Rough starting idle

[magimerlin]

Ok y'all. Been trying to diag this but nothing come outta ot and everything tests good. See if any of y'all have an idea or test I may have missed. On cold start up it acts like it has a large cam, very loppy. It's all stock, internally by the way. She doesn't/won't idle up at cold start up either. She smells very rich as well. I manually idle her up to about 1500 and after a small amount of time she will smooth out then idle like a rock at about 750ish. If I try to take off before the idle smooths out she will sputter and die. Let the idle smooth out first and she is good to go.

1994 k1500 5.7 4L60E 4x4
--Shortie headers to stock style exhaust
--Tbi mods(all)
--Vortec fuel pump
--Adjustable regulator set at 13 pounds and does stay at pressure under load
--tbi unit rebuilt with new diaghram
--very recent tuneup (full with all filters)
--new- coolant temp(computer one), map sensor, tps,
--timing set at 2 degrees
--injectors have a good cone pattern, verified with my timing gun and did not see them drip after shut off.




Truck has never really idled up when cold since I've owned it.



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[magimerlin]

Also has new 3 wire O2 sensor installed.

So trying to diag and fix both the crappy rich rough cold start idle and also trying to find why she won't idle up by herself on cold start.

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[letitsnow]

Does that truck have an IAC?

 

[Schurkey]

1. "Smells rich" does not mean that it IS rich. Even LEAN misfire will put hydrocarbons in the exhaust--which "smells rich".

2. What does the data stream tell you about coolant temperature, O2 sensor voltage and crosscounts, commanded idle speed, and short- and long-term fuel trims?

 

[magimerlin]

Does that truck have an IAC?

Yup... sorry forgot to mention that is newer as well and I did test it and it ohms out great.

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[magimerlin]

1. "Smells rich" does not mean that it IS rich. Even LEAN misfire will put hydrocarbons in the exhaust--which "smells rich".

2. What does the data stream tell you about coolant temperature, O2 sensor voltage and crosscounts, commanded idle speed, and short- and long-term fuel trims?

Yes, that's why I mentioned it "smelled rich" as I'm not sure if rich or lean.

All the info looked normal last I checked, was not to long ago. This has been having these symptoms for a while now. Keep in mind my truck is obd1. I do have a snapon brick(mt2500 for those that don't know its knick name) that I'll try to remember to hook up on my next day off and record the info. It'll be sometime next week.

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[delta_p]

What does it do on cold start, when you pull a vacuum line somewhere after the throttle plate and try to control the idle with you thumb on the line?

 

[Stillruns]

Ok y'all. Been trying to diag this but nothing come outta ot and everything tests good. See if any of y'all have an idea or test I may have missed. On cold start up it acts like it has a large cam, very loppy. It's all stock, internally by the way. She doesn't/won't idle up at cold start up either. She smells very rich as well. I manually idle her up to about 1500 and after a small amount of time she will smooth out then idle like a rock at about 750ish. If I try to take off before the idle smooths out she will sputter and die. Let the idle smooth out first and she is good to go.

1994 k1500 5.7 4L60E 4x4
--Shortie headers to stock style exhaust
--Tbi mods(all)
--Vortec fuel pump
--Adjustable regulator set at 13 pounds and does stay at pressure under load
--tbi unit rebuilt with new diaghram
--very recent tuneup (full with all filters)
--new- coolant temp(computer one), map sensor, tps,
--timing set at 2 degrees
--injectors have a good cone pattern, verified with my timing gun and did not see them drip after shut off.




Truck has never really idled up when cold since I've owned it.



Sent from my SAMSUNG-SM-G891A using Tapatalk

My bone stock 94 k1500 recently started doing the exact same thing... your description sounds like you are talking about my truck!

I have eliminated any possible issues from plugs, wires, IAC, O2 sensor, distributor, reluctor, ICM, TPS, EGR valve, EGR solenoid and coolant temp sender, I have not been able to find any vacuum leaks... still not sure about MAP sensor, but...

I have found one clue... if I unplug the brown wire under the glove box, (for setting timing), the truck starts cold perfectly... no stumble, no big cam lope and no rich smell... it idles about 900 and is rock steady. After a couple of minutes of idle, I shut the truck off and reconnect the brown wire and restart... no issues and idles at 700 like normal warm idle.
I'm no expert but I'm leaning toward a possible ECM issue. Once I connect with my buddy and view fuel trims in real time and see what else the scanner says, I will hopefully know more.

 

[Steve A]

My bone stock 94 k1500 recently started doing the exact same thing... your description sounds like you are talking about my truck!

I have eliminated any possible issues from plugs, wires, IAC, O2 sensor, distributor, reluctor, ICM, TPS and coolant temp sender, I have not been able to find any vacuum leaks... still not sure about MAP sensor, but...

I have found one clue... if I unplug the brown wire under the glove box, (for setting timing), the truck starts cold perfectly... no stumble, no big cam sound and no rich smell... it idles about 900 and is rock steady. After a couple of minutes of idle, I shut the truck off and reconnect the brown wire and restart... no issues and idles at 700 like normal warm idle.
I'm no expert but I'm leaning toward a possible ECM issue. Once I connect with my buddy and view fuel trims in real time and see what else the scanner says, I will hopefully know more.

Might be the "Spark Retard" module pulling timing or a bad knock sensor too.

 

[magimerlin]

Ok y'all.. sorry took so long but here are a few numbers...lol.. only thing I see besides the idle jumping when it's running rough is the iat temp, it reads -40F but a 94 TBI does not have a iat unless I'm missing something in my thoughts here. Keep in mind both sets of numbers are in open loop. Ok here uah go.. rummbers in red on left are while it's running rough at start up and loping like is got a huge race cam. Numbers on the right are after the idle stabalizes and runs just fine.

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