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Login to post Forums » Grassroots Motorsports » Help! Car won't rev past 2K (Jaguar Xj6)

May 20, 2011 8:03 a.m. Jamesc2123 Reader

So, my friend and I have a free 1990 Jaguar XJ6, which we figured would be fun to undertake an operation of extreme lightening. Unfortunately, the car has decided that extreme Run-like-crap would be a better fit for a free British tank (Shocking I know, right?).

Here's what I know: The car won't rev past about 2K rpm. Sometimes this goes up to 3k or so, sometimes 1500 is all it will go to. It sounds like an ignition cut, like the car is hitting the rev limiter at 2000 rpm. Further making me think its a deliberate action by the ECU is that the car idles beautifully, and pull strong and hard up to its predetermined point, then just cuts out.

Its like its in permanent severe limp mode. It was throwing an EGR valve code, but we traced that to a dead solenoid, which we replaced and now there's no code, but still no revs. I read that a faulty Throttle Position Sensor might also be scaring the ecu into limp mode, but there's no code for that.

Ugh, anyway, long post, annoying british car, need help. Is there any way to manually bypass limp mode, or any other ideas as to what might be causing this?
May 20, 2011 8:09 a.m. Dr. Hess SuperDork

I'd check the fuel filter or pump. Put a pressure gauge on the rail.

What's more expensive than a cheap Lotus? Free Jaguar.
May 20, 2011 8:12 a.m. iceracer Dork

When you check the pressure, check the volume also.
May 20, 2011 8:54 a.m. Graefin10 Reader

Take time to look for all the obvious sources of problems: clean air intake, filter, air flow meter, sensor connections, ignition system componants, fuel sys tests (as mentioned above). If that doesen't reveal the problem google 90 Jag. XJ6 troubleshooting guide. If you find nothing there, start calling parts suppliers until you find a really knowledgeable person that will give you some advice.

I worked at JRT dealerships for 3 years before opening my own shop. I never accepted a Jag. with a computer in it into my shop again. My ex wife and a friend of hers drove me nuts wanting one. I made so many negative waves about them that the wife didn't get one but the friend, much to her dismay, did. My ex understood then.

If the above info. doesen't help, call in a psychic.
May 20, 2011 9:00 a.m. Dr. Hess SuperDork

Check the pressure when it is missing. If you have to run the pressure gauge taped to the top of the hood when you drive it, whatever it takes.

Guy in my car club had a repair shop in NJ for a long time. He got to where he refused to work on V12 Jags. He'd fix them, they'd drive down the road, die, call him, he'd go to pick them up and the ignition module would have cooled down by then, the car started and the people left.
May 20, 2011 9:02 a.m. 93EXCivic SuperDork

Recommended procedure before taking on a repair of Lucas equipment: check the position of the stars, kill a chicken and walk three times sunwise around your car chanting: "Oh mighty Prince of Darkness protect your unworthy servant."
May 20, 2011 9:04 a.m. Tom Heath Web Manager

I had similar conditions once. Completely different car, but it turned out to be a clogged catalyst. Good luck!
May 20, 2011 9:22 a.m. Jamesc2123 Reader

Would a lack of fuel pressure (or volume) make the engine run perfectly up to a certain point then just BANG cut out instantly every time? To me it would seem that fuel delivery issues would be more progressive, like it would rev high but then start sputtering and slowly lose the ability to rev.

Its a good idea though, since it did run fine briefly last fall before it sat all winter. Even though we drained the tank and ran new gas, it could still be clogged somewhere.
May 20, 2011 10:01 a.m. Graefin10 Reader

A customer had a car w/electronic fi towed to my shop and said it would start but not run and that he had just replaced the fuel pump and it had good fuel flow. Initial testing showed that it had consistent spark and "good fuel flow" but about 7psi pressure. Sure enough, he had replaced the fuel pump . . . with a generic, over the counter pump for a carb. I called with the price of the new pump and he said _ it, I'll come back and get it.
May 20, 2011 10:06 a.m. tuna55 SuperDork

I am betting on exhaust as Tom mentioned. It seems fuel flow, as James mentioned, would not be RPM dependent, but load dependent - 6k at idle, 1k going up a hill. I could be very wrong, though.
May 20, 2011 5:08 p.m. sanman Reader

Solution to the problem....Push the car onto the closest well traveled street you have and pop the hood. Get a nice hair cut, shave, put on some slacks, a blazer , and a crisp shirt. Then tell the first woman to stop that you are so sorry to trouble her, but you broke down and left your wallet in your other pants. Could she get you a tow and the garage bill and you will buy her dinner sometime? It'll work cause you got a Jaaaaaaag!
May 20, 2011 7:31 p.m. Wally SuperDork

Is there enough smoke in the wiring harness?
May 20, 2011 7:40 p.m. conesare2seconds Reader

Tom Heath wrote:

I had similar conditions once. Completely different car, but it turned out to be a clogged catalyst. Good luck!

Me, too.
May 20, 2011 8:14 p.m. MostExaltedPotentate Reader

In reply to Wally:

I snorted!
May 21, 2011 1:25 a.m. amg_rx7 HalfDork

Is there some kind of throttle positions sensor?
May 21, 2011 8:00 a.m. noddaz New Reader

3rd (or 4th?) vote for a clogged exhaust. Drop the exhaust and start it up. Or hook a vacuum gauge to an intake port and slowly raise the rpm and see what happens...
May 21, 2011 9:07 a.m. iceracer Dork

Ah , the much forgotten vacuum gauge.
May 21, 2011 9:46 a.m. Jamesc2123 Reader

Actually, count Lucas is laughing in his unholy grave, since the EGR solenoid that failed was a DENSO part....Also, I've learned that all the wiring in the car is connected with these giant barrel shaped connectors that are the easiest to take apart that I've ever experienced (obviously, they knew people would be replacing lots of electronics, somehow....)

We checked the TPS and adjusted it , but that made it run worse, so we ruled that out.

It is/was definitely some sort of fuel system clog. That or its just messing with us for kicks, because we went out to check it yesterday evening and it now runs perfect. Now we can continue with the plan and take out as much weight as possible (we'll weigh everything in a post when we're done, I'm rooting for 1000lbs.) and maybe Lemons it when we get bored with it.

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