I'm a longtime owner of a beloved '98 XJ (only 110k miles - it's been around the whole country but I live in the city and don't drive to work), with no experience in car repair.

Problem #1: Occasional lack of throttle responsiveness after a cold start. The car would intermittently fail to rev or accelerate when I stepped on the accelerator, but after warming up for a few minutes, it ran fine. This gradually became more frequent over 4-5 months of very intermittent use. I initially took it in to a shop, and they said they couldn't find a problem.

Problem #2: Driving on the highway 1 month ago, engine temp suddenly shoots up to max. I pulled over, looked under the hood and the coolent wasn't boiling, fans were working fine. I waited a while, got back on the highway, and the same thing happened. I pulled off again, then drove slowly on back roads to my destination, then drove home on the highway with no further problems.

I took it in to a shop and they replaced the throttle position sensor and thermostat. I now find that the throttle problem has gotten worse - the throttle non-responsiveness happens every time, and now while stopped at lights the engine revs intermittently on its own and occasionally stalls. The engine temp has shot up to max one other time - this time during slow city driving. Worst of all, now I find that even when warmed up, the throttle will stop responding on any incline - uphill or downhill.

My shop guy's most recent suggestion is that the "computer" might need to be replaced. Having now done some research, I assume he means the PCM. Could that be the cause of both problems (throttle and temperature)? Could it have something to do with the O2 sensor or another cheaper component? Help much appreciated!

 

Replacing the computer is a "diagnosis of exclusion", meaning that EVERYTHING else must be verified, tested, swapped or ruled out methodically before you condemn the PCM. It is very often blamed for problems that it is not responsible for. And what is happening to you doesn't sound like a computer problem.

It is very likely that you have 2 separate problems.

1. Clean your throttle body. They get carboned up over time and miles. Always review tuneup hardware when symptomatic. Fresh Champion plugs gapped to .035, new plug wires, distributor cap and rotor. If the above is all completed and still symptomatic, verify your fuel pressure WITH A GAUGE. For a 98, you should have 49 psi, plus or minus 5 psi. Testing fuel pressure only takes a couple of minutes if you have the gauge (which can be rented from many big box parts stores).

2. What is the history of cooling system maintenance? How old is the radiator, water pump, fan clutch, radiator cap? This stuff doesn't last forever. Caps are only about $5.00 and often overlooked. It is also possible that the spike you are seeing is not real. If it spikes up in a second and goes back to normal temp in a second, that would be a clue that you have an issue with the signal between the coolant temperature sender (located on the thermostat housing) and the gauge. In that case, I'd replace that sending unit. You can carry an infrared point and shoot thermometer with you to verify any temperature issues. Point it directly at the thermostat housing for a good reading.

 

Thanks - I'll look into those things. Any chance the acceleration problem could be the transmission? I didn't think so at first, but the new problem with hills has me worried...

 

I don't totally agree with the mechanic... It could be the o2 sensors, but idk, there is a few electrical components that are possible.. It doesnt sound mechanical.. Any other work done to tune it up?

 

^x2 tjwalker!!

 

A dirty ground issue? Maybe the idle issue is the IAC motor?