Engine stall if AC is turned off

[Boldster]

OK, this little baby still driving me crazy...

Engine start (rough idle) but stop as soon I put the transmission selector to any other than P or N.

Not very rare? Well, wait for this: If I turn on the AC, everything run normal... Any ideas????

 

[franklin2]

Have you cleaned your throttle body and is the idle screw adjusted correctly? The computer monitors the A/C system, and when it's turned on it bumps the idle speed up. That is making your engine run better. So you have idle control problems.

P.S. The idle air control valve needs to be cleaned also.

 

[Boldster]

Hi, thanks for your fast response. Yes I clean the throttle body and also try several different adjustments of the idle screw ( not sure how it works since the computer come with a pre set idle speed). When the transmission is on P or N, when I turn on the AC the idle speed go down like 50 RPM... Now I discover another vacuum leak on the booster, so will fix that (replace the booster) and see what happens...

 

[rusty ol ranger]

Do not mess with that screw.

Clean your IAC and check for vaccuum leaks. I

 

[franklin2]

Do not mess with that screw.

Clean your IAC and check for vaccuum leaks. I

I know you re not supposed to mess with that screw, but you are all too late, by this time all of them have been messed with, just like his has been. Ford originally adjusted that screw, I don't see why we can't adjust it properly.

To the original thread starter, once you get all the vacuum leaks fixed, get the engine running and fully warmed up, the top hose will be hot because the thermostat is open.

With it sitting there idling, unplug the idle air control valve. When you do that the engine should slow way down. You should be able to almost count the fan blades going by. It may run like that for a little bit and then stall. If it is still idling with some speed to it, turn the screw back to slow it down. Turn it down till you get the idle speed very slow. Once it's doing that, plug the idle air control valve back in.

Once you plug the idle air control valve back in, it will be idling very fast. And then it will start trying to work itself out. It will take a few drive cycles for it to work itself out and learn the new idle settings.

What are you doing when you set this screw? Ford designed it so a certain portion of idle air comes through the main butterfly in the throttle body, and the rest of the required idle air comes through the idle air control valve. This gives the computer full control of the idle over a certain rpm range.

 

[rusty ol ranger]

Theres actually a bit more to readjusting the screw then that. I think it involves running it at 2000rpm for 2 minutes or so, unplugging tht iac, then unplugging the pvc hose, blocking off 1/2 the hose hole or so...then setting the idle.

@adsm08 knows the full story,

 

[franklin2]

I can photocopy and post the info from the 86 factory emissions manual if you would like. Just paraphrasing it;

-If the idle is too high make sure to pull the codes and make sure everything is working.

-Place tranny in neutral, A/C-heat in the off position

-bring the engine to operating temp and turn off

-disconnect the idle air control valve

-start engine and operate at 2000 rpm for 120 seconds

-let engine idle and check the rpm. It's supposed to be 750 for a manual, 700 for a automatic

-turn the screw to adjust it if it needs it.

-turn the engine off, then restart. Check the rpms again and adjust if necessary.

-shut engine off and reconnect the idle air control valve

-verify throttle is not sticking in the throttle bore.

Then they say if it idles really high, to turn the key off and then back on again. My experience has been it takes a day or so of driving it before it starts acting correctly.

I noticed in the above procedure for the 2.9, the target idle speed with the idle control disconnected is very near the plugged in idle spec. In the same book, the procedure for the 5.0 is a little different. For the 5.0 the plugged in target is 675 for a automatic, 775 for a manual. Unplugged they tell you to set it at 600 rpm or 700 rpm, then you stop the engine and turn the screw back an additional one-half turn. Then plug it in and restart.

So in my other post, I may be setting the v6 a little bit slow in rpm.

 

[sgtsandman]

Hi, thanks for your fast response. Yes I clean the throttle body and also try several different adjustments of the idle screw ( not sure how it works since the computer come with a pre set idle speed). When the transmission is on P or N, when I turn on the AC the idle speed go down like 50 RPM... Now I discover another vacuum leak on the booster, so will fix that (replace the booster) and see what happens...

If your idle speed is dropping when you turn the A/C on, something isn't right. It should go up to compensate for the load of the A/C pump operating.

 

[Boldster]

Thank you all guys! I just replace the brake booster and eliminate all vacuum leaks (cannot find the right air cleaner temperature sensor (MAT?) which was creating a vacuum leak but for the moment I plug the front vacuum tree till I can find a new one). Thank you Franklin2 for the info!!! will do the idle screw adjustment during the weekend...