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[terry paget]

I need to trace an air leak causing a fast idle. I believe spraying the right thing about the engine at idle will cause an audible change in idle rate. What is the right spray?

[Nick W]

Easy Start. Carb cleaner.
Anything like that, but whatever you use, do so sparingly.

[cam.in.head]

If you squeeze flat the air pipe coming from the icv does the idle speed drop significantly. If it does then it suggests a faulty icv valve. Can be confirmed by substitution of a known good valve.
Big o rings under plenum usually survive many times but there are many possible leak points on top of engine,namely servo elbow,breather pipes at rear,top hat seal,throttle plate o rings ,etc. Sorry terry if ive mentioned something you have already checked

[TheBoy]

Check the butterflies in the throttle close absolutely completely, and are clean and free.

Sometimes, mongrels adjust them in order to adjust the idle, because they don't understand anything newer than carbs.

[Enceladus]

I mentioned such in his other thread, but Terry didn't respond. Since the cam covers were removed to fix the seals he likely removed the throttle linkage? Might it be that the clip that adjusts the slack was put back in the wrong place and now the butterflies can't fully close. There should be quite a lot of slack when closed but the butterflies should be fully open just before the pedal reaches the floor.

And one of the notorious oil breathers exits in front of the butterflies as I recall. This can deposit a nice layer of carbon on the butterflies and the throttle body surface which can let air leak past. Carb cleaner and an old toothbrush?

Check the butterflies in the throttle close absolutely completely, and are clean and free.

Sometimes, mongrels adjust them in order to adjust the idle, because they don't understand anything newer than carbs.

[terry paget]

I mentioned such in his other thread, but Terry didn't respond. Since the cam covers were removed to fix the seals he likely removed the throttle linkage? Might it be that the clip that adjusts the slack was put back in the wrong place and now the butterflies can't fully close. There should be quite a lot of slack when closed but the butterflies should be fully open just before the pedal reaches the floor.

And one of the notorious oil breathers exits in front of the butterflies as I recall. This can deposit a nice layer of carbon on the butterflies and the throttle body surface which can let air leak past. Carb cleaner and an old toothbrush?

Check the butterflies in the throttle close absolutely completely, and are clean and free.

Sometimes, mongrels adjust them in order to adjust the idle, because they don't understand anything newer than carbs.

My apologies. I did check, and always check, that there is slack in the throttle cable at idle. I cleaned the throttle when I had the plenum on the bench too. Before I removed the plenum engine idled at 500rpm hot or cold. After various procedures, and fitting a new ECU temp sensor, it still idles at 500rpm cold, but 900rpm hot.

[terry paget]

Thanks to all for advice. Tried squeezing the pipe to idle control valve, did it with a cable clamp, when virtually flat idle rate decreased but I could not stall the engine.
Sprayed Easy Start all the obvious places, engine note never changed.
Tried pressing on the plenum. There was a noise, I thought a squeak, but clearly it was a hiss. Pressing on the plenum top with a cloth, rear left, and bringing it in to the breather bridge, hiss stopped. Moving it left and right, with cloth against the breather bridge, hissing stopped. Engine note remained the same, 900rpm. Pic shows where holding the cloth stopped the (quite loud) hiss. I had removed and cleaned the breather bridge during overhaul. I did not change the top hat gasket.

[Doctor Gollum]

You shouldn't have to change it if you remove the bridge correctly...

1. Disconnect the four pipes from the rear of the breather bridge.
2. Unbolt the throttle body and pull forwards.
3. Remove the bridge cover and the bolt beneath it.
4. Gently, and vertically, pull the breather bridge away from the Plenum.

Any other way and the top hat rapidly becomes a single use item

[terry paget]

You shouldn't have to change it if you remove the bridge correctly...

1. Disconnect the four pipes from the rear of the breather bridge.
2. Unbolt the throttle body and pull forwards.
3. Remove the bridge cover and the bolt beneath it.
4. Gently, and vertically, pull the breather bridge away from the Plenum.

Any other way and the top hat rapidly becomes a single use item

Thanks Doc. Removed the breather bridge this morning, and removed all the seals, see pic. The top hat gasket doesn't look too bad but I will replace it, and the front pipe O-rings. There was no O-ring on thr central mounting pillar; I had presumed the screw went into a blind hole in the plenum - not so. I suspect that was my leak.

[Enceladus]

If the top hat seal isn't damaged then it can be reused. New ones seem to be only available in the USA, none to be had in UK ur Germany. (And another used one might not be any better than the one you have.)
 
As I recall the only other seals are the two small o-rings between the front spigots on the breather bridge and the throttle body. I'd replace those if there's any sign of damage or hardness. I don't think there is a seal for the securing screw. Might it be that one of the four tubes that connect to the rear has split, or isn't connected at the other end?

I'm not trying to get you to redo work already done, but if pressing on the plenum cured the hiss then might it be that one of the six big o-rings on the air intake underneath got displaced during re-assembly?

[terry paget]

If the top hat seal isn't damaged then it can be reused. New ones seem to be only available in the USA, none to be had in UK ur Germany. (And another used one might not be any better than the one you have.)
 
As I recall the only other seals are the two small o-rings between the front spigots on the breather bridge and the throttle body. I'd replace those if there's any sign of damage or hardness. I don't think there is a seal for the securing screw. Might it be that one of the four tubes that connect to the rear has split, or isn't connected at the other end?

I'm not trying to get you to redo work already done, but if pressing on the plenum cured the hiss then might it be that one of the six big o-rings on the air intake underneath got displaced during re-assembly?

Thanks. That thought occured to me too, so I have removed he plenum. All were well seated in their grooves. I did spray Easy Start towards the other ends of the 4 hoses, without result. I agree that the securing screw should itself provide enought of a seal, and yet, pressing my cloth in at the side of the bridge did stop the whistling. Perhaps an O-ring, perhaps a flat washer, might make a better seal there.

[Enceladus]

The tapping for the securing screw isn't open to any of the intake ports. So sealed or not shouldn't matter? However there's plenty of stuff at the rear of the plenum. Perhaps something there is displaced or not sitting on it's bracket and is fouling and preventing the plenum from sitting evenly on the six o-rings?

[TheBoy]

That screw hole securing the breather to the plenum should not be open to the intake.  Check its not. Also, the breather system is also not open to the hole.

If they are, a suitable repair/replacement needed.

[terry paget]

That screw hole securing the breather to the plenum should not be open to the intake.  Check its not. Also, the breather system is also not open to the hole.

If they are, a suitable repair/replacement needed.

You are quite right, TB. I have turned the plenum over, and observed its clever shape, dual ram at the back, then I see the screw peeping through its hole at the front where it is clearly divided into 2 chambers. The breather bridge similarly gives no access to its interior where the location screw goes.