XJS - New XJS Purchase - Troubleshooting

XJ-S
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Over the summer I made an impulsive decision: I purchased a v12 XJS with minor cosmetic damage to replace the 1989 which was totaled last year (hit when pulled over on the shoulder while changing a tire). I bought the car because it was (1) converted to a 5-speed transmission; (2) appeared to have the right maintenance (new distributor and fuel hoses, …); (3) reasonably priced; and (4) was a v12 XJS convertible in signal red! I learned how expensive bodywork, paint, and convertible work are after the purchase. You live and learn.

After enjoying it for a season I am now guessing it sat for a while before the ‘standard’ maintenance was performed. I found a few flaws to work on over the winter and would like guidance from the group on the root causes and resolution approach:

  1. Occasionally the car will not rev over 2,500 RPM. It pulls strong and the experience on reaching 2,500RPM feels like fuel cut-off or hitting a rev-limiter. The RPMs drop and the car behaves again (until RMs reach the 2,500 mark again). Turning the car off and back-on resolves the issue so my theory of a clogged fuel filter, fuel pump, or pressure regulator are probably off. Exhaust is coming from both pipes and the car appears to be firing on all cylinders. It will keep misbehaving until the car is turned off and on. Any suggestions?

  2. The car has a visibly oscillating idle (idle bounces between 800-900 RPM when warm and 1200-1300 when cold). The revs go up and right back down. This issue is persistent every time the car is turned on. The experience here seems to indicate the auxiliary air valve. Is this where I should start?

  3. The fuel gauge doesn’t work. The resistance on the sensor is constant (and incorrect). Can I avoid draining the fuel tank by running the car out of gas and replacing the fuel sending unit?

Thank you for the help!

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Were you ever able to clean the stained undershorts???

:persevere:

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As much as I like my xjs…I would not buy another one…I would move on but I hope you have good luck…I would probably get a new pump, filter and both regulators and make sure fuel lines are not separating inside…of course check for flow first if lyou are able to…the aav valve can be bypassed…look in the “book”…running a car out of gas is not good as you can have other problems such as dirt in stuff…again see the book or on this site for how to drain tank…

1 Like
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I was actually transported out from the scene by an ambulance. I was fine with with the exception of a few cuts from shattered glass and a bit of shock/surprise. The lesson learned is to call a tow truck if any issues arise on the highway and wait past the guardrail to protect from drivers who do not pay attention to the road (my car was at least 4 feet past the rumble strip).

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I’d start with a fuel filter. When is the last time it was changed?

Gordon

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also: buy a pair of those hand operated clamps to pinch the fuel lines. they kind of look like scissors bent at an angle and are not sharp. use these when changing filters etc.![ there is a screen in your sump tank also and you could bypass that temporarily but have a filter in its place so you don’t get junk in your pump…s3

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The first thing I thought of was the clogged fuel or air filter.
After that the Fuel Pressure regulator is notoriously DOA.
I didn’t have any AAV issues but don’t know what that would be like.
Another thing to check is the Throttle Position Sensor.
If you have a pair of digital volt meters you can balance it.
Otherwise you can find a PDU to do it.
If you need one don’t pay the $400 Jaguar is asking.
My spares bin has a used one and a NOS one for cheap.
Drop me a PM if you need one.
The fuel gauge sensor can be removed from the rear of the tank if you are below half.
Got one of those also.

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All - Thank you for the advice. I’ll tackle the fuel filter and and gas gauge first. The gas gauge is a must for drive-ability and the fuel filter looks well used. I don’t think this is the cause of the RPM issue (turning the car off and on again immediately resolves it) but worthwhile maintenance regardless.

I checked the TPS with a volt meter and the reading was aligned with specifications in the book (0.35 - 5V). It is the red unit and looks to have been replaced already by the PO.

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Tomasz,
I thought your car was a facelift(94 & up) 6.0.
That is the wrong procedure for the your TPS diagnostics.
So everything I told you is wrong.
Please update your signature so we don’t make the mistake again.
It’s always a good idea to change ALL fluids & filters unless the PO gave you documents otherwise.
The fuel sensor I have is a USED, lo miles, 94 and known good.
I keep trying to put together a list of parts but I get bogged down by It’s magnitude.
The list got seriously smaller for you, now that I know what car you have.
Can you easily let me know what projects you have in mind?
I’m currently having demands on what’s left of my brain power.
One other thing, please use PM for buying and selling, as per forum rules.

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Your loss of power at 2500 RPM sounds like a coil to me

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Thank you for the valuable advice. The basic maintenance is now behind me (TPS calibration, fuel filter, fuel pressure regulator, fluids, and the fuel level sender). The oscillating idle is still present (but appears to be at a lower amplitude). The fuel level sender was straight forward to replace and the gas gauge now works. The rev-limiter like issue still persists and shows up occasionally. I will check the coils.