Something is about to give? (Intermittent no start with good crank)

My 1988 XJ-S V12 has been very dependable for the last 3+ years, I’ve replaced most everything fuel and ignition related.

Yesterday, for the first time, it would not start. Not even trying to start. Like there was no spark, or no fuel. It had been driven to work in the morning with no issues, and simply sat in parking lot for about 8 hours. I had to go somewhere, so just left it. Came back 2 hours later, and it started up right away.

Driving home, no issues at all, except about 20 minutes into the drive, from a stop I gave it a lot of gas. As it accelerated, for about 1/2 second, no power. It was like someone flicked a switch off, and then it suddenly flicked back on. Didn’t choke or anything, just zero power.

I am going to look at all connections for ignition and see if something is loose? I’ve never replaced the coaxial wire to the ECU, perhaps it’s time to inspect. Otherwise, I may have to wait for whatever is about to give, to fail? The GM Module is only a year old. There ‘could’ be a fault, but when hot, it works no problem.

Sounds like my car last summer when I replaced my ignition coil. Turned out the coil was a dud. I put the old one back in never a problem since. My new one has been on order for 8 months from the Jaguar dealership.

Yeah, I wonder about that too. I had bought one of the new DAC6093 single coil updates years ago, and it was faulty so sent it back. I then bought same coil from somewhere else, and it’s been fine for 3 years. maybe?

I also purchased an MSD 8207 coil as a backup. Maybe it’s time to install that.

Don’t forget the CTS. Carry a paperclip with you. Also the condenser inside the ignition amp can kill the engine if it shorts to ground. I removed mine.

Ha, paper clip and extra GM Module at all times on this car.

Yes, if I can get the no start condition again, I’ll try checking CTS. I removed my condenser a long time ago.

Was thinking what have i changed over the last two weeks. As a hot soak test, i relocated the air temp sensor from B bank air intake housing to an area at front of engine compartment. I assume it can make a slight difference in fuel mixture, but not cause no start or no power? It ONLY measures temp, right? Not flow?

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@gregma - I’m interested in this also, as I am looking to relocate that sensor also due to building out more intake air flow.

FWIW, relocating the air sensor did not make any difference to hot start issue.

I did build 3" air intakes out of PVC, and simply drilled a hole in it for air sensor.

If you have gone through the normal checks wires Fuel pressure ect i suspect it will be the coil ,If you replaced it with a Lucas single coil they are prone to failure I’ve I have run many V12 jaguar on a M.E.D blaster COIL they give a very high spark which improved performances as well . The V12 just love a strong spark …

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Man, @gregma, you got me thinking now. I was gonna use honeycomb aluminum to fab a housing , instead of the trumpet cover, that would allow more air through the honeycombs, but now you got me thinking about how to adapt a 3" slip hub with a 3" PVC elbow with K&N filters… Just what I need another easy sounding project :grin:

I got rid of my K&N filters, i think it was causing the bit of oil I was finding in both airboxes. My paper filters do just fine.

So I kinda did what was suggested in The Book, I cut off the trumpets, got some 3" PVC pipe, and used a ton of JB Weld to attach to the metal air intakes. Sanded best I could, and painted them.

Definitely get a bit more throttle response, and the sound is amazing at full throttle.

Other things that have changed recently, I’ve upgraded the alternator from 100Amps to 120Amps. Could this affect/ruin an ignition coil? Or maybe the Jaguar stock coil can’t take that much power?

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It’s simple enough to take a resistance reading on your
Coil, mine was reading about 4000 ohms on the secondary
I think you need to be up around 8-12000 if I remember correctly.

I don’t think upgrading the Alternator would have any effect on the coil. The alt only supplies the car with sufficient current to satisfy the cars needs.


Alright, more data! I went ahead and fitted the MSD coil i had laying around. Started fine, went for drive, no issues. Parked, came back fifteen minutes later, no start. Exactly same as yesterday. Tried twice, no dice.

The third time i cranked engine, i also tapped gas pedal on/off. Started right up nice and strong, car ran fine on way home.

Is there something triggering injectors at start up VS while running? I know tapping gas with ignition on will spray injectors, correct? I hope my ECU isn’t going bad.

Those intermittent no start problems are a real PITA to solve…
I think it’s imperative to determine if it’s spark or fuel, or rather the lack of it, that causes the no start.

If spark, ignition amp would be my first choice, coax cable and maybe the ECU.
You eliminated the coil.
Check all your connections, the other day mine would not start because the spade on the coil was loose.
Then it would not rev above 5.000RPM because the spade to the secondary coil was broken inside the spade boot.

When my ECU went bad it would sometimes take forever to start, engine was rough but suddenly would clear and everything was perfect. It would start again no problem until next morning.

When my new (non GM) ignition amp went bad it would not start sometimes until it wouldn’t start at all.

The 120Amps Alternator has no effect .
The engine will start even with the ATS disconnected, it will be just a bit rough.
If the CTS is bad the engine might not start but I think that the CTS either works or it doesn’t, no intermittent problems, unless there is a bad connection.

An other time it wouldn’t start because a piece of debris blocked the fuel pump.

I would really look at the FPR, when mine went bad, it would have trouble starting sometimes until it would not start at all.

Hope that helps.

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Could be lack of fuel (FPR…?) but could be coincidence.
You have to repeat the test to verify.

If it is Lucus ignition system, try cleaning the tips of the Star wheel and the Pickup contacts with very very fine sand paper and contact cleaner.


Thanks for the replies. It all may be a coincidence, but I’m also racking my brain trying to figure out how I could have introduced this with the subframe removal/replacement. Never had this no-start until now.
Like you said, I could have knocked a wire. Will double check everything.

Another thing, or I could be way off, I filled up the tank in early December, but I did put some StaBil into the gas tank. That gas has been sitting there since then, through a cold humid Seattle winter. It was fine starting last couple weeks driving with a full tank, but I’m now down to 2/3 tank. Could some water in the fuel be messing with me? May be worth topping off and getting it mixed around?

Your hitting the throttle may be a clue. Is a wire or cable getting touched by the throttle linkage? Could the throttle position sensor be on its way to failure?