Low boost issue, need to know what to do next

[guru944]

All oem intake manifolds leak because of a design flaw.....get it fixed/modified. Then work from there. Vacuume /boost is off until properly addressed.
Pm me for details. Later.

[Mike Goebel]

Originally Posted by DannoXYZ

That's weak enough that exhaust pressure alone will open the wastegate at high-RPMs with zero pressure in the diaphragm (wastegate hose clamped shut). That test itself will give a clue if the wastegate's leaking exhaust. It doesn't rule out collapsed wall in the crossover.

I wasn't getting more than about 1.4 reading on the stock boost gauge so just did the wastegate hose clamped shut test. Took it easy and the gauge read above the usual 1.4 BAR. So I decided to giver her a little bit more and the gauge did not go passed 1.8 at full load/throttle. Does this pretty much guarantee the wastegate has gone soft? I just put on the down pipe about 200 miles go and it looked fine! Plus all hoses, all vacuum lines etc have been replaced.

Thanks
Mike G.

[DannoXYZ]

Anything after turbo doesn't matter as much. Exhaust issues is typically between header and turbo. Dual-layer crossover section can get clogged by inner-layer melting. Usual test is to see if golf-ball can be rolled through. Many 951s have collapsed crossover.

Also clogged cat can reduce boost, but usually only 1-2 psi. Test max-boost by driving with catalytic section removed.

You'll also want to test wastegate as well. Remove it and apply pressure to diaphragm (bicycle hand-pump w/gauge) and measure two values:

1. pressure where wastegate valve just begins to open
2. pressure where wastegate valve is fully open.



These numbers will tell you condition of wastegate. There are actually several ways to increase spring-rate of factory wastegate if it opens too easily. I've used factory turbo & wastegate up to 25psi without any issues (custom chip-mapping or aftermarket EFI system needed).

[Mike Goebel]

Originally Posted by DannoXYZ

1. pressure where wastegate valve just begins to open
2. pressure where wastegate valve is fully open.

I can't read the values on the gauges. What should they be for those two conditions?

Thanks
Mike G.

[DannoXYZ]

Measure actual values on yours and we can compare.
Max-opening is about 13-15mm on wastegate valve.

Also test your CBV - compressor bypass valve. Simple test is to blow through each big port. Should not flow pressure in either direction. If it has ruptured diaphragm, you'll hear fluttering or turkey-gobbling sound when building boost or shifting. Replace with part# 993.110.337.50 which is little stronger with brass-reinforced diaphragm.

[Mike Goebel]

Originally Posted by DannoXYZ

Measure actual values on yours and we can compare.
Max-opening is about 13-15mm on wastegate valve.

5 psi for lift off
11 psi full open

Thanks
Mike G.

[DannoXYZ]

Originally Posted by Mike Goebel

5 psi for lift off
11 psi full open

Thanks
Mike G.

Ok, that great testing! Yes, perfectly good wastegate. Weak ones tend to start opening at 2-3psi and fully open by 5-6psi, so yours is in great shape. To regulate to stock 1.6-bar boost, wastegate only needs to open halfway, about 5-6mm.

I suspect combination of issues:

1. Cycling-valve is most likely culprit as confirmed by your pinched-hose test. By default, cycling-valve is fully-open and letting all pressure to wastegate and dumping boost. Working cycling valve is controlled by KLR and diverts pressure back to intake (away from wastegate) and allowing boost to build. Getting 1.8-bar with pinch test shows either KLR or CV is not working. You can test KLR’s CV control line with noid-light and see if it’s sending signal to CV

2. CBV - apply vacuum to little port. Can you see plunger opening bottom port? Does it hold vacuum and keep bottom port open?

3. Collapsed crossover pipe. This is next most-common source of low boost as exhaust is not hitting turbo hard enough to generate boost. Might be only slightly collapsed as you’re still able to get 1.8-bar boost.

4. Clogged cat is also very possible. Easy to test by removing catalytic, and holding rear exhaust up with coat-hanger. Test pinched-hose test again in remote area (loud) and see if you get 2.0-2.2 bar boost. Not likely as DME will cut off fuel at 1.8-1.9 bar. But i’ve seen many that won’t do that until 2.1-2.2 bar

5. Worn turbo. I’ve seen many of these. Do you have oil pooled up on back of intske-valves when car is stopped? Do you get grey/blue smoke when getting on boost? Most common cause of worn turbos is inadequate cooling. Does turbo water pump work to cool turbo for 2-5 minutes after stopping? Have you verified that water-port for turbo on water-pump has been drilled out? I’ve seen many, many of these in default 944NA blocked-off state.

Try to feel and measure radial and axial-play of turbo-shaft at compressor wheel nut. If it’s more like than 1.25-1.50mm in any direction, turbo’s kaput!

[Mike Goebel]

Originally Posted by DannoXYZ

I suspect combination of issues:

1. Cycling-valve is most likely culprit

Haven't tested yet.

2. CBV - apply vacuum to little port. Can you see plunger opening bottom port? Does it hold vacuum and keep bottom port open?

