snickers

My 87 4000Q stumbles badly when trying to accelerate, but only when it is cold (running less than 10 min.) It starts and idles fine and will rev OK in neutral but when I put it in gear and try to accelerate it acts like it's starving for gas. Any ideas?

Katman

Search for a post by me on that..

Try pulling the coolant temp sensor connector before starting, and see if that helps. If so, bad sensor.

snickers

Thanks Frank...I'll give it a try.

Gary

Driveanhourfixfor4

BTDT several times - If it starts and idles OK and runs fine in closed loop, I'd hesistate to condemn the temp sensor. Likewise, be sure your basic tuneup items and common vacuum leaks are squared away before you go on a snipe hunt. Assuming CIS-E, cold acceleration fuel enrichment is provided in response to a signal from the airflow meter position potentiometer (that hard to reach plug located on the right side of the airflow meter). If the basic relationship between the airflow meter and the fuel plunger is slightly lean, (as it tends to become over time) the enrichment circuit doesn't have enough dynamic range to provide the required extra fuel for drive-away. Checking/setting up the potentiometer properly requires an electrical adapter and a special fixture, but if you can see continuously changing resistance throughout the airflow meter's range of motion it's probably OK. Ditto if you have a trip computer and the mpg readouts are reasonably accurate. If the potentiometer is OK someone with a CO meter could check/set you basic mixture setting and get you back on track. Having spewed that steaming mound, (keeping in mind that this advice is worth exactly what you've paid for it), try cracking the mixture screw about 1/8 turn counterclockwise and give it a try under the same conditions. I've done that whenever my '85 4KQ has done that exact same thing - the "cure" lasts for a couple seasons.

Katman

I'd be with you on possible problems with the flap pot, except for the fact that he said that it runs well when warmed up.

Easy enough to pull the coolant temp sensor connector and see if it helps. If so, then a measure of the cold resistance of the sensor is in order, if for nothing else than to rule it out as bad or good.


On the other hand, *I'm* having slight miss problems at part throttle, that kind of go away when I let off and stand on it again right away. You've just given me another idea of a possible problem, AND a solution!

Driveanhourfixfor4

True - Easy solutions first. If 'snickers' listens to me and turns that mixture screw counterclockwise he's gonna hate me - clockwise is richer in this case. Too many carburators...