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Help! High Idle and stumble

ZAKS VETTE

Active member
Joined
Jun 24, 2020
Messages
29
Location
Raleigh,NC
Corvette
1992 C4 green drop top
Hello all Zak here,
I have been working on my 92 Lt1 for some time now. Just finished all new brake pads and rotors along with a new master cylinder and new brake fluid. Got new tires put on and cleaned up the rims. This Vette has been neglected for so long I have some list of things to go through before I can really enjoy it.

I took off the throttle body and cleaned it as best as i could and put on new TPS, IAC sensors. New gasket set and air filter.

Now I've turned to my high idle problem and looking through the new shop manual I bought it says to pull any codes, trouble is I don't have a check engine soon light. Manual has me checking things such as fuzes which are ok.
It says to check CKT 19 however following the manual I'm not so sure how to do this? Key off disconnect ABCD plugs on the ECM. Key on check CKT 19 for Voltage? 0 voltage?
Could also be a bulb out.
I'm also checking for vac leaks found one already at the cruse control.
Symptoms are: high idle, hesitation at start the bad stumble at 3000 RPM I can get through that but will stumble again.

Any thoughts from anyone?

Thanks for any suggestions...
 

Hib Halverson

Technical Writer for Internet & Print Media
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Jan 10, 2001
Messages
13,453
Location
CenCoast CA
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71 04 12 19
Turn the key on but don't start the engine.
Do you see the Check Engine Light come on during bulb check then stay on when the engine is not running?
 

ZAKS VETTE

Active member
Joined
Jun 24, 2020
Messages
29
Location
Raleigh,NC
Corvette
1992 C4 green drop top
Hunted some vac leaks idle went down to 1000 RPM at start up then right back to 2000.
Installed a serpentine belt and finished the plugs and wires on the drivers side. That's some job I don't want to do again...
 

compyelc4

Well-known member
Joined
May 1, 2002
Messages
600
Location
Dayton, Ohio
Corvette
'95 LT1 Coupe, Comp Yell; C6 Roadster, Vel Yell
Suggest you look in your manual to see how to pull codes and then read them. The C4 can run all kinds of codes without setting off a Check Engine Light.
1. What did you do for that BAT light?
2. Did you have that high idle before all that intake and idle circuit work?
3. Do you get the high idle on a hot start too?
 

ZAKS VETTE

Active member
Joined
Jun 24, 2020
Messages
29
Location
Raleigh,NC
Corvette
1992 C4 green drop top
Suggest you look in your manual to see how to pull codes and then read them. The C4 can run all kinds of codes without setting off a Check Engine Light.
1. What did you do for that BAT light?
2. Did you have that high idle before all that intake and idle circuit work?
3. Do you get the high idle on a hot start too?

1. i can't pull codes nos ses light?
2. I do have manual.
3. I have the bat light only with key on no start.
4. Yes high idle before the throttle work.
5. Yes high idle on hot start.

Thanks for your help...
 

compyelc4

Well-known member
Joined
May 1, 2002
Messages
600
Location
Dayton, Ohio
Corvette
'95 LT1 Coupe, Comp Yell; C6 Roadster, Vel Yell
1. i can't pull codes nos ses light?
2. I do have manual.
3. I have the bat light only with key on no start.
4. Yes high idle before the throttle work.
5. Yes high idle on hot start.

Thanks for your help...
Zak, thanks for the answers, but I don't understand where you say, "i can't pull codes nos ses light?" Does this mean you tried to pull codes and there are none, or do you mean you have no SES (check engine?) light so you think this means there are no codes set?
 
Last edited:

ZAKS VETTE

Active member
Joined
Jun 24, 2020
Messages
29
Location
Raleigh,NC
Corvette
1992 C4 green drop top
Zak, thanks for the answers, but I don't understand where you say, "i can't pull codes nos ses light?" Does this mean you tried to pull codes and there are none, or do you mean you have no SES (check engine?) light so you think this means there are no codes set?

I have no light on the ses so how can I pull codes? Key on I only get a bat light. when i jumped A and B I get nothing no light.
 

Hib Halverson

Technical Writer for Internet & Print Media
Joined
Jan 10, 2001
Messages
13,453
Location
CenCoast CA
Corvette
71 04 12 19
1. i can't pull codes nos ses light?
2. I do have manual.
3. I have the bat light only with key on no start.
4. Yes high idle before the throttle work.
5. Yes high idle on hot start.

Thanks for your help...

In the Factory Service Manual you should find some diagnostic info on what to do if there is no check engine light at bulb check or no check engine light and codes which are set.

Also, check the bulb in the DIC behind the "Check Engine". Could be the bulb is inoperable or has been removed.

Finally, you may want to consider purchasing a scan tester.
 

ZAKS VETTE

Active member
Joined
Jun 24, 2020
Messages
29
Location
Raleigh,NC
Corvette
1992 C4 green drop top
In the Factory Service Manual you should find some diagnostic info on what to do if there is no check engine light at bulb check or no check engine light and codes which are set.

Also, check the bulb in the DIC behind the "Check Engine". Could be the bulb is inoperable or has been removed.

Finally, you may want to consider purchasing a scan tester.

