964 Turbo boost problem

runehar · 05-06-2009, 02:34 PM

Taking my 91 964 turbo out for a spin this evening i notised that full boost is reached at least 1000 rpm later than normal (4000/4500 rpm compared to 3000/3200).

I drove the car 200km on the race track last weekend and didn't notice anything then, but i guess something has happend.

I'm not familar with turbo cars (had it for three weeks), but my wild guess is that it either is something wrong with the turbo or the hose between the turbo and the engine (as the hose between the intercooler and the turbo was changed last year). The car has 95000km on it.

Any "expert" suggestions on what might be the cause and what i need to check?

Thanks
Rune H.

911boost · 911boost's Garage

What do you mean by full boost ins't acheived?

Is the wastegate stuck partially open?

mark houghton · 05-06-2009, 04:07 PM

The wastegate valve could be stuck partially open, but my guess would be an air leak after the turbo somewhere. Check your hose connections to the intercooler, as well as intercooler O rings (assuming the 964 intercooler mounts up the same as the 930's). A small boost leak will definitely delay or even prevent reaching full boost.

Worse case scenario is you sucked something into the turbo and damaged the turbin blades, or the thing is starting to seize up due to lack of oil or severe coking of the shaft.

If your car is CIS injection and if the leak is large enough you might also be experience rough running due to over-rich conditions (any boosted air escaping before making it to the intake manifold will be that much less air going to combustion = more fuel than needed for the given amount of air = rich). Remember your air-to-fuel ratio is pre-set by how far the fuel metering plate is deflected when air is drawn across it. All that air must make it to the combustion chamber or the mixture will be off. But if you're running EFI, then.....NEVER MIND.... this does not apply!

lucittm · lucittm's Garage

I'm no "expert" but there are several things that would cause this condition. I would do a complete engine "nut and bolt" check first. Then, I would inspect the exhaust system at my friendly Porsche dealership who gladly puts my car on a lift early in the morning for a box of doughnuts (Charity lifts are great).
An exhaust leak on our cars is like a stuck wastegate.

Then I would close off all the ports on the intercooler and test drive the car again.

If the boost onset is still late, I would check the intercooler for blockage, replace the o-ring at the throttle body, remove and test the blowoff valve, and finally, get a smoke test of the entire intake system at a reputable shop to check for air leaks. Finally, I would pull the wastegate and check it for smooth operation and inspect the valve seat.

Good Luck,
Mark

staffanbe · 05-07-2009, 07:46 AM

The difference in rpm for full boost is big.

I would check to see if the cat is blocked, could happen.I

runehar · 05-07-2009, 12:48 PM

I disconnected the air control line that opens the waste gate and tried with compressed air into the wastegate. First with low pressure air is passing through (out the pipe??) applying more air pressure the wastegate opens with a click.

can i by this conclude that the wastegate is function well (and not stucked half way open) ?

Rune

mark houghton · 05-07-2009, 01:09 PM

Quote:

Originally Posted by runehar

I disconnected the air control line that opens the waste gate and tried with compressed air into the wastegate. First with low pressure air is passing through (out the pipe??) applying more air pressure the wastegate opens with a click.

can i by this conclude that the wastegate is function well (and not stucked half way open) ?Rune

Nope, ya gotta look inside to see the valve in it's seat. The "bench test" you just did only confirms that the diaphram isn't ruptured and that the valve is probably opening completely. The question is...is it closing completely?

The sound of air passing with low pressure is usually just the small amount that leaks between the valve stem and it's guide. More or less, depending on it's condition/age. Not an issue as long as the WG opens when and as quickly as it should under normal operations.

Another simple test is to stuff a wad of paper in the WG dump pipe, then start the car and rev it up a bit. If the valve is sticking open, exhaust gas should be venting and will push the paper out.