87 Suburban 454 idle raises in gear

[panoman88]

My 87 Suburban 7.4L idles normal until I put it in gear. Then the idle slowly raises to about 1500 rpms. Replaced ECM, prom, Park/neutral switch, idle air control valve, throttle position...all no help. Anybody got any ideas? I have had it to two mechanics and we are now all stumped? Please help!!!!

[amac209]

does it raise rpms after it's in gear or when you push the brake pedal to put it in gear? if so i'd be looking at the brake booster. otherwise might be a problem with the transmission vacuum modulator if you have one.

[panoman88]

Thanks for the reply. I can put it in gear with the parking brake on and the same thing happens. Initiallly when it goes in gear the idle goes down lower than it should..even stalling sometimes...and then its like the ecm starts telling the IAC to increase the idle to compensate but it doesn't stop at the factory specs. I thought about the modulator but tranny shifts fine..other ideas One other fact. You put it back into neutral or park and idle is about 1500rpm and will not reset to normal until you shut it off to reset IAC. Any ideas?

Originally Posted by amac209

does it raise rpms after it's in gear or when you push the brake pedal to put it in gear? if so i'd be looking at the brake booster. otherwise might be a problem with the transmission vacuum modulator if you have one.

[amac209]

look for a vacuum leak at the tbi base gasket. these things leak all the time.

[panoman88]

Thanks for the suggestion...I finally have given up for now..just unplugged the idle air control valve and turned the idle screw a half turn so that it won't stall when I put it in gear. Should be okay for the summer. Just won't have fast idle when it is cold.. Funny though, check engine light doesn't come on when I unplugged IACV!?

Originally Posted by amac209

look for a vacuum leak at the tbi base gasket. these things leak all the time.

[dirty dan]

Do you have an EST control or vacuum advance on your distributor?
'87 was a borderline year. Do you have a MAP sensor?

[panoman88]

I have EST control and Map sensor. Any ideas? Thanks for your response...





QUOTE=dirty dan]Do you have an EST control or vacuum advance on your distributor?
'87 was a borderline year. Do you have a MAP sensor?[/quote]

[dirty dan]

I would start with what amac said and check for leak at base of the throttle body (and where intake manifold meets the heads). Get some throttle body or carb cleaner spray and spray it around these joints with the engine running. If you have a leak, the engine will stumble when you hit it with the spray. While you're there, clean out the throttle body and make sure your throttle is not binding at an idle. Be careful with the TBI adjustment screw, it's a real pain to set back right.

[panoman88]

Thanks for the suggestion...I'm on it!

QUOTE=dirty dan]I would start with what amac said and check for leak at base of the throttle body (and where intake manifold meets the heads). Get some throttle body or carb cleaner spray and spray it around these joints with the engine running. If you have a leak, the engine will stumble when you hit it with the spray. While you're there, clean out the throttle body and make sure your throttle is not binding at an idle. Be careful with the TBI adjustment screw, it's a real pain to set back right.[/quote]

[dirty dan]

panoman88,
I just thought of a couple of other things that will up the idle on these old burbs.
First is low alternator output. If the ECM sees it's not getting enough voltage, it will open the IAC to increase the idle. It would make sense that when you put your truck in gear the engine RPM lowers a little. Your voltage may be dropping off.
Another item, though less likely is that your coolant temp sensor for the ECM is shot. A quick scan should pick that up easily.