# [SOLVED] 1990 chevy silverado 5.7L will not idle



## cmorphis (Oct 9, 2010)

I have a 1990 chevy silverado 5.7L that will not idle. It will start if you feather the gas but will surge and die. If you try to give it more gas it will not take it. But if you unplug MAP sensor it will run better. Replaced MAP sensor but that did not fix it. Can not find any vac. leaks. Any ideas? Thanks


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## octaneman (May 13, 2009)

*Re: 1990 chevy silverado 5.7L will not idle*

Hi cmorphis

Remove the IAC solenoid and test it for dead spots ,the plunger is most likely stuck on one position. When your done with the IAC hook up a fuel pressure tester and test the fuel pump, the readings should be somewhere between 12-15 psi.



post back your findings. 







post back your findings.


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## cmorphis (Oct 9, 2010)

*Re: 1990 chevy silverado 5.7L will not idle*

I checked and cleaned the IAC valve to no avail. Don't have way to test fuel pressure but it seems to be injecting enough fuel just by looking. It did idle better after cleaning valve but would not accelerate.
Thanks


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## octaneman (May 13, 2009)

*Re: 1990 chevy silverado 5.7L will not idle*

You need to do an ohm test and then hook up a 9V battery to check the IAC, the plunger has to expand and retract as you reverse polarity. By visual inspection you only see carbon build up but and clean the idle port hole thats about it, you need to find out if the IAC motor works properly.


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## cmorphis (Oct 9, 2010)

*Re: 1990 chevy silverado 5.7L will not idle*

I ohmed the IAC out . It showed to be good(52 ohms on each side. I hooked a 9 volt batt. to the IAC and then rev. polarity, I could feel the plunger move slightly but you can't see it move. The plunger is sticking out 5/8" from threads. Does no movement in plunger mean IAC is bad?
Thanks


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## kjms1 (Jun 4, 2010)

*Re: 1990 chevy silverado 5.7L will not idle*

yup
if you cant see a noticeable movement its not working


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## octaneman (May 13, 2009)

*Re: 1990 chevy silverado 5.7L will not idle*

cmorphis

Kjms1 is right, if there is no movement the IAC is gone.


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## cmorphis (Oct 9, 2010)

*Re: 1990 chevy silverado 5.7L will not idle*

Can a bad IAC valve be the cause of the whole running rough problem?
Thanks


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## octaneman (May 13, 2009)

*Re: 1990 chevy silverado 5.7L will not idle*

cmorphis


Yes it can and it can and allot worse.


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## cmorphis (Oct 9, 2010)

*Re: 1990 chevy silverado 5.7L will not idle*

Replaced the IAC still have the same problem. I don't know what else to check. Thanks for all your help!


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## octaneman (May 13, 2009)

*Re: 1990 chevy silverado 5.7L will not idle*

cmorphis 


I've seen this problem before, when you disconnect the MAP sensor it runs OK and when you put it back on the engine stalls. On 5.7's the fuel pump gives out along with the IAC. You will have to check the fuel pump with a fuel pressure tester. Please go to your nearest auto parts supplier and rent out the tester if you don't have one. Hook up the gage on a T-fitting and take the readings it should be between 12-15 psi, and i'll bet dollars to donuts your pump readings will be 3-9 psi. 


post back your findings.


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## kjms1 (Jun 4, 2010)

*Re: 1990 chevy silverado 5.7L will not idle*

pull the spark plugs and look and see if their super clean or black ...


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## cmorphis (Oct 9, 2010)

*Re: 1990 chevy silverado 5.7L will not idle*

Checked fuel pressure and it was around 5 psi. Also checked voltage at fuel pump connection and was 8.5 v. Should it not be 12 volts going to fuel pump?
Thanks


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## octaneman (May 13, 2009)

*Re: 1990 chevy silverado 5.7L will not idle*

cmorphis

The pump pressure is very low it has to be replaced.


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## cmorphis (Oct 9, 2010)

*Re: 1990 chevy silverado 5.7L will not idle*

You were right. Replaced new fuel pump and it runs fine. Thanks so much for all your help. If you are ever in Winston-Salem,NC I owe you lunch!


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## SABL (Jul 4, 2008)

*Re: 1990 chevy silverado 5.7L will not idle*



cmorphis said:


> You were right. Replaced new fuel pump and it runs fine. Thanks so much for all your help. If you are ever in Winston-Salem,NC I owe you lunch!




Thanks for the return visit with the outcome....this one is now marked solved. Stop back any time and view our other areas.


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## octaneman (May 13, 2009)

*Re: 1990 chevy silverado 5.7L will not idle*



cmorphis said:


> You were right. Replaced new fuel pump and it runs fine. Thanks so much for all your help. If you are ever in Winston-Salem,NC I owe you lunch!


 


Awesome cmorphis !!


:4-cheers:


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