Hi all,
I recently bought a 1988 Jaguar XJS V12 HE and have been going through some basic maintenance. So far, I’ve replaced the spark plugs, plug wires, distributor cap, air filters, fuel filter, and fuel lines. I also pulled and cleaned all the injectors — they’re working well and flowing properly.

After all that, the engine runs extremely smooth. However, I’ve run into a weird issue:
When I shift into drive or reverse the engine won’t rev past 1000 rpm No misfiring, no surging — it just hits a hard ceiling like it’s in some kind of limiter. But if I shift into neutral or park, I can rev it freely all the way up (even to 6000 RPM) with no hesitation. to be clear this was not happening before I changed spark plugs and distributor cap.

There are no warning lights on the dash, and it doesn’t feel like a mechanical issue. It almost seems like a “limp mode,” but I’m not aware of the 1988 ECU doing that without a code or light. I’ve double-checked firing order and vacuum connections, and everything appears in order.

I’d really appreciate any insight from other owners who may have seen this before.
Thanks in advance!

 

What happens if you go down into first or second?

Does the engine speed drop when you go from park to drive? I'm wondering if low transmission fluid could cause this, although I've seen that expressing itself as revving without movement, so not the problem you have here. Strange.

There won't be a limp mode, or any codes, its too basic a system for that. I don't think the ECU can send error lights either. I wonder about the wiring for the ignition amplifier, it may have been disturbed while you were working on things. But then, if a wire broke, its more likely that the car just wouldn't run.

 

The transmission still shifts smoothly, and the fluid level looks good. I’ve tried 1st and 2nd gears, but the issue persists across all drive positions the engine won’t rev past 1000 RPM in any of them. When shifting from Park into Drive, the engine speed drops slightly, maybe around 100 RPM, but that seems within the normal range for load.

My first thought was the fuel pump, but it seems unlikely. If it were a fueling issue, I’d expect hesitation, surging, or inconsistent power delivery not such a clean and consistent RPM ceiling. Its definitely a weird issue and I haven't seen anything online about it happening to anyone else.

 

It would be worth checking that the Throttle Position Sensor, under the capstan, is adjusted and working.
Also clean the connectors on the car loom to injector loom plug and on the Resistor pack.

 

I reckon that be normal.

ASSUMING the car is stationary, Left foot on the Brakes, Right foot on the Loud Pedal, the torque converter has a predertimined "stall rev", usually about 1500 rpm, so your 1000 may be a tad out.

You stomp on it, the engine revs and hits a brick wall,. NO MORE than 5 seconds at a time and 5 minutes cool down between tests.

IF this issue is ON the road, I have NO idea, and will continue to drain the beer fridge while thinking.

As said, there will be NO codes, flashing lights, bells and whistles, that stupidity in not available in a 1988 XJS, at least Down Here.

 

I will definitely check out the TPS sensor and clean the connectors/resistors later today. Interesting about the torque converter having an idle cutoff when stationary as that does seem related.

 

To be clear, are you saying that, foot off the brake, selector in Drive, the car will start to move forward but will not pull more than 1000 rpm?

 

That is correct. When moving forward or reverse in gear foot off the brake the car will not exceed 1000 rpm.

 

So like you're on the road of your neighborhood, put the car in 1st gear and pressing the accelerator will never produce over 1000 RPM while driving down the road? Even if you drive the car, keep it in 1st and just hammer down?

 

Pretty much exactly like that. When going downhill in first gear the RPM does go past 1000 rpm but no amount of flooring it or manually turning the throttle can get it past 1000 rpm on level ground or going uphill. I have tested it a number of times and removed the battery to reset everything but no change.

 

I'm leaning towards a failed torque converter

Cheers
DD

 

I don't see how torque converter would cause that problem.

if the converter is massively slipping, the engine would rev but the car wouldn't accelerate.

if the converter locked up, the car would rev and accelerate, but slowly. This is a non-electric, three speed TH400 trans, not some ECU controlled trans.

Doug

 

I will check and see if I can find any signs of torque converter failure.
Are there any good ways to test for a failed converter?
I checked the TPS and it seemed to be in good condition (smooth voltage increase as the throttle was opened)

 

Well, I know nothing about torque converters, but what is at the back of my mind is this paragraph from the first post above:

After all that, the engine runs extremely smooth. However, I’ve run into a weird issue:
When I shift into drive or reverse the engine won’t rev past 1000 rpm No misfiring, no surging — it just hits a hard ceiling like it’s in some kind of limiter. But if I shift into neutral or park, I can rev it freely all the way up (even to 6000 RPM) with no hesitation. to be clear this was not happening before I changed spark plugs and distributor cap.

I think a peep inside the distributor is called for. Also, try getting the rear wheels in the air (VERY carefully ensuring the cage is fully and foolproofedly supported) and see what happens when the wheels are off the ground when trhe car is in drive.

 

OK, now I got it.

I have some suggestions based on the car being "normal" before you got at it.

You dont mention a Rotor?, OK, I do cap and rotor as a pair, ALWAYS. NOT YOUR problem I think?
Distributor innards, I will assume a few drops of ATF were dribbled into the shaft when the rotor was off?. If NO, then I suggest that, and ensure the mechanical advance snaps back to zero when tested by hand and released.
Check the cap, on the top adjacent to ONE HT post is a Number 1. This is the position for #1A HT lead, then anticlockwise around the rest in the firing order is simple.
Look at the carbon brush up inside the centre post of the cap, they fall out, mongrel things. It will run without it, but has no *****. Been there too long ago.

The fact t has "flight revs" is good, but really means little. Loaded revs id what is missing, and the above is my findings over the years.

Basically I am suggesting :go back and check EVERYTHING you touched last. It ran fine, now it dont.
There is no mystery to these engines, and the Lucas system that runs it, unlike more modern beasts.

 

Alright, I figured out what was going on — and it turned out to be a pretty simple (and admittedly dumb) mistake. I was referencing the Marelli distributor wiring diagram instead of the correct Lucas diagram for my car. Once I sorted that out, everything fell into place. Needless to say... pretty new to distributors and these V12's.

The issue is fully resolved now, and she’s running beautifully!

Huge thanks to everyone who offered advice — this is an awesome community, and I really appreciate the help.

 

Goodo.

Forgot, it happens. Dont laugh, your time will come.
CREDIT for the suggestions above goes to Greg in France, as we discussed this during our phone call,

Rule #1 with a V12

FILL THE BEER FRIDGE FIRST.

Works every time.