Hesitation question ????

[alamantia]

I have a 1992 Chevy Tahoe 350 TBI, just noticed the other day that when i rev the truck a little when its in gear once i let off the gas the engine stumbles and runs rough for a few seconds and then pops right back up to smooth idle.

I first noticed it when i was stoped at a light on a hill behind another car and i moved up a few feet, once i let off the gas it almost died and shot back up to normal idle. The trucj had been runnig for a while and the engine was at operating temp.

I put new plugs in it, the wires cap and rotor were all done last summer so only have abiut 10-12k on them.

I pulled the IAC valve, it looked good but I cleaned it out with carb cleaner anyway. I installed a new fuel pump with filter last fall. I checked all the vaccume lines and they look good. I did not yet pull the EGR...

No check engine light came on either.

This just started a few days ago and I am perplexed. Any ideas?

[chrisfrom nj]

my 88 gmc 1500 pick up does the same

[alamantia]

DId it always do it?

I would like to pinpoint what the issue is before I randomly replace sensores and chit, so I would like to figire it out...

[NJSPEEDER]

Because of the PCV layout in the TBI cars, the first thing I would check in the IAC. It sits in a little ditch on the pass side, rear of the TB. The hole it sits in is a terrible crap collector. Just unplug it, remove it (15/16's if memory serves) and clean off the IAC and clean out the hole as best you can.

If the IAC is clean another popular trouble maker is the module in the distributor. Small cap HEI's heat up pretty bad and it can start acting up when switching. A stumble off throttle can be an early sign of trouble.

Also check the other little stuff, fuel filter/plug wires/ and the other little ticky tack crap that likes to drive car guys nuts.

-Tim

[alamantia]

would an ignition coil get weak with age?

[NJSPEEDER]

It can, but they usually show signs of missing sparks all the time when they get bad, not just under decel conditions.

[alamantia]

Gotcha.

I just went out to the driveway and started it with the hood open, i set the emergency brake and had my brother sit in it with his foot on the brake and in drive. I got it up to operating temprature and it was ideling very smoothly in drive at about 650-700RPM's it was running great. I put my hand on the throttle linkage in the side of the TBI and gave it a little gas, opening the throttle plate up only a few degrees to get the RPM's just up over idle, maybe to 850-900 or so and thats when runs really rough. In gear just over idle. At idel it runs great, over 1000RPMs it runs great. It seems to be that transition in between idle circut and open throttle. Only for a range of 200-300RPMs between idle and throttle blade open it runs extreemly rough. Im not sure if its IAC, EGR, O2, MAP, MAT, ignition module, bad gas etc etc. I dont want to start replaceing componunts one by one in holes to get it. I will probably bring ot to my fathers dealer in efforets to scan it tomorrow, I was just trying to avoid that.

[alamantia]

http://www.race-dezert.com/forum/showthread.php?t=11375

[alamantia]

it was the EGR vaccume solenoid, i replaced it and everything works well again.