Ok i was able to get code C-41 no comm signal by jumping G and A so I have trouble with my ECM...

Also checked Vacuum was at 21 inches steady...
 

Hib Halverson

Technical Writer for Internet & Print Media
Joined
Jan 10, 2001
Messages
13,453
Location
CenCoast CA
Corvette
71 04 12 19
It's possible you have a bad ECM, but not likely. That code is a CCM fault which set if the CCM cannot communicate with the ECM. The first thing to check is wires and connections between the two controllers.

Also, I apologize for not mentioning CCM diagnostics (FSM Sec 8D-1) as a way to get ECM codes when the MIL isn't working. As I sold my '95 several years ago, I've gotten a bit rusty on the C4 diagnostic stuff.
 

compyelc4

Well-known member
Joined
May 1, 2002
Messages
600
Location
Dayton, Ohio
Corvette
'95 LT1 Coupe, Comp Yell; C6 Roadster, Vel Yell
Ok i was able to get code C-41 no comm signal by jumping G and A so I have trouble with my ECM...

Also checked Vacuum was at 21 inches steady...

NO, NOT NECESSARILY!!! I thought that code 41 sounded familiar. I just looked in my chassis manual for my '95, and notes I took when I was troubleshooting that code. In my book it says that DTC 41 will set when voltage on circuit 423 exceeds 4.6 volts and engine speed is less than 1500 RPM. Circuit 423 is the wire between your ignition control module and the ignition control tap on the PCM. I think it is white. If that circuit has higher than normal voltage it means that there is most likely an open circuit between the ignition control module and the coil. I can't find anything that points to a bad ECM yet. I believe your problem is that you have a partially or totally broken wire/connection between the ignition control module and the coil or between the ignition control module and the PCM (I doubt the latter). Can you locate your ignition control module and coil? In my 95 they are mounted on the pass. side of the car, below and in front of the pass. side valve cover. That is a very hot nasty area for those two important devices and it is quite probable that underhood heat, age, normal engine vibration, and just disconnecting and reconnecting wire bundles and connections in that area has contributed to a wire being partially broken. Also, your optispark directly affects signals going to the PCM and then to the ignition control module and coil. Do you follow? I'm saying it could also be related to a bum optispark.

Here's a description of your circuit: The distributor ign. system provides two timing inputs to the PCM, a high resolution signal and a low resolution signal (these are created by the Optispark). The PCM uses these two signals to determine individual ignition spark timing for each cylinder. Once the PCM calculates ignition timing, the timing signal will be sent to the ignition control module on the IC circuit. Each timing pulse received by the ignition control module will trigger the coil module to operate the ignition coil. Secondary ignition voltage ranges from about .5v to 4.5v. I hope you can find this circuit and trouble-shooting diagram in your manual. If you can, study the circuit, measure voltages, answer questions in the diagram, and this will get you much closer to finding out what is causing your problem. One final thing I would check, and yes, do this first: check all of your grounds running from your engine to the chassis. There are a few of them and it is necessary that they all are making good connections. Oh, I almost forgot, I found the ignition control module on my 95 and started making voltage measurements. I found out the DTC 41 was being set because I had a broken lead in the wire bundle going into the ignition control module, and yes I had measure more than 4.5v on that ignition control module at connection B. You should see that connection point in your shop manual. I had to peel off all the protective electrical tape to trace back each wire until I found the guilty wire. Sliced it and my engine came back to life and is now running like new. It's going to take some time and patience so good luck.

Finally, if you are having problems finding the DTC 41 circuit troubleshooting pages in your manual (don't know why not), let me know and I will scan the pages and get them to you.
 
Last edited:

compyelc4

Well-known member
Joined
May 1, 2002
Messages
600
Location
Dayton, Ohio
Corvette
'95 LT1 Coupe, Comp Yell; C6 Roadster, Vel Yell
Thanks. I'll go back and look when I can keep my eyes open.
 

ZAKS VETTE

Active member
Joined
Jun 24, 2020
Messages
29
Location
Raleigh,NC
Corvette
1992 C4 green drop top
Sorry went to the beach for a week just seeing your post now.. great stuff thank you I will follow this out till i find the wire..
 

ZAKS VETTE

Active member
Joined
Jun 24, 2020
Messages
29
Location
Raleigh,NC
Corvette
1992 C4 green drop top
Ted
thanks so much for your reply and all the info...

I have some Q's if you will..

1 what is a D.T.C? and where is it I can't find it the shop manual...
2 do you mean ECM or PCM if its PCM what's that? I can't find that either...

I have located CKT 423 it's under the ECM heading. the page reads ICD CADB B being 423 wht A12 EST am I correct?

I'm deep into the manual now trying to figure this out could be open CKT to the ECM still looking.
Funny car still runs and drives until 3000 RPM then stumbles.

Thanks again for your expert help...
 

ZAKS VETTE

Active member
Joined
Jun 24, 2020
Messages
29
Location
Raleigh,NC
Corvette
1992 C4 green drop top
Ok I did some home work and now know what DTC stands for also PCM stands now your post makes sense. I will look for the poss open in the wires from ICM to the ECM.
Thanks again...
 

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