Tass 928

I'm baffled...here is the background. I just finished installing the supercharger on my 79' 4.5. Car started up fine and ran great with no adjustment required.
I took it up the street for a test ride, went less than two miles round trip. I never had it above 4K rpm and even then only when shifting. It "shut off" when I pulled back onto my street and I was able to coast all the way back into the garage. I went through all of the usual quick stuff ( added gas, made sure I have spark, verified fuel was getting to the motor). Looking through the vent holes I could se that the belt was in place and not broken. Then I did a compression test and one after the other all of the cyls were coming up 30-50 psi. I actually checked my tester on one of the other cars just to be sure it was calibrated.
Suspecting a loose T-belt had jumped some teeth, today I tore off all of my S/C stuff and opened up the front. The belt is still tensioned correctly and the cam timing marks are spot on.
I bought a leak-down tester and if i am using it correctly, it just shoots right over to 100% leak.

What can you possible do that would cause all 8 cyls to go flat in 2 miles of driving??? It looks like I may be motor shopping

RET

I don't have any experience with troubleshooting supercharger installations, so I think that this is offered as my best guess...

I think that there might be a problem with the dynamic compression test if the supercharger is connected to the throttle body while the test is preformed. This test is normally done with the throttle plate completely open and no intake restrictions. I think that the manifold from the supercharger should be removed from the throttle body.

I would guess that the compression results would be extremely low if this is not done and there are any restrictions to intake airflow, which I presume there would be since the supercharger is not turning very fast while this test is performed..

FWIW, the piston of the cylinder undergoing a leak down test must be at top dead center on a compression stroke with the valves closed and the crankshaft kept from turning while air pressure is applied in order for the test to be valid. Each piston must be brought to TDC, the engine locked, and the test performed. I have never done this on a 928, so I don't know the correct procedure and technique or tools required to lock the crankshaft.

HTH.


Bob

Daniel Dudley

Are you saying that you have done the leakdown, and air is shooting out the exhaust or back through the intake with no restriction ?

Tass 928

Yes, that is what it seems to be doing. I started with number 1 cyl and the timing marks at TDC. Also put a dowel in there to watch it come up to TDC.

I have all of the S/C stuff off so that is out of the equation. I cannot hear air leaking into the intake or the exhaust.

Garth S

If no apparent air loss to either intake or exhaust during the leakdown test, that leaves rings or ... gulp .... holes burned through pistons ; difficult to see all 8 cyls having a headgasket issue simultaneously. Anyway, on the serious side, it does not sound as if it ran on boost, so I take it you have discounted lean A/F and timing issues that could comprimise pistons and head gasket.
On the #1 @ TDC, you might try a spoonful of motor oil to seal the rings, and see if pressure builds.

mark kibort

are you sure you are on the compression stroke?? sorry , I had to ask.

If not, rotate the engine 360degrees and try again.

im a little worried about the 50psi compression test. this isnt right. the restriction doesnt really matter much, as the flow is very slow. It just might take a couple of more cranks to get the same final reading. (ie throttle open, vs throttle closed)

mk

Tass 928

I have to sign off for tonight. I have a couple of things to try in the morning and will report back.

Thanks to all who responded, I'll keep you posted.

Carl Fausett

Kibort is correct - when using a cylinder leakdown tester make sure you are on the compression stroke for each cylinder - not the exhaust stroke. A common mistake.

How old are those head gaskets? I wouldn't "start shopping" for a new motor. Not yet.

...and you never exceeded 4000 RPM? Thats a pretty light load on that motor too.

Mrmerlin

ahh can you do a compression test?" Are you sure the cam belt is moving with the crank? could the timing marks be lined up on the cam and the crank is just spinning? (IE a sheared tooth on the T belt)

Paul D

Is it possible for the cam gear to spin in relation to the cams? If so, this might explain how the belt timing looks correct but the cams (and valves) may no longer be correct. I certainly hope nothing bad has happened.

dprantl

I don't think that's possible, especially on the early cars. The cam gears are keyed to the camshaft.

Dan
'91 928GT S/C

jcorenman

It's gotta be something simple. Sudden shut-down, low (not zero) compression in all cyl's, leakdown shows zero in all cyl's, that all makes no sense. The probabilty of suddenly-blown head gaskets or holed pistons or whatever on both heads and all 8 cyl's is right around zero, right? My guess is that something is blocking the intake, and the (new) leakdown tester is defective or not set up correctly or maybe like Mark said it wasn't at compression TDC.

Before attributing anything to the leakdown tester wouldn't it make sense to run it on a known working car? That would verify that the tester and the procedure were OK.

I also don't see how it can be the timing belt. Dave said that it is in place and the marks all line up. The only possibility is that it is literally not turning as MrMerlin said, and the cam marks just happen to line up. Unlikely, but accept it for a moment- but then the valves wouldn't be moving. Which means the compression would be zero, not the 30-50 that Dave reported-- no valves, no compression, right?

So if we eliminate the unlikely then we're left with either the likely or the impossible...

The likely causes for sudden shutdown are fuel or spark but those are both apparently OK. What about air? That's the third piece of the puzzle, what if the SC failed or something suddenly blocked the intake?

I don't know squat about SC's but an intake restriction is one thing that could cause low compression on all cyl's. Does it make sense to re-run the compression test with the SC-to-intake pipe opened up, and WOT? That's how it's done on a non-SC car, and would be a good verfication.

Dave, are you really going to make us wait all night for an answer? I really hope it is something simple, fingers are crossed.

Jim

mark kibort

broken cams????

WICruiser

It doesn't seem likely that anything associated with either cylinder bank is the cause if all 8 cylinders are exibiting the same results. That pretty much gets it down to something on the intake before seperation to individual cylinders. Maybe a shop rag sucked into the throttle body severly restricting air flow?

Stan.Shaw@Excell.Net

Can you hear where the leakage is when you are using the leak detector? You should hear air escaping, right? intake, exhaust or into the crankcase? this would identify the loss to intake valves, exhaust valves or pistons/rings.

Does the motor spin easily by hand, as would be true with no compression?
You are sure the crank and cams are spinning together when you turn the crank bolt?