turbo roached?

[Fast Freddy 944]

Ok bro's I know this question has been asked a lot, but I'm too lazy to dig up the archives. I have a 86 951, I noticed the other day whilst driving, I wanted to open er' up. Usually when I do it the boost needle moves and I get the nice little "warp speed" effect. I tried it the other day and the engine revved out but no "warp speed" feel. The boost gage worked perfectly. I looked under my hood and everything seems connected. But I'm cornfuesd. Any help would be greatly appreciate. Hopefully a simple fix.Fredster!

[RRRacing1970]

Boost By-Pass Valve stuck in open position?
Ron...

[Fast Freddy 944]

Originally Posted by RRRacing1970

Boost By-Pass Valve stuck in open position?
Ron...

Ill have a look, thanks Ron.

[Ga 951]

Copied from Clark's garage / 951 FAQ well worth reading the whole FAQ ...hell, the whole site is well worth it


6.2 Turbo Does Not Boost Correctly


It boosts to a level and will not go any higher
It does not produce any turbo pressure at all
It produces varying boost depending upon the gear you are in. (See 6.2.5 Other Mechanical Problems below - the kinked throttle cable problem.)
Diagnostic Suggestions: Clamp off the line from the cycling valve to the wastegate and drive the car. This will close the wastegate and direct all the exhaust to the exhaust turbine. Now drive the car but do NOT fully depress the throttle. Just go over 3,000 rpm to see if the turbo boost is working. (If it is and you depress the accelerator a lot then the turbo could spin too fast and over-boost the engine causing damage.) If the car then boosts OK it means that the cycling valve and line are where the problem lies. The wastegate is not being controlled properly. This could mean that the cycling valve is being given bad information due to a faulty DME, knock sensor or 'control units' or is faulty itself.

If the car doesn't boost properly then look for:

A mechanical problem with the turbo
A leak or other problem in the air intake system
The wastegate sticking open
A leak in the exhaust system
Other mechanical problems
Engine computer problems
These checks should point you in the right direction.

Other diagnostic checks:

Does the car only boost up to 0.2bar or 1.2bar absolute?
If yes, then it's a likely problem with a connection on one of the various sensors (TPS, airflow meter, etc).

Hook up an after-market boost gauge to get an idea of what the turbo is actually doing. (The turbo could actually be working but a slipping clutch could absorb all the turbo boost and make it seem as if there is no turbo effect.) The after-market boost gauge measures turbo boost pressure before the air enters the intercooler.

[944 Ecology]

Make a tool to pressurize the intake system and check for boost leaks using soapy water. Don't forget the two rubber connectors to the intercooler.

[Fast Freddy 944]

Excellent, ill give it a go. Is the boost by-pass valve the same as the waste gate? If it isn't where is that puppy located?, Ill defo take all of yall's advice. Thanks for the help. I pretty sure my clutch is ok, I know my turbo is in good shape. Thanks again. Now to find that Boost by pass valve.

[RRRacing1970]

It's #7
Ron...

[RRRacing1970]

Worse case Scenario... Input compressor wheel (alum.) tried to digest a bolt or washer dropped in the intake box when u changed the air filter!
Ron...

[Fast Freddy 944]

Originally Posted by RRRacing1970

It's #7
Ron...

Thanks for the help Ron. I know where that little bugger is, now is it supposed have a vacuum while its running? or is it a on/off kind of critter? When it was running today it had a nice vacuum while it was running. The turbo seems to be whirring quietly, and I sometimes hear the psst noise from the waste gate. The clutch grabs like Fred Sanford at his heart, and I have plenty o' recipts from the previous owner.

thanks again Ron!

[Roger 911]

Originally Posted by Fast Freddy 944

Ok bro's I know this question has been asked a lot, but I'm too lazy to dig up the archives. I have a 86 951, I noticed the other day whilst driving, I wanted to open er' up. Usually when I do it the boost needle moves and I get the nice little "warp speed" effect. I tried it the other day and the engine revved out but no "warp speed" feel. The boost gage worked perfectly. I looked under my hood and everything seems connected. But I'm cornfuesd. Any help would be greatly appreciate. Hopefully a simple fix.Fredster!

Are you sure the boost gauge is working properly? Since the gauge is reading pressure from the manifold, it is virtually impossible for it to see pressure without your cylinders seeing the same pressure. When the waste gate fails, you will have a nice normally aspirated car that will run just fine, but makes less than 150 hp. But you should also not see any boost pressure.

[RRRacing1970]

FF944...
Your next thing to ck could be #8 (Turbo Cycle Valve)... it may have gone south for the winter (ck all (3) plastic ports for broken ports or leaks)(also I have seen the top plastic cap on top of #8 come out of it's socket and then the electric (2) terminals that feeds elec. signals to KLR and DME. will not work!)... and may or may not be the problem? The line #31 that goes to your by-pass valve is a turbo boost pressure line (not a vac. pressure line!) that when it get to high (boost press.) it pushes down on the (spring)(diaphragm) in the by-pass valve and lets out over boost pressure back into the intake port to the turbo charger! This may or may not cause your by-pass valve to fail or stick in the open position.
Ron...

