XJ8 - sticking throttle

Can anyone help. I have an onging problem with my XJ8 4.0 (yr 2000 uk model) that Jaguar cannot and are not interested in solving.

This is the story.

Earlier in the year the car developed two symptons - Firstly, when starting the engine it would rev up to 2500/3000 rpm and it wouldn't always return to the proper level and one would have to put the car into Drive (with the foot firmly on the brakes!) before it would settle. Scary stuff in the car park!

Secondly, following the occasions when the car didn't start properly, it would start to develop a shudder and stutter. At lows speeds the car literal jerks and surges before the engine revs fall back. At higher speeds the Car doesn't want to slow down. If you take your foot off the accelerator the car just drives on as if the Cruise Control is on (which it isnt). The symptons become increasingly frequent, violent and ultimately dangerous. At speed 60mph+ this causes a lot of shudder and the traction control and stability control to fail. All the lights come come and warning test come up on the dash. The car would then go into fail-safe mode (5mph! in a hurry) and on one occasion it stalled in the fast lane of the M23! Afterwards you just wait a minute, re-start and all is well again for 10 mins or another few days.

Jaguar initially couldn't find anything wrong - no negative readings on the computer. After encouraging the dealer they contacted the engineers at Jaguar and suddenly they fit, at no cost outside the warranty period, a new throttle housing. (Evidently this is silent recall). They say the throttle had been sticking.

All was well for a week when the symptons returned - they are much less pronounced but I am concerned that they will worsen.

I have tried to isolate when it happens and there seems to be two circumstances that set it off a) Taking the car out of the Garage and putting it back later without going anywhere. b) The wet and damp. There seems to be some correleation with the weather but this is not 100% scientfically consistent. (It seems to be the change in the weather not the actual weather! i.e dry to wet not simply wet).

My dealer who, when all is well, is excellent simply doesn't want to know now there is a problem.

Has anyone had, or know of, a similar exprience/fault and can anyone suggest a cure please!

Thanks. Ian.

Reply to
Ian
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After reading your post I'm of the (uninformed) opinion that your engine's problem may be related to its "cold start" system. All gas engines need additional fuel to start cold and run initially while still cold. There are several components of this system: an engine temperature sensor, a temperature related timer relay (longer ON with lower temperatures) and an electric valve that sprays fuel into the engine's intake manifold. I suggest that one or another of these components is intermittent - works as designed - or fails and over fuels the engine.

When the engine is warm and is overfueled the effect is as if you were stepping on the throttle - but there's a problem: too much fuel for the air being allowed into the engine by the throttle that's controlled by your foot. So the O2 sensor sends a "too rich mixture" signal to the engine's computer and it tries to correct the condition. This may be the reason for its stumbles, erratic behavior and rapid restart.

These thoughts are theoretical, not from my "hands on" experience with this engine, but since you're seeking a hypothesis to approach the problem it's the most plausible one I can think of. I hope this helps cure the problem for IMHO the car is too dangerous to drive in its current state.

I'm disappointed by your dealer's attitude, one doesn't need to buy a luxury car to have that lousy kind of ownership experience.

Reply to
T.G. Lambach

"T.G. Lambach" wrote in news: snipped-for-privacy@NoHamOrSpamcomcast.net:

Many thanks for replying. Sounds like a good idea to follow-up with the dealer.

I have had two very unique experiences recently where the car won't start early in the morning. You have to turn the engine over a long time

20seconds or so maybe before it eventually coughs into life.

Another chap suggested it could also be an air/fuel mix problem, this time too much air getting in.

It gives me something to work on. Thanks again. Ian.

Reply to
webrat uk

I thought about your car's problem again, re-read your description and change my hypothesis to electrical or electronic but suggest one test that you can easily make to ensure one possibility is excluded.

The test is, after driving a mile or two, to turn the cabin heat to MAX. If the heat becomes unbearable then you know that the engine is operating at its specified temperature and the possibility of a "cold engine" is excluded. But if there's little difference, then the "cold engine" possibility exists, having been masked by the automatic climate control. A stuck open thermostat is the cause and it should be replaced. The problem may be solved.

The steeper hill is to find the input sensor that apparently confuses the engine's computer control - even takes it off its "map" and engages its crawl home mode. This could be a temperature sensor, throttle position sensor, O2 sensor or even the wire harness that connects these could have a short. The lack of diagnostic codes indicates an uncommon problem so these guessing games get expensive as this is replaced and that's replaced in an effort to uncover the culprit.

Suggest you give the car to the repair shop for some days for them to use on a daily basis so they will experience the problem first hand and you try to come to an understanding with them that expensive replacements are installed on a trial basis and returnable if the problem isn't solved by that part. Unfortunately such a trial and error method is the only way to find such intermittent defects.

After a lot of effort and cost the whole thing may come down to some $10 sensor!

Reply to
T.G. Lambach

Dear Ian,

I am a firm believer in starting with the basics.

These engines are known for leaky intakes. Find a good port on the manifold and install a vacuum gauge. If you have no or little vacuum at idle then you've found a ported source; try again.

Steady needle Normal reading (usually 17-22" Hg. in stock engines) Race engines with big cams have little vacuum. Intermittent fluctuation at idle - Ignition miss, sticking valves, lifter bleeding off. Again a big cam can do this too. Low, but steady reading - Retarded timing, low compression, sticking throttle valve, intake or manifold leak. (Remember big cams have little vacuum!)

Drifting needle - Improper mixture setting or minor vacuum leak Fluctuating needle as engine speed increases - Ignition miss (plugs/wires), blown head gasket, leaking valve or weak or broken valve spring. (Pull out your wallet!) Steady, but needle drops/bumps in a rythym - Burnt valve or incorrect valve setting (too tight). (needle will fall when its the bad valve's turn in the cycle.) Gradual drop at idle - Clogged exhaust,plugging Cat/filter, excessive backpressure. Excessive vibration that steadies as RPM increases - Worn valve guides

Reply to
DieInterim

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