Surging idle on cold start

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K-Man

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Hello everyone, my first post! '94 5.7 TBI with 145,000. Truck is original and last year developed a miss/bad idle in gear. I replaced the IAC, TPS and EGR, plugs, wires, cap, rotor, still had the problem. Checked the fuel pressure, good, 13 psi. No codes. Next, I noticed that the distributor was making noise, bad bearings. Replaced the distributor and problem solved, the truck was running great! All parts replaced were Napa/Echlin. Now the temperature is dropping, and on cold morning idle surges badly for about two minutes, then smoothes out and runs like normal. I have tested the coolant temp sensor and it checks out, and still no codes.

This seems to be a common problem that has been discussed on many forums, but I cannot find a forum with a resolution or an answer!
Any help would be greatly appreciated-

Kevin Showalter
 

K-Man

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Also, I did not install a new ignition coil. How is the timing/spark advance controlled on these trucks, past the computer?
 

PlayingWithTBI

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I did not install a new ignition coil. How is the timing/spark advance controlled on these trucks, past the computer?
After you set the initial timing to zero with the EST bypass wire disconnected, the pickup coil in the distributor sends DRPs (Distributor Reference Pulses) to the ICM. It converts the Sine Wave pulses into DC digital square wave and sends it to the ECM which, in turn, based on your SA tables and other conditions, it sends Spark Timing pulses back to the ICM which grounds out the Ignition Coil accordingly. If the EST Bypass wire is disconnected, the ICM simply converts the Pickup coil references and sends them directly to the Ignition Coil.

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Cheap ICMs have been known to fail almost immediately. A bad coil has been known to kill an ICM in no time at all. This may or may not effect your poor idle when cold :33:
 

1997

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Hello everyone, my first post! '94 5.7 TBI with 145,000. Truck is original and last year developed a miss/bad idle in gear. I replaced the IAC, TPS and EGR, plugs, wires, cap, rotor, still had the problem. Checked the fuel pressure, good, 13 psi. No codes. Next, I noticed that the distributor was making noise, bad bearings. Replaced the distributor and problem solved, the truck was running great! All parts replaced were Napa/Echlin. Now the temperature is dropping, and on cold morning idle surges badly for about two minutes, then smoothes out and runs like normal. I have tested the coolant temp sensor and it checks out, and still no codes.

This seems to be a common problem that has been discussed on many forums, but I cannot find a forum with a resolution or an answer!
Any help would be greatly appreciated-

Kevin Showalter
by the sounds of it my 95 has the same issue. It’s a strange one, starts, idles fine, give it some gas, coughs and sputters to about 2k rpm, sputter goes away. Runs fine if you let it idle for about 5 min before driving. no issues when weather is warm.

no help yet, but it’s on my to do list shortly.

check for codes, sometimes mine sets 45.
 

1997

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from my search on possible causes for code 45:

This is the description of a Code 45 as set on GM products It is an indication of a rich mixture as indicated by the oxygen sensor readings.

Circuit description: The ECM supplies a voltage of about .45 volts between the appropriate terminals of the sensor. (If measured with a ten megohm DVM, this may read as low as .32 volt). The O2 sensor varies the voltage within a range of about one volt, if the exhaust is rich, down thru about .10 volt, if exhaust is lean.

The sensor is like an open circuit and produces no voltage, when it is below about 360 degrees C, 600 degreeF. An open sensor circuit, or a cold sensor, causes Open Loop operation.

Test description:

1.) Code 45 is set when the O2 sensor signal voltage remains above .7 volts for 30 seconds or more and in "closed loop".

2.) Engine time after start is one minute or more.

3.) Throttle position between 2 % and 20 %.



Diagnostic aides:

1.) The code 45 for rich exhaust is most likely caused by one or more of the following:

a. fuel pressure - system will go rich, if pressure is too high. The ECM can compensate for some increase. However, if it gets too high, a code 45 will be set.

b. Leaking injector or rich injector - perform an injector balance test. Check for fuel contamination in the oil.
.

