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Jaguar Repair/Jag XJ6 Cold start issue



We spoke late last week about my 1985 XJ6 having a cold start issue and your last post to me was a series of tests of the wires at the thermal/time switch and the cold start injector's spray pattern, etc. I began to test the white/green and white/purple wires at the thermal time switch last Friday evening, but my helper kept turning the key all the way to start, and the car kept starting on first try. In fact, I never got readings that I was satisfied with, and need to re-visit that actually, but I have to admit I quit on account that the car is now starting on the first try, every time. Every single time. In fact, I have tried it a couple of times a day, every day, and it'll start on the first try every time now - with the thermal/time switch unplugged. Do you have a known reason for that?


ANSWER: Hi Michael,

All engines require extra fuel for cold starting. If your car starts first time, every time cold without the cold start injector operating then either you have the car garaged and it is not that cold or the engine is receiving enough fuel from other sources. Like a leaking cold start injector or any one or more of the 6 regular injectors might be leaking. You could also have a broken diaphragm in the pressure regulator and it is drawing fuel from the vacuum hose.

There are many possible sources for extra fuel. But it is a fact that all gasoline engines require extra fuel to start when the engine is cold. That is why carburetor cars have chokes and all injected cars have some method to supply extra fuel for cold starting.

If your car is parked outside and it starts first time every time now with the cold start injector disabled, You definitely have a fuel source.

Unless you have CO/HC test equipment available do this. Take a volt meter set on a low volt scale and unplug your 0-2 sensor (if it is a one wire sensor) in the exhaust manifold and start your engine and set the RPM at about 1500 RPM and hold it there for 30 seconds and check the voltage of the wire from the 0-2 sensor (not from the car) The 0-2 sensor produces voltage when heated to about 600 deg. and the output should read about .5v if the fuel mixture is normal. If the mixture is rich (extra fuel) the reading will be from .8v on up to 1v. If the mixture is lean it will read .3 or below.

If you see a rich mixture you need to start looking for the cause in some of the items I listed and even high fuel pressure. Normal engine operation is close to 32 PSI and open throttle or lose of vacuum will cause about 42 PSI. A failed pressure regulator can cause high fuel pressure which causes a rich mixture.

A clear indication of a failed diaphragm in the regulator is fuel droplets in the vacuum hose on it. To test the regulator itself you need to put a "T" in the fuel line at the fuel rail and run the engine and read the gauge. (be sure to put clamps on all hose connections)

Let me know,


---------- FOLLOW-UP ----------


Ok, Fuel pressure checks ok, and regulator checks ok. I re-tested the white/green and white/purple wires to thermal time switch tonight and got just what I got the other night - that I wasn't satisfied with, which is zero v. Checking both separately to ground, there is nothing there. There is fuel pressure of course at the hose to the cold start injector, but with it removed, and in a jar, thermal time switch wires connected, flap in throttle body open, etc., there is no spray from the injector. At all. BTW, voltage from the o2 sensor was 0.95v at idle.

The car is in a garage, but temp was about 50 degrees tonight and has been about hovering around 60 in there the past few nights. I have to rule environment out as the car was doing this in July when the ambient temp in the garage was often 80 degrees. I just became motivated to deal with this here lately.

Thanks again,

Michael, From what you said you didn't do any of the tests I said. I guess I am not making myself clear so lets do only one test at a time.

1. Remover the two wires (white w/green# and #White w/purple# from the thermal/time switch #large front switch on the coolant rail#

2. Connect the #-# negative probe of the volt meter to a good ground.

3. Connect the #+) positive probe of the volt meter to the White w/Green wire.

4. Turn the Ign key to the "Start" position and read the volt meter. If it does not read 12v then go to the starter relay on the firewall (The large Relay) and locate the "W-2" terminal with the two White w/Yellow wires and check the voltage on those wires while the key is in the "Start" position. You should see 12v.

If you see voltage there and didn't see it on the White w/Green then you have a disconnected wire between the White w/Yellow and the White w/Green.

Let me know.


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Howard M. Fitzcharles III


Jaguar from the XK 120 to XJ-6 ser. 3 1987


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

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