surging and low idle speed
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Joined: Feb 2001
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From: Nacogdoches, Texas
surging and low idle speed
help me out guys, this is a 90 350 tbi. It idles very low and surges up and down. I replaced the coolant temp and did not affect it. Could I have a grounding problem?
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Joined: Jun 2000
Posts: 180
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From: Manassas, VA, USA
Check the minimum idle speed screw setting:
Run the engine until it is at operating temperature.
Remove the air cleaner so you can see the throttle body. Ground the diagnostic terminal for the computer (the FAQ I believe tells you how to do this, it just takes a paperclip). Now turn the key to ON but don't start it yet.
Look at the passenger side of the TBI unit. There is a little funny-shaped hole next to the throttle bores, and sticking out the side is a large thing with a 4-wire connector. This is the IAC (Idle Air Controller). If you look down the hole, you should see the end of the IAC sticking into the hole. This means the IAC is completely closed (it may take 15 or 30 seconds after you've turned the key to on). Now disconnect the connector for the IAC, leaving it closed.
Take the paperclip out, ungrounding the diagnostic terminal.
Now start the car. You may have to press down slightly on the gas to get it to start. Slowly let off the gas and see what speed it is idling at.
It SHOULD be around 500 RPMs +/- 50 RPM. If it is any lower (or won't even idle at all) you need to adjust the minimum idle, and this could be part of your problem. If it is any higher, you still probably ought to adjust the minimum idle but it is most likely not causing the problem you're having.
To adjust:
Inspect the front driver side corner (front side, not the top) of the TBI unit. There is a small plug there (a little less than 1/2 inch across). You need to remove it. I managed to pry it out with a screwdriver, a small sharp punch can work too. You don't have to put it back (you can if you want) so don't worry about removing it intact.
Behind it is... the throttle adjustment screw. It is, of course, a torx head. I think it's around a T-25. Turn clockwise to increase the idle speed, counterclockwise to decrease. It doesn't take much turning to make a difference. If the car wouldn't idle, try turning it a half or whole turn then starting it. Once you have it idling at around 500 RPM, turn it off, reconnect the IAC, and see if your problem is any better.
Note: You are adjusting the MINIMUM idle speed, the computer will normally keep the idle (by using the IAC) closer to 600 or 700 RPM.
Run the engine until it is at operating temperature.
Remove the air cleaner so you can see the throttle body. Ground the diagnostic terminal for the computer (the FAQ I believe tells you how to do this, it just takes a paperclip). Now turn the key to ON but don't start it yet.
Look at the passenger side of the TBI unit. There is a little funny-shaped hole next to the throttle bores, and sticking out the side is a large thing with a 4-wire connector. This is the IAC (Idle Air Controller). If you look down the hole, you should see the end of the IAC sticking into the hole. This means the IAC is completely closed (it may take 15 or 30 seconds after you've turned the key to on). Now disconnect the connector for the IAC, leaving it closed.
Take the paperclip out, ungrounding the diagnostic terminal.
Now start the car. You may have to press down slightly on the gas to get it to start. Slowly let off the gas and see what speed it is idling at.
It SHOULD be around 500 RPMs +/- 50 RPM. If it is any lower (or won't even idle at all) you need to adjust the minimum idle, and this could be part of your problem. If it is any higher, you still probably ought to adjust the minimum idle but it is most likely not causing the problem you're having.
To adjust:
Inspect the front driver side corner (front side, not the top) of the TBI unit. There is a small plug there (a little less than 1/2 inch across). You need to remove it. I managed to pry it out with a screwdriver, a small sharp punch can work too. You don't have to put it back (you can if you want) so don't worry about removing it intact.
Behind it is... the throttle adjustment screw. It is, of course, a torx head. I think it's around a T-25. Turn clockwise to increase the idle speed, counterclockwise to decrease. It doesn't take much turning to make a difference. If the car wouldn't idle, try turning it a half or whole turn then starting it. Once you have it idling at around 500 RPM, turn it off, reconnect the IAC, and see if your problem is any better.
Note: You are adjusting the MINIMUM idle speed, the computer will normally keep the idle (by using the IAC) closer to 600 or 700 RPM.
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