chouwalker

I need help with a baffling problem I've encountered on my unmodified 1986 944 Turbo. The car is no longer generating full boost and as a result, doesn't go like it used to. For example, under full throttle in second gear, the car will generate 1.5 bar on the gauge up until about 4200 rpm when the boost starts to drop. By 4500 rpm, the boost gauge is down to 1 bar.

Many things have been done to diagnose and correct this problem, all to no avail. Here's the list:
1) checked for vacuum leaks and thoroughly checked all vacuum lines and connections
2) replaced the dump valve
3) replaced the rear cycling valve
4) removed the waste gate and both tested it and physically inspected it
5) verified turbo run-out is within spec
6) checked main boot between turbo and airbox
7) checked intercooler and connections
8) removed the exhaust from the manifold back, verifying that the down pipe wasn't internally collapsed and the catalytic converter was intact
9) banged head on wall repeatedly

Does anyone have any ideas on what the problem could be or suggestions on what to do next (other than just going back and checking for vacuum leaks for the 47th time)?

Thanks in advance for any help you can provide.

David Floyd

Clamp the line going to the wastegate shut, being very very careful go drive the car and see if boost will build now. BE VERY CAREFUL BOOST, SHOULD BUILD VERY FAST !!!!!!

Frank@Work

I am having a similar problem! Can you hear boost pressure escape somewhere? I can. I suspect my exhaust (from the exh. manifold to the turbo) is broken. That is the only part I was not able to check. Next week I am going to pull that part of the car for inspection.

chouwalker

Thanks for the responses.

The test with vacuum line to the waste gate clamped off has been performed and it made no noticeable difference. As mentioned in my original post, the waste gaste has also been removed from the car for physical inspection and testing (the gate opened and closed as it should in response to vacuum).

There is also no sound of escaping boost pressure. I wish there was, it'd give me something to pursue!

Any other ideas besides just converting the car to NA (which is where it's at right now)?

Claus Groth

If you have clamped the wastegate signal line and did not experience over 20 PSI boost, then the wastegate spring is letting the valve open before it should. Worn out wasteagte. You have ruled out other possibilities. BTW, I have a large file on boost problems. Provide your e-mail and I'll send it to you.

chouwalker

Claus,

My e-mail is chouwalker@netzero.net. Please send your file on boost problems, I need all and any help I can get!

Thanks for your help.

Eyal 951

long shot, but check that test tube that comes off from where the headers meet the crossover pipe, the thing with the little rubber cap on top that fishes to the top of your engine bay. I once blew that off at WOT. it sounded like escaping boost and I lost some boost. Then i realized it was exhaust related and though i cracked something. i don't even know how i figured it put, but i just screwed it on, very relieved and problem solved.
~Eyal

Frank@Work

Thanks for the reply.
I had already checked that part. During the rebuild I replaced the test tube with a cap because for me there was no reason to keep the test tube.
The cap was still in place.

daniel951

if u aren't building boost like u were before and u checked all the above items then it could just be your turbo is wore pretty bad and can probably use a rebuild. I know the k26/6 run out of steam on the top end and yours just got worse so that is were your problem is. Looks like u are just going to have to upgrade

chouwalker

Updates on my low boost problem (which is still not fixed):
- Put a known good waste gate on the car and checked the boost - the low boost problem is still there.
- I don't think the turbo is bad. I can generate 1.5 bar of boost, but then it drops above 4200 rpm. If the turbo was bad, would the boost would drop at higher rpms? Also turbo run-out has been tested and is in spec, as stated in my original post.

Claus - Thanks for the turbo boost document - lots of great stuff on low boost issues. I'm still working through all the information in the document, so maybe the silver bullet is in there.

I'm still open to other ideas and suggestions, if anyone has any.

Thanks again to everyone has helped out.

J Chen

Hi Claus,
Can you please e-mail me the info too.
Thanks in advance.
e-mail : jeoffrey@mail.apol.com.tw

hosrom_951

This should help, found it on Clarks Garage:



6.2 Turbo Does Not Boost Correctly.................

all the way to section 6.2.6

Hope that helps...........


http://www.clarks-garage.com/951faq.htm

Dagar1

I have the same problem on my '87. I've come to the conclusion that the turbo is just tired and needs a rebuild. I have a shimmed wastegate and weltmeister chips, so at these boost pressures the WG never opens. In fact I even pulled a vacuum on the WG and still go no more boost. Full boost also comes on later than it should, so my guess is that the bearings got coked. Trying to bite the bullet for a turbo rebuild. If you find anything else, let me know, but I suspect we need turbos.

boston944turbo

I had a similar boost issue as this and it turned out to be a bad TPS. If you have checked all of the mechanicals out (and it looks like you have) I would run a blink test on the control unit. That is how I found out I had a bad TPS. This was causing the system to run in protected mode and the car ran like it had a potato on the exhaust pipe.

This may be a long shot but a homemade blink tester is about 2.00 and then you can rule out any fault codes.

FYI I tried clamping the wastegate line just like you did and I could not build boost either. That is what led me to the fault test.

mdex

I have seen the inner section of the down pipe collapse and cause the same type of problems.