Bought it couple years ago, had 30,000 original miles, 1 owner car. Well maintained vehicle.

Doesn't happen often, but today after parking it for an hour or so, it started but ran roughly and kept stalling. Had to restart several times with engine revving before it finally settled down.

Had it checked out at our Jag repair shop last year, they couldn't find source of problem. Always use premium gas.

It always starts right up the first time, only when running and then stopped for a while when it happens. But it's so infrequent that shop can't replicate. Very hot here today but it's happened in cooler temps as well.

Any ideas greatly appreciated!

RB

 

Sounds like hot fuel rail/ vapor problem. Cycling the key to ignition on for a few seconds then off several times without cranking may help as this will pump cooler fuel through the rail.

 

Which model, which engine?
Greg

 

V12, Convertible

 

Is the fuel cooler still connected?

 

My guess is the B bank fuel pressure regulator is getting iffy. Quite a worthwhile precautionary change, anyway.
Greg

 

1992's had no fuel cooler...
it was a better design...


you could be looking at the beginning of COIL failure...
A or B


a hot coil is not a happy coil....


had bank B coil fail (fully finally) and replacing that cured all mis-firing and setting of codes... improved power and gas mileage by 33%

 

1992's only have one fuel pressure regulator....

 

As I have a 1992 for coming on two years now...
your center console should have the newer LCD trip computer and information center... and a check engine light (if it is not burnt out)
the things that are happening should have set a CODE... these codes will be displayed on the center LCD... just turn the key to the off position and remove it... place the key back in and turn the key to the on position with out starting the engine... and wait for the LCD to display the code it thinks is the most important... after your event what code is displayed?

they are in the format of FF##
we have the 26cu fuel injection computer the 36cu is for the 6.0L but they are compatible... I use this PDF for the codes and trouble shooting

are you clearing the codes and then driving the car?

 

Thanks guys, will follow suggestions. Can drive car for 6 months before it happens, but once it happens seems like it's a relatively long time before it happens again which makes it difficult for shop to replicate.

 

Is your hot start circuit present and working correctly ?

I suffered from this issue (but it was worse than yours) on my 92 car and the main cause was the delay valve in the hot start circuit was the wrong way round and was not allowing the fuel pressure to increase above 40psi, which is what the system is designed to do to help hot starting.

This is what the vacuum lines should look like....



and this is the delay valve, which was in the wrong way on my car....



and this is a description of how the circuit works.....





My car has a fuel pressure gauge fitted to it, and when the hot start circuit is active, I am getting well over 50psi (can sometimes get up to almost 60psi) during a hot start. Normal fuel pressure is 40psi

 

Today while at a red light car was idilying roughly, so on a hunch I turned off AC. Bingo. Instantly ran smooth. AC on, rough, off smooth.

Could that be tied into the problem? Although last time I had AC recharged I was advised the compressor might need replacement.

 

It is just possible the aircon compressor loom, when live, is somehow interfering with the ignition. This, I suppose, could cause poor idling and starting problems, but it does not immediately seem related to the no-start problem.
Next time it will not start, turn off the aircon and see if it makes any difference. Meanwhile examine the low tension 12v feeds to the ignition system and see if there appear to be any cracked/broken/worm insulation on any of the wires concerned.
Greg

 

Very possible. Extra mechanical drag by a failing parasitic compressor.
Or, a weak engine at idle. Added compressor load beyond it's capabilities. Or a feature that increase idle RPM when the Ac is on, is not functioning..


Carl.

 

Carl, Sorry my post was not clear. I completely agree the compressor load could cause a poor idle, or indeed a compressor on the brink of seizing. I meant to make the point that whether this is related to the OP's no-start problem is possible but not that likely, outside a loom problem somehow disrupting the ignition 12 v feed.
Greg

 

Thanks guys, but to clarify, it starts each time but then dies multiple times, rough running till I get home but then months go by with no problems. Took it out today and with ac off ran smoothly. After returning to car after an hour, started right up but ran roughly on way home until ac turned off. Things that are inconsistent with no discernable pattern complicates diagnosis.

 

So "does it do this after startup if you switch the aircon off?" is the question. You have established switching off the aircon fixes the problem if it appears on the run. What about immediately after startup?
Greg

 

Intermittent problems I'd be looking at the loom and grounding points. As a bad ground gets warmer its resistance will go UP and can cause all sorts of intermittent issues.

 

My initial reaction was - fuel filter. When it get's clogged - all kinds of strange things can happen. Just a possibility. If your filter is new - it's something else.
Leaky vacuum connections can have similar effect, vacuum leaks confuse ECM.

 

Next time this happens I'll turn off ac and see what happens.