Hard cold start V12...running out of ideas!

1989 V12, Marelli. I’ve been working away at sorting a variety of age related niggles and have the car running well. The biggest improvement in what was a very lumpy idle was to replace the CTS which made a huge difference. Mind you, it was waaaay off spec!

The main outstanding challenge is a variably hard cold start. This is from cold only: absolutely no issues starting if even slightly warm. So, first start of the day and it can take multiple 7-8 second cranks to get the engine to fire and it generally stumbles very badly for a second or so before catching and running smoothly. Runs well at all other times.

What has been done/tested: AAV all good (removed, tested, cleaned, refitted - idles at around 1000-1100rpm cold before settling to 750 once warm). FPR’s good, fuel pressure 36psi (with a slight injector leak admittedly, but my Jag Whisperer, Brian Turner from Jagland in Brisbane reckons it’s not bad enough to be a factor), fuel filter has maybe 1000km on it, CTS, ATS both new. Plugs in good condition, wires, cap, rotor, coils, amplifiers all new. ECU fuel trim at centre. Throttles adjusted, throttle bodies clean. Exhaust is balanced, vapour, no fuel smell. No cold start injector on this vintage unit.

I prime the fuel rail a couple of times before turning it over…although opinion appears divided whether this ‘should’ be necessary: I like to think that’s not how they planned it in Brown’s Lane…

What really bugs me (engineer!) is that it’s variable. Not intermittent: it does it every time, but the amount of time to catch (and indeed the speed at which it ramps up) varies seemingly without logic. I’m down to suspecting the fuel pump: not the pump as the pressure (and normal running) is fine, more the built in check valve which might have failed letting the rail pressure drop rapidly after the 2sec run on key turn. However, I can’t help but think this is an unlikely failure mode, so reaching out to the wider V12 wisdom for suggestions…TIA!

The pump should also operate whenever the starter is cranking the engine.

Does it?

Cheers
DD

It should not. If it is, the check valve is leaking. A leaky check valve is common on the XJ6, but the XJ-S has the check valve built into the fuel pump outlet and is reportedly very reliable.

My cars are Lucas ignition, so Marelli is a little foreign to me.
I would check you have a decent voltage on the +ve side of the coil or coils when cranking.
It should be very close to the battery voltage when cranking.

If it is not then it could be hard to start cold, but as soon as the alternator spins you have more voltage anyway. Once the car has warmed up even a few minutes it should be easier to start.

If you get it to start from cold, run it for maybe 5 seconds then try to start again what happens ?

Another trick is to use a spare 12V battery for the coil +ve while cranking. If it fires up easily then you eliminate a lot of other possible causes.

Does the v12 have a cold start injector?? If it were carburator controlled, we know to look at the choke. Just wondering.i had a fuel injected car with a bad cold start injector and it was a bear to start cold.

put a 13V battery in , i had the same problem , new battery , measured 12.99 V yesterday morn, was 28F around 8;00 AM !, unheated garage. NO charger on it, cable disconnected

.
yeah i know(no such thing as 13V car battery), just maybe modern technology has changed things !
a world of difference starting, maybe more coil Volts cranking, i also use a relay to put full battery to coil (when crankin).

I have a 89 xjs v12 with Marelli and have the same problem in the morning. This is my first Jag V12 so I thought maybe because of the age and a V12 it was a characteristic of the car.

This does not mean that is correct.
I would investigate that if I was you.
When you start the engine you are on closed loop, so, the trim is important.
Incorect/worn plugs or cables could also be a possibility.

I think it might be worthwhile to check some voltages, notably at the ignition coil + terminals and on the power feeds to the EFI ECU. During starting, of course.

Doug, yes, the pump does run during cranking.

Richard, I’ve checked voltages: good at battery and coil

Thanks Thomas…no cold start injector on this vintage of car

Aristedes, I’ll do some more investigation here. I had a bunch of work done on the car a while ago and the trim was adjusted back from ‘quite a long way clockwise’ to centre which may well be at least in part to blame for what I think is worse starting performance from that time (it’s a bit subjective though!). The car has 196k km on it so I’d expect the trim could have been adjusted leaner. Now the challenge of digging through the forums for the correct set process (6 clicks anti-clockwise from max was the factory setting IIRC)!

Yes, got that.
You say once started the car is immediately 100% O.K.
That suggests just about everything is working well.
And when warmed up, easy to start which is generally the case anyway

I did suggest starting the engine cold and running for only 5 secs or even less.
The engine will still be cold.
Then is it hard to start again ?

Two things make life more difficult for an engine to start.
The battery voltage drops on cranking, less energy in the coil.
Once spinning the alternator increases available voltage.
Inductive pickups give a signal proportional to RPM and the gap.
If the gap is a bit too big might not give a good signal at low cranking RPM.

I mentioned on the forum a while back both my V12s will not start first go if they have not been driven for a month or so. The battery is always charged for a day before cranking,
It might take 3 or sometimes 4 cranks 4 seconds long at 5 second intervals.
No obvious reason for this. Provided the car has been driven at least 4 or 5 days previously they usually start first up with a 2 or 3 second crank. Never a problem starting a warm engine.
Is this typical ?

Richard…sorry, forgot to mention that yes, once it’s caught it will start again immediately even if only run for a few seconds. And this stays true even after about half an hour. Apologies for being slow but what inductive pickups are you referring to here?

Alastair, that is a little bit like my Lucas HE V12s.
In my case they are a little hard to start if not used for about 4 weeks +, not as hard to start as yours, and once run for even a few seconds easy to start after that.
Otherwise never hard to start when cold.
One of the V12s is stock standard, one has a totally different aftermarket EFI system.
An eternal puzzle.

The gap mentioned is for the inductive pickup to trigger ignition. Lucas has it in the dizzy, Marelli has it on the front of the crankshaft. Worth checking but if you can start a cold engine a second time easily after an initial start not a prime suspect.

I would still try a separate fully charged battery on the coil +ve. If that makes starting easy something in the ignition system is suspect, something really marginal.
If it does not make it easy, back to the drawing board.

Thanks to all for the great suggestions/advice. I’m not in a position to test them at the moment as the car is off getting the aux fan replaced (flogged out around the centre mount) at the moment.That’s another lovely job that the repair manual indicates is an easy-in, easy-out…until you realise the whole fan shroud/cowling has to come out first. Interestingly, when I took it in, the car fired up perfectly, first time. Gotta love old Jags!

Sorry to butt-in.

I didn’t know the pumps were different. I’ve been interchanging them over the years. Can you tell by looking which has the check valve? Or do I check part #'s? I have a box of used ones. I guess the V12 type is best for the 6 or 12.
Thanks,
Rob

As I understand it, the pump for the 6 – or perhaps for the saloon, 6 or 12 – has a separate check valve that screws into the pump and serves as the outlet hose barb. The pump for the V12 XJ-S has an integral check valve built into the pump outlet end. You’d think replaceable would be better, but reports are that the separate check valve needs replacing all the time while the integral check valve rarely malfunctions.

I dunno if both pumps have the same capacity. I do know that running a V12 with an undersize pump would most certainly result in piston damage.

The XJ6/XJ12 had two tanks, two pumps, two lots of fuel line plumbing, and a bunch of extra solenoids and electrical complication. Yay! Well done Jaguar.

I’m glad the XJS has fixed that. Wait, what’s that you say? My XJR-S also has two pumps? So a chance of twice as much to go wrong, and possibly break my pistons, which are made from unobtainium. Thank you Jaguar, thank you so much.