Cold running/stalling issue

Hi All;
New to the forum here and need some help. I have a 1985 XJS - HE with 5.3L V-12 that was running pretty well, but now won’t take throttle when cold, until reaching normal operating temp. I thought it was a fuel delivery problem, as it will idle, but when throttle is increased it stalls. I installed a new fuel filter, removed the injectors and had overhauled and bench flowed back to 100%, and reinstalled with new hoses. Also replaced press and return hoses to/from fuel rail. I tested the pump & press reg while at it and it has a solid 40PSI. I also tested the aux air valve, and it seems to be functioning as designed.
So now I think it must be in the electronics. Any suggestions on where to start trouble shooting there? Thanks, Steve

It’s not supposed to have a solid 40psi. The pressure should vary with manifold vacuum, and since manifold vacuum is high at idle, fuel pressure should be low. You should be able to watch the fuel pressure jump as you blip the throttle. Might wanna check your vacuum connections to your FPR’s.

Easy enough to test your Coolant Temperature Sender and Throttle Position Sensor with a voltmeter just to make sure.

Also, your AAV could be stuck closed.

I would look at the coolant temp sensor CTS very cheap and easy to change out.

I have checked the AAV and it is open when cold, and as warms up mostly closes. I understand the small amount still going though it is for the ideal. I’ve already changed the Temp sensor/sender. You’re rite, it was cheap and easy. Made no difference. Good idea to check the press regulator connections.
Thanks guys!

At this point it couldn’t hurt to check the Throttle Position Sensor for correct setting and make sure it doesn’t have any dead spots. Any irregularity…fueling lag in this case… would probably be intensified on a cold engine.


Thanks DD; I may as well check/replace spark plugs and distributor condition/settings as long as I’m at it. Does anyone know what the symptoms of a bad computer (ECU) are? Will it even run if that goes bad? Sure hope I don’t have to replace that!

ECUs can fail in so many different ways, and the 16CU is pretty hardy.

But mine did. My suspicion was that it barely ran, and disconnecting the CTS or vacuum line to ECU made no difference.

Stevek, check spark plug wires while you are at it. There is a certain ohm reading per linear inch of spark plug wire. I don’t know the value off the top of my head but I believe this a few youtube videos on the subject. Good luck!

BTW, mine has this issue the first 30 seconds of running, especially when it’s cold outside. Tonight, I stalled it giving some gas right after startup. Although it is 32F, and my AAV is not fully open when cold, so it’s idling qute low (400 rpm)…

How well do these V12’s run when stone cold in freezing weather? It reminds me of my old carburated cars that needed a good minute to warm up when it was cold out.

Good suggestions. I’ll check plugs, wires, and cap etc. while I’m in there. Before I start that, from what I’ve been reading I think I’ll check and or replace the throttle pot. It’s easier to get to and apparently the early ones (my year) are known to be unreliable. When I get this problem solved I’ll be sure to post it here for everyone to see.
Thanks; Steve

How long did your car sit? And how did the PO drive it?

I’ve replaced almost everything on my V12 (1988 XJ-S), and although it runs very well and has plenty of power and excellent compression, it doesn’t idle 100%, runs a bit rough when cold (first couple minutes), and isn’t getting great gas mileage (10.5 mpg in city).

Many on here have suggested an ITU (Italian Tune Up) as the final step. I’m almost sure the first owner of my car didn’t drive it hard with only 48,000 miles and living on an island (lot of short trips), and it failed smog twice with the PO running super rich. (I’m the third owner) Which means a lot of carbon/varnish build up on the valves, tappets, etc. Your engine could have the same thing?

I’ve ran some sea foam twice, and am currently running Risone in the oil, but an ITU is overdue this spring. You may want to try it once you make sure your ignition, fuel system and compression is in good shape.

