GEO147

Hi fellas. My fd has very recently developed a high idle of 1200rpm.

I immediately checked the hose from the servo to the plenum and cant see any visual cracks leaks. I also tried adjusting the idle adjusting screw (loosened it) and that did nothing, and there is slack on the throttle cable.

I have been told that my problem is most likely an air leak, but I have no idea of where, what or how to find it!!

I have a PFC and checked the tps voltages VTA1 and VTA2 are both perfectly in spec.

The car idles at 1200rpm no matter what, and when I have been driving and then turn the lights off it starts to hunt. I believe this is a symptom of the constant high idle but not the cause.

I would really appreciate any help here as the car is due for maping and it will be a waste of time if I dont have this idle/air leak issue solved beforehand.

The car has always idled perfectly until now, and is still running strong and showing strong boost/vacuum readings.

Calling all gurus and knowledgeable 7 owners!!

Thanks.

GEO147

Bump...


I could really do with some help here guys. Someone must know how to sort the above problem!!

DaveW

There are so many possible air (vacuum) leak sources, we don't know where to start.

There are dozens of vacuum hoses attached to the engine, plus other things that can leak.

What you have to do is look around and listen, look for what may be leaking.

Search for "vacuum" and you'll find all sorts of stuff.

Gotta get back to work.

GEO147

A couple of other things i have noticd recently.

I think the accelerator pedal has more play on it then before but cant be sure. I think ive got about an inch of play? As if the pedal is slightly depressed!!
I tried adjusting the idle adjust screw (but only anticlockwise) but this did nothing!!

Also, I am pretty sure that the brake pedal is softer! When pressing hard on the brake it seems to make the idle drop!! Especially when comming to an abrupt stop!!

Any more help guys!!

DaveW

1. The throttle plates may be sticking open, or the throttle cable may be binding. Have you tried lubricating the cable, linkage and the throttle-plate shafts? How about cleaning the throttle plates and their bores? Is the pedal catching on something and binding?
2. Anticlockwise would make the idle faster (clockwise closes down the air bleed).
3. Softer pedal may mean more vacuum (possibly due to the faster idle)
4. There is a normal interaction between the brake application and idle speed - the vacuum assist for the brakes needs to have its vacuum replenished each time the brakes are depressed. However, this would normally raise the idle briefly as more air was sucked out of the vacuum assist chamber. A leak in the vacuum assist diaphragm could cause a fast idle as the engine vacuum was constantly trying to suck enough air from the brake vacuum chamber.
5. In your initial post, you mentioned "hose from the servo to the plenum". Do you mean from the brake vacuum assist to the UIM (upper intake manifold)?

That's all I can think of for now.

Dave

GEO147

Hi Dave and thank you for that super post...i laerned a lot from it..

5. In your initial post, you mentioned "hose from the servo to the plenum". Do you mean from the brake vacuum assist to the UIM (upper intake manifold)?
YES, thats exactly hat I meant

I also take your point about the soft brake pedal being a symptom of faulty diaphram.

I have also been told that it could be the check valve within the hose from vacuum assist to UIM or the hose itself...I am leaning toward the vacuuum assist or some component thereof.
How would you attack this issue for diagnosis?
And, caan the diaphram be replaced??

Thank you again for the lesson!

George.

DaveW

1. Faulty diaphragm would make the brake pedal harder (to push) and the brakes would seem less effective - you'd have to push harder to get enough stopping power.
2. Check valve will not cause constant high idle - there is only a fixed volume to evacuate in the vacuum assist chamber, and once it was at the proper vacuum, the idle would go back to normal (assuming the chamber/diaphragm was not leaking).
3. Plug the line from the UIM that goes to the vacuum chamber. If the idle goes back to normal, the leak is somewhere closer to the vacuum chamber from where you plugged it.
4. I don't know for sure. I THINK you'd have to buy (get) a whole vacuum assist unit - I don't think you can get the diaphragm by itself, but I could be wrong about that.

Dave

GEO147

I am definately having to push the brake harder/further to get the same stopping power.....the pedal seems to be softer aswell tho (but could be imagining it!)

Also, when its idling at its new high idle of 1200rpm warmed up, if I depress the brake pedal the engine will start to die but not quite cut out ie...the rpm will drop dramatically temporarily!

Does this help in diagnosis of problem?

I feel we are getting close to the issue now!

Also, if I just pop the vac hose off the UIM and plug/cover the UIM with my finger/hand and the idle drops back to normal then it has to be either hose or vacuum chamber related....if it doesnt then its keep looking....am I correct??

DaveW

1. I'm not sure I understand this description...it seems to contradict itself somewhat.
2. I think this is a sign of a severe leak in the chamber/diaphragm. The additional air being sucked into the engine (when you depress the brake pedal) is making the mixture way too lean. The ECU relies on the throttle position and vacuum reading to control the amount of fuel injected, and the vacuum leak is large enough to confuse it.
3. See #2
4. Yup - I think the problem is in the brake-booster assembly

Good luck. I've got to sign off for now.

Dave

GEO147

Dave,

Thank you very much I really appreciate it and will let you know how I get on....gona tackle it 2moro and see what I can figure out!!

Regards,
George.

GEO147

After eliminating the brake booster assembly earlier today I took the car to a mechanic friend of mine and we went at it with with carb cleaner to discover that the Lower Intake Manifold Gasket is screwed.

F**k.

Looks like a relatively big job to sort it out aswell...any comments?

And thanks again Dave.

George.

DaveW

You're welcome (even tho my diagnosis was wrong...). Sorry it didn't turn out to be an easy fix, but at least now you know where the problem is. The biggest problem with the LIM gasket, IIRC, is all the other stuff that needs to be removed to get at it.

Dave