Here´s the situation: 2002 XJR 70K miles. After sitting overnight (engine cold) the car starts normally and drives normally (so far so good). The problem is, after driving then shutting off the engine for anywhere from a few minutes to an hour or more (engine warm) the car “starts” (after a fashion) with unusually slow engine speed (very low r.p.m.) and after a few seconds the engine will die. I´m curious to know if this description is diagnostic of possible causes of the problem. Thanks for any suggestions, etc.

 

It's very possible in your case, the jag has a small vacuum leak, as a little unwanted
air can get inside the engine. When you start a cold engine, the ECU will command
a mixture with more fuel and the air leaked in won’t make any significant impact on
the combustion reactions because the added fuel has made up for it. However when
the car warms up, the ECU will cut back the fuel and balance the air/fuel ratio. At that
point the impact of the air leaked in is more noticeable. The engine will lose power
due to a lack of fuel. In some cases I have found vacuum leaks that are sensitive to
heat. When things are cold, the crack is hard and tight but as the the engine and thus
the engine bay gets hotter, the gaps open, and the air leaks in.
If your OBD2 scanner throws the code P0171 or P0174 (system too lean) and no
other related codes are found, a vacuum leak is likely to be a problem.

 

I would check your temperature sensor (the one in the cross-over coolant pipe). If stuck, the signal to the MAF detects a cold engine; you start and run great; as it warms, it thinks the car should still be in cold start mode over enriches the fuel mixture . . . for really old timers, same effect as forgetting to push the manual choke back in . . . What kind of gas mileage?

Randy's thoughts on P0171 and 0174, if you are showing those codes, would be the first choice.

How old are the plugs; the air filter; the fuel filter; how dirty is the MAF sensors; the throttle plate and bore . . .?

 

It may be useful to determine what brand ATF was used during the replacement of the fluid. At 75K miles I had the entire pan assembly replaced with an OEM ZF parts by a shop around here that specializes in Euro and British cars. They filled the new pan with Pentosin which does not meet the OEM specifications set forth by ZF Friedrichshafen AG. Several members pointed that out.

I lifted the following information from a post made by Don B just a couple of years back:

“the only three fluids known to be correct in the 6HP26 are ZF Lifeguard 6, Ford Motorcraft Mercon SP, and Shell Spirax S4”.

So, the shop in question drained the Pentosin and refilled with the Mercon SP . I now have +110K miles on the odometer and no hunting for gears or surging.

So there you have it. If there have been any changes among ATFs that now meet ZF specs it would be great to get feedback from other members.

 

As Jim said, the temp sensor would be the place to start. I have solved the hot starting problem in a couple of friends cars that way.

 

Re: original post of Sep 2024. Thanks to RandyS and Jhartz for their input. Had both fuel pumps replaced professionally for a lot less than I thought. (I suspected fuel pumps from the get-go, but intermittent starting threw me.) Everything is good now – starts effortlessly, better idle when cold, smoother acceleration. So, the fuel pump performance had deteriorated before starting problems were manifest.

BTW, when I first got it back with the new fuel pumps, I could smell gasoline inside the cab after making sharp turns. Changed the fuel filter (suspected back-pressure from dirty filter). No more gas smell. Apologies for tardy feedback.

 

If you look under the gas pedal, you will see some kind of stop bump. You can check that it is pushed to the max. On my 3.2l it can rotate freely but it can also go too high/take it out if you turn it anti clockwise.

On my car it happened and it was causing that I could not fully press the pedal therefore you never reach max power.

Edit: oopsiee wrong thread. Sorry!

 

Maybe the wrong thread: but still a valid consideration

 

Cool idea this - will check also on mine

 

If you see 100% you are good, my point does not apply then.

 

How about collapsing catalytic converter?...tell your technician to check them.

 

Have you read then codes? OBD2 readers are cheap and very useful.

 

In my case it was dirty injectors, sending them for cleaning fixed the issue
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