I have a 1998 XJSport 3.2L
Had some severe problems in the summer, with car failing to start and having to be towed TWICE!
Problem was eventually fixed although we don't quite know how it was solved; throttle body was the main suspect.
However, since then, car chugs at about 1500 revs, corresponding to the ranges 35-40Mph and 45-50Mph (you know; it's as if the car is running out of petrol, or else can't decide what gear it wants to be in).
Problem occurs when trying to maintain speed within these ranges, or else trying to gradually accelerate through them;
Rev counter needle jumps a bit, within a couple of hundred revs (though occasionally much more than that).
Problem disappears if stronger acceleration is used, or if driver takes foot off accelerator completely.
However, when I use the gate to manually select a gear (4th) it makes no difference; chugging persists, so I suspect it is not a gearbox problem.
Olil filter had been changed quite some time previously, and is not thought to be the problem.
Any ideas?

 

John, I would venture to guess that you have a problem with either your camshaft position sensor or your crankshaft position sensor. What it sounds like your car is experiencing is a fluctuating load calculation, which is then causing the computer to get confused (seeing the motor under extreme load, no load, extreme load, etc). This will cause the car to adjust the amount of time that the fuel injectors are open, therefore leading to your loss of power/chugging. With you saying that the car was having issues starting, that would tend to lead me towards the crankshaft sensor since that is what ultimately controls the timing of the engine (gotta love a multi-toothed gear with a tooth missing).

I would venture to guess what your issue is stemming from is a capacitance issue with the system and at the RPM talked about, the system is "ringing" and messing up the ECU. Is to where that problem lies, I would start with the easy thing which is replacing the crankshaft sensor. Then see how things go from there. Unfortunately, all the standard checks for that type of sensor are going to pass since you can make the engine run at other RPMs. If replacing the sensor don't work, then I would be leaning towards a new ECU. If you have a friend with a similar car, you may want to see if you can borrow his ECU for a little bit and see if you can drop it into your car and make the problem go away.

 

Thermo, I kind of suspected something along those lines, but I have to say I am stunned by your expertise.
I will pass you comments along to my mechanic to see how it works.
Thanks.

 

John, I am a self taught mechanic when it comes to this stuff. A lot of the vehicles these days are using the same basic technology. It is all in how you program the computer. Give me some time with that and I might be a dangerous man. I had to get into it fairly deep when I tore apart the top half of my truck motor and ended up messing the timing up. That was not a good point to be at. But, a little bit of searching the internet, some background with electronics, and it is amazing what you can figure out.

 

Kudos to Thermo, but I'm not out of the woods yet.
My mechanic has seen Thermo's suggestions and reply thus;
"I have read this with interest, but we have ruled out crank/cam sensor's as they in our experence fail completely or work. Any issues with them can be picked up with a scanner and we have scanned your car 4 or 5 times already with no sensor problems. However !!! I am around long enough not to rule anything in or out without changing it first,I will look at the car on friday again."

 

My mechanic (very dedicated guy!) has checked all of the sensors by driving the car with his laptop connected, right through the "chugging" phase, and all sensors read as perfectly normal.

He has replaced the throttle body (at no small expense!) but there has been little if any improvement. When I went to pick up the car, it failed to start first time! so I am back where I started.

This is a real mystery.