'79 XJ6 4.2 doesn't start like it should


Here are the symptoms, cranks and starts (shakes) for a few seconds, then dies.
Compression is ± 8 on each cylinder
Fuelpump in trunk is new, with a good amount of fresh gas.
Sparkplugs are new, points on distributor cap are clean.
I’ve removed the cold start valve, and while cranking, a small amount of gas is being injected.
But when jumping the valve with 12v directly from the battery (with plug on injector removed ofcourse), it sprays great.
I checked voltage on the plug and its around 10V while crank.

Any help is greatly appreciated!


I had a similar problem in an an '84. The culprit was the Mass Air Flow Sensor. Here’s the test: Take off the air filter assembly and manually move the flap as you start the car. If you can find a position where the car will run while you manually move the air sensor flap – replace the entire assembly.

Good luck. On my car, I diagnosed or replaced nearly everything else and sure enough, when I replaced the MAS it ran like a new car.

Often you can buy them on an exchange basis – Rock Auto for instance has a rebuilder.

Thank you for the answer, though it seems it has a new one on it. 2 used ones were laying in the trunk.
Bought it like that. So maybe the previous owner had issues with it anyway.

First; remove the fuel relay and, on the socket, jumpwire between white and white/green. Various faults may cause the fuel pump to stop when key is released from ‘crank’ to ‘run’ - the jumpwire will bypass such faults; the pump will always run in either position…

The cold start injector only sprays very briefly during cranking - then plays no further part once the engine is started. Another cold start feature is the Auxiliary Air Valve, which provide increased cold idle, required for cold starts, and indeed the idle set-up - an engine that cannot idle cold will not run while cold. Remove the top hose on the AAV and verify that the slide valve is about half open when cold - move the spring loaded slide with a small screwdriver to loosen reposition the slide. Later to be verified closed with the engine hot. Also remove air duct at the throttle body and check that throttle gap is 0,002" - and that the throttle body is clean.

If any of these quick checks improve matters, further test can be done to localize the exact problem. If there is no change - we’ll look deeper; identical problems may have entirely different causes…:slight_smile:

And welcome on board!

xj6 85 Sov Europe (UK/NZ)

And welcome on board


Thanks for the welcome and answer.

I just jumped the wires, but no improvement.
Though when i took off the AAV hose i saw the valve is nearly closed… opened it with a screwdriver but it shuts (with a small spline) when releasing.

About the timing order, #1 is near the firewall, correct?

Slide is spring loaded (bimetallic spring), Abby, but with the engine cold it should definitely be about half open - something is not right with the AAV. If it is closed; the engine doesn’t get enough air to overcome cold engine drag - and may not idle. Also verify that the pump runs with ign ‘on’ and the relay socket jumpwired as described…

The #1 cylinder is indeed closest to the firewall - #6, frontmost, is the usual reference cylinder.

Questions arising; did the starting difficulties appear ‘out of the blue’, or after you have done some work on the engine or surroundings? The symptoms described is indeed also common with ignition mistiming among other things…so…?

xj6 85 Sov Europe (UK/NZ)

Thanks for the reply Frank,

Concerning the AAV, i checked this morning and it is closed… so indeed something is wrong with that part aswell.
The socket is jumped and the pump runs.
I saw they switched the firing order with #6 as #1, i corrected it but still not like it should.
Also, i measured resistance in the plug wires and 2 have nothing while the other 4 have between 10 and 15 k ohm… so not good anyway. I’m gonna start with a new set of these.

The firing order was not changed, Abby - it’s just that ‘1-5-3-6-2-4’ is sort of conventional way of referring to ign sequence…:slight_smile:

For any engine work the engine is to be set to TDC with #6 at the firing stroke, and set like that; the ignition sequence then reads 6-2-4-1-5-3 - which is absolutely identical; only the starting point differs. (You may even say 3-6-2-4-1-5; it makes no difference - except for the starting point…)

The crucial point is that the rotor must(!) point to the cylinder at TDC on the compression stroke - ready to fire. Ie, the first step is to verify which of the #1 or #6 cylinder is at its compression stroke - then verify that that cylinder’s plug lead is put into the dist where the rotor is pointing. One way of verifying compression stroke is to remove the #6 spark plug and turning the engine over toward the TDC mark with a finger on the plug hole - pressure is felt as the piston comes up towards TDC…

To wit, when the head is removed and replaced (engine always set to TDC mark) and using the cam setting tool - #6 automatically becomes the cylinder on the firing stroke. and the rotor should point to #6 cylinder plug lead. Hence the use of the #6 cylinder as the ‘reference’ cylinder - also because it is more accessible.

Jaguar, for whatever reason, numbered cylinders from rear as #1 at the time, contrary to the usual practice of counting from the front - hence some confusion…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

Still no improvement…
It runs badly for a few sec. than it shuts.
Maybe the pressure regulator or injectors are the cause of it.

Is the AAV still closed, Abby - it should be half open…?

Are you sure the firing order is correct - it’s not something to arbitrarily switch plug leads around. Verify tat you removed the #6 spark plug, turned the engine towards TDC while feeling pressure build-up at the plug hole (indicating that the piston is coming up on the compression stroke) - and set the engine at TDC.

Check rotor position and ensure #6 plug lead in is in that dist lid hole. Verify/place the other plug leads counterclockwise in the firing order; 6-2-4-1-5-3. Double check…

Check that the injectors are clicking while cranking. Jumpwire the fuel relay base white to white/green - to ensure the fuel pump runs with ign ‘on’…

If you doubt the fuel pressure regulator; you need to measure fuel pressure…

xj6 85 Sov Europe (UK/NZ)

Now the starter fails…clicks and nothing more. (Battery fully charged, not even with my Noco jumpstart) I think i’m gonna sell it as is to an enthousiast.
I have too many projects i need to give priority.
If anything changes i’ll give a heads up.
Thanks anyway for all the help!

Oh, well, Abby - it’s likely something trivial, and easily fixed…:slight_smile:

xj6 85 Sov Europe (UK/NZ)