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Jaguar S-type oddities

I have a 2001 Jag S-type, about 125K miles, a perfectly reliable car overall. I recently had a problem with misfiring; the spark plugs and valve covers were replaced. Also, some vacuum hoses were replaced. The car had renewed power, but when I brake, the car goes through strong cyclical vibrations, as if it were still misfiring, though when it’s going forward it seems fine. Also, the car died on me when I was making a right turn in town, going only about 5 mph or less. My foreign car mechanic, a guy with an excellent local rep, seems mystified. I might add that the symptoms seemed lessened when I turned off the AC. I tested the car using the “regular” drive and the “sports mode”, and I thought that it performed better on S-mode, thought that’s pretty subjective. Anyone know what’s going on?

I assume that you have the six cylinder (3.0L) since you said a vacuum hose was replaced. It’s a common problem on the V6, but pretty much unheard of on the V8.
Anyway, you need to replace all six COPs (Coil-On-Plug). You should also replace the plugs again, since you’ve been running them with the bad coils.

I don’t have the invoice from the last job in front of me, since it’s in the car, which is in the shop, but I believe he replaced the coils,or some of them anyway, along with the plugs and valve covers. I remember him saying that at least one of them had been fouled with oil, due to the worn valve covers.

Another thing to check (only applies to the pre 2003 ones) is the IAC. It could explain the stalling at idle.
Also, all coils need to be replaced, not just some of them. Trust me on this. The way these coils fail is very difficult to catch, and then only if you have a scope.

I’ll pass this on. By IAC, do you mean the air conditioning system? What is it about the pre-03 models that makes it a problem?

No. IAC is Idle Air Controller, the device that maintains idle speed set by the computer.

OK, thanks. I’ll pass that on also.

"What is it about the pre-03 models that makes it a problem?"
The fact that they have one. The 03 to 08 are throttle by wire and don’t have an IAC.