My '84 XJ6 seems to have developed a warm start issue where it idles rough after I've driven her awhile, then stopped for a little while to run an errand.
Usually she just idles rough (not hunting, just rough) for a minute (around 500 rpm) and then stabilizes (around 1000 rpm) but today, just once, she caught, stumbled and died. I restarted her, she idled rough for a minute, then ran fine.

Mostly I drive her to and from work, so I rarely have a warm start and this doesn't come up during the week but it comes up on the weekends whilst running errands. I tried running the A/C for a few minutes prior to shut down, just in case there's fuel cooling functionality like on the V12 in my XJ-S and, it may be placebo, but it seemed to help but I realize it could just have been a fluke.

It's not a show stopper yet, but I'd like to resolve it before I get stranded.

Any ideas what I should check first?

'84 series III

Thanks in advance.

 

The 'could be' list is long. This is one of the cases where you hope the problem gets much worse.....as it would probably be easier to narrow it down.

Both the ignition coil and ignition module are known to give trouble after absorbing engine heat. Check the spark at the plugs next time the engine acts up....is it weak and yellow or strong and blue?

A couple drippy injectors could be flooding their cylinders as the car sits. A cold engine would like the extra fuel but a warm engine wouldn't.

The coolant temp sensor may be giving skewed reading and giving a 'cold' signal when the engine is actually warm-ish. This would cause over fueling. Unplug the connector and jump the terminals with a paper clip....which you'll keep in your pocket, ready to spring into action! This trick gives fixed 'warm' signal to the ECU.

Others will chime in with more

Cheers
DD

 

Thanks. I believe the coolant temp sensor & ignition coil are both new (replaced by SARC before he sold me the car) but I have many paper clips so I'll give that a try and check the plugs...

Thanks again,
R

 

hi doug! i seem to have the same problem too, except mine just dies out/stalls at stop lights, stop signs, gear shifts (reverse to drive). after i drive it for a while, it will be just fine when i am not on the accelerator. it does just fine if it is cold (as in the morning) or if it has been parked for more than an hour and a half. same trouble shooting guide? btw, my indie jag mechanic suggested "T-ing" the fuel tanks together to solve the problem because it had something to do with fuel not being able to drain back when parking the car for short periods when warm. didn't sound "kosher" to me. what do you think? thanks in advance, william

 

Yes, but start with checking the coolant temp sensor. Or, first, the easy paperclip trick.... to see if anything changes.

Post back.



Hmmm. I guess he's saying that lack of fuel return is creating excessive fuel pressure.....which can be confirmed by a fuel pressure test. Did he perform one?

If there's a problem with the return system it shouldn't be hard to fix. I can think of just 3 things that would prevent proper fuel return: faulty pressure regulator, a faulty return valve (left of right), or a kinked/clogged return line. Tee-ing the tanks together on the supply side, if that's what he is talking about, would do nothing to solve any of the problems. But.....maybe he identified one faulty return valve and wants to bypass it....leaving one operational return valve.....and then tee the supply side? That's sounds plausible.

Cheers
DD

 

one trick not mentioned that could fix other symptoms in your 1984 is to add a ground strap to the 1-wire oxygen sensor. (this only works in early Series 3, 1986 and 1987 XJ-6 came with separate ground wires, also known as 2 or 3 wire O2 Sensors).

I added one to my 1984 and it made a difference making the idling more stable, and also helps with starting the engine.

 

Hello Jose,

Where do you buy such a strap?

Thanks,
Rhett

 

hi Rhett,

I made my own years ago. Then made a few for other owners. I could make you one since I have all the parts to make them including the large ring terminal shown in the picture and a new crush washer, but I am away from where I have all my supplies.

however, if you go to a junkyard where they have salvage XJ-6, look for the ground strap for the radio, one side of which is bolted to the driver's side of the transmission tunnel, (you have to raise the carpet to see it), and the other side of the strap is the side which is normally bolted to the rear of the radio.

then you can drill a hole large enough on one end of that strap for the O2 Sensor to fit through. the other end is attached to one of the bolts of the Cruise Control Bracket.

another option: get a thick insulated wire and wrap it very tight around the base of the O2 Sensor, the other end of the wire goes to any nearby bolt that attaches to the body.

the O2 Sensor gets very very very hot, and it will melt the insulation of the wire. You can do it just to test if it improves anything in your car.

the idea came from a reputable mechanic who used to contribute at another forum in the 1990's and who said that as the manifolds get rusty and greasy in these cars, the grounding of the O2 Sensor becomes less "positive", so adding a dedicated ground helps the sensor send a better signal to the computer. I tried it and it works, regardless of the skeptics.

 

Not sure this makes any sense but about a month ago, the fuel pump started emitting a high pitched wine loud enough to hear in the cabin through the back seat, so, seeing as SARC had graciously included a new unit when he sold me the car, I had the fuel pump replaced and the warm start stumble hasn't recurred since.

It was intermittent to begin with, so maybe it's too early to say but I do drive the car nearly every day and, so far, so good.

Does this seem plausible?

 

yes, sounds like it was a bad pump. Funny my 1984 pump has never given me trouble (knock on wood).

 

Just thought I'd follow up and report that since the fuel pump replacement in March, the warm start stumble, which happened almost every day, hasn't recurred even once. The car starts like a champ every time so I'm going to call this resolved.

Thanks every one.

 

Every moving part on a car eventually wears out, we just can never be sure when ! Manufacturers are engaged with new cars, so knowing the symptoms is only ever found out from experienced shops and forums like this.

 

Agreed! I wouldn't own one old Jag, let alone 3, without forums like this one.

It also didn't hurt that the previous owner, a veteran of this forum, included a brand new fuel pump with the sale of the car. You know, just in case. :-)

 

I have been having a similar problem. Mine on low speeds would die out or when I would park somewhere it would not start back up (usually during summer). The air duct hoses (or whatever they are called) were not tight. When I tightened them the problem went away, but at low speeds it still accelerates real slow sometimes. I still have the constant fear that it will die when accelerating from a traffic light, but it hasn't happened for like 6 months now.

 

Mine wouldn't die, it would run fine until I turned it off when warm and would try and restart it. On the bad days, it would fail to start a few times before, finally, catching (with a bit of throttle from me).

Now it starts every time without hesitation.

 

I'd be checking the fuel tanks for rust, or at least fitting a pre filter before the pump, a very common issue is that the tanks rust out which in turn damage the fuel pump due to foreign matter, there is no real pre filters on these cars and it is a good piece of mind, especially that you now have a new pump in place.