Hello all, I’m a new member but I’ve spent many hours browsing here over the years. I’ve been having intermittent issues at idle with my daily-driver Jag since September. Every time I send her to the mechanic, she comes back running great but then relapses after a couple weeks. Because it’s a long complicated saga, I’m breaking it down into a list of symptoms and the “fixes” that resulted in temporary relief.

The car in question is a 1987 XJ6 (Series III), 4.2L engine, with 120k miles -- I’ve put about 10,000 miles on her since purchasing in June 2016. The past two Iowa winters have been trouble-free even during very cold spells.

Last autumn the car developed an unsteady idle (both during warm-up and after), and sometimes the revs would surge between 0 - 1000 RPMs. Once or twice it actually stalled while waiting at a light, but I was always able to throw it into Neutral and restart the engine immediately. This didn’t happen every trip, and putting the car in Park at long red lights would prevent the surging and stalling when it did happen. Other symptoms eventually cropped up as the weather got colder:

• After a cold start, the car would hesitate, stumble and misfire while accelerating, until the engine warmed up to 50C.
• A few weeks later the car developed black sooty exhaust during warm-up, even in Park (this was solved permanently after fuel injector cleaning).
• Once the weather was regularly below freezing, I couldn’t use the headlights or front/rear defrost while the car was warming up without the revs surging wildly (the headlights would dim and blower fan would surge in time with the revs). In order to drive home in the dark after work, I first had to wait for the engine to warm up to 50C.
• Most recently, the car has started stalling while in motion with no warning (no surging first) when I let up off the gas, both during warm-up and even after reaching operating temp. This happened just before replacing the IAC valve last month, and recurred this week after the valve was replaced.

Here are the interventions we’ve tried over the past months, in chronological order, each of which resulted in a couple weeks’ reprieve:

• All new spark plugs (Champion copper, gapped to .35), ignition wires, distributor cap and rotor -- I did these replacements myself, primarily because a coolant leak onto the distributor cap caused no-start on damp mornings, and because I didn’t know when these components were last replaced. Ignition coils were not replaced.
• Nothing -- the first time I brought the car to the mechanic for this issue, she spontaneously healed herself in their parking lot overnight before her appointment Their diagnostics revealed she was “running a little rich” and the MAF sensor was operating slightly out of spec, but not enough to warrant replacement.
• Fuel injector cleaning -- the next time she went to the mechanic, they decided the misfires and black exhaust were caused by overfueling due to gunked-up fuel injectors sticking open during warm-up. This did permanently fix the black exhaust, but not any of the other symptoms.
• IAC valve (Aux Air valve) replacement -- I got a used but “tested and working” replacement from everydayxj.com which was installed a couple weeks ago.

Which brings us to today. I’m hesitant to take the car back to the shop without some theory of what’s actually wrong. I get the impression that the Jag tech at my shop thinks it’s foolish to try to drive this car year-round, and that I should expect these sorts of performance problems during deep cold (nevermind that this all started back in September, doesn’t improve during warm spells, and didn’t happen the past winters).

Apparently none of the interventions so far were the actual problem or solution, since the symptoms would seem to resolve for a couple weeks but then come creeping back. I’m starting to think that disconnecting the battery for service was what actually caused the idling problems to clear up temporarily each time -- is that crazy?

Any other guesses about what could cause the issues described, which could resolve temporarily before failing again? A loose/dirty connection somewhere? Problem with the ECU? Is there anything you’d suggest that I try to check on my own before taking her back to the shop (bear in mind that I’m learning as I go)?

Miscellaneous notes: to my knowledge, the shop has not checked for vacuum leaks and hasn’t re-checked the MAF sensor functioning, nor checked the O2 sensor or the TPS. Through all of this the CEL has never come on, not even when I could smell uncombusted fuel in the exhaust. The catalytic converter was just replaced last winter (it was probably original to the vehicle and was rusted through in some spots).

Thanks in advance for any insight or advice -- I’m at my wit’s end and I miss my trusty kitty!

 

my wild guess is the AFM

you might go over the following 6 pages by Roger Bywater of AJ6 Engineering: Specifically Number 9 and Number 17 on page 6.

Fuel injection and the Jaguar XJ6 4.2 Series 3 / AJ6 Engineering

 

Ask them to do so. It's a good, basic step. And you might hit paydirt.

Air Flow Meter, not MAF.

I'd be curious to now how they are checking it.

Should be checked on general principles; easy to do. But my gut says it ain't the problem. On these cars the O2 sensor just 'trims' the fueling. What you're describing sounds like a more severe fault

No TPS on this car, at least not in the usual sense. It has a throttle position switch to enrich fueling at large throttle openings. I'll do some memory refreshing to clarify the full extent of the function


And it never will. Because the Series IIIs don't have a CEL



The 'could be' list might get a little long.

Before getting too far into the weeds, though, here are a few very easy steps/basics:

-Clean the throttle body bore and make sure the throttle blade is set to the proper .002" gap. Problems here can cause idle surging and roughness, hard starting.

-Make sure the ducting between the Air Flow Meter and the throttle body is leak-free and secure

-Make sure the coolant temp sensor is operating within spec and the associated connector and wires are in good condition. The CTS plays a major role in fueling, especially cold fueling; a fault here can cause problems for sure. I can get the specs for you....but the shop probably already has them. It's a rather generic sensor. One trick is to jump the terminals in the connector and see if symptoms change. A cold engine will complain at this but a warm engine won't mind. If a warmed-up engine behaves poorly with a bypassed CTS, you know it isn't the CTS causing the problem

- Check/clean/tighten the bundle of ground wires at the rear of the coolant rail. These are the grounds for the entire fuel injection system

- Coil and ignition module. These can cause a variety of ills. Most commonly they give problems when hot....but that isn't carved in granite. There are tests for these but they're useless unless the testing is done while the problem is actually occurring.....or until/unless a hard failure occurs. The module is AC Delco D1906. For the coil Iike the Lucas DLB170 but we'll have that discussion later, if needed.

- Ballast resistor on the coil. You car was built with one....but it might've already been tossed away. If not, I recommend doing so, if suspect

- Carefully check connections at the coil

That's all I can think of at the moment.

Cheers
DD

 

 

you may be right Gregory.

get a new one and install it yourself, they are inexpensive enough. Also Doug has a way to test them with a paper clip.

 

First. Numbered paragraphs would assist me and possibly others in composing a response.

Your third: The electrics are another issue. Light brightness, heater fan speed, etc.
These suggest voltage swings. More than one might expect. A cold electrical joint?
High resistance. Improves with added engine heat?
and perhaps low volts induce a misfire, and as such the rpm swing...

Point. Clean each end of the 4 large cables. At the battery, across the fire wall and don't forget down and under. The oft missed braided cable at the point where the transmission and engine are joined...

Carl