924Board.org

[drewnashty]

I was recently getting a slight rattle from what I was told by PO as the spring in the POV or BOV or w.e. was going out.

Well I went to start my car the other night when I was out of town and it sounded funny, as though it was running on 3 cylinders with a loud rattle (like a light metallic object was getting flopped back and forth, and then the car died. I thought holy shiet broken valve, but then I remembered the BOV was making a simliar sound but a much quieter version. I started it back up and had to feather the throttle to keep it alive. I noticed my boost gauge was going nuts not necessarily spiking but just wobbling in a general spot really fast. If I put it around 3500 it was putting out it's normally boost it was just wobbling like crazy at about 6-7 psi. It runs ok as I get it going, a slight loss in power once going but very noticeable at low rpms, seemed like the engine couldn't breathe like the BOV was causing major backpressure.

I drove it down to a local porsche repair shop that I interned at out of high school and the owner seemed to have thought it sucked a valve at first. He took off the valve cover and tested for any play on the lifters... seemed normal as he put it. Had me fire it up and he watched the lifters, and he said they seemed normal. He then thought maybe it was the #1 rod bearing or the rod itself has gone, at that point saying it's not worth the repair for this car and that the engine is toast if be the case. I mentioned to him about the BOV situation and he didn't think it was the case while not denying it could be.

My question to you guys is can the BOV going out cause the engine to act like this? To me it seems possible as an engine will die if you plug the exhaust. So I'm thinking thats why it dies at low RPM's cause theres not enough pressure to push through the messed up BOV. Or I might be dellusional and be in denial as I really don't want it to be a broken con rod or sucked valve.
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81 931

[drewnashty]

I'm just hoping this is not my "Blue Screen Of Death"
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81 931

[Lizard]

without hearing the noise it is hard to say,
the blow off valve is on the intake side of things and if it fails it would simply lower the amount of boost actually made,

I would pull the plugs and do a compression check and measure vacuum at idle to give some ideas as to what could be happening,
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3 928s,

[emoore924]

You can take the BOV apart with the turbo in the car and check the gaskets on either end of the little BOV piston. If the BOV was making noise before your starting/running prob's, it is likely that fragmented gaskets on either side of that piston are the source of the noise.

Not sure whether that problem is all that's going on for you, but at least you can diag/repair that with the engine together.

Do a search and you'll find the procedure. I think the gaskets are stock 930 parts...

[-nick]

Drain the oil, you'll know really quick if it's a rod bearing - the oil will have lots of chunky metallic particles in it. However, the weird boost gauge readings don't really point to any internal engine havoc.

Will it idle at all? If so, is the idle vacuum reading the same? (assuming your gauge is vacuum & boost).
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1980 931S
15psi boost, MS-II, EDIS, 951 IC, custom intake, Ford 5.0L throttle body, Forge BOV, WB o2, G31 w/LSD, 964 wheels, 968 rear sway, Bilsteins, 200# Welt. springs. A laptop, and a partridge in a pear tree.
1991 964 C4 Cabriolet

[Smoothie]

What you're calling a BOV or POV is actually a recirculation or bypass valve. It helps to call it what it is to understand how it works - it doesn't actually "blow off" or "pop off" anything.
What it does is recirculate intake air from the outlet back to the inlet side of the turbo's compressor to prevent boost spikes that might damage the compressor wheel or throttle and throw off the air:fuel mix and slow the compressor (the entire turbo actually) between shifts. It's spring holds it closed when the engine is shut off and while running when the throttle is sufficiently open to cause vacuum to drop low enough for the spring to overcome the pull of the vacuum, resulting in closing of the valve. In other words, manifold vacuum sourced from intake runner #1 pulls the valve open when vacuum is high (as in idle and between shifts when you let off the pedal and close the throttle), then when vacuum is low (when accelerating), the spring closes the valve, which allows the full boost pressure that's produced by the compressor wheel to flow into the engine. If your recirc valve were stuck closed, you see boost spikes between shifts. If stuck opened, you have very little to no boost at all. Either way, a recirc valve malfunction wouldn't cause a backpressure problem. A quickly fluctuating vacuum/boost gauge suggests something more like a valve problem to me. Is it a boost-only gauge or combo boost-vacuum?
I'd also check for vacuum leaks mainly at that large line from the bottom of intake runner #1 to the recirc. valve. [-And at the wastegate control line that runs from the rear-center of the main charge tube to the wastegate. There's also a large spring in the WG that can rattle.]
A quick diagnostic of the recirc valve is to "blip" the throttle while still and at idle - you should hear a click (or "clack") sound that would be the recirc valve closing when you hit the pedal and opening again when you let off of it.
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"..it's made in Germany. You know the Germans always make good stuff."


'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

[drewnashty]

Yes it is a boost/vacuum gauge, and it seems to get the normal ratings it's just wobbling at those ratings. And a quick blip doesn't do much as the engine isn't that responsive at this point. I will not be able to take a look at anything until I get back to Orange on Tuesday, I'm pretty much trying to soak up many different perspectives and solutions as possible so I know what to look for and or how to fix it. So I don't sit there scratchin my head the whole time.
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81 931

[drewnashty]

Vacuum was about 15-20 Hg at about 600-1000 when feathering the throttle to keep it there.

It's not idling, if it is warm it won't die as quickly it will try to idle and eventually dies. I had to do some fancy footwork to get it to the shop during traffic hours of Orange County. I'll check the oil and filter for metal, do a compression check and check the recirculation valve system and hoses.

[Smoothie]

See if you can troubleshoot it with this -
http://www.earlycuda.org/tech/vacuum2.htm

Don't waste much time looking at other things until you've completely checked for vacuum leaks.
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"..it's made in Germany. You know the Germans always make good stuff."


'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

[drewnashty]

Well I got lucky that I happened to intern at DeVito's Auto Repair, a Porsche repair shop that specializes in 928's in Orange, for my Auto Shop class in high school, as they let me use there tools and pick their brain every now and then. And showed me that repairing these vehicles is more like miracle work than mechnical.

Anyhow! Couldn't locate any vacuum leaks so I went on to do a compression test of which all cylinders were good except for #4, and guess what... no compression at all. NONE. So dropped the oil and found no metallic stuff. Then I proceeded to disassemble for head removal. Found a lot of loose bolts and a lot of frozen bolts. When I got to the throttle body there were tons of little metallic flakes inside. GREAT!

Removed everything to get to the head, what a freakin nightmare. I'll be so much more appreciative when I have to work on my 383 in the Blazer now. The exhaust was a real pain in the ass as some of the heads just wanted to strip so I decided to try and pull the head out with the manifold attached and just unmount the manifold from the turbo and the pipe at the rear of the manifold of which I'm assuming is the downpipe? Well when I thought the head was ready to be removed I did not pay attention to this oddball thick steel bracket looking thing just sitting right there in the way, serving what purpose who knows. So I put some 2x4s between the head and block to get more clearance to the exhaust bolts under neath, removed the bolts took off the manifold. Now the head should slide out... oh but what's this the back side of the head just slides right into a wall. So I tried jacking the motor up to get some clearance. Close but no cigar. Then I had to come back home.

Do I have to remove the Head studs to take out the cylinder head or does the entire engine have to come out.
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81 931