Lately I have noticed that my 1986 XJ6 S3 gets harder to start as the temperature drops...the colder the temp the harder it is to start. The other symptom is that I can often get it to start by "bumping" the starter instead of cranking it. If I bump the ignition switch enough for the engine to turn over once or twice, it will sometimes catch when the key is returned to the run position from the crank position. Once it starts, the car runs great with no misses or rough running. Two years ago I upgraded to a Pertronix ignitor and coil upgrade along with new plugs and wires. When cranking, I am seeing about 11.8 Volts on the coil's power connector, so it appears to be getting enough power to the coil. I didn't use the resistor bypass that are sometimes used to reduce the coil's voltage during normal running, so it should be getting full voltage at all times (and seems to be according to the cranking voltage readings).
The unusual symptom of starting when the key is changed from the cranking position to the regular running position is important, I think. The one thing I haven't done yet is to look at the spark when it is being cranked to insure it is hot enough to do the job. My mechanic thinks it had something to do with the injectors, but I disagreed due to the unusual starting habit of firing on the key return to the run position. It usually takes a combination of cranking, pumping the gas pedal like a madman, and bumping the key in the crank position before it will catch. Today it was about 20 degrees F and I had a very difficult time starting it. The battery is good and cranks normally...I can't wait until it gets below zero!!
Any ideas on this one?

Jim M. - Looking for a hot spark on a cold day

 

This engine and it's management system is not an area of expertise, if I have one at all.


1. I doubt that pumping the throttle has any effect. In carb'd engines, yes, the "acceleration" pump injected added fuel.


2. But, the point that struck me is that the engine would not fire with the key ion crank, but, on release to run, it might!!!


A. Lots of starter drag eating volts and as such little left for ignition.
B. An ignition switch issue. It fails to provide volts to the coil in the crank position.
C: A wire issue, the circuit from ignition switch in the crank position
is open.


Diagnostic: Has helped solve a lot for me. Jump 12v from the battery + post to coil + post. Crank. It might just fire right up!! Or even jump from a different 12v source!!!


Carl

 

Hi Carl,
Thank you for the response. I checked the voltage at the coil when cranking, and I got almost 12V (11.8 to be exact), which is pretty much normal with the load that the starter creates.
Another tidbit of info that may/may not have any significance, is that the car is more difficult to start in a direct proportion to the outside temp. On 70F days, it fires right up. 50F days it takes a little more pumping and cranking/bumping. I know that pumping the pedal doesn't accomplish much in a fuel injected car, but old habits are hard to lose. The temp shouldn't affect the ignition switch and I AM getting power when cranking (to the coil), so I don't think it is a switch issue. A wire issue falls under the same line of reasoning in that it shouldn't be temp related, and it appears too predictable to be a bad/loose connection.
I might try a direct wire to the coil. The only correlation I can imagine that is temperature dependent would be the battery. I put a load test on it and it does well and doesn't drag at all when cranking. I'm beginning to think that the coil might be sensitive to slightly lower voltages, which should be lower (from the battery) when the temp drops. Even though I am getting voltage to the coil upon cranking, perhaps it is just a tad too low to have the Pertronix coil/ignitor working properly. I really think I need to look at the quality of spark at the end of a plug wire when cranking...that should tell me right away if the problem is spark or fuel related.
I'm sort of talking out loud and appreciate the ideas and troubleshooting. I've thought about this a lot and the temperature and battery voltage are the only components I can think of that could cause an issue like this...unless there is something hidden away in the Jag's wiring that connects it to an outside temp sensor.
I think a spark check should be the next step....

****After checking out some related posts, could this possibly be a Thermotime switch issue? I didn't realize that there might be a cold assist circuit (although I alluded to it above). OK...check out the spark quality and determine if the Thermotime switch/process is working...if my car even has one!

Jim

 

Yes, you have a thermotime switch which controls the cold start injector. The cold start injector gives a spurt of raw gasoline into the manifold which cranking....if the engine is cold. The duration of spurt of gas varies according to coolant temperature. It lasts 6-7 seconds in sub-freezing temperatures....but gives no spurt at all at coolant temps of 100ºF or so.

In moderate temps of 60-70-80ºF an inoperative cold start injector might go entirely unnoticed. But it would certainly make a big difference in starting at 20ºF as you describe

The most basic check is to simply remove the injector from the inlet manifold and have a helper crank the engine. If the engine is stone cold you should get a strong spray of fuel. ...which you need to be ready for! Note that the system self-defeats after a few seconds on cranking so as to avoid flooding.... so make this check after the car has sat for a few hours

Here is some info:

Cold Start Circuit Checklist, Jaguar XJ6 Series III

Cheers
DD