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ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

Soooo...while I'm waiting for Paeco to rectify the lost distributor debacle, I've been conducting some tests to see if I can isolate the persistent and elusive apparent vacuum leak on the 1981 NA. I'm using a mighty vac hand pump.

I have isolated and tested most of the major components in the engine bay (decel valve, brake booster, thermo switch, AAV, charcoal cannister, etc.) by creating a closed vacuum circuit on each component and testing them individually. So far, those all check out fine except for the thermo switch, which demonstrates a very slight vacuum leak if I pull 15 or more Hg. It's possible that the lines themselves might not be sealing tight enough on the two connectors. FYI, it is part number 058.131.851.A, callout 52A on page 105-05 of the parts catalog.

I also tested the line that goes back to the gas tank (the one that connects to the charcoal cannister), and it seems to leak pretty severely. Is this normal, or should that be air tight?

Now, for the more important question. No matter what I do, when I attempt to draw vacuum on the intake manifold / throttle body / plenum assembly, I can't get any vacuum whatsoever. I've removed all of the vacuum tubes that go to the decel valve, distributor, aav, brake booster, etc. and plugged them. So effectively, I've attempted to create a closed circuit including the plenums, TB, intake, and head. For the life of me, I cannot get *any* vacuum. Is this what I should expect when the car isn't running, or does this indicate a massive vacuum leak? For example, how do I tell for sure if any of the plenums are leaking (they appear to be in great condition on visual inspection)? Or the intake gasket? Or the TB gasket? Or the cold start gasket? Or is there some other way for me to determine what's going on?
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erstwhile owner of just about every 924 variant ever made

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11733 Location: PacNW

Your last test would only work with the cam removed. There is (almost?) always one cylinder on "overlap," with both valves open.

rasta
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Toofah King Bad

WeiBe (1987 924S 2.5t) - 931 S3

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

So short of removing the cam, is there anyway to effectively test for all possible vacuum leaks in the plenum / intake / TB / head area?????
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erstwhile owner of just about every 924 variant ever made

!tom · Posted: Sat Mar 31, 2007 7:48 am Post subject:

Not only that, but the throttle never completely closes, so short of sealing over the intake somewhere, air will always make it past the throttle valve, via the idle air bypass, or even through the throttle valve (since it doesn't form a seal).
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78 924 NA
5-lug

Lizard · Posted: Sat Mar 31, 2007 9:34 am Post subject:

there is another way, get a fog machine (party fogger) and fill the intake system with fog, and look for losses that way.

how do you know it is a vacuum leak though?
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3 928s,

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

Because I can't get the car to produce more than 12 Hg of vacuum at idle, when it should be producing about 18.

The history on this dates back to my top end rebuild. See this post for more background. Car ran great at first, pulled 18 Hg consistently. Over the next year, the vacuum has gotten progressively worse, so something somewhere is failing. I have replace EVERY vacuum line, and have EVERY vacuum line secured with a clamp or a zip tie (on the small 4mm lines). I have replaced the rubber boot that attaches to the TB. I have replaced the TB gasket twice. I have replaced the CSV gasket twice. I have checked the brake booster and charcoal cannister multiple times. I have checked valve timing countless times. I have checked ignition timing multiple times. So in addition to not being able to produce more than 12 Hg, the other thing that convinces me is that the problem wasn't there at the beginning but has gotten slowly but progressively worse, which doesn't sound like incorrect timing or valve clearance or other installation errors.

One theory was that the intake manifold gasket is bad (even though I installed it new during the top end rebuild). Another theory was that the bladder in the ignition distributor was bad, which is one of the reasons I sent it out for rebuild.

Bear in mind, the ignition distributor is currently OUT and LOST (thanks a lot, Paeco

) so I can't run the car right now. I'm trying to do as much troubleshooting as I can in its current state.
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erstwhile owner of just about every 924 variant ever made

Smoothie · Posted: Sat Mar 31, 2007 11:05 am Post subject:

Try to get a look at the lifters (aka- buckets) through the oil filler either directly or with a dental mirror. -Just a wild hunch as we run out of good guesses, but it's too easy to do, so why not...

While I'm doing wild hunches....
The extra-high compression vs. extra-low vacuum thing has me wondering - could the crank pulley pin have gotten sheared?, then the crankshaft drifted slightly out of time in relation to the cam?. To check, you'd have to finally find that crank pulley tdc notch (we all have trouble finding it, but come-on now, it has to be there

) -then make sure both the crank pulley notch and the flywheels' tdc mark line up at the same time.
I'm thinking something like this might account for the higher compression with lower vacuum, plus it's something that could have gradually worsened over time..

