Erratic idle again!

[supersnipe1979]

First off, I apologise for broaching this subject again.
I'm looking for suggestions as to other possible reasons (other than the aux air valve is knackered) that the idle on my 1986 5.3 might be hunting (between 600 and 1500 rpm) when cold then settling at about 1000 rpm when warmed up.
i've been right over the engine and can't find any air leaks and all the various pipes etc seem to be correctly connected. The idle speed screw doesn't seem to make any difference whether it's right in or out.
I removed the AAV and checked it in hot water. It fully closed at about 85 deg C and opened fully in cold water so I WD 40'd it and put it back (no change).
One thing I find strange is that if I cover the air filter inlet on the LH bank I get a noticable suck and reduction in revs (as you'd expect) yet when I cover the RH inlet nothing happens (no suck or rev change) I've set the throttle butterfly valves at 2 thou and using a modded carb balancer ensured that there is an equal amount of depression across both banks.
I'm running out of ideas.
Finally, if I have to replace the AAV which one do I get? My engine number is 8S71648SA which is a replacement for the engine that was in it when I got it. The original? engine i believe was also suffixed with SA. I presume the 2nd S denotes standard compression. Does the A indicate the emission controls spec? I ask because my parts catalogue lists two AAV part numbers EAC2273 and EBC1198 (and i won't rely on the number stamped into the AAV presently fitted as that came from yet another engine) Any help would be gratefully accepted.
Last of all kuddos to Brake Buster on his restoration of his motor. Be nice to see how it's getting along. Mine was in pretty much the same condition five years ago (the rear suspension collapsed into the car on the tow home) but is nearing fruition.
Good choice of colour too.
Regards Mark

[dhdove]

Mark

Sounds like the quickest way to get a solution to your problem is to drop David Marks an e-mail, I'm sure he will be able to provide an answer.

Best Wishes
Tony

[ptjs1]

Mark,

Can't really give you any advice on the general problem. I would endorse Tony's suggestion and give Dave Marks a call.

I can tell you that EBC 1198 is the later type air valve that was also fitted to the 6 litre engines. It was fitted to 5.3 litre engines after engine number 57762 so I'm guessing this is the one for your replacement engine. Sadly, it's twice the price of the earlier valve!

The A / B in the SA / SB engine designation does not refer to the emissions type of the engine but the size of the pistons. Jaguar seemed to have some problems with consistency of the size of the liners in the engines and had two grades of pistons. The pistons were sized to best match the size of the liners. So there are "Grade A pistons" & "Grade B Pistons". It's as basic as that.

Good luck with sorting the problem.

Paul

[almcl]

The hunting is caused by the revs getting above 1500 with the throttle closed. The ECU cuts off fuel at this point, which is why it drops back again.

The cause is a bit more tricky. Clearly too much air is getting in. The AAV is fed by the left bank air filter and from your description it could be that this is opening too far and not closing enough (a common complaint with older valves.) Kirby Palm's book suggests that although the valves change in shape and size many work in different applications, have a read. By the sound of it, at some point an old, knackered, AAV was replaced by another old, knackered, valve. Spring for a new one (I know they're horrendously expensive.) But before you do, try blocking off the air feeds to the valve when the hunting is happening and see if this stops the problem. You should be able to get a warm idle of ~700 rpm.

[kelsham]

Hi It sounds like an air leak. As stated once the revs reach 1500 the engine will begin to cycle as it switches off, and then back on.

If you block off the aav completely and there is little difference you can prove the fault does not lie with it.

There are lots of vacuum pipes that can become loose, start there after blocking the aav.

Depend on it air is getting in somewhere. all the air filter housing bolts are in place I hope, if they are removed. a massive air leak takes place.

Keep us informed. Regards Kels.

[JED1961]

I had the same problem with mine .....it turned out to be the inlet manifold gsket which had split,even though all the nuts were tight the easy way to check for this is either with a piece of pipe to your ear and the other end place near the gasket one at a time with the engine runing you will hear a loudish sucking noise iff any leaks are present

[jonrome]

I'm having the same problem. It was fine when I drove it into the garage 18 months ago. Now it starts easily & runs OK until the temperature guage starts to move off its end stop. At this point it starts to hunt, nearly dying & then racing. It only does this in park & neutral. In all other positions its fine except that the tiickover is very fast. I cant say how fast because the revcounter has packed up!
I haven't done much looking yet except to find that the air temp sensor is o/c. I will try substituting it with a 300 ohm resistor. (My experiment with wires to two probes stuck into damp earth with a multimeter measuring the reisitance were inconclusive!)
I can't find any evidence that whatever built a nest behind the aircon compressor, out of the bonnet insulation, has chewed through anything.

[masterb]

Just a reminder for you 5.3er's
But if you are having problems with your AAV's you can always chuck it in the bin and modyify your car to a manual choke arrangement.
This is available to view on bernard embdems site see below.

bernardembden.com/xjs/index.htm