For Greg and everyone else following my plight, here is some history. First, I am the previous owner, having owned it for over 30 years. Actually I’m the second owner, getting it when just a few years old. I dove it very regularly for nearly 20 years including some long trips and wasn’t afraid to use full throttle now and then either. Then gave it to my wife to drive around locally for another few years. Then she got a new car and no one was driving it, although by that time it had accumulated just over 103,000 miles. I thought it was too nice to give away but there wasn’t any interest or value in these cars then, so I stored it inside and covered for several years. Oh, I kept the battery charged on the bench and once or twice a year would put it in and drive around the block a few times, but had it “non-oped” with the CA DMV and also suspended the insurance.
I got it out of storage last March (2019) insured it again, passed the CA smog test, and relicenced it. At first it ran pretty good, though a bit cold blooded, and since has been running worse and worse. Knowing that just sitting isn’t good for any car even with fuel stabilizer, I started out replacing the battery and all tires. Then the fuel filter, and removed all 12 injectors and had them cleaned/rebuilt by “Mr. Injector” who I highly recommend by the way if any of you need that service! Only one tested good when he got them but after cleaning and reassembled they all tested 100% on the bench. I reinstalled them with new fuel lines, and also replaced the press and return lines under the hood.
Oh, and since getting it out of storage I’ve run two or three cans of SeaFoam through it, and at least the same of Chevron Techron. Now I’ve been using Marvel Mystery Oil in each tank so hopefully that will help with any buildup on the valves and cylinder heads. And now it’s just over 104,000 miles.
A couple more things about the storage, the first thing I did was to remove the brake master cylinder reservoir, clean it, and flushed - bleed the entire brake system with new fluid, so I could trust them. I then flushed the cooling system and refilled with 50/50 mix of new coolant. I discovered the heater valve had frozen up, so replaced that while I was at it. And all of this to get it good enough to put back on the market! Hope I haven’t rambled on for too long here, but hope that gives everyone a sense of what I’m doing. Thanks to everyone that’s offered some suggestions!

I never had any cold running problems with my '88 XJS. Started easily, never stalled, no bogging-down. Even in sub-freezing conditions. For the record, the AAV was inoperative on that car for most of the years I owned it.

I have had some cold running problems with my present V12…and '88 engine transplanted into an '85 Series III sedan. Specifically, bogging-down on acceleration.

The CTS, which was only 2-3 years old, wasn’t giving consistent readings. Replacing it did help…but some bogging-down remained. An Italian tune-up took care of that.

Since then…a year or so ago…no problems at all.

For the record I don’t have an AAV on this car. I ditched it entirely. I have no stalling problems when cold…although I do have the base idle set a bit high, about 900 RPM in “P”. I just start the engine, drop it in gear, and go. In sub-freezing conditions I let her run for 30 seconds or so before driving off. it just seems like the right thing to do :slight_smile:


Without AAV, how do you set idle rpm?

I plumbed-in a valve for that, vaguely along the lines of Bernie Embden’s arrangement.


Hi Doug, I’m back, had some things to do earlier. I have done the same as you. My AAV is still installed. What I did was silicon a washer in the metal tube in the air cleaner housing and I modified the hole to get the correct idle. I stopped at 900rpm idle speed and I get 750rpm with tranny in any gear and the AAV idle adjust totally closed. No start up or running problems at all just like you. Extremely smooth idle.

Sonora/CA, 90 XJS-V12 conv, United States

I did away with both fuel pressure regulators & installed an adjustable one along with a fuel pressure gauge on the fuel rail. Why, because after changing over to a manual transmission with aluminum flywheel & getting the ECU reworked via Bywater the car stalled very easily with a hot engine restart/driving within the first 5 mins , Probably vapor lock of some type. With that said I adjusted the fuel pressure to 40 psi from 35psi. Hot stalling problem solved, Gas mileage/runs good, etc

I’ve now replaced the TPS with the later style “red” one. My original “black” one did have a dead spot in it, so needed replaced anyway. However it hasn’t solved my problem, and in fact it’s worse. Before diving into the ignition system, I pulled the center wire from the distributor and cranked it over. It has a very strong consistent spark and I see no reason to suspect the timing has changed. The wires and cap are not that old and are in near new shape, so I’m back to looking at the electronics.
About the only thing left now is the Central Processing Unit. I see some on e-bay, but would like to have this one tested before buying something I may not need. Does anyone know of a person/place that tests or repairs the computers? Or some way for me to test it?

I had a similar problem about a year ago where I tried everything.

It ended up being the ECU. My suspicion was that unplugging the CTS barely change the idle. Nor did disconnecting the vacuum to ECU. When warm, unplugging the vacuum should make the car almost stall, and unplugging the CTS should stall the engine right away.

I sent it off and sure enough the ECU needed fixing.