Hello everyone!.. I have a 1991 that wont idle cold and idles about 400-500 rpm hot.


So here is what it does when it's cold I start it RPM goes to 3,000 RPM.. after 10 secs or so it drops to 1,500 then about 5 sec later it stalls unless i keep the rpm above 1,000 rpm untill I it gets to operating temp then it idles low around 4-500rpm
I have lots of test equipment here is what i have done so far.

-Took BAC valve off and tested re-installed. checked wiring with scope.
-Hooked up smoke machine to intake and found a small vac leak and fixed it still runs the same.

-Changed plugs and wires
-Used scope to check currant ramp on both coils are good
-Adjusted TPS
-Took intake off and adjusted fast idle cam as to workshop manual (no change)
-compression test got 3 pules on each bank and total of 105 PSI each bank
-adjusted idle screw almost all the way out still idles low
-New oxygen sensor

any idea's of what to check next?

 

What happens to the idle when you unplug the BAC's plug? If it stays the very same then perhaps the ECU is not sending the proper signal to the BAC or the unit is clogged. Did you clean the BAC after you had removed it for testing?

 

I cleaned it with TB cleaner and q-tips .. You do notice a change if you pinch the feed hose for the BAC from the snorkel. ill try unplugging it when I get back to the shop tomorrow.

Someone also mentioned that it might be a water thermo sensor causing the ECU to think the engine is hot when it's actually cold causing the BAC to not open enough? make any sense?...

Do you think timing might cause this issue too? seems to work great besides the idle issue.

 

The BAC is basically for helping out the idle when load is placed on the engine so I would not think that the Water Thermosensor is a player here. If the sensor gave the ECU a hot reading when cold then how would you explain how the idle is still too low when the engine is hot? W/o load the idle should not need assistance of the BAC. Do you know what the voltage output of the alternator is for if its too low it would cause the idle to drop some. And did this problem just happen recently or what?

 

I found the problem this morning by luck one of the check valves for the vac line was leaking. Thing since it only flows one way it didn't show on the smoke test. i could just hear a faint hiss. Ran my hand across all the vac lined and found it was leaking where the 2 halfs of the check valve were mated.

took the check valve out for now let it sit for 3 hrs and tried it again and it started up and idles fine cold now.

man what a pain in the ***!..

I appreciate your time thank you

 

I might have the same problem. I mean at starts up cold 3,000RPM's it would stay there for awhile. Sometimes I would rev it up then it drops to 1,500RPM. then after 5 minutes warming up it will be at 1,000-1,200 RPM. When the Motor is really hot driving it quite a while it idles at 800-900RPM. is that normal? I be paranoid sometimes as if It wants to die. One day I was doing maintenance and with the engine running and hood popped up, I hear a hiss for like 3 seconds then it goes away and it repeats the hissing sound in 15 second intervals. I just want to make sure its not a vac leak. Haven't done no tests so I can't tell.