I just finished replacing the IAC and followed the tech article for adjusting the minimum air and adjusting the TPS. Everything looked good when I fired her up, idling great, right where it should be. Then I shut the engine off and a few minuets later started it up again, it idled at around 1500 in park and didn't drop at all after a few minuets. I thought I did something wrong so I went through step by step all over again, and had the same results. Has anyone had this problem before? Any ideas? I'm going crazy....

 

Another question. I was reading up on adjusting the minimum air and came across this link Tech Articles - Adjusting Your Minimum Air
if I'm reading this right it's saying to leave the ALDL grounded during the adjustment. It was my understanding that the engine should never be started when the ALDL is grounded anyone know if this is safe to try?

 

I've had no problems using the GM manual's minimum idle procedure on my '88 350 TPI .
The ALDL is still jumpered in step 5 when you're instructed to start the engine.
Don't forget to reconnect the Set Timing Connector.


Idle speed is speed controlled by the ECM through the Idle Speed Control motor and is not adjustable.
The minimum air rate is adjustable as follows:

1. Ground diagnostic terminal, turn ignition on, but do not start engine.
2. Wait 45 seconds for IAC to fully seat, then with ignition still on disconnect IAC connector.
3. Disconnect the distributor set timing connector.
4. Start engine and allow engine to go into "Closed Loop".
5. Remove ground from diagnostic terminal.
6. Adjust idle speed to specifications using throttle stop screw.
7. Turn ignition off and reconnect IAC connector.
8. Adjust TPS voltage if necessary: With ignition "ON" use a scan tool or 3 jumper wires to adjust TPS sensor to correct voltage specification. Tighten screws, then recheck reading to insure adjustment has not changed.
9. Start engine and check for proper idle operation.

 

I'm trying this as soon s I get out of work.

 

I tried and still no change. I adjusted the minimum air before leaving work. It ran great all the way home, but again as soon as I turn off the engine the next time I start it up the idle is back up to 1500 rpm

If anyone has had a similar problem or knows what is wrong or what I am doing wrong please let me know.

 

Mine does the same thing, however, after 2-3 minutes it goes down to 600rpm.. Isnt it the Cold Start option thing?

 

I just went out and started it up and let it run for about 10 mins. no change in idle speed. I'm still stumped

I also tried swapping the old IAC valve back in after cleaning it, same exact results.

 

I suspect a vacuum leak downstream of the MAF.

 

Have you used the idle air control diagnostic chart?
I'd do that before looking for vacuum leaks.

 

Your engine may also have a dirty and/or worn throttle body. That can greatly affect idle speed. A dirty throttle bore or plate may not always close fully and repeatably, allowing extra idle air despite the position of the IAC. A worn throttle shaft bore can allow the throttle plates to bind in the bores, effectively doing the same thing.

A sticky PCV valve or damaged PCV hose can also allow varied amounts of idle air into the plenum, as can a leaking power brake booster.

 

well i find it kinda odd how it idled fine, until he restarted it a few minutes later.. unless a vac line melted or something...

 

Thank you all for your help. I think I figured this one out. Due to my own stupidity I believe that I damaged the throttle body air passages. I used a stiff wire brush when cleaning instead of a soft brush

A friend of mine gave me a spare TB and after putting it on the engine is now running and idling great.




Steve

 

what does open loop or closed loop mean?

 

Open loop is when the engine is controlled without feed back from the Oxygen sensor, like when the engine is cold ( and WOT,) once it warms up it goes into closed loop and the Oxygen sensor is used to control the air/fuel ratio, that's the simple answer.

 

oh ok so another way of putting it is waiting for the engine to reach "operating tempterature" which is when the oxygen sensor starts to be in use?

 

That's correct, except that the O² sensor will be active well before the system has eached full closed loop mode. ANY adjustment should be made with the engine at full, normal operating temperature unless the directions for a procedure specifically state otherwise.

Engine and intake temperature have an effect on internal friction, oil pumping losses, compession, and fuel atomization (even in a port injected system), so full normal operating temperature is important.