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Idling surging and dropping once car is warm

Old 09-01-2015, 08:34 PM
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Idling surging and dropping once car is warm

Hello everyone, I'm having some strange symptoms with my idle. After it warms up it will randomly rise and drop and basically the ECM goes nuts trying to keep it running. When I ran the 9495 free scan, I kept seeing the engine closed loop and fuel trim enable coming off and on. Although they were all jumping while trying to keep it going.

It's a 94 Z28 with 102k
Auto
New (last year) TPS IAC MAP
Longtube headers
MSD vented opti conversion
New plugs and wires
New coil
ICM
CAI
electric cutout
Dyno tuned.
I just replace the Opti and water pump, but I'm convinced it's not related at this point. The temp sensor and T-stat were also replaced in this process. Could this be an O2 sensor??
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Old 09-01-2015, 10:07 PM
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Re: Idling surging and dropping once car is warm

If you post a link to the data log I'll take a look at it.
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Old 09-02-2015, 06:45 AM
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Re: Idling surging and dropping once car is warm

Originally Posted by Injuneer View Post
If you post a link to the data log I'll take a look at it.
Thank you that would be a big help. I will figure out how to post the link and have it up by 4pm eastern time today.
Edit: I came home for lunch on a gut feeling to check the grounds to my ICM and they are super easy to twist. Could that nut not being tighten down to the ground wires cause such anarchy?

Last edited by tdigger9899; 09-02-2015 at 11:36 AM. Reason: Adding information
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Old 09-02-2015, 02:53 PM
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Re: Idling surging and dropping once car is warm

Originally Posted by Injuneer View Post
If you post a link to the data log I'll take a look at it.
I warmed it up a few minutes ago and this time at full temp (while starting to surge and sputter) it threw a code 15 for the coolant temp sensor. The car surged with the old sensor, and now the new sensor also. This would indicate a wire issue correct? I wonder if I got antifreeze on it when removing the pump. I tested my old temp sensor and it read around 316 ohms at 179 degrees. So I thought it was fine but replaced it anyway since I'm not confident on my troubleshooting skills. Plus it was cheap enough to replace. Is there a way I can test the connector and wiring for this sensor? Is it normal for the car not to start after it's at full temp and stalls from the problem?
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Old 09-02-2015, 03:45 PM
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Re: Idling surging and dropping once car is warm

If the coolant temp sensor is showing the engine is cold, and the engine is hot, it's probably going to flood, because the PCM richens the A/F ratio for cold start.

Shoebox has a guide to testing the coolant temp sensor and circuit:

4th Gen LT1 F-body Tech Articles

Since you mentioned getting coolant on it, I assume you are testing the sensor in the water pump housing. You need to make sure you have 5V at the coolant temp sensor harness, key on, connector off the sensor.

The coolant temp affects the PCM going into closed loop/fuel trim enable. Temp has to reach a minimum of about 120*F before that can occur. When looking at your data log, see if the coolant temp is going south at the same time it drops out of closed loop.

If you can't find an FTP site to upload the Scan9495 file, you can e-mail it to me. I'll send you a PM with my e-mail address.
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Old 09-02-2015, 04:03 PM
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Re: Idling surging and dropping once car is warm

If you don't mind just sending your email, that would save a lot of frustration for me. I am far from computer savvy. thank you.
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Old 09-06-2015, 06:55 PM
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Re: Idling surging and dropping once car is warm

Update: replaced sensor in the pump and nothing has changed. Still shows a code 15 (high voltage low temperature)with the temp reading like -40 on the scan. The harness had 5v and no continuity between the two wires. I'm stumped! Any ideas? The sensor and harness are both testing fine.
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Old 09-06-2015, 08:18 PM
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Re: Idling surging and dropping once car is warm

Do you have access to another PCM? Might be the pull down resistor in there.

Did you at least think about the possible influence of the othe devices grounded by the same pin B6 on the PCM. Did you check the black wire and the yellow wire, measured to the engine block? Did you jiggle both the CLT sensor wires and the TPS wires while testing the CLT sensor wires?

Since this appears to be a complex electrical I'll see if I can get GaryDoug to take. Look.
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Old 09-06-2015, 09:53 PM
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Re: Idling surging and dropping once car is warm

Originally Posted by Injuneer View Post
Do you have access to another PCM? Might be the pull down resistor in there.

Did you at least think about the possible influence of the othe devices grounded by the same pin B6 on the PCM. Did you check the black wire and the yellow wire, measured to the engine block? Did you jiggle both the CLT sensor wires and the TPS wires while testing the CLT sensor wires?

Since this appears to be a complex electrical I'll see if I can get GaryDoug to take. Look.
In the meantime I will follow those wires back and try to find the issue. I do have another ECM, but it won't give me any codes or data. That is why I replaced it. If all else fails I could at least swap it out and see how it runs at full temp to determine if my current ECM is junk. I hope not I just got it tuned !
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Old 09-07-2015, 10:56 AM
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Re: Idling surging and dropping once car is warm

Originally Posted by Injuneer View Post
Do you have access to another PCM? Might be the pull down resistor in there.

Did you at least think about the possible influence of the othe devices grounded by the same pin B6 on the PCM. Did you check the black wire and the yellow wire, measured to the engine block? Did you jiggle both the CLT sensor wires and the TPS wires while testing the CLT sensor wires?

Since this appears to be a complex electrical I'll see if I can get GaryDoug to take. Look.
I received your email and thank you for the response.
I replaced the harness in the hopes that I wouldn't have to chase wires, but after a few heat cycles, the surge came back. This time however, it did not throw a code, nor did the scan go crazy. I sent you the log I am referring to. I shook the wires of the sensor, tps, and even iac to see if it would change, but I couldn't confirm any deferences when doing so. I will have to start chasing that ground next I guess.
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