It is hot here today; about 98 with high humidity. I had just left the interstate and all of a sudden my base '89 XJ 6/40 started running very rough and surging. I went about 200 yards before I had to pull over because the car would not maintain speed or accelerate. Then it stopped running. I started the car and it still would not rev or continue to run. As I was waiting for a tow truck (20 min) to retrieve me, I tried to start the car.... and surprise, it ran fine! I drove about 8 miles with no problem before the same thing happened again. Waited about 5 minutes and the car started and ran fine, but after 300 yards; same problem. This time I waited 30 minutes before starting again and was able to make the 5 miles home with the car running fine. One guy that helped me suggesting that it was vapor lock. I thought that problem went out with pressurized fuel injection. Someone else seemed to think it was a transmission problem. I think it is a fuel problem. Could this be ethanol vaporizing before the gas causing a gas bubble in the fuel line? The trans has new/clean fluid and appears to be fine and full. Has anyone else had this kind of problem? I haven't had this car very long. I this a case of a car designed in a cool climate having trouble with a hot one? Any advice would be appreciated.

 

Hi dskul1,

I can confirm that temps and humidity in TN have been brutal.

To answer one of your questions, yes, vapor lock is possible in an XJ40 with pressurized fuel injection. The key word in that sentence is "pressurized." Any sudden loss of pressure can allow the fuel in the rail to boil to gas, creating a type of vapor lock. The injectors continue open up which just compounds the problem because they allow some of the gas to escape into the combustion chambers, reducing pressure in the system further. The only thing to do is to allow the engine to cool until the fuel in the rail condenses back to liquid form.

Common causes of this condition on an XJ40 is a fuel pressure regulator (FPR) that is failing or has failed and can no longer hold the proper pressure in the rail (in the range of 40 psi). Hard hot starting is one clue the FPR is not holding pressure. You can also pull the vacuum line off the FPR and check for wet fuel, which indicates the diaphragm has failed and is allowing raw unmetered fuel into the air intake leading to rich running.

If the fuel pump stutters or chatters a sudden loss of pressure can occur, leading to the scenario in the first paragraph above.

All of that said, a known problem on the 1988 to 1990 XJ40s was that in hot weather the fuel pump may chatter excessively causing frequent engine stalling in hot weather. Jaguar's solution was to add a second, parallel fuel pump when an owner complained of hot weather stalling.

Since your car has made it this long without the second fuel pump, I would check out other possibilities first, starting with the fuel pump electrical supply to be sure there isn't a loose or corroded connector, a failing fuel pump relay or corrosion on its socket and/or spade terminals, a problem in the fuse box that contains the fuel pump relay fuse, etc.

Often these sorts of symptoms are due to multiple issues, such as an overly-obstructed air filter, contamination of the MAF sensor and/or its electrical connector, an Engine Coolant Temperature Sensor (ECT) that is no longer operating properly, corrosion on ground studs, especially those used by the Engine Control Module (ECM) such as the two that attach to intake manifold studs and the one on the firewall behind the cylinder head, tracking across the ignition coil tower, corrosion on the ECM electrical connectors or water ingress in the ECM, failing critical relays or corrosion in their sockets (EMS Main relay, Ignition On relay, Fuel Pump relay, all located on the firewall in the engine bay on your car).

Hopefully others will have some additional suggestions.

Please keep us informed.

Cheers,

Don

 

I posted a short while ago that my car would occasionally surge and you, I think, pointed to the FPR as a possible source of the problem. I will be focusing on that for starters and then look at some of your other suggestions. I think the car was ok when I had a high volume of air carrying away the heat, but when I left the interstate, the heat built up and overwhelmed the cars fuel systems where they could not operate properly. Thanks!