Hi to all, this is for a 1984 XJ-S V12

The car is having a very sluggish take off, it idels fine and behaves well after it has gained some speed, only when takin off from total stop is when you feel as if the car has no power...
I have checked all the usual places and the only thing I can find is that the TPS reads 0.44 volts and not the 0.32 to 0.36 volts that it is supposed be, my question is if this extra 0.1 volt difference in the TPS is enough to cause a sluggish take off?

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No, it is not, if anything, slightly higher TPS at closed voltage would help takeoff.

Some off the top of my head things to check:

• idle rpm, should be about 650 to 700 in drive
• fuel pressure low, worth changing the B bank FPR
• Fuel filter (behind spare wheel in boot) clogged
• fuel swirl tank sock filter clogged
• timing a bit off, you could try advancing it a touch using the adjuster on the side of the dizzy

Good luck.

 

Greg,
Thanks so much for your reply, I will leave the TPS alone for now and address that once I solve the main issue. I will look into the timing, this might me the point to solve...

Alfredo

 

Is your take off in a low gear ?
At what engine RPM and what gear do you think it seems like it has full power response ?

The TPS should give a smooth increase in voltage as you turn the throttle capstan, better check that.
0.44V at idle is a bit high but would not cause a low power response at takeoff when you have plenty of power at higher RPM.
The vacuum signal to the ECU might be slow to change when you take off. Check the vacuum line is not clogged or kinked.
The other thing is to check the pulse pickup clearance so you have a good pulse signal to the ECU through all the RPM range.

 

Is this a new development?

At some time in the past it had more power?

Cheers
DD

 

The take off is at 1st gear, the transmission is shifting fine and I have checked manually going to 1st gear.
After take off, at about 20 mph, the car runs fine...
The car had been working fine, this behavior was noticed just after a brake rehab, master cylinder and 4 calipers... Brakes are working properly now and I can push the car in neutral with one hand...
I will chech timing today

 

Yesterday I checked the timing and it is fine, 18º btdc at 3000 rpm with the vacuum advance disconected, and just for the records, 0º at idle.
What I found is there is no vacuum going to the capsule on the distributor, so no vacuum advance, this makes me believe that I have found the problem, my understanding is that from a total stop, and 0º there is not enough "advance" until the engine speeds up and the mechanical advance comes into play, am I on the right track?
I now have to try to understand how this vacuum source works, what the diferent timers, regulators, solenoids, etc. do, and how to fix this, or solve my problem some other way. Anyone coul explain how this works and if there is a common failure that I should look at, or any lead for a writeup on this matter? The car is a 1984 XJ-S V12 NORTH AMERICAN MARKET.

 

10 out of 10 finding but beware as that vacuum connection is disabled for the first 45 seconds after starting. I don't know if this helps the vac system is an abomination and collection of fixes for different issues that Jaguar encountered - wall to wall sticking plaster solutions - well that's my opinion.

The V12 Ignition Advance ....

 

Your vacuum advance could be toast. It has been 40 years
Kind of a time consuming repair as your have to take the distro apart down to the base. All the plug wires come off the distro, distro, rotor, flash shield (watch out for the 4 little bitty screws), and then the pickup. Will need to punch out the stop rod to remove the vacuum unit. Installation is just the reverse.

I am told the vacuum advance module is actually a Chevy part so you might want to check cross referencing the part # first. They are fairly pricey thru Jag suppliers IIRC. Make sure to get the little foam seal as well. The old one is likely brittle and cracked.

 

It could be that your vac capsule is toast. To prove this, if you can suck air through a vac tube connected to the vac capsule port, the capsule needs chnaging. This could explain why your car suddenly had this symptom.

If the capsule is OK, then if your legalities allow it, disconnect all the timer/vac rubbish and run a vac tube direct to the vac capsule from the small spigot under the A bank throttle body, or any other spigot on the end of the intake manifold.

 

The vacuum capsule is fine, just changed it a few months ago but tested it yesterday again with a handheld vacuum pump... It is working properly...

 

Yes, no problem with emissions lagalities for this car.
On the A bank throttle body there is a port on the top and another in the bottom, I think one is "ported" and the other "manifold" vacuum, which one should I use in case I go this route?

 

I tested the advance with the car fully warmed up, so the 45 seconds were over, unless this os the problem and the 45 s gizmo is not opening as it should...

 

Problem solved, it ended up being a clogged vacuum hose from the intake manifold to the 45 second valve... A new hose and now the car takes off with no issues...
As always, thanks to all for your help...

Alfredo

 

Well found. FYI in answer to your which port question: The one under the TB should be connected directly to the vac capsule, if you want to do away with all the rest of the vac stuff and the 45 secs. Easy to try it out before you remove anything.

 

Thank Greg, now that I know how thill all works, I will soon try that on my 88 v12, just to be ready in case something fails in the future, or just to see if I like it better...