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Jaguar Repair/1985 XJ6 cold start problem



You may very well not remember, but I last asked you about a knocking in the rear end after having replaced rear wheel bearings, radius arm bushings, shocks, etc. Well, it was the u-joints after all. I replaced all four last spring and all is smooth and quiet now. I read recently your answer to someone on here who had a no start issue on another series 3. I'm having a cold start problem myself. If not driven for several days, mine may require a dozen attempts to get started and running. Once running, I can go out for as long as I wish and run errands or go sit at dinner for an hour. Either way, it'll start on the first try every time the rest of the day or evening.

I had an assistant turn the key while I checked voltage at the cold start injector tonight and that showed about 9.75 volts. An ohm reading at the thermal time switch was 50 ohms. What else can I measure to perhaps determine exactly what the issue is here? The evidence would suggest the thermal time switch, but I'm not guessing on a $60 part.


ANSWER: Hi Michael,

With the engine cold remove the two wires off of the Thermal/Time switch. One is white w/green tracer and the other is white w/purple tracer. With your ohm meter test the two pins on the switch. One should show some resistance to the engine block and the other should show a direct short to the engine block. The pin that shows a direct short to ground (engine block) should get the White w/Green wire. That checks the switch and the wiring to the switch.

Now with your meter set on DC volts have someone turn the key to the start position while you test the voltage on the White w/Green wire and the White w/Purple wires (not connected to anything) You should see 12v + to both. That tests the power applied by the "Start" position of the key.

Now, remove the cold start injector (you will need a piece of fuel hose to put in place of the short hose on the fuel rail that feeds the cold start injector and long enough to have the injector so you can point it into a jar or can.

Now, remove the air filter can from the airflow meter and open the flap inside and lay a screw driver or anything to hold the flap door open.

Now, With the wires on the thermal/time switch and the injector and pointing the injector into a jar or can have someone spin the starter motor. You should see the injector spray in a cone shaped pattern. If it does not spray then you either have no fuel pressure or the injector is bad.

To test the injector by itself connect two jumper wires to the injector and with the airflow meter still wedged open turn on the ignition key to IGN not Start. You should hear the fuel pump running. Carefully ground one of the jumper wires from the injector and just touch the other to a + power supply. (Keep in mind that you are now spraying fuel into your jar or can so you want to make the connection far away from the jar or can because it will make a spark)

The reason it starts easily an hour after you have run the engine is because the coolant and engine block is still warm and the thermal time switch has still not activated the cold start injector as it is not needed. The fuel from the injectors is enough to start long after the engine has been shut down. (except in really cold weather)


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

QUESTION: Thanks, Howard.

I'm at work this morning, so I cannot do the testing you just described until this evening, but it did occur to me after I sent my question last night to try jumping the connector for the Thermal/Time Switch to close the circuit. The car started at the turn of the key that time - dead cold. I shut it right off, but left the jumper in place and came back about 45 minutes later and it started right up again. This is in a sub - 60 degree garage. Would that give you any clearer indication of a fault with the thermal/time switch, or would you still recommend the testing you outlined in your first response?

Thanks again,

ANSWER: The trouble with your test is that if you put a  jumper between the two wires of the time switch you did nothing, because they are both hot wires and a hot wire jumped to another hot wire does absolutely nothing. So, unless your wiring is messed up you completely eliminated the cold start injector.

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

QUESTION: Point taken. That leads me to expect to maybe find a leaky cold start injector that is flooding the intake, but when disabled, allows the engine to start quickly. Just a hunch, I'll do all the testing you described later this evening.

Thanks again!

Symptoms and logic can sometimes lead you to a fix but working in dealerships we had to use more positive methods to arrive at a 100% fix the first time because we were working on commission and any "Come-Back" had to be done free. Diagnostic by logic and symptoms was occasionally a quick fix but only worked about 25% of the time. Diagnostics by testing was a 100% fix rate. Thus over the long run much quicker. Another factor was no electrical part can be returned to the parts dept. even by a dealership mechanic. This is still true today in all car dealerships. So many cars today get fixed but only about 50% of the fixed cars today were fixed with the minimum parts necessary. As many mechanics today diagnose by logic and symptoms and only test as a last resort. To add to the problem the code system in computers today only spot a failure and that is pure symptom diagnosis.

I watched a mechanic check for codes in a local Ford dealership I worked in as an engine builder and it said xxx valve failed so the mechanic drew that valve (an electrical valve) from the parts dept and put it on the car and cleared the codes and road tested the car and it set the same code. Now he had to test each of the seven wires going to the xxx valve. He found a break in only one of the wires which fixed the car but that customer had to pay for the new xxx valve and the wire repair because parts departments will not take back an electrical part once it had been installed on a car.


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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.

Associate member SAE EAA member

Import Car magazine

ASE Master Auto with L-1 certification up to 2000

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