1988 GMC Sierra 1500 5.7 V8 High Idle,EGR not hooked up

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buddha131
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Joined: Sun Nov 22, 2015 11:14 am

1988 GMC Sierra 1500 5.7 V8 High Idle,EGR not hooked up

Unread post by buddha131 » Sun Nov 22, 2015 11:28 am

My 88 gmc runs good but has high idle, While a New EGR has screw in it. When I hooked Egr up, truck runs, Idle does come down, but when gas is applied the motor wants to stall or miss fire while driving.
Can my timing be off, Could use help finding this problem Please.

 

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carriedi
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Joined: Wed Apr 10, 2013 12:42 pm
Location: Oregon

Re: 1988 GMC Sierra 1500 5.7 V8 High Idle,EGR not hooked up

Unread post by carriedi » Sun Nov 22, 2015 5:58 pm

how high is your high idle? Most high idle problems are vacuum leaks. Unmetered air that causes the O2 sensor to go lean so the computer adds fuel to bring up the mixture that is no longer lean. So, more air and more fuel = higher rpms.

Small vacuum leaks will affect idle but are not usually enough to affect driveability because you are opening up the throttle so the vacuum leak is negated. If the engine runs fine with the egr plugged but runs poorly when the egr is hooked up to run normally, you should look at your egr circuit. There are different types of egr valves. Some egr valves hold vacuum, some do not. Some use exhaust back pressure, some do not. Installing the wrong type of egr valve will make the system operate incorrectly. Too much exhaust back pressure will cause some egr valves to be pushed open when they would normally be closed. Do a backpressure test to verify exhaust is clear.
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