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350 TBI idle problems

N

N22AJ

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What is the best way to start trouble shooting a low idle on a 350 TBI? It will idle fine at first start of the day, but as it warms up and sits it will just barely idle. Turn on the A/C and back out the driveway and by the time I get it in Drive, it quits. Goes fine up and down the road. Idle seems to be the problem.

Where do I start?
I assume that I cannot change the idle on a TBI and that all of it is controlled by the MAP and other sensors.

I apologize in advnace, my knowledge of the TBI is VERY limited.

Vehicle is a 87' V20 Suburban 4X4.

Thanks to all those who can help.
 
Another thing to look for is a vacuum leak. My map sensor vacuum line likes to pop off my 89. When this happens I can tell because when I come to a stop light it wants to die.
 
How do I pull the trouble codes? I've never done that before, but I might as well learn now.
 
A few common TBI problems...

The coolant temp. sensor is what the ECU uses to determine the idle speed. You may have a bad sensor, or a problem with that circuit.

A faulty MAP sensor or a leak in the vacuum hose that goes to the map sensor could also contribute to low idle problems.

Next on the list would be the EGR valve. If the valve is sticking partially open it will idle rough and tend to stall when you decelerate. A sticky EGR freequently will not trip the check engine light unless the there is a problem with the EGR solenoid, which is rare in my experience.
 
don't forget the fuel filter... If it gets clogged up you could have this problem... also look at the injectors while its running to make sure they are working right.
 
The biggest thing to look for and check is the IAC motor which is what controls the idle speed. As said above check for any codes and see if any of them pertain to the IAC motor. The IAC is an electric solenoid which makes a controlled vacuum leak to raise or lower the idle RPM's. I would also check the TPS voltage and adjust or replace it as needed.
 
What about an idle screw? TBI doesn't necessarily mean no idle screw. Don't know about the stockers but my holley has one.


Its on the driver side by the vacuum lines.



How does it run when you give it a little throttle?



Do you have a tac or know what rpm it idles at?
 
on <highjack> does anyone have a wiring diagram for the data link connector to the ecm and fuse box. I had to pull one from another vehicle. I need to know where the ones I had to cut from the fuse box need to be.<highjack> off

TBI does have an idle screw. You've got pull out a little freeze plug like thingy to get to the torx head idle screw.
 
Yes, there is an idle screw adjustment but it is recommended that it not be adjusted until eevrything else has been checked and adjusted first and then you could adjust it up a little but then you will need to recheck and set TPS switch and IAC motor back to spec.
 
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