TBI Idle stumped!

[chevymotocross]

Had this problem for a few years now and finally I want to get to the bottom of it. My truck is an 87 R10 with the 305 TBI. When the air conditioning is turned on, the engine will stumble and stall at idle while in gear. Also stumbles and occasionally will stall when first started in the morning. Idles great and runs great when warmed up. The engine turns the A/C compressor fine at higher RPM.

Here is what I've done:


I swapped a MAP sensor from my s10 blazer which is equipped with TBI. No change whatsoever. Eliminated that and glad I didn't waste $30+.

Checked fuel pressure at TBI: 13 psi

Checked Timing: 0 degrees

JUST replaced IAC: no change

Checked, double checked, and triple checked for vacuum leaks: None

Checked to make sure EGR is functioning properly: It is

Spray pattern from injectors: Good

Truck has had a full tune up which consisted of all new plugs, wires, cap, rotor recently.

When issue first started to occur I can't remember exactly, but I've had the issue a few years.

Once again, symptom is cold idle too low and may stall if started and moved around too quickly. Also, stalls with A/C or defrost whether engine is cold or warm. Idle does not kick up like other vehicles when the A/C is applied.

Are there vacuum lines under the dash for the A/C. I'm wondering if that should be an area to check?

[Tazman]

How about your PVC ,fuel and air filters?Are they okay.

[chevymotocross]

Good, good, and good

[topp]

AFAIK, When you turn on the A/C it, lets the computer know it's on, and the computer compensates.  It is B-8 on the computer.  The Dark Green wire from the compressor goes to the computer.

[frotosride]

A few things off the top of my head. You mentioned a few things that you checked but when was the last time you replaced your 02 sensor? If it is old then the ecu will not get the proper reading when the a/c load is placed on the engine. As the sensor gets old the pulse width spreads out and when the ecu doesn't get a specific ration it will quess in order to keep the engine running.
IAC-idle air control valve...this controls the bypass air around the throttle valves to control idle speed while preventing stalls due to changes in engine loads. The position of the IAC is determined by a number of factors during idle like coolant temp, batt voltage, load and rpm. The IAC valve is located on the back passanger side of the throttle body.. it almost looks like a small electric motor. Check to make sure that all the passages to and from the IAC valve are clear but you will need new gaskets and seals for the throttle body because you will have to break it down to a degree.   Also the coolant temp sensor for the ecu that helps control the IAC valve at idle is located by the thermostat housing(the only thing with two wires in that area of the intake manifold)...good idea to replace it if it is still the original. even if it doesn't solve your problem it will save you in mpg...hope this gives you a new avanue to look down...hit me up if you have any questions.

[VileZambonie]

Check your fuel pressure regulator and spring.

http://forum.73-87chevytrucks.com/smforum/index.php?topic=6233.135

[jimmyjames]

I have a 87 chevy wrangler r10 2wd, and was having the same problems. I unplugged the valve under the air filter on the rh side, Haven't had an issue since. Egr valve, if i remember correct.