Racing Engine at idle. WHY>>

Jag has sat for a while waiting for a new Vacuum Brake Vac Booster. Jag worked fine before siesta. Now when I started it, the engine raced like crazy and sounded like the engine was going to fly apart. Well after numerous attempts and pulling the vacuum from the Booster unit and plugging it off the engine still wants to idle at approx 1500rpm. WHY?? What should I look for?? What just sitting could have caused this problem? What controls the idle and why would it change? From a distance perhaps not easy to help but any
help you can give will surely be appreciated. Have no idea of what to look for.
Thanks/Lee Noga

Lee, what engine are we talking about here?

86 or 87 4.2 XJ6 US spec.

Does it run even faster with the booster connected normally?

Stuck throttle?

That’s my first thought.

Lee, the throttle cable enters the engine bay right below the booster. Maybe it got kinked/bent/damaged while removing the booster. Or perhaps just incorrectly routed?

Take a peek


I’ll do that Doug. Sure strange. My son took the booster out and put back in and he is pretty diligent and careful, but doesn’t take much sometimes. Other things are the temp control,etc, and on a non-carburated engine I don’t know where that stuff is at. Guess I will have to get what Jag books out that I have, and get my brain educated again.

Doug I have the engine running okay now with the vacuum to the booster disconnected and plugged. But the engine is still idling too fast, close to 1500rpm.
The manual says to adjust the engine to idle at 800 rpm and the idle screw is
on the air distribution block… Where is that Doug?? Is that idle screw adj note
meant for the non carburated engines?

You checked the cable? It’s OK?


1 Like

Sure is an EFI feature, Lee - it is well hidden underneath the AFM and screw is difficult to access…:slight_smile:

Idle should always be set with the engine fully hot - when cold; the AAV adds air to increases idle, and 1500 rpms is ‘normal’ for a cold engine…

If idle is that high with the engine hot; first check the AAV. Disconnect the air hose to the AAV and peer inside; the slide should be fully closed with a hot engine. While cold; the slide is about half open and, if stuck stays open - which may happen with the engine standing. If so; use a small screwdriver to ‘exercise’ the slide - it is spring loaded and sometimes gets stuck by accumulated gunk. And may sometimes fail for reasons of its own…

xj6 85 Sov Europe (UK/NZ)

Good info Frank. I will have to spend a little time getting acquainted with this engine again however to find all the parts you mention. As yet I have no explanation of why the engine had such a high rpm when first started. Still have the hose disconnected from
the brake booster. I doubt if that is the problem however since it was just replaced. Still higher than I would like. It is getting up into the 70’s & 80’s here now so idle should come down with running a short time. Where exactly is that idle screw Frank for the '86 XJ6?
Also the AAV mechanism. Not sure where it is but will consult my literature to see if I can get an idea.
Thanks much Frank/Lee

Yes the cable is okay Doug. It goes over to some contraption located on the passenger side of the engine, kind of on the manifold or whatever. Don’t know what it’s function is but it seemed to be free. Even the literature does not give you
a very good idea of where things are. Sure would like to know where that idle screw is hidden.
How is everything with you Doug? We are all getting older. I am 93 now but still doing okay.

It acts on the shift cable and also tells the computer what‘s happening at its end.
The idle screw is very hard to reach and sits on the intake plenum forward of the throttle. Aluminium housing with bolts and hoses.

Here’s a pic of the Air Distribution Block where the idle speed adjustment is made


The adjustment is a little awkward and not likely to be your problem, Lee. The failure mode is more commonly that the passageway in block gets clogged…which wouldn’t increase your idle speed

More likely, IMO, is the AAV sticking as Frank mentioned. Or the hose underneath the AAV is leaking. The hose is very hard to see.

The AAV is bolted to the underside of the water rail and looks like this


Doug’s pictures should identify the air distribution block (idle screw) and the AAV, Lee.

Both are bolted to the inlet manifold, the AAV with a square base and a ‘tower’ leaning upwards/outwards - the idle (on air dist block) screw is 6mm hex.

Presuming that the brake booster hose is securely clamped or blocked; the high idle is either the AAV malfunctioning (not closing when hot) as described or incorrect throttle gap. With vague possibilities of an air leak - or a displaced seat on the idle screw. Best bet is the AAV - easiest checked…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

Thanks Frank & Doug again. You guys are always so very helpful. I will rule out the very high fast idle I got the other day after we got the brake booster replaced. Want to make sure that was not in any way a problem with a vacuum leak. Will hook it up again today.
As stated the Jag sat for about 2 or 3 weeks outside temperatures before a new booster was installed. The way the engine raced at such a terrific rpm sort of indicates what you are describing as problems in the AAV perhaps. Something must have been stuck as after several tries, the rpm finally started at around 1500rpm.

High idle happened again on me. Shut the engine down and restarted. It started and came back to 1500rpm right away. Seems like something is sticking all right

The AAV cannot race the engine this way, Lee - its capacity is limited, but 1500 rpms is well within this limit…:slight_smile:

Only a stuck open throttle or metered air ingress can raise rpms to such high levels - possibly a temporary glitch resulting from the repair process?

Do check, by pressing on throttle the lever, that it abuts the (tiny) throttle stop screw. Giving the proper 0,002" throttle gap - which should also be verified downstream, as a matter of routine…

xj6 85 Sov Europe (UK/NZ)

Okay all you XJ experts, really need your help or even a hint of what the connection is between the vacuum brake booster and the engine revving up. We installed a new booster and have the problem after that. Today I went out again today to see if I could shed any more light on the subject. I disconnected the vacuum from the booster and starter the car. Idle was approx 1500 rpm and pushed the floor gas pedal and engine came down to the normal idle with vacuum line disconnected. Then while engine was running I connected the vacuum line to the booster. No increase in idle was evident. I then pushed the floor gas pedal down and the engine immediately raced to 3500rpm and stayed at that speed until engine shut off. I simply at this point cannot
figure out what is happening here. I hesitate to remove and replace the new booster w/o first understanding what could be happening here. Makes little logic. Anyone???


in strange situation I tend to ask strange questions: could it be a mechanical problem such as the floor carpet making the loud pedal stick … or the gas wire sticking? After all, the gas wire exits the bulk head around where the brake booster is located and was replaced.

Good luck


75 XJ6L 4.2 auto (UK spec)

I had that happen to a car that came into my workshop, spent a lot of time stripping the carb down without success to cure the high idle, went into the cabin and sure enough, a heaven rubber mat was covering the loud pedal.

As Jochen says, Lee; check that gas pedal linkages returns to idle when pedal is released - engine cannot race without the throttle being opened/held open…

At the throttle body, while ‘racing’; physically check/push the lever down to the throttle stop screw - that should return the engine to idle. follow up with checking all linkages between pedal and throttle butterfly!

There is really no relationship between the throttle and brake booster, but somehow, or independently of the work done; your throttle is sticking. A fault like this is usually related to a frayed throttle cable, ut other causes, like a floor mat, sure is a possibility…:slight_smile:

Point is to eliminate this as a factor - then address further anomalies…

xj6 85 Sov Europe (UK/NZ)