Scott Dunavant

So I replaced about everything on this car after siting for 20 years. I did't replace the blow off valve. I went under boost to about .6 bar, then then it feels like all the boost is going some where or recirculating. Wasn't going out the waste gate as I would have heard that.
The car runs perfect even running the RPM's up not under boost. Should I invest in a aftermarket one? I'm just running 1 bar.

urquattro20Vt

So it's the factory bypass valve, right? You can pull it off and and blow into it - if the diaphragm is torn air will go through.
Personally I don't see a need to go aftermarket. Kayser now makes the one I used to get from Bosch.
I once had a K24 (old Audi) turbo stop making boost - was really weird - it would make boost for about 15 minutes then...nothing. Looked perfect. still no idea what happened - I decided to upgrade the turbo...
I assume the exhaust sounds normal - no big leak at the headers...

Brandon
'91 Turbo

Scott Dunavant

Yes, Exhaust is Good. I watched a video where one stock valve leaks air. Wasn't sure if that's possible?
I'm considering a promotive upgrade.

fritz k.

A new wastegate will not solve your problem. Almost the only thing that can fail is the diaphragm and the effect would be a higher boost pressure because the bypass no longer opens or opens later.

Most likely the boost pressure is being blown off somewhere, the usual suspects are the elbow hose at the supercharger inlet, the o-ring on the throttle valve casing or the funnel-shaped pressure hose to the intercooler. There are also various possibilities for leaks at the intercooler.
I would check whether all connections are tight and the hoses are in order.
Fritz

Scott Dunavant

Fritz, Thinking about it again. The boost is totally lost at .6 bar. I guess I'll pull the intercooler this week. What about the Recirculate valve?
Could that be bad? I also like the idea of a adjustable blow off valve. I don't like the jerking pressure of quickly letting off the throttle. Any thoughts on those?

fritz k.

Based an the description i suspect that the pressure is held until 0.6 and then suddenly blows off due to leaking. I had a similar effect when a clamp on a hose came loose.
The changeover valve could have the same effect if i.e. the connection on the intercooler is loose. If the pressure were to recirculate in boost mode, a lower boost pressure would build up, but that feels completely different than a sudden drop at 0.6 bar.

I use a control valve to influence the boost pressure, but that requires a constant, correct display of the boost pressure for monitoring. I also monitor the air fuel ratio by a lambda probe.

Fritz

urquattro20Vt

did you test the bypass valve yet? (aka blow off valve (but blow off valves vent to atmosphere - not back into the intake system like ours - hence bypass valve or recirculation valve)).

Do you have access to smoke tester?

Brandon
'91 Turbo

TMWTP

I've had similar issues. Pull the intercooler and check all vacuum hoses, boost lines etc. The dreaded o-ring on the throttle body is a likely cause. I've also blown the hose off the diverter valve a couple times. If any of the clamps are not sealed perfectly, they could work fine until .6 bar is reached.

Scott Dunavant

Found it. Clamp at the turbo slipped off.
Scary powerful with short gears.

cobalt

Interesting. My experience is if there is a leak in any of the areas suggested the car will not run period. Especially a minor leak at the o-ring from the I/C to throttle body. More likely a bad clamp at the silicon hose from the turbo to I/C. Sometimes the worm gear tooth on a clamp will fail and will only tighten so much. Once boost pressure exceeds a certain amount the air finds a way to escape but not enough to blow the hose off. Also the jerking of the car when lifting abruptly is a result of the CIS. The sudden loss of air flow over the metering plate will have this effect. Not sure there is much you can do about that other than going EFI.

fritz k.

I have also experienced the engine not starting if the O-ring on the throttle valve has slipped out of its seat at one point. But then the leak is already present; if a clamp is loose, the hose is usually still on the connector and the leak only occurs at a certain pressure.
The jerking when suddenly lifting the pedal gas is actually effectively prevented by a functioning shut-off valve.

Fritz

cobalt

I had a hose clamp that held the hose to the bottom of the IC that was missing a tooth years ago. It would seem to be tight but wasn't. When pressure built it was enough of a leak for boost to be lost but not enough for the hose to be blown off. I have seen some situations where the hose came completely off and the car just stops running once the throttle is lifted.

So regarding the jerking motion when lifting. You are saying the fuel shutoff valve (i'm assuming you mean decel valve) is not functioning properly when the metering plate closes abruptly and cuts the fuel?? This could be a bad vale or just needing adjustment? I have to admit it has been a long time since i had to diagnose CIS issues as most of my 964's are motronic although I had always experienced this with any of my CIS cars including a relatively new SC I had in the early 80's, so I assumed it was just part of the quirkiness of the system and even more so when boost is involved. I would be interested in hearing more. I had been told by some Porsche techs that there was no actual controlling this just minimizing it. Once the metering plate slams closed it cuts the fuel. But it would make sense if the decel valve isn't allowing the air flow to be diverted properly. I plan to pull my engine this year as soon as I get the other 2 engines finished which is quickly approaching. I will look into the decel valve and see if it is functioning properly unless you have other advice?

fritz k.

The designation shut-off valve (not fuel....) comes from the PET, I think diverter valve is more appropriate.
The purpose of the valve is primarily to prevent the turbo from delivering against the closed throttle valve when lifting off the pedal and is therefore loaded and braked. This is achieved by a bypass between the suction (big suction hose) and pressure (IC) area which should be opened by the diverter valve. As a result, the turbo delivers in a circle for a certain time and a side effect is that the metering plate does not close abruptly.
In "normal" or as you like forced driving, this works very well; my car doesn't experience any jerking when I release the gas.

The diverter valve is very simple and can be easily checked with a vacuum pump. Remove it and apply vacuum to the small connection, then the valve must open visibly and completly . If it is ok check the vacuum hose, which is routed via the vacuum limiter into the inlet manifold underneath the throttle. When the throttle is closed and the engine revs a little higher when coasting, the vacuum is at its highest there and the valve must then open the bypass.

Fritz

urquattro20Vt

Different Bypass Valves AKA Diverter Valves have different spring pressures - which affect when/how it opens when you go from on boost to off throttle. Softer spring means it opens up sooner (requires less vacuum pressure) - so less abrupt. This is ONE thing that can affect the car jerking when quickly going from on boost to off throttle.

Brandon
'91 Turbo

Scott Dunavant

I have to pull mine and test it. I probably need another one as everything associated with this car has lived it's life and died years ago. Resurrecting the dead here as i poked the speedo today on the car's 6th test drive.