It be too damn hot in Greenville. Hit 101 F today. But my Vette doesn't care since it's still in the shop (until tomorrow).

The plot thickens.....
I'll follow the intent of everyone's advice thus far and report back.

 

I had the same problem with surging when hot on my ’94 TA. My datalogs using Datamaster showed no codes. So I changed the Opti-Spark, Ignition Coil and Module, plugs, wires, fuel filter and fuel pressure checks out under load. But I never fixed the problem.

The last thing I can think of in my case would be my ECM acts up when it gets hot. But then I wonder, if my ECM is going bad and acts up when it is hot, is it possible that DataLogs look fine with no codes and still have a bad ECM?

Sorry I can’t really contribute any suggestions, I am just in the same boat you are and I really hope you find your problem and you post it here.

 

See, another damn Texas 94 Corvette...I'm tellin ya, the Bowling Green guys said, "This one going to Texas?" "Ya, put the special idle problem on it".

 

That's the god's honest truth too.
Well, I just got the Vette out of the shop. Here's what was done for/to it (perspective thing):
a. New water pump
b. New front cam seal
c. New spark plugs
d. New plug wires (originals replaced)
e. Opti spark cap kit
f. New ignition module
g. Coolant
h. New radiator (original was caked with grass and debris)
i. New speed sensor (replacing old one that was supposed to be replaced for the dealer, and wasn't)
j. New cruise control servo
k. New cruise/turn signal column switch

The car runs like a raped ape. Then when it stops to idle it still spikes while the engine is running at no more than 218 F (sometimes lower) with AC on. The idle spiking doesn't cause the car to lurch forward, but spiking it continues to do at a much lower coolant temp.

Either I don't know how to use the cruise control, which seems to be very straight forward, or that function still doesn't work.

It is horribly hot out there right now. I'll wait until Friday afternoon or Saturday morning before I play with the scanner.

 

If it's only with a/c on and there's no cruise, sounds like a vacuum problem/leak - hole in the tank, broken check valve under the rail cover on the driver's side are things to look at. Also, if that source is next to the MAP, check that connection also.

 

AC on or off doesn't matter regarding the idle spiking. Cooling temps are very good right now. The tech told me that the cruise now worked, but that was before I took the car on I-30 and tried it out. The cruise didn't work; the car goes back tomorrow after work for the tech to check it out. I am NOT leaving the car in their hands though. The tech did suggest that there might be a bad ground. Ground 106, per the service manual book 2, section 8A, seems to be common with the ECM and cruise control, to name just two systems. Well, it'll be one day at a time, one problem at a time.

 

i recommend going to corvette corner in rowlett, tx. these guys really know what they're doing. ive taken mine in a few times and always been very happy with the service. 30 min down the road

 

I have certainly heard of this shop and it comes highly recommended by another Vette head I know at work.

 

Yesterday I took the Vette back to the shop so that the tech could chase down the cruise control malfunction. It turned out he had broken a brittle vacuum line during the course of his working on the car. Apparently this happened AFTER he checked out the cruise control system after making repairs to it. He found the broken line, showed it to me, and rerouted a new one in its place. The cruise control system works like a champ.
And with the vacuum line replaced along with a water pump and radiator allowing the car to run about 25 degrees cooler, I think the idle problem has been resolved. As I recall there is a school of thought on this thread that once my cooling problem was fixed then my idling problem would be solved as well. That may be the case. I drove it 11 miles to work this morning. This route takes me down a state highway with a posted speed limit of 60. After I pulled into my parking spot, I checked the temp, which was at 209, and let the engine idle for about a minute. The idle hung right in there with no burping. After work I'll take it out for another spin and see how the idle does then.

 

I hope it's a vacuum issue, at least then I'll know where to keep looking. I've thought I had this idle issue beaten several times, and yet it comes back, so don't count your chickens yet. In fact, after one change I made, it didn't show up for at least a week and I'm thinking woo hoo, finally got rid of it. I don't want to be too pessimistic, but I'll wait to see if it returns after another week of driving it. In the meantime, I think I'll start checking my cruise control (I don't think I've ever used it).

