My 1994 E-250 won't go if I push on the gas pedal at all. I have to just touch it a tiny bit and let it slowly gain speed while holding it in low gear, until its close to 35 MPH, then shift it into second.
If I push the pedal down at all it will flood out and pour gas down the exhaust. When I let the pedal up again it runs smooth and continues on. If I push the pedal down quick and off the pedal again it will backfire out the exhaust. I can do this on demand and scare who I want.
I have replaced: MAP, EGR, Fuel pump, Fuel filter, Fuel Pressure Regulator, Distributor, The thing on the side fender that controls the injection based on the timing at startup, TPS, Air Intake temperature sensor and am about to try an Air Bypass Valve. I am also leery of the throttle position sensor although I unplugged it and had no change in running condition.
This has been intermittent and it would give a cough and run perfect. Then it would cough and stop running. Back and forth from one day, or minute, to the next. Now it doesn't want to kick in at all.
Anybody else ever have this problem?
Thanks for the good word.
Grant

 

If unplugging the TPS had no effect at anything other than idling it's bad.

Sounds very typical of TPS failure based on your description.

At idle the IAC admits air to the engine, not the throttle blades. Anything above idle with a faulty TPS and the engine doesn't expect the influx of air from the opening throttle blades because it can't "see" the throttle opening due to faulty TPS so when you press the pedal, the blades open, here comes lots of air the engine wasn't expecting and you go WAY lean and it stalls. By letting the engine gradually gain speed the mixture is being richened based on load, not throttle position so the faulty TPS isn't a factor in driving this way. The engine probably just thinks it's sitting at idle (since it can't see the throttle being opened) and a BIG load is being run like AC or some such so it's feeding in more fuel to compensate for that, not throttle position.

Same thing (in reverse) happens when IAC sticks open. Engine gets lots of air and has to richen the mixture to compensate so idle goes way up. You can drive quite quickly without touching the pedal because the engine is getting enough air (inspite of closed throttle blades) to run maybe 1,000-1,300 rpm at light to moderate loads. When this happened to me I could drive about 50 mph without touching the pedal (3.08 gears).

 

I posted that I replaced the TPS but I actually tried a used one from another van that I had parted out. I will get a new one and put it in.
I also unplugged the Air Bypass Valve while it was running and it quit immediately. So I believe it is working correctly. What else would make the idle fluctuate?

 

"IAC unplugged = stall" sounds just right. With it powered off the engine should receive no air and thus, not run. If an IAC fails it's either stuck open (high idle, unplugging causes no change) or it's stuck closed (won't run at idle, unplugging causes no change).

Other idle fluctuations could be vacuum leaks, which would raise idle (like IAC stuck open) and thus aren't likely your problem, fuel pressure regulator could be bad and since it's vacuum responsive might cause the issues you described (not raising pressure as vacuum falls with throttle opening).

Fuel pressure gauges are cheap and easy to use, especially on the 4.9 in a van as the schrader valve on the fuel rail is right your knee when seated in the passenger seat with the doghouse removed. If the fuel pressure regulator is the problem it's outside their normal mode of failure, which is diaphragm ruptures which flood the engine with fuel through the vacuum line.

I suggest you test your TPS(s). Easy to do with a VOM and then you'd know for sure. They're just a potentiometer like a light dimmer. Throttle closed (like at idle) the light would be very dim and as you press the gas the voltage goes up so the light would get brighter. The TPS has 1 connector with 3 wires. 1 wire is 5v all the time (key on, that is). One other is ground (all the time). Between the two is the signal wire. Check that you have 5v and a good ground in the harness then check that resistance between the ground and signal pin of the sensor goes down progressively without dead spots or spikes as you rotate the sensor.

Easy and safe way to do it is key off, TPS unplugged, meter set to continuity and one probe on battery (-) post or other known good ground, connect to either outer pin of the harness till you get strong continuity. That's the ground. Set meter to vDC and turn the key on. Connect probes to batter (-) as before and now to the opposite outer pin of the harness. Should see 5v. The center pin is signal and should be somewhere between 0 and 5v DC coming from the sensor depending on throttle position. Good practice to test sensor while tapping it in case the pot has come loose inside. They sometimes test fine with the engine off but give an erratic signal when the engine is running due to vibration.