Hi JT

Red Lines at 7000 (I think?)

But as previously mentioned I was only doing between 4000-5000 rpm when the problem occurred

 

Take this with a grain of salt but 7,000 RPM for an engine from that era would be really rare, not withstanding an engine designed for racing. Your average American V-8 pretty much runs out of air at about 5,000 RPM. My current AMG V-12 will not do that and it is much newer. You get serious valve float in that range as well as float in the points if still equipped with a distributor. What it really sounds to me is a rev limiter is kicking in to save the valve train but unsure if the old Jags even had some rudimentary form of this. When you shift to drive, the revs fall off and engine returns to normal. I have a car with a Lotus-designed V-8 that truly redlines at 7200 RPM and the sensation when the rev limiter kicks in is exactly as you describe. The rev limiter essentially cuts the ignition to keep the engine from becoming little shards of metal.

 

Fine to put Seafoam in the gas tank. But a better thing to try is injecting into throttle body while the engine is running. It comes with a special thin tube with a hook or kink in it to facilitate this.

 

Hi Guys

Thank you for all your replies

But just to be clear I'm only doing 4000 to 5000 rpm in (2) when the problem occurs and don't go anywhere near the red line or rev limiter, although unfortunately I cannot edit the Headline of my Post unless a Mod can change it for me to 4000 to 5000 rpm

'Grant' reckons it could be the Fuel Filter for the reasons that he has explained within this post and I agree that this could be the problem

New Fuel Filter EAC 3112 Should be arriving Tomorrow, so before I try anything else then I will try that, as trying anything else at this stage will only confuse the issue

 

Your V12 has a rev limiyer set st 7K RPM.IT WILL CUT OUT AT THAT LIMIT IN ANY GEAR. This protects your engine from tearing itself apart, shift just before it cuts out.

 

you have to "blow it out" every now and then.

 

I have a 2012 R and experienced a similar problem under hard acceleration in any gear. Example, run up to redline on an entrance ramp. Greatly reduced power, minimal acceleration and remains so until car is turned off and sits for a few minutes. Dealer technician says it is a fuel supply issue. Apparently there is a dual fuel supply, one in the tank and then upstream. I think back in the day we called it a fuel pump. Anyway, he said the problem is in the tank and it is a major repair. I have made sure I manually shift before the red line and have not experienced any problems. I felt it was not worth a major repair to extract the last .001 % of power out of a car that is already overpowered for normal usage.

 

Interesting, that would be the lift pump youd think a new car wouldnt have such issues

 

Here’s your problem, all Jaguar models have a fail safe so that you can’t over rev your motor. For instance, when in park or neutral you’ll notice that your engine wont rev over 3000 RPMs.
you really don’t have to use manual shift, (drive) is best and if you use your traction control just leave her in drive and you’ll be just fine. I have an S Type Sport and mine also has the sport mode for on track fun. Lol

 

there is a block of ground wires on the engine close to the firewall check them for being loose

 

I think you car may need a tune up. When those car were new the way to see if it was running properly was to put the car in first gear & go to the floor. The governor in the GM 400 would force the car to shift into second gear around 6200 rpm. The rear wheels would break loose on the 1-2 shift. If it up shifted @ lower rpm's you checked for sticky weights in the distributor or a partially blocked catalyst. When they face lifted the car in 1992 we had a press car @ our dealership. Of course we had to test drive it. We were quite surprised when driving the car that they no longer up shifted like the early cars. Needless to say we hit redline. In those days it was common practice to remove the catalysts & install 1974 XJ12 down pipes & just tape back the oxygen sensors.

 

Hitting a rev limiter?

 

Yes, a rev limiter. Since the advent of electronic ignition systems (late ‘80s), engine designers realized that they could build in a revolution limit into their engine control modules. The theory is simple. If the ECM can limit revs above a certain point (different for every engine), resultant catastrophic engine failure can be avoided. Prior to this, if you over-revved, the valvetrain, crank journal bearings, wrist pins, connecting rods, even the block would fail and the engine would tear itself apart before it could be shut down. In my car, if I hit 7,200 RPMs, the rev limiter kicks in, altering the spark plug firings, and the engine can’t rev higher. It feels like the engine is suddenly misfiring which it actually Is (by design).

 

sounds like you're hitting the rev limiter

 

I have not seen mention of a rev limiter function in any of Jaguar's technical documentation for an XJS. I have seen no evidence there is an active rev limiter in terms of cutting fuel or spark at a particular RPM.

 

Nor have I, FWIW.

Never felt one, either, although I don't venture beyond 6500 RPM

Cheers
DD

 

That's because there is NOT one.

These ARE cars where the Driver had control, AND, knew what he/she was doing. NOT relying on some "gizmo" to sort out things.

The Driver actually Drove the car, oh the forgotten art we now are forced to accept as "normal".

Thats why I LUV my MKVII, not a computer in sight, and only 2 (TWO) fuses in the whole damn car. Reliable, HELL YES, Goes like stink, stops??, well sort of (drum brakes), but SATISFACTION at the end of a journey, PRICELESS.

 

I think some people are jumping to conclusions based on the title without reading the thread. If they had read the thread they'd know that the problem is around 4500 RPM, not 7000. Since the redline is at 6500, we can discount the non-existent rev limiter and actually try to solve the problem.

If the plugs/leads/coils/distributor/fuel filter haven't been serviced recently, that's the first thing to do. If they're all in good condition then its something else. I still suspect combustion deposits, since that exactly matches the symptoms described. Easy enough to sort out, keep your foot down in 2nd gear. It will get better.

The buildup of combustion deposits is something Jaguar noticed while developing the HE engine, its particularly common with leisurely driving.

 

Hi Guys

The New Fuel Filter has just turned up (hope its the right one?) which I will hopefully be fitting today and will cover the whole process on my 'Cherry Blossom' Restoration Thread to show how easy or difficult it is to do and hopefully this will fix the problem

Although I'll have to leave draining the Swirl Tank and the Swirl Tank Filter until the Weather improves

Also Please Note that I wasn't going anywhere near the Red Line but somewhere around 4000 rpm thanks to my 'Typo' in the Title of this Thread, which I unfortunately cannot edit

 

Today I learned a good way to get your post noticed is to have 7000 RPM in the title.

Taking notes for next time....