Here you can just barely see the pointer on the oil pump and the notch on the pulley just as you'd see them in real-life (just barely) - http://pg.photos.yahoo.com/ph/smoothies931pics/detail?.dir=bdb3&.dnm=ff6b.jpg&.src=ph
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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

Lizard · Posted: Sat Mar 31, 2007 1:41 pm Post subject:

The other thing I would be checking is the injector inserts, the ones that screw into the head, and the corrosponding O-rings,

have you also verified the timing?
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3 928s,

leadfoot · Posted: Sun Apr 01, 2007 11:16 am Post subject:

IIRC a bad valve guide/seal etc causes fluctuating vaccum loss, unless all your valve parts have worn considerably or your cam is flattened then it would assume your issue is with fuel/ignition or vaccum loss somewhere else upstream...
The exception to this is with worn piston rings and you would need to do a leak down test to confirm this.
So timing can be verifyed by using a pyrometer or a dwell meter from memory, I'm pretty sure endwrench used to use a dwell meter....
Air/fuel ratio is a little harder but there are hand held units that have become cheap recently that might be able to give you a good enough reading for diagnostic purposes....
As for testing system/control pressures, firstly when was the last time you replaced your fuel filter??.
This site is cool... http://www.secondchancegarage.com/public/186.cfm
Leadfoot
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1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...

924RACR · Posted: Sun Apr 01, 2007 10:31 pm Post subject:

Agreed - I'd suspect a valve problem, most likely sealing. Might have to pull the head and relap a valve or two. You need to run a leakdown.

Sounds like you might need to borrow my leakdown tester!
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

Picked up a used distributor from a local junkyard yesterday. Holds vacuum fine, shaft seems to be pretty good. Still no luck, but at least I can resume testing. When I try to start the car, it will not catch at all. When I spray starter fluid into the TB, I can get it to cough and produce almost a mild backfire out of the exhaust, but that's it. Bear in mind, I drove the car to my garage, but haven't been able to get it to run since.

Yesterday, in addition to installing the replacement ign dizzy, I also replaced all of the 4mm vacuum lines with some NOS OEM 4mm vacuum lines I had acquired earlier...the silicone 4mm didn't seem to hold vacuum as well on some of the small plastic hard lines, so I replaced all of those joins with the OEM stuff. Shouldn't be any vacuum leaks now, at least from those lines.

924RACR · Posted: Wed Apr 04, 2007 12:09 am Post subject:

Oh, man, you've got a whole list of things to work through.

Refresh my memory, where are you living now? Sounds like you need a heavy day of diagnosis - need to schedule a 924RACR intervention!

Unfortunately for you, I'm a little backed up on racecars to fix; mine (prep for race season start), Chris's ITB 924 (formerly Jeremy's) prep for race school (going there at lunch), Steve's ITS 924S (assisting in new build for this season), and Erik's 944S2 (due for new rod bearings, belt job, alignment, etc). But I guarantee we need to spend at least a half-day debugging/diagnosing your car... I think you're wandering off track.

Since you already have my pressure tester, I assure you you have already the correct fittings to measure pressure.

A valve job is a relatively simple/straightforward DIY job, once you've got the head off.
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

I just figured out the CIS tester...didn't realize I needed to take BOTH fittings off of the fuel dizzy. D'oh!

Anyway, I'm getting 5 bar. Haynes says 5.4-6.0 bar, so it seems I may be a little low...but it still doesn't explain why I can't get the car to fire with starting fluid. I'm going to test spark later today.

I'm still in Plymouth...Ann Arbor & Beck Rds. I'll send you my addy and we can work out a "labor swap". Let me know when and where you need help with the race cars, and I'll be there (unless I'm out of town).
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erstwhile owner of just about every 924 variant ever made

924RACR · Posted: Wed Apr 04, 2007 12:47 am Post subject:

That's your supply pressure, right? You know how to check control pressure too?

Correct, I wouldn't expect that to fundamentally cause a no-start, not on its own. That said, it wouldn't hurt to get the pressure up to spec. I don't remember if you need to add or remove shims to get that effect - I'd guess add? Do you have any spares to grab shims from?

You don't have a bad ground for ignition, or something unplugged or corroded, do you? DITC car or no? I've run into problems with the crank sensor connection (at the DITC) before...
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

I might be able to find a spare shim from a 931 setup I have laying around. I'll read up on the control pressure and check that too.

Regarding grounds, I don't suppose it can hurt to check them all again. Good thing to do while the car is down anyway, I suppose...another dose of dielectric grease while I'm at it

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erstwhile owner of just about every 924 variant ever made