1998 Taurus, 98K. Car runs fine for the first hour or so, normal operating temp and then starts to stumble/hesitate after coming to a stop and taking off again, as if it's getting no or a reduced fuel flow. Turn ignition off for about 10 seconds and it's seems to clear the problem for a while.
Replaced fuel filter. Used/using injector cleaner (Lucas) for the last tank and a half. It seems to have helped some, there were times that it felt as though it pulled itself out of the stumble/hesitation and drove fine, but now it's back again. Appreciate any info that's available.

 

An additional bit of the puzzle. When it goes into this hesitation/stumble mode, there is no response from the gas pedal.

 

One of the first things I'd check to be certainis install a fuel pressure gauge in the fuel rail and monitor the pressure during a stall/hesitation.. Will it accelerate easy or pull 'past' the stumble if forced? Does the CEL come on during the stall?

 

One of the first things I'd check to be certainis install a fuel pressure gauge in the fuel rail and monitor the pressure during a stall/hesitation.

I don't know if it will occur with the vehicle just sitting at idle speed. It usually takes some driving time for it to occur and when it does happen when driving, there's no response from the gas pedal if I've just come to a stop. If I'm moving at a low speed, 30 to 40 mph and it happens, it has kind of pulled through it, but still there was no real response from the gas pedal, I just kept the pedal steady. One thing I just did notice is that the tank is now under half full and it seems to be happening more frequently now. Nothing comes on during the stall/hesitation, it just kind of winds down in rpm's until I pull over, shut it off for a couple of seconds, start it up and it drives fine for a while.

 

A fuel pressure gauge can be installed and left visable through the windshield so the car can be driven under load at normal operating conditions until the stall occurs.. If the pressure never falls off during the stall, then chances are it's not fuel delivery.. Even a restricted fuel filter would show up as fuel pressure loss since the available fuel volume/pressure would be exhausted in the fuel rail during the stall..A throttle position sensor if it were to have a 'drop out' or circuit issue could at times have the throttle go 'flat'with the 'check engine' lamp coming on for a short while.. But not always.. If the car's PCM (computer) doesn't reada varying/sweeping voltage from the throttle position sensor as the throttle is increased from idle position (low voltage at idle, higher ramping voltages as the throttle is applied) and looses or recieves a 'low voltage' (during a dropout) outwhen your driving at cruise speeds or wide open throttle, the car will loose power or can feel like it's starving for fuel or stalling..

 

Hayapower, appreciate the response and sorry I haven't gotten back sooner. Lots of lousy weather have kept me from attempting a fix. From every indication, it seems your assessment is correct. Now my problem is trying to get the 2 screws that hold the TPS loose. I've already broken 2 screw inserts that go with the socket set. Got any ideas or tricks of the trade for getting those screws loose?

 

Did you check for a good fuel pressure as well? Helps to elliminate the possible causes, or the ones you can, since there can be multiple reasons for stumble/stall concerns.. Fuel supply being one of the big causes.. You might try using a 'soft' punch against the face of the screws with a solid whack,, or install the driver bit and give it a love tap before trying to back them out.. If they do crack loose but won't back out, use some penetrant and rock them in and out working them until they back out completely.. Carefull,, they can/will break..

 

I'm giving up, going to have to put it in the shop. What's so frustrating is that I can't get those screws loose, to replace the TPS. There's only about 3 or 4 inches between the TPS and the firewall, so there's no room to give them any love taps to try and loosen them. Everything that's been pointed out, as far as the TPS goes, is what's happening. The CEL comes on and flashes some time times and the idle speed drops from 1000 to 500 with no throttle response. I feel it is the TPS that's the problem, but the darn screws now are stopping me cold.

 

Just a general update. Cars been in the shop now 4 days. They have attempted to trace the cause and they have now settled on replacing the fuel pump. They took a pressure reading and it was between 28 - 30 psi, with a bounce of about 2 psi. Pressure should be between 30 and 45 psi from what they said. The pressure dropped down to around 10 psi when the hesitation started. I didn't think a fuel pump could stagger like that. I always though, it's either good or bad. All I know is that it's getting expensive. Where up to around $6 to $7 hundred bucks right now between parts and labor and were still not sure this will fix the problem.

 

Revised update: Pulled the gas tank to change out the pump and the tank was filled with crude and rust. So now it looks like the root of the problem has been found. Now it's a new tank and a new pump along with all that shop time and labor. Looking right at $1000 as of now and I do hope that's the final tally..............