Too much boost?

[Mr_John111]

Sue's car has developed a strange symptom.

When first up and running the boost gauge was quite slow to respond and we ran out of road / guts before the gauge got anywhere near 12 o'clock which seems to correspond to 7.5 psi boost (manual says max is 8 psi as its a LC engine)

Being new to the engine I figured that whilst I had repaired the stuck wastegate at the time I reassembled the engine, I did not comprehend the importance of the valve seat in the manifold adapter and have a vague recollection that it wasn't in good shape. My initial reaction was ''aaah, perhaps thats where the boost is leaking away''

Anyway fast forward to now. After a recent oil and filter change (thats all I fiddled with I swear!) All of a sudden the boost gauge leaps like a greyhound for 12 o'clock and I am having to back off before hitting top revs as I am sure that the old girl won't thank me for overboost (the car not Sue )

When should the wastegate cut in? Not having driven a petrol turbo before, I'm not convinced that I would know what happens when the wastegate does open.

The engine is completely stock/standard with no intercooler (and was fully rebuilt by me following DPO melting two pistons due to skimmed head and stuck wastegate) - If I do ''floor it'' in an attempt to get the wastegate to open I'm assuming its best to do it on a cold morning when air will be denser / cooler?

And how worried should I be if it gets to 10-12 psi and I can't tell if the wastegate is open??

Presuming that I am in danger of detonation again?

Any advice / similar experiences ?

Cheers

John

[USAndretti42]

Assuming no pipes have come off, it sounds to me that you need a hand pump and gauge like the ones you can get for pressurising a cooling system to check for leaks. Connect said pump to the wastegate capsule and pump it up noting what the pressure is when the wastegate starts to open. That way you can check if it is stuck or not.

[910Esprit]

John - can you hear the sound like a rubbery neighing horse!! when you change up? Also does the throttle response feel normal or do you feel the revs continue to build after you lift off? If you cant hear the wastegate as much as usual and the throttle respose is 'odd' you may have a split diaphragm. My boost gauge rises to 12 very quickly but not much further.

Steve

[Mr_John111]

John - can you hear the sound like a rubbery neighing horse!! when you change up? Also does the throttle response feel normal or do you feel the revs continue to build after you lift off? If you cant hear the wastegate as much as usual and the throttle respose is 'odd' you may have a split diaphragm. My boost gauge rises to 12 very quickly but not much further.

Steve

Yes a neighing sound when changing up (has always been there)

Throttle response feels normal (but nothing to compare to other than how its always been)

Revs die off when lifting off as does boost gauge

Might have to bite the bullet and just keep my foot buried and see what happens

Thanks

USAndretti42:-

How would I tell if the wastegate opens? there seems to be no external signs that its open or closed?

[910Esprit]

All sounds normal to me, keep your foot on the throttle and see how far it goes (within reason....)

[Simon350S]

It should rise rapidly and then stop.

The fluttering sound is the blow off valve under the fuel pressure regulator and nothing to donwith the wastegate.

The valve seat in the wastegate is not an issue unless it's completely destroyed and you then can not reach full boost. If you can not reach full boost then it may be an issue buys it's not very common. The most you will get is a reduction in the speed of build up of pressure due to the leak past the valve but you'd not notice it I'm sure.

A short burst of overboost shouldn't cause any concerns at all, I'd say try it and report back your findings.

[molemot]

If you disconnect the pressure line from the turbo to the wastegate, get a bit of rubber tube over the end of it, and blow down it....you will quickly know if the diaphragm is intact, without risking bursting the engine under high boost pressures. If you blow down it and you keep blowing down it until you've run out of air, there's a leak somewhere...if the pipes are airtight then that'll be the diaphragm, then!! You should be able to blow down it, build up the pressure, stick your tongue over the end of the hose, wait for a bit and then remove your tongue and note the pressure exhausting from the pipe into your mouth. Not particularly sanitary, I suppose, but effective!! Putting an airline on to the hose - carefully! - enables you to put puffs of pressure into the wastegate ... and you should be able to hear the valve clack back against the seat as the pressure goes. By the time you've done these tests, you'll know if you have another duff diaphragm!!