As you know, I’ve been a stickler in trying to get my 1988 V12 XJ-S to idle as smooth as possible, and I’ve pretty much pulled it off!
Getting the two throttle plates set perfectly seems to be a big help. But I’ve noticed it sometimes slowly moving away from that smooth idle now and then. So today I tried giving both throttle bodies a spray of throttle body cleaner, and idle is suddenly back to smooth!
My PCV system is working perfectly, and I am getting no oil accumulation in the air filter housings. But I do detect oil vapor in there, which is perfectly normal.
So perhaps a monthly cleaning of the throttle bodies is a maintenance thing?
2 Likes
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
2
I wonder if a different brand of gasoline might help? They’re forever advertising their additives help keep your engine cleaner. I’ve always presumed they were bollocks.
Gas additives won’t help the throttle bodies. Top tier gas can help keep injectors and intake valves cleaner. They have more cleaning additives over and above the government mandated ones in non-top tier gas. The government minimum levels of cleaners/detergents came about after severe levels of carbon buildup on port injected intake valves in the early 80s caused rough running and higher pollution. Believe it or not, Costco gas in the US is top tier.
What will help throttle bodies is a better PCV system to keep oil vapor from going thru them. An oil/air separator will help. The gauze under the pig snout doesn’t do much except get clogged and cause high crankcase pressure.
I installed an oil catch can. That separates out the oil from the vapor.
It does trap a lot of water condensate in the winter time.
Are you still having an issue after installing the oil catch can?
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
10
Perhaps we’re looking at this the wrong way. Presumably all cars get deposits around the butterflies, but not all cars develop a rough idle from it. Maybe we should be thinking about why such deposits seem to cause a rough idle on the Jaguar V12. Perhaps it plugs up the vacuum taps?
Or perhaps it just results in an unstable flow of air past the nearly-closed butterfly, like it’s burbling or something. If we had parts to play with, just for grins I’d suggest cutting a tiny notch in the edge of the butterfly right at the bottom, so there’d be a little clear passage when closed. I’d think this might help suck the deposits on through, and also might help keep the flow consistent when the throttle is closed. Of course it’d raise the idle a bit, so either the butterflies would need to be adjusted farther closed or the adjuster on the AAV would need to be closed a tad to keep the idle at spec.
Of course, my theory could be total coincidence. But I do notice every time I clean throttle bodies, the car seems to idle better.
Of course, if it is gumming up the throttle bodies differently, since more PCV oil vapor gets sucked into B bank rather than A bank, and so the B bank was getting a wee bit less of air at idle, I would think the O2 sensor would sense that and richen the B bank. So???
But doesn’t the balance pipe make sure both are getting plenty of air?
I just believe that the HE is designed in such a way that it has to run on a very delicate flow of air to keep the leanness, and it’s more prone to any tiny fluctuation in flow.
I have noticed if i set the throttles to the tight side of .002" and let the balance pipe do it’s job with the idle screw, it idles a bit worse than if I set the throttles to the loose side of .002" and close the idle screw accordingly.
The Balance Pipe delivers air via the rear of the intake manifolds, but the throttle bodies deliver right in the middle.