Leaks slightly from large port to large port. Won't even come close to holding vacuum.

3. Collapsed crossover pipe.

I hope not!

4. Clogged cat is also very possible.

Cat is only 1000 miles old, but...

5. Worn turbo.

Rebuilt turbo with approx. 400 miles on it

Can the BOV by itself be the cause of the low boost problem?


Thanks
Mike G.

[DannoXYZ]

It's possible bad CBV is sending boost around to turbo-inlet instead of into engine. Simple test is remove it and plug vacuum-hose with bolt and two big hoses with cork or something. Then gently try some test-runs. Don't let up throttle too quickly as shock-wave bouncing off closed throttle-plate can shatter turbo's compressor-blades without CBV installed. If it's not holding vacuum, then definitely needs replacing anyway.

Cycling-valve and collapsed crossover most likely culprits at this point.

[Mike Goebel]

Originally Posted by DannoXYZ

KLR or CV is not working. You can test KLR’s CV control line with noid-light and see if it’s sending signal to CV to it.

Applied 12V to the CV and it clicks!!! DOH! Need to get Noid light next. Can this be done with just a test light?

Thanks
Mike G.

[DannoXYZ]

It may be locked up inside and still click. Really need to test flow-conditions of CV. Be really careful with plastic nipples on CV. Pull hoses straight off, do not bend them or they'll break nipples off CV. Might be safest to cut hard crispy hoses off with dremel and replace with new soft hoses. I like to lay triangular filet of epoxy where nipples join body of CV to reinforce. Can attach hose to port C and blow in it and see where it comes out. Power on CV no more than 5-10s at time to avoid overheating it. To test CV:



1. With no power-applied to CV, confirm you have flow between ports C and W

2. With power-applied to CV, confirm you DO NOT have flow between ports C and W

3. With power-applied to CV, confirm you DO have flow between ports C and R

If you have defective CV, don't need to use crappy old plastic part. Get modern all-metal boost-solenoid:
https://www.aemelectronics.com/products/sensors-connectors-accessories/boost-control-solenoid
Which is actually just MAC control valve https://www.ebay.com/itm/153357750198


As for testing signal, not sure duration is high enough to light up test light (it's PWM signal like injectors). Wouldn't hurt to try. Noid-light has duration-extending circuitry to ensure it lights up with shortest of signals. Oscilloscope would also work.

[Mike Goebel]

Damn! I wish I didn't have to work today!!

Thanks
Mike G.

[Mike Goebel]

Originally Posted by DannoXYZ

1. With no power-applied to CV, confirm you have flow between ports C and W

YES! There is flow.

Originally Posted by DannoXYZ

2. With power-applied to CV, confirm you DO NOT have flow between ports C and W

NOPE! There is flow.

Originally Posted by DannoXYZ

3. With power-applied to CV, confirm you DO have flow between ports C and R

YES! There is flow.

Thanks
Mike G.

[DannoXYZ]

Originally Posted by DannoXYZ

2. With power-applied to CV, confirm you DO NOT have flow between ports C and W

Originally Posted by Mike Goebel

NOPE! There is flow.

Well, that confirms it, CV is not working. When activated by KLR, it should stop flowing pressure to wastegate and return it to intake to allow turbo to build boost. Easy enough to replace with MAC valve.

Still possibility your crossover is slightly restrictive as well.

[Mike Goebel]

Originally Posted by DannoXYZ

2. With power-applied to CV, confirm you DO NOT have flow between ports C and W

I'm not convinced this is true. I tested a brand new Porsche CV and it behaved the exact same as the one on the car!!! Flow still occurred between C and W.


Thanks

Mike G.

[Mike Goebel]

Got this from a German Forum!

The timing valve has no diaphragm, it clocks a piston. The line C-W is always open. Via the piston, the line R is released intermittently to lower the pressure in C-W.

Test the timing valve

You should blow in "R". If the cycle valve is closed, "R" is also closed (almost). The full charge pressure is therefore on the wastegate. If 12V is applied to the timing valve, "R" is open. The boost pressure is for the most part back into the intake area and the control pressure on the wastegate is minimal.

Thanks
Mike G.

[Mike Goebel]

RE-tested CV just to make sure and the old CV is fine. New BOV and sealed the throttle shaft. Did a test run still only 1.4 BAR. Hmm! Going to check the boost press with a gauge now. Wonder if the stock boost gauge has a way to be calibrated?

Thanks
Mike G.

[Mike Goebel]

Checked boost pressure with crappy Harbor Freight test gauge that has a max press of 10 psi. Looked like about 9 psi max in 3rd gear. Pinched off WG hose, again, and the gauge read 1st-8.5, 2nd-10ish, 3rd-pegged at 10+psi.

What does the vacuum typically read on these beasts?

Thanks
Mike G.

[nize]

it should be -18 to -20 inHg vacuum at idle.

[Mike Goebel]

What kind of boost do most see when pinching off the WG hose with a stock engine?

Thanks
Mike G.