Rough idle on cold start only

Asked by Twiztidnative Jan 14, 2021 at 02:36 PM about the 1993 GMC Sierra 1500 C1500 SLE Extended Cab LB

Question type: Maintenance & Repair

I've had this truck for 2 years. Once I had it
home I did a tune up on it plugs wires cap
rotor and oil change. After I did that I had a
rpm surge. After replacing multiple parts
multiple times this year I solved the rpm
bounce. New distributor came with a faulty
icm. Drove around for a year with faulty part.
Now my only problem is a rough idle at cold
start up only. If I hold rpm at 1500 and wait
for the idle change it will run smooth with out
my foot on the gas. Without my foot on the
peddle it sometimes wont even stay running.
I have replaced alot. Some more than once.
Most recent fuel pump/filter starter battery
full tune up cap and rotor included. Ignition
control module and idle air valve. I messed up
putting starter in hot wire didn't get hooked
up to holding connector on the frame and it
melted and arked causing alot of sensors to
go bad. I've triple checked for vacume leaks  
leaks last year had new intake gaskets put on
and got throttle body cleaned up. I've done
alot of chasing my own tail. Went as far as
removing egr with a plate and blocking all
vacume lines. Only code I'm getting is 34

4 Answers


I read your post, I can see your frustration. I'd be frustrated also. I'm almost afraid to talk to you because of your frustration. Don't want you to take it out on me, I'm just trying to help. This is for code 34. You can see map sensor wire colors. The gray wire is ref voltage, 5 volts, goes hot with key on. The purple wire is ground. The other wire is signal wire, the signal wire voltage varies with engine load, about 1 volt at idle, around 4.6 volts at w/o throttle. The map sensor is attached to engine vacuum, that how it determines load. If this is high mileage engine, the vacuum may be effected, so the sensor signal is effected. So with high mileage engine, the idle may never be exactly as you would like. Just my opinion. good luck. A lot of people here answering questions, smarter than me, maybe they'll jump in?

1 people found this helpful.

Thank you I did need that info. I was playing around today unplugging this and that. When I unplugged that tan wire that gets unplugged for setting the timing my problem went away. I did put an elcheapo distributor in last summer I'm wondering now if I should buy a Delco. Will I hurt anything by driving around town with this wire unplugged? It seems to drive ok with out it. My girlfriend will be driving it so I'm trying to make it as smooth as I can for her. information- ebook/dp/B08SKM2GBK/ref=sr_1_1? dchild=1&keywords=jesus+roberto&qid=1610667843&rnid=2941120011&s=digital-text&sr=1- 1

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