[Fast Freddy 944]

Originally Posted by Roger 911

Are you sure the boost gauge is working properly? Since the gauge is reading pressure from the manifold, it is virtually impossible for it to see pressure without your cylinders seeing the same pressure. When the waste gate fails, you will have a nice normally aspirated car that will run just fine, but makes less than 150 hp. But you should also not see any boost pressure.

It appears to be working good, but ill check its connections, and make sure there is no issues with it. Thnx Roger.

[Fast Freddy 944]

Originally Posted by RRRacing1970

FF944...
Your next thing to ck could be #8 (Turbo Cycle Valve)... it may have gone south for the winter (ck all (3) plastic ports for broken ports or leaks)(also I have seen the top plastic cap on top of #8 come out of it's socket and then the electric (2) terminals that feeds elec. signals to KLR and DME. will not work!)... and may or may not be the problem? The line #3 that goes to your by-pass valve is a turbo boost pressure line (not a vac. pressure line!) that when it get to high (boost press.) it pushes down on the (spring)(diaphragm) in the by-pass valve and lets out over boost pressure back into the intake port to the turbo charger! This may or may not cause your by-pass valve to fail or stick in the open position.
Ron...

Ill check that next, are numbers 8 and3 easy to access under the hood?

[Fast Freddy 944]

Ok I figured out the boost gauge, the needle swings when I "goose" the throttle from idle, but during slow climbing revs, the gage shows no swing? I wonder if its the waste gate? Is it hard to fix? And the boost is intermittent, some days it boosts, some days nothing?

[Roger 911]

Originally Posted by Fast Freddy 944

Ok I figured out the boost gauge, the needle swings when I "goose" the throttle from idle, but during slow climbing revs, the gage shows no swing? I wonder if its the waste gate? Is it hard to fix? And the boost is intermittent, some days it boosts, some days nothing?

The factory gauge reads BAR, where atmospheric is 1 BAR. When you hit the throttle, you are essentially changing the manifold pressure from less than atmospheric (aka, vacuum), to close to, or at atmospheric. If you are making boost, you should see greater than 1 BAR under acceleration, up to 1.8 BAR.

It sounds like your wastegate might be toast. You can test this by going out and driving around for a while, not necessarily hard driving, but enough to heat up the exhaust. Stop and reach under the car and touch the dump pipe, coming from the wastegate. If it's warm/hot, good chance the wastegate is stuck open. I think places like Lindsey Racing sell rebuilt factory wastegates. You could also go aftermarket, like Tial, but if everything else is stock, and you intend to keep it stock, you should probably stick with the factory set up.

[DannoXYZ]

Most common source of low boost is:

1. Weak wastegste spring - test with pinced-hose test. Takes all of 5-minutes, including the test drive.
2. Collapsed exhaust between downpipe and catalytic. Remove and see if you can roll golf ball through.

These two contribute to 99.9999% of low boost problems. Don't bother wasting your time troubleshooting anything else until you've tested these two areas.

[Fast Freddy 944]

Originally Posted by Roger 911

The factory gauge reads BAR, where atmospheric is 1 BAR. When you hit the throttle, you are essentially changing the manifold pressure from less than atmospheric (aka, vacuum), to close to, or at atmospheric. If you are making boost, you should see greater than 1 BAR under acceleration, up to 1.8 BAR.

It sounds like your wastegate might be toast. You can test this by going out and driving around for a while, not necessarily hard driving, but enough to heat up the exhaust. Stop and reach under the car and touch the dump pipe, coming from the wastegate. If it's warm/hot, good chance the wastegate is stuck open. I think places like Lindsey Racing sell rebuilt factory wastegates. You could also go aftermarket, like Tial, but if everything else is stock, and you intend to keep it stock, you should probably stick with the factory set up.

Excellent, ill take it out for a spin, and check it out. thanks Roger.

[Fast Freddy 944]

Originally Posted by DannoXYZ

Most common source of low boost is:

1. Weak wastegste spring - test with pinced-hose test. Takes all of 5-minutes, including the test drive.
2. Collapsed exhaust between downpipe and catalytic. Remove and see if you can roll golf ball through.

These two contribute to 99.9999% of low boost problems. Don't bother wasting your time troubleshooting anything else until you've tested these two areas.

Which hose? forgive my ignorance. I'm just a normal aspirated dude learnin' the wisdom of the turbo.

[DannoXYZ]

Do a search for "951 vacuum diagram" to find picture of hoses. Might even be on Clark's Garage site.

1. Find hose leaving CV-cycling valve (boost controller) that goes to wastegate (or trace from wastegate back up)

2. Wrap rag around expose section of hose, usually between rear of intake manifold and firewall

3. Clamp hose shut with needle-nose vice-grips. Be careful not to tug hose too much or you'll break the plastic nipple it's attached to at CV

4. Do a series of runs with increasing throttle as RPMs increase, 20-30%, 30-40%, 40-50%


Note if boost increases more and more each run with increased throttle. At about 40-50%, it should be hitting 1.8-bar on gauge (10-11psi). DO NOT give it any more throttle than this!!! With the boost-controller cut off, you can easily hit +25psi boost and blow up the engine!!!! We are just testing wastegate here. It it stays shut and holds boost, the problem is somewhere else. Test for collapsed and blocked exhaust next.

[RRRacing1970]

It's hose #16 in the above vac. system print!
Ron...