HEI shielding - an open ground ckt 453 may result in EMI, will result in induced electrical "noise". The ECM looks at this noise as reference pulses. The additional pulses result in a higher than actual engine speed signal. The ECM then delivers too much fuel, causing the system to go rich. The engine tachometer will, also, show higher than actual engine speed, which can help in diagnosing this problem.

d. Canister purge - check for fuel saturation. If full of fuel, check canister control and hoses.

e. MAP sensor - an output that causes the ECM to sense a higher than normal manifold pressure (low vacuum) can cause the system to go rich. Disconnecting the MAP sensor will allow the ECM to set a fixed value for the MAP sensor. Replace the sensor if the rich condition goes away when the sensor is disconnected.

f. TPS - an intermittent TPS output will cause the system to go rich, due to a false indication of the engine accelerating.

g. O2 sensor contamination - Inspect oxygen sensor for silicone contamination from coolant, lead contamination from fuel or improper RTV sealant. Silicone contamination will be indicated by a Code 45 and lean drivability conditions and a powdery white deposit on the sensor. The ECM will then reduce the amount of fuel delivered to the engine causing a severe surge drivability problem.

Check for a leaking fuel pressure regulator by checking the vacuum line to the regulator. If there is fuel in the vacuum line, replace the regulator.
 

PlayingWithTBI

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HEI shielding - an open ground ckt 453 may result in EMI, will result in induced electrical "noise". The ECM looks at this noise as reference pulses. The additional pulses result in a higher than actual engine speed signal. The ECM then delivers too much fuel, causing the system to go rich. The engine tachometer will, also, show higher than actual engine speed, which can help in diagnosing this problem.
I had this issue when my ICM and Ignition Coil went bad after only 2000+ miles. IDK if the coil took out the ICM or visa versa. I caught it when data logging and captured the high RPM while cruising. Changed them and my surging went away but, this was when the engine and ICM were warmed up.

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honkon

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I had this issue when my ICM and Ignition Coil went bad after only 2000+ miles. IDK if the coil took out the ICM or visa versa. I caught it when data logging and captured the high RPM while cruising. Changed them and my surging went away but, this was when the engine and ICM were warmed up.

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This is OBD1 data stream? What software did you use for this?
 

thinger2

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Hello everyone, my first post! '94 5.7 TBI with 145,000. Truck is original and last year developed a miss/bad idle in gear. I replaced the IAC, TPS and EGR, plugs, wires, cap, rotor, still had the problem. Checked the fuel pressure, good, 13 psi. No codes. Next, I noticed that the distributor was making noise, bad bearings. Replaced the distributor and problem solved, the truck was running great! All parts replaced were Napa/Echlin. Now the temperature is dropping, and on cold morning idle surges badly for about two minutes, then smoothes out and runs like normal. I have tested the coolant temp sensor and it checks out, and still no codes.

This seems to be a common problem that has been discussed on many forums, but I cannot find a forum with a resolution or an answer!
Any help would be greatly appreciated-

Kevin Showalter
My 94 k2500 5.7 has developed the same problem again.
I thought I had it cured after a tbi rebuild and new bushings.
No problem above a certain air temp.
The thing Ive noticed is that my fuel pump seems to have a pretty long run time before shutoff.
Like 20 seconds or so.
Key on for 3 seconds in the summer and fires right up.
I didnt even listen for pump shutoff.
In cold weather, that doesnt work.
So as an experment for the last week or so, I key on untill the pump shuts off even though it is way late and it starts normal.
But, I can smell raw gas on the passenger side.
But nothing is leaking.
For sure temperature related.
I think it is one of those "all of the above" problems.
A little bit of everything is making it not work.
Not just one thing
Ill dig into it deeper cuz it is just irratating and fouls plugs.
As soon as the ice water and 50 mile an hour winds stop.
Its been a ratty month in the PNW
 

1997

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I had this issue when my ICM and Ignition Coil went bad after only 2000+ miles. IDK if the coil took out the ICM or visa versa. I caught it when data logging and captured the high RPM while cruising. Changed them and my surging went away but, this was when the engine and ICM were warmed up.

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curious how this would point to ICM, Ignition coil issue?
thx for your help.
 

1997

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I changed my ECU about a year ago to repair my code 82 issue. That’s when it first started, but went away in the warmer weather, so kinda figure it might be an issue with the ECU, working on confirming.
 
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