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MG MGF Technical - Intake manifold pressure

Someone suggested my Intake Manifold Pressure was too low (it is 29kPa). What is likely to cause this and would it reduce performance of my car?

A friend with a ZR showed me his today and it was 38kPa.

Thanks in advance

Anton
Anton

Not sure what the normal range of pressures within the inlet manifold is, but unless you've got forced induction, the pressures are negative.

If you have a vacuum leak (leaking inlet manifold gasket, leaking throttlebody gasket, leaking breather hose etc, the effect is to drop the inlet vacuum. This is less diasterous on a fuel injected engine than it is on a carb fed motor, in that the fuel intake is not dependent upon the inlet vacuum, but if you loose the pressure pulses within the manifold, less air is drawn in through the throttle, and this means less power...
Rob Bell

Thanks Rob. Is there any way I can check the above easily, or is it a garage job?

I have an enclosed K+N connected to air vent. Not sure if that counts as forced or not?

Can I spray something around the TB, manifold when engine is runnig (e.g. WD40 or water) to see/hear if I can see any leaks?
Anton

You can spray anything that will burn, but I find that propane from an unlit propane torch does a great job and isn't messy. If you have a vacuum leak the revs will increase noticeably as soon as the propane is drawn in.
tom

When was the 29kPa mesaured? at idle?
David Peters

I've not tried the propane approach Tom, but that sounds like a great approach. What I've done in the past is to spray a non-flammable liquid onto various joints and look for evidence of a leak - but this is not a good way of doing it as it is nearly impossible to approach the underside of the manifolds etc...
Rob Bell

Actually - I'm not sure if power is reduced for an EFI; depends on the EMS.

The biggest vacuum in the plenum is at idle, or when the engine is being driven on a closed throttle.

Air leaking into the manifold provides more air for the engine to consume (bypassing the TB), so you'd expect idle to be higher than normal. Maybe the idle runs a little weak or is corrected by the EMS.

At full throttle, there's very little vacuum in the manifold as the TB is wide open, and so operation of the engine e.g. power output, will not be affected.

What I'd do is: give the manifold a gentle pull in different directions at idle: if the engine speed changes, this sugests a crack in the manifold or more likely a perished inlet gasket (assuming the manifold nuts are reasonably and consistently tight). I also use WD-40 to look for leaks, in the manner described up the thread.

For older engines, poor vacuum may also be due to worn piston rings / bores meaning that at idle, they can't suck well enough to give a high vacuum.

BTW - when I bought my (complete) engine, the inlet vacuum was low, and idle lumpy; it was better a revs. But it was far better when I fitted an inlet gasket, missing from the engine as sold(!)

Anthony
Anthony Cutler

Cheers guys.

The 29Kpa was indeed at idle. Sometimes it is 30 but usually 29.

I will try wiggling the manifold around if the weather improves at the weekend.

What are the chances of worn piston rings etc on a 47k miles car?


Not sure I entirely like the idea of pouring propane into a hot engine bay !!
Anton

HI Anton,

I have a similar manifold pressure on mine. Usually 29kPa. I also have poor performance in a random fashion. Seems to be flat spotting between 3 and 5 K (but not ALL the time). Wonder if the IMP is the problem?

I havew got some new breather hoses to fit too as one of mine was perished at the end. Mind you when I rmeove them completely it does not seem to affect the IMP.

Is youre performancea problem?
I Brown

According to my manual, 40kpa should be measured at idle in hot engine status.
David Peters

40!!! Oh dear. Then it is somewhat low.

Anton

I tried the butane trick aropund the manifold and there was not chamges, though I was pretty worried letting too much gas down there!

GUess I can rule out a leak
Iain Brown

This thread was discussed between 24/11/2006 and 18/12/2006

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