Engine stumbles off-idle after warmed up

mnunn

Here's my TPI question:

Engine is fully warmed up

I'm at a stop light (daydreaming with the Moody Blues again) and the guy behind me beeps his horn to let me know the light changed.

I press the gas quickly and the engine stumbles and gasps, sometimes even dying. If I press the gas pedal slowly under the same circumstances, it revs up normally and takes off, w/no hint of the problem.

Problem doesn't seem to be getting any worse. It does seem to be related to the speed with which I press the gas pedal, not the amount I press it.

If it were a carb, I'd be looking at the acellerator pump.

Where do I start looking? The TPS? The IAC? The MAP/MAS sensors? The computer?

If it helps, the car is a 94 Ford Winstar (but it parks in front of my Elky so it's an Elky question.) Okay?

Still, the basic TPI theory/troubleshooting should be similar to a GM.

Any Ideas?
Thanks
_________________
70 Elky (Non-SS) 454CID, TH700-R4
69 Chev C-10 Pickup (recently Upgraded to 454) w/TH400
29 Model A (Recently Sold).

kettbo · Posted: Fri Mar 17, 2006 3:15 am Post subject:

Mike,

I had a 91 Bronco from '93----'05 and an 89 Crown Vic from '98---'04.
Found On Road Dead, FORD. At least the Crown Vic was free from the in-laws.

Timing
Fuel filter clogged
Bad load of gas (fuel at a different station)
NE fuel mixes
Rotor and cap
Air Filter

I found the basic car stuff to check usually resulted in me finding the problem. Very seldom did the Fords stump me.

George
_________________
George Kettler
Lakewood, WA

68 Vette 427 Cp owner/builder the last 16 years
70 ElCo 350/Th350 cowl, buckets, console, 12-bolt, pdb. 15.1@91.25, budget 454 almost done

Harald K · Posted: Fri Mar 17, 2006 1:54 pm Post subject:

Actually you are thinking along the right track, accelerator pump, the only thing is that with fuel injection the TPS assumes the job of an accelerator pump it tell the ECM where the throttle is and when you step on the gas it send the signal that more gas is need and that timing needs to be adjusted. I would still check all the obvious things too such as vacuum leaks fuel pressure timing, etc. See if you have a code set. But as far as your stumble off idle it points to a TPS out of adjustment. Harald K

mnunn · Posted: Fri Mar 17, 2006 2:15 pm Post subject:

Thanks, George and Harold. I probably should have mentioned it is a Ford Windstar in the front of my post so as to avoid someone thinking the problem is in the Elky's accelerator pump.

The Windstar is a full up EFI system, complete with computer controlled timing as far as I know, so I don't think bad timing is the culprit.

I'll check the basics: filters, for broken parts, air in the tires, etc. It isn't bad gas (bought gas from a variety of places). I'm told the problem was there before and migrated with the long block replacement about 20K miles ago (before I bought the vehicle). So that kinda leads me to think it's a re-used sensor of some kind or low PSI on the fuel pump.

Out of whack TPS sounds promising. But would that only behave badly with rapid throttle movement and not with slow movement of the throttle plate?

If it were low fuel pressure, that would seem to make more sense (computer telling injectors to go full open while not enough gas going through them, leaning the mixture). But then why doesn't it happen when the engine's cold (in open loop?) requiring even more fuel to run than when warm?

My plan at this point is to check for low fuel PSI, replace the TPS and look for codes. I'll repost after I've done that.

Thanks!
Mike
_________________
70 Elky (Non-SS) 454CID, TH700-R4
69 Chev C-10 Pickup (recently Upgraded to 454) w/TH400
29 Model A (Recently Sold).

greywolf · Posted: Sat Mar 18, 2006 5:54 pm Post subject:

Responding to slow throttle movement probably means the O2 snesor is making up for the TPS being out of line. It just takes a moment to run some air through the engine when you open the throttle, then the sensor to read lean, and the computer adjust for it. When you move the throttle slowly the system can keep up, but kicking the throttle open needs the TPS to give it a head start.

That's my guess as why it might work that way.