Good luck.

 

I'm not counting chickens yet. If the idle comes back or not, the car is running much better than it was. That is one chicken I can count. Plus the cruise control works. That's another chicken to count. If there is a lingering idle problem, I expect it to show up after I take it out on I-30 after work and drive about 15 miles to the Commerce exit. By the time I get back to Greenville and sit at my first stop light, the idle problem should be back if it is still there. At least now I'm down to one chicken left to count.

 

here's to hoping you kill the last chicken and then tell me how you did it.

 

The idle spike is still there although not nearly as pronounced as it once was. But there nonetheless. The last chicken lives on to die another day. This weekend I'll slap the scanner on that car and chase down that chicken.
Even with this one small problem, the car runs like a raped ape.

 

Bah... I'm originally from San Antonio, but I'm currently stationed in San Diego, Ca. With the exception of one day when it was cloudy, it's been 105 / 106 the last 6 days in a row at my house (Alpine, Ca. High desert/ mountian area), so I don't know what you guy's are complaining about. Oh yeah, the d@mn A/C in the house is broken too!

 

What type of scanner do you have? If it is a real time scanner you should be able to see what happens to the sensors when the idle spikes up as it does.

 

I have an Actron Super Autoscanner Model CP9145.

 

I seem to remember a C4 owner that solved this surging problem by replacing the fuel injectors. Apparently the problem is some injectors get gummed up and don't work well at low flows. When the engine is cold it runs "open loop" and is basically running rich. When It gets hot enough, the engine runs "closed loop" and uses the oxygen sensor to reduce the fuel rate, but because one or more of the injectors don't put out enough fuel, the engine starts to stall so the ECM increases fuel rate and the engine rpm increases. The ECM reduces fuel rate and the problem cycles.

 

Before I did the scan, I attempted to install a new TPS in case that might have been my idle problem. Upon installing it, the idle ran too high, around 1200 rpm or more. I put the old TPS back in and the idle decreased to about 1000 rpm. But now I have an SES light and Service ASR light with the old TPS installed. I assume that I buggered up the ASR's old readings when I put in the new TPS, since I didn't have the Tech 1 tool to teach the ASR the new "voltages".

Then I hooked up the scan tool. Here's a partial set of readings. If a parameter was at 0, I did not record it in most cases.

Codes were present during the Engine Off and Engine On scans. That code was 22, history and current. That code is TPS voltage too low.

Engine Off results:
IAC position: 101
Idle requested: 3188
Fuel Trim L/R: 128
Loop: Open
MAF (gr/s): 0
MAP (" HG) 29.65
IAT: 134
O2S left: 18
O2S right: 44
TPS (V): 0.04

Engine On results (at idle with AC off):
CYL ID: 47-87
IAC position: 54
Idle requested: 625
Knock sensor: 3
Loop: closed
MAF: 10
MAP: 9.16 to 9.49
O2S left: 71-852
O2S right: 71-844
Spark adv: 36
Throttle (%): 5
TPS (V): 0.0-0.02
Eng RPM: 1082-1100
TCC control: ON

Engine On (idle with AC on):
IAC position: 74
Idle requested: 675
ST Fuel trim L/R: approx 120-135
Knock Ret: 0 degrees
Knock sensor: 7
Loop: closed
MAF: 11-12
MAP: 9.81-11.01
IAT: 138 degrees
O2S left: 89-904
O2S right: 62-852
Spark adv: 37
TCC brake sw: on
TCC control: off
TCC solenoid: off
Throttle: 6%
TPS sensor (v): 0
Engine rpm: 1032-1112 at idle
TPS voltage remained at 0 with rpm increase

The old TPS was used in all of the above scans.

Before each scan run, I would erase all codes (22). As soon as I started the scan, the code returned. I ran NO scans BEFORE this TPS fisaco this morning.

Maybe this info will make sense to someone out there.