Boost issues, too high, then too low

[FirstDotLast]

Hey guys,

I have been having some issues with my boost pressure and after searching around for a while, I think I have less of an idea of what is going on now. I'll preface this by saying I'm certainly no expert in these cars but I'm relatively mechanically inclined. The car is a low mileage, completely stock '86 930.

The other day, my car was idling funny and I noticed that the AAR was unplugged. I plugged it back in and adjusted the idle to a solid 950 RPMs. Later that day I was driving and noticed the boost gauge was moving between .4 bar at idle and .8 under full boost.

At this point I'm thinking the boost gauge sensor is acting up. I park the car and after a few hours, go for another drive. Now the gauge is at 0 bar at idle but under boost it will only go to .4 bar and no further and there seems to be a lot more lag. Doesn't feel like it is kicking the same any more or at all really.

So now I'm thinking a boost leak...I already swapped out all the intercooler o-rings in case there was a leak there but still seeing this issue. The earlier issue with the pressure being too high at idle hasn't happened again since. So here's where my head is right now:

The car runs well, starts right up, idles at 950rpm with no bounciness and is smooth all the way through the RPM range. Just low boost.

1) Possible wastegate issue...perhaps valve or worn spring? This would not explain the first issue but I guess it's possible the gauge was being flaky and corrected itself. Btw, is there some special tool to getting the wastegate off that I'm not aware of? I can't find a socket or bendy socket that fits the two nuts near the exhaust pipe bend. I can get an open-end wrench on them but not without fear of rounding the nuts. Beginning to think I'm going to have to just cut them off and replace them.

2) Could a bad boost sensor still cause the behavior I'm seeing? Initially boost pegged high and now cutting out at .4bar?

3) Something I'm overlooking/haven't thought of yet. Although I feel like a lot of other things would manifest as poor running/idle/etc.

Anyone seen this before or have any ideas? Oh, and here's a pic because I hear these help!

[T77911S]

the stock boost gauge is not very reliable. get a good reading of the boost before you tear things apart.

if you swapped the Orings before you noticed the increased lag then you likely created an air leak.
i have had orings roll out of the groove and even break.
make sure ther IC is bolted down good. it will blow off.

[Akila]

Have you checked your wastegate?

[mark houghton]

If you don't find a boost leak, then I too would look next at the wastegate. If you car doesn't seem to have the boost punch it should have, then intake leak or wastegate stuck open could be the culprit.
As to the gauge showing .4 bar at idle, then magically fixing itself, the sending unit may be suspect....either going bad or just a bad ground...or the gauge itself.

Easy enough to check the sending unit; take it off the car, attach a regulated air line, and apply varying amounts of "boost" while taking resistance readings with an ohm meter. I have a graph of boost against ohms but it's a pdf document and can't be attached here. Basically, you should see around 20-30 ohms at 0.0 boost, 45 ohms at .2 bar, 135 ohms at 1.0 bar, and 190 ohms at 1.5 bar. Draw your own graph and extrapolate the slope

[FirstDotLast]

Alright, some progress...

If I disconnect the wastegate hose from the intercooler and plug the hole in the IC. Then go for a drive - carefully to not overboost - I get normal boost and the boost gauge shows the correct reading all the way up to .8bar. So that eliminates any electrical goofiness with the boost sender and boost gauge along with intake leaks and turbo spooling issues.

Pretty much narrows it down to the wastegate. I pulled the wastegate off (blow torch is necessary for those bolts by the way). Valve is seating fine, took it apart, diaphragm is fine with no tears, spring visually is fine and this is a pretty low mileage car. If I put some air in from the wastegate hose, I can hear the wastegate opening at just about 1bar and see the pressure drop rapidly which seems correct given that the car was not running. So The wastegate seems to be operating normally. When I apply air through the hose, If I stop before 1 bar and the wastegate isn't open yet, the air slowly leaks out. Is this normal or should it be holding pressure? Pretty much narrowed it down to either the wastegate hose or something that I'm not seeing about the wastegate.

Does anyone know the part number for the rubber wastegate control hose? I couldn't find it anywhere. I'm guessing I can pick up any vacuum hose that is the same also, does anyone know the inner/outer diameter for this hose by any chance?

[T77911S]

you can also pull a vacuum on the diaphram if that is easier. with air pressure you have to be sure there are no leaks in what i am assuming is the hose coming off your air compressor.

you said " I can hear the wastegate opening at just about 1bar and see the pressure drop rapidly"
what pressure did you see drop?

here is how i tested mine.
on the bench of course.
i pulled a vacuum on the both ports to see if it would hold.
then i put air pressure into the boost input (like you have done). i could feel and hear air coming out of the "exhaust port" of the WG.
my valve guide was bad. low miles WG too.

[FirstDotLast]

Ah sorry, should have been more clear. I stopped applying air once I heard the wastegate open and watched the pressure drop on the gauge I had hooked up to the WG line.

I feel like the hose will be cheap to replace if I can find out the specs of it. If that doesn't fix it then it might be time for a new WG. When I took the WG off the car, everything seemed fine including the valve seat but looking at how it operates, I wonder if its possible for the diaphragm to stick and not close the valve all the way causing these symptoms.

[T77911S]

regulate your air down to around 15-20psi. put that into the boost input of the WG (on the bench).
thenn check for air coming out of the exhaust port when the valve opens.

if the diaphragm is good you should be able to pull a vacuum on it and it hold.

[Ruf930]

Hi FDL m you mentioned the AAR's Plug was off?,Is it loose much movement in the connection?
have you got good continuity? would this make the symptom you refer to?

Merry Xmas Everyone

[OSI930]

The hose between the intercooler and wastegate is 8 x 14 x 380 mm according to the Porsche PET CD.