i have a 1991 xj40. idles fine but falls on its face when i give it gas. i have changed spark plugs, wires,coil,fuel pump, fuel filter, MAF controller,and the crankshaft position sensor. the compression is good on all cylinders. the cats are not blocked. the distributor rotor is aligned... i'm at a loss i have no idea what to do next other then get rid of the car and thats not what i want to do.

 

Have you done a fuel pressure test? Does it sound like it's starving when you try to give it gas?

 

i've tested fuel pressure in the line after the filter but i do not have the right adater to test it at the rail. when i give it gas it boggs down and backfires though the intake (sounds like popping) has no power what so ever.

 

the plugs are covered in soot. not even 2 miles on the car since i got it. and i've cleaned the plugs twice now.

 

Have you looked at the air side of the intake system?

Check/replace air filter. Clean throttle body.

Generically it sounds like not enough air versus too much fuel.

Just my thoughts.

 

yeah the air filter is good. i even ran it without the filter and air box just to see. it didn't a bit of difference. but yeah thats what it seems like..

 

Sounds horribly like a TPS (Throttle Position Sensor) going bad, located on the underside of the throttle body/housing.

They do fill up with gunk via the throttle spindle. Sometimes a good clean with solvent will give them new life, sometimes not.

It is about all you have NOT touched as per your original post??.

 

Ok i'll try cleaning it. there was oil build up in the intake tube before the throttlebody.

 

if the TPS was going bad it would cause carbon fouling on the plugs?

 

Maybe, coz if it is giving strange signals to the main ECU, fueling etc will be all amuck.

I do know that on my V12's when they went AWOL, the engine overfuelled like crazy on the XJ-S, and just did odd stuff on the XJ12. The throttle switch (old school) on the Pre HE just did its own thing at whatever time it wanted, but the engine was bogging down bad, however that system is waaaaaaay different to what you got.

I am just saying that this sensor/switch is a priority signal provider to the ECU, and in all my years, if it is NOT giving a "smooth steady" signal to the ECU, that same ECU gets confused, and does equally strange things, mainly to fuel/injector operation.

 

there is oil in my intake. and i don't mean a little. my oil looked like it was over filled. by a quart.

 

But you don't tell us if cleaning all this out, cleaning the breather tube, reattaching the belows etc, has made a difference.

It should have done.

 

i cleaned out the air tube, sprayed off the throttle body and drained out the oil overfill so that the oil is in the hatchmarks now. no differnce. i'm going to do a fuel service on it today i hope. i'm praying that this is the issue.

 

That's a pretty old car you have there, so it could benefit by the fuel injectors being checked out individually and cleaned. If the spark plugs are black with soot, there must be too much fuel going in or something. Fuel injection specialists normally are able to clean and rebuild injectors, especially those of 1991; maybe not those on later models.

Other thing is the two biggest inputs to fuel injector pulses from the ECU is engine temperature and mass air flow; the other inputs are trimmer inputs

 

well with the two MAF sensors that i have used it has acted the same. haven't touched the temp sensor. but the CEL isn't on. still stumbles hard under accell.

 

It's a case of elimination, I'm afraid. I would suggest checking out the coolant temperature transmitter. These can fail but the thermistor resistance has to be checked at cold and when hot against the correct values. It's DIYable, but a bit awkward as you need an AVO meter. If this is OK, I would be looking at each of the injectors. All this assumes your fuel rail pressure is OK

 

you know i want to say that the PCV (part throttle orifce control housing) valve is the issue. all that oil in the intake would cause that kind of problem... i just got to figure out how its getting in there.

 

From the engine breather, yes ? My Rover 75 V6 also got a lot of oil into the throttle body housing via the same route. It didn't stop the car, though.

 

Well i cleaned it out , nasty. couldn't even blow air throuh it. still falling on her face. if the coolant temperature transmitter was bad wouldn't throw a code? yeah. At this point i'm almost willing to try anything. if rubbing her on the back fender and telling her how pretty she is would work i'd do that.

 

Believe me, I know how frustrating these kind of problems can be. I had the model before yours, the XJ6 Series 3 which had EFI, but this was Bosch L-Jetronic analogue technology - no faults codes or anything. Maybe it was better this way, as I sorted it out in the end, but it took a long time. If the fuel is there, and the spark is there at the right time, it has to run !