Jaguar Repair: My 1985 XJ6 - strange starting today, tie rod ends, coolant temp

Question
Howard,

First, thanks again for doing this. I always appreciate your insight. My XJ6 restoration is moving right along, having completed re-bushing the front end this week, including ball joints, shocks, and tie rod ends, and the ride rivals 20 year newer vehicles. With the mild TN winter we are having, I have had my XJ6 out all day today, but a little miffed that the normal - first try start - was not the case all day today. Several times, the first attempt at starting produced a quick puff of smoke from under the hood, specifically, belched from the air intake horn. Second try and it always started right up, and ran well with no other issues on back roads, or highway speeds. Any thoughts on what is causing this backfire from the intake on first start attempt, when the second attempt, it starts just fine?

The only other recent work I have done is replacing the exhaust manifold gaskets earlier in the week, tightened the bolts last night after having driven with the new gaskets in.

Thanks,
Michael

Answer
Hi Michael,

Good to hear the project is progressing. First, are you sure the puff of smoke was from the intake? If so, most backfires out of the intake on a cold start is due to a lean condition. Especially if the throttle was opened. The series III Jag should not require the throttle to be opened when starting. As soon as the engine starts (throttle closed) it should go to a fast idle and usually on flat ground you should be able to back up with no throttle at all. Most people don't take the time to warm the engine up before driving (which they should do). It should only take a minute or two for the engine to drop to a normal idle even though the engine temp has not moved. The removal of the fast idle is not done by coolant temp but is done electrically.

If you get any backfire into the intake when the engine is warm, you need to run a compression test or a leak down test to see if you have a leaking intake valve. Also check the ignition timing. If all this is ok, you might try to turn the mixture screw on the air flow meter a little to a richer mixture since it is colder and the coolant temp sensor for the injection may not be doing enough. Also test your fuel pressure with vacuum applied to the regulator and with out vacuum. About 32 PSI with vac and about 42 without vac.

Howard