incorrectly. I would run some tests on it, or take it back to factory default and see if that fixes the problem At least that would give you a starting point.

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Michael Iverson
"Just A Fat Kid With Broken $h!t!"


"Another one Bites the Dust."