Newbie here, I know enough to be dangerous..

I just bought my 3rd Jag, but it's my first V-12. This car does start and run, but it runs poorly. I guess I'm looking for a starting point, although I know the coolant temp sensor is one of the first things I need to check. Here's what I know so far:

- It starts pretty normally.
- Idles fairly smoothly in Park, but sounds more like a cammed V8 car than a smooth V-12.
- In drive, still idles, but rougher... around 550-600 RPM.
- Revs hesitantly at first, but after a few tries swings cleanly through the RPM range.
- Same goes for driving. Rev its guts out, then put in gear and it runs fine, until you try to maintain a cruising speed (any cruising speed) and it cuts in and out as if you're letting off the gas. Very reluctant to accelerate after that starts unless you "clear" it out by revving again.
-Trunk smells kind of like gas. (I know, probably not related )

There's a myriad of other little things like the A/C not blowing cold even though the compressor is running, but that stuff can wait... I just would like to not set the damn thing on fire... which I know is always a possibility when they run this way, Marelli ignition or not.

Any opinions? You guys generally have answered my questions on previous cars without my even having to ask... thanks in advance!

 

A few things I forgot... of course, the exhaust smells rich. The dollar bill trick didn't lead me to believe it has a stuck valve, and it makes no strange valvetrain noises. Thanks!

 

Sigh... sorry, triple post fail. The distributor cap, wires, and plugs are all new. (Invoice in the glove box from 100 miles ago.)

 

I recently got my own 1986 XJS, and it was running "rich" smelling as well, and didn't seem to be having any misfires. However, checking the plugs revealed some really bad connections that would indeed misfire occasionally, even though the "dollar bill trick" didn't seem indicative. Swapped out all the plugs and plug wires, and became 100% better. The closeness of firing on the V12 does make it really hard to detect occasional misfires, unlike on an 8 or 6.

So, given that experience, I'd say replace all those plugs. You're going to want to eventually anyway, so might as well do it now.

Edit: Posted before your third post. But I still stand by checking all the plugs and plug wires. Just because they were replaced doesn't mean they were replaced correctly, or that a plug wire isn't loose and thus not always sparking correctly.

I've also heard the throttle position sensor can cause the roughness on acceleration at lower temps, so might be worth checking that too.

 

In addition I would also recommend adjusting the throttle so that both sides open at the same time. It made huge improvements for me. In fact In the next 2 days I will be repeating the procedure as something has become misadjusted. I felt it when it happened. The procedure is fairly simple and can be found in Kirby's book.

Good luck!
Rob

 

Adding to the list from my experiences.

The large pipe across the back of the engine has a vac hose attached, and this same hose attaches at the ECU in the boot/trunk. This vac signal controls fuel richness (basically), and if it is leaking (common) the ECU senses LOW vac, so MORE fuel is supplied, follow where I am going, hope so. Check that hose, remove it at the engine, suck on it, ensure it holds vac.

The TPS, under the throttle capstan, has a specific setting for idle fuel proper running, and if it is out of whack, I bet it is, the overfueling, rough running, no acceleration, etc is there to stay.

Remove the plug from the CTS (coolant sensor), bridge the 2 terminals in the plug (dont let it short on metal nearby). This tricks the ECU to "think" it is dealing with a HOT engine, so will lean things off a tad.

Send me your email via a PM (if you want to), I have many "word" docs on my findings/fixes for all this. Some very large files, and am happy to send to you on ONE rule. Post back the findings here so that the archives are kept updated, and the moderators stay all warm and fuzzy.

The 2 fuel pressure regulators do leak fuel internally, and that introduces raw fuel into the inlet manifold/s, and rough crazy running is the normal issue. remove the small vac hose and see if fuel is present. If it is, the FPR is toast. The RH FPR can be eliminated (as in removed). The LH is the actual controller here, BUT, the RH CANNOT be fitted on the LH side.

The list goes on.

 

Update - checked the vacuum line from the pipe to ECM, it's good and holds vaccum when disconnected from the pipe and could blow through it when disconnected on both ends. Disconnected the CTS, tested with meter. Tests good. Pulled vac line off of the LH FPR, no fuel or vapor present.

Plugged everything back in and went to move the car back into the garage... ran perfectly. Drove it about 3 miles, still perfect. I'm guessing the CTS harness? I didn't touch anything else other than vacuum lines, and they were securely in place before removal.

 

AHA.

The fiddle factor, thanks to Doug, fixes most V12's.

It may last for years like that, or minutes.

Good suggestion on that harness, as it is well documented to cause grief.

 

Okay, back to the drawing board. Running well lasted about 15 minutes.

After looking at that CTS, I realized the connector is the same as a GM fuel injector. Purchased a new harness, and replaced it. Same issues. Now, I suppose it could be the wiring... but...

What I noticed, was that when it ran properly, the MPG portion of the trip computer was functioning. As soon as it began to ran poorly again, the computer reads 0 or 99 for instant, and 0.1 as the average. They have to be somehow related, as it didn't work at all before I fiddled with anything either. Any clue what feeds this part of the trip computer? I guess it could be an ECU issue, though the ECU in my car is already a replacement. (I assume, from the "warranty void if this label removed" sticker that's visible on it.)

Vac line from the rear pipe to ECU reads 11-12 idling in park, and about the same in drive. Using a vacuum pump on the line feeding the ECU to create any more than 14 in. of vac results in the car dying. I Seafomed it in both banks, and it did smooth it out quite a bit... but the issues still remain.

Does any of this scream an obvious solution to anyone? Thanks again!

 

A quicky before heading to work.

Alongside the 2 relays in the boot is an interface (black box) that has V12 on the label. that is the interface that controls the trip thingy. Simply unplug it, which will remove it from the scenario. Memory is it picks up a pulse from 4B????.

Failing that I am leaning toward engine bay wiring being waaaaay past its use by date. mainly the EFI loom in the bottom of the "V". About 10 years sees it getting toasty, and things happen.

 

Well, this gets more strange. I was talking about the problem with my neighbor, and showed him the new CTS harness. He pointed at what I can only assume is an air intake temp sensor on the LH intake horn; the connector is the same, and the rubber was stained green from corrosion.

I unwrapped the electrical tape which a previous owner had put on it to find the wires exposed and corroded. Some fiddling with this yielded positive results. It looks like it runs into the same harness as the CTS, just further up.

I unplugged it altogether; it smoothed out and idled perfectly. A test drive with it unplugged and it ran nearly perfect, the only complaint was that it was fairly lethargic until around 3500 RPM... but no cutting out.

Does this sensor work just like the coolant temp sensor? I'll hit it with the multimeter tomorrow if so, and if not I guess I'll just rewire that connector too.

 

The air temp sensor on the intake just trims the fuelling via the ECU. Removing the plug enrichens the mixture about 10%. I would clean up the loom for BOTH sensors (careful not to mix them up) as they have a common 5 volt feed and try again. It is highly likely that fiddling with the plugs made an iffy loom function on the coolant sensor, without which the car will not run.

greg

 

Any news on this issue? Just purchased an 86 XJS and I'm having similar issues. It's very frustrating...

 

Just a quick update, I was rechecking everything I thought I had already checked and I think I found my problem. The vacuum hose going to the ECU "appeared" to be ok. It held vacuum, was clear when pushed air through it and was attached. I looked more closely at the last 1/2" or so of the tube as it connected to the ECU and it had become brittle and was letting too much vacuum leak out. I trimmed the piece back to the softer region of the hose and reattached. Runs and drives much better. Next issue for me is leaky gas tank.

 

Good work, Jason

Cheers
DD