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[SOLVED] 1990 chevy silverado 5.7L will not idle

This is a discussion on [SOLVED] 1990 chevy silverado 5.7L will not idle within the Automotive Support forums, part of the Tech Support Forum category. I have a 1990 chevy silverado 5.7L that will not idle. It will start if you feather the gas but


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Old 10-09-2010, 03:15 PM   #1
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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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Old 10-09-2010, 06:36 PM   #2
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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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Old 10-10-2010, 09:41 AM   #3
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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.
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Old 10-10-2010, 07:39 PM   #4
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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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Old 10-11-2010, 02:20 PM   #5
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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?
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Old 10-11-2010, 08:27 PM   #6
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yup
if you cant see a noticeable movement its not working
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Old 10-12-2010, 06:46 AM   #7
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cmorphis

Kjms1 is right, if there is no movement the IAC is gone.
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Old 10-12-2010, 12:58 PM   #8
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Can a bad IAC valve be the cause of the whole running rough problem?
Thanks
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Old 10-12-2010, 02:58 PM   #9
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cmorphis


Yes it can and it can and allot worse.
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Old 10-14-2010, 05:10 AM   #10
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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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Old 10-14-2010, 06:04 AM   #11
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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.


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Old 10-14-2010, 10:01 PM   #12
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pull the spark plugs and look and see if their super clean or black ...
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Old 10-16-2010, 11:09 AM   #13
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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?
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Old 10-16-2010, 01:22 PM   #14
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cmorphis

The pump pressure is very low it has to be replaced.
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Old 10-17-2010, 02:07 PM   #15
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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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Old 10-17-2010, 02:48 PM   #16
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Quote:
Originally Posted by cmorphis View Post
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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Old 10-17-2010, 10:05 PM   #17
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Quote:
Originally Posted by cmorphis View Post
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 !!



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