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waterhouses

Manifold air leaks

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Just fitted a new metering unit from Prestige to my 73 CR TR6 - all working as expected however my tickover is now alot higher - I suspect air leaks. When I fully close the bleed screw the car still ticks over at c700 rpm. I've sprayed wd40 around all interconnecting pipes to see if I can pinpoint the leak but I can't. In changing the MU for a new one could it be possible that the seal between the new MU and its engine mounting is allowing air in. If not where else can I look. As I understand it the car should stall if all is set OK when you fully screw in the bleed screw.

Edited by waterhouses

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As I understand it the car should stall if all is set OK when you fully screw in the bleed screw.

 

Ideally, but only if the butterflies are closed or pretty well, and there are no leaks in the spindle bearings.

Could you stall it before?

 

No air will get in through the m/u.

 

If the vacuum hose between m/u and manifold is leaking that can lead to overfuelllng but would then expect more lumpy idle rather than high idle.

 

What else did you do at the same time?

 

Having said all that a 700rpm idle is too slow anyway, so not really something to worry about.

 

Ivor

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Ideally, but only if the butterflies are closed or pretty well, and there are no leaks in the spindle bearings.

Could you stall it before?

 

No air will get in through the m/u.

 

If the vacuum hose between m/u and manifold is leaking that can lead to overfuelllng but would then expect more lumpy idle rather than high idle.

 

What else did you do at the same time?

 

Having said all that a 700rpm idle is too slow anyway, so not really something to worry about.

 

Ivor

 

Thanks for the reply Ivor - I agree that 700rpm is too slow for the TR but I can only achieve this with the bleed valve closed. Before the MU change I could stall the car when valve closed so not sure why I can't now. I thought (and was hoping) that maybe with the MU change air could get through if it wasn't properly seated but reading your response this doesn't seem possible.

I haven't made any other changes to the car other than replacing the MU.

If I disconnect the brake pipe servo and put my finger over the outlet on the manifold to the brake servo will this illiminate any leakage through the brake servo?

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If I disconnect the brake pipe servo and put my finger over the outlet on the manifold to the brake servo will this illiminate any leakage through the brake servo?

 

Yes, but if the servo were leaking that badly it wouldn't really work anyway. You can test the servo by leaving the car with engine off for five minutes, then press the brake pedal a couple of times. If you get a swoosh from the servo it's holding vacuum.

 

The only way air can get in from the m/u direction is if the vacuum pipe from the m/u is leaking due to perishment or leaks at the unions, but in that case there would be excess fuelling, and anyway without a major leak I doubt you would get enough air to keep the engine going.

 

Presumably you changed the m/u because it was worn? As they wear the fuelling drifts, perhaps you could stall it because the idle mixture was sub-optimal.

 

Take it for a run check the plug colour.

 

Ivor

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It should be simple enough to identify where the air leak is coming from. Whith the air bleed fully closed (check by sticking your thumb over it to be sure it genuinely closes) then:

1)Clamp the hoses 1 by 1 - manifold to servo & manifold to MU.

2) With the plenum removed put your palm over each of the butterflies - if one or more isn't closing fully then you'll feel your hand being sucked to the manifold & note the tickover slow as your hand seals it.

3) Check each of the injector "O" rings to check they are not the cause of the air leak.

 

If none of the above are the cause of the air leak then the remaining possibility is the manifold gasket isn't making an adequate seal between the manifold & cylinder head - but that's relatively unlikely.

Worn spindle bushes can let a little air in but not enough to tick over at 700rpm but may contribute to difficulies adjusting the butterflies.

 

The likely culprit is the butterflies being out of adjustment.

Edited by andymoltu

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Vaguely recall that you were replacing bits of worn linkage? The difference between butterflies open and shut is the proverbial gnat's crotchet, you might unknowingly have disturbed them.

If you get into balancing, one needs an airflow meter and quite a lot of time. It's what Holmes used to call a two-pipe problem.

 

Ivor

Edited by 88V8

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