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Ask GM Tech (Cam) Your Own Question
GM Tech (Cam)
GM Tech (Cam), Chevy Technician
Category: Chevy
Satisfied Customers: 15181
Experience:  GM Grand Master Technician 2007. 14 years experience.
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Need some help with idle issue 1994 s10 blazer cpi 4.3.

Customer Question

Need some help with idle issue 1994 s10 blazer cpi 4.3. Idles up and down.
Submitted: 12 months ago.
Category: Chevy
Customer: replied 12 months ago.
As I was adding. All gaskets from heads up are new. No missing now and it still searches for idle and dies. You can unplug the new iac sensor when the pintel is fully extended and it will run good, no up and down idle but woth it like that after it cools down it will start and stall
Expert:  Reggie Whiddon replied 12 months ago.

Hello you are saying that you have a dead skip or miss in emgine, and it want idle. Tell me about what tools you hAve do you hAve a scantool? Fuel pressure gauge, voltmeter? Does the skip go away with rpm or under a load? Do you know what cylinder? Do you have a VAcuum gauge

Customer: replied 12 months ago.
It no longer misses. Was a blown headgasket. All I'm going through night now is idle issue. It goes up to 1500 and down to 500 up and down and dies. Have checked codes on it using a jumper wire as it is obd1. No coeds have been stored. The only codes I have gotten are for the sensors I have unplugged to check for differences. All sensors were purchased new. The idle air control valve works. If you unplug it where the pintel is fully extended and the motor is warm it will idle perfect no issues. But if you plug it up a few seconds later it starts surging and dies. But no service light.
Expert:  Reggie Whiddon replied 12 months ago.

Try disconnecting both battery cables tie then together for few minutes then reattach and crank thr vehicle at no point do you touch the throttle. If you have to restart it's okay let it crank and run on own after a minute drop in drive foot on brake and have no accessories on then after idle evens out test drive

Customer: replied 12 months ago.
Okay I'll try that. I did try a relearn procedure for iac on it and it seemed to help but started back doing same. I'll try that and get back with you
Expert:  Reggie Whiddon replied 12 months ago.

Do it exactly like that it should take care of you

Customer: replied 12 months ago.
Okay I will thanks for the info. I'll get back with you as soon as I get to do it
Customer: replied 12 months ago.
Just tryed this and it did idle good for about a minute then idled down some then started going up and down up and down then died. Didn't touch anything started it back and it done the same thing. Up and down then died
Expert:  Reggie Whiddon replied 12 months ago.

I know it's aggravating but in extreme cases it has to be done several times. Try it a few more aND try driving it for a few miles after doing it. Let me know in the morning. Sleep well we get it tomorrow

Customer: replied 12 months ago.
Okay I will. Thanks and you too
Customer: replied 12 months ago.
I have tryed this process you told me to 3 different times. It starts up fine idles perfect for about 10 seconds then starts going up and down. Hasn't died yet but the idle is up and down sometimes it almost dies. When it idles down it sucks air through the intake like it's gonna die. Do I still need to continue this process
Customer: replied 12 months ago.
Update also. The more I kept doing it after it died it will start and then idle very low and die. Now it starts and dies
Expert:  Reggie Whiddon replied 12 months ago.

Okay I'm sorry it didn't work that is best idle learn procedure. Did you say you have replaced iac valve? And you replaced head gaskets, do we know the valves are adjusted properly? I would like to know the fuel trims in the data. Do you have a scan tool? VAcuum gauge?

Customer: replied 12 months ago.
I'm not aware that you can put a scan tool on obd1 yes new iac and the valves are adjusted proper. Motor will idle of you unplug iac valve at operating temperature
Expert:  Reggie Whiddon replied 12 months ago.

Yes sir they make scan tools for obd1 I have one. What about vacuum leak? And you could have a bad iac I've had that.

Customer: replied 12 months ago.
No vaccune leaks checked mutiple times. It's had 2 new iac valves. The odds of 2 new ones bad from different part stores are unlikely. I know it's getting signal to the iac probed the wires running to it. I know if you unplug it while the motor is running the service light does not kick on. Every once in awhile it will
Expert:  Reggie Whiddon replied 12 months ago.

Okay I agree with you. Sounds like you may have a bad Drop the ecm down its behind the glove box and check powers and grounds. I know yo I have checked for codes it would really help if we had a scan tool that way we could confirm no vacuum leak and also tps and other sensors aren't causing problem.

Expert:  Reggie Whiddon replied 12 months ago.

I have though about it and don't know a cheap way for u to get scanner. One thing u could do is unplug each sensor one at a time and see if the idle straight out. The reason I thought ecm is you said the relearn. Would work for min and then revert back. One thing we use to do and don't laugh was called the "tap test" drop ecm down and with it running take the ooen part of your hand and slap the ecm pretty good if it cuts off or changes running at all its bad

Customer: replied 11 months ago.
Sorry just getting back to you. You can. Unplug the iac and it will idle fine either high if engine is cool or low if it's at operating temp. I have another ecm to put in it just haven't gotten around to it. It is the same service number as it came out of one identical to my blazer just a different color.
Expert:  Reggie Whiddon replied 11 months ago.

Have you installed the ecm yet? I hope and feel like that will take care of your problem.

Customer: replied 10 months ago.
Sorry for the long delay. Been a rough few weeks. I did install the ecm woth no change. I did however find thag if I unhook the brake booster vacuum line I have no issues except for hard brakes? Any ideas on that. I replaced the check valve and booster. Connected the line and it does the same thing. Idles up and down then dies. If I unhook vac line to new booster it's perfectly fine
Expert:  Reggie Whiddon replied 10 months ago.

Good to hear from you, Sounds like you have found the problem but you say you replaced the booster and same thing. That is interesting. And you close the line off no issue so we know we are losing vacuum in that booster, let me ask you when you undone the master cylinder from the booster was there fluid there? Maybe master is leaking brake fluid in booster causing leak? another thing comes to mind perhaps brake switch or cruise switch are adjusted to tight putting slight pressure on booster causing it to operate when at idle? Make sure pedal is pulled all the way back up, good job on finding that by the way