924Board.org

[ideola]

Last week at Tweeks Fun Fest, I put the NA on a dyno. I was surprised to find that the car is producing less than 80 HP (at the rear wheels). At the same time, I had an O2 test performed. It is running about 9PPM, which is a little rich...should be more like 12 according to the techs. Also, on the way down, we measured mileage and I'm only getting about 16.5MPG on the highway. All of this points to a vacuum leak, confirmed by my vacuum gauge which consistently runs about 14HG at 950RPM idle speed.

The weird thing is, the car is running exceptionally well. It runs very cool, never even touched the second tic mark on the temp gauge, oil temp never exceeded 90C. The car feels good, although it does not pull quite like I think it should, which I attribute to the 30-40 lost HP.

Keep in mind, this is a clean top end rebuild (only 2500 miles since), all new gaskets, all new injectors & seals, vacuum lines. I've even gone back over every vacuum line, installed clamps just about everywhere, temporarily bypassed the brake booster, and done everything else I can think of to find a leak. I can NOT get the HG to go any higher than 14HG at normal idle speed. What am I missing?
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erstwhile owner of just about every 924 variant ever made

[dpw928]

Brake booster or intake valve gaskets could be leaking. Had the same problem with my 80NA and didn't find the blown intake gasket until I pulled the head. To check the brake booster, pull the vacuum line and check valve, plug the valve and recheck the vacuum. If it has gone up you''ve found your leak. BTW my 81 turbo doesn't get full vacuum until it is at operating temps. Are you sure your Tstat isn't stuck open? How old is your O2 sensor?

Dennis
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81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

[augidog]

I'm not sure if this applies to your 81.
I always,-always, always ran no more than at 14hg on my 78.
16hg only obtainable if I would advance my timing.
Then I disconnected all my emmision systems and now run about 17 to 22 depending on my timing and if I have my decel valve on or off.
The decel valve gave me more vacuum disconnected but the back fire was annoying and made the car sound like junk.
The vacuum retard on the distrib was a huge vacuum reducer at idle because of the atdc timing .
The systems were not functioning properly and I passed inspection with them all not working.
Btw I would have left them on if they worked. I'm all for clean air.
I planned to upgrade them all if I did not pass.
IMO I don't think 14 is low with all your emmision systems on.
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1978 924 95 mile daily driver.
Audi TB/POR174M/High Flow Cat/2.25" exhaust
I knew that positive thinking thing wouldn't work.

[ideola]

Thanks for the tips and ideas guys...

...I was getting 17-18 HG at idle just after the rebuild, but it has slowly degraded to about 14 over the past 2500 miles. The manifold gasket is new, as is the t-body gasket and the thermostat. I've already pulled the line from the brake booster and plugged it, no difference. I also removed the vac lines from the distributor and plugged them, no difference. I have not replaced the O2 sensor, but I don't see how that would impact vacuum?
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erstwhile owner of just about every 924 variant ever made

[dpw928]

Agree that the O2 sensor probably wouldn't affect your vacuum but it could be affecting your air/fuel misture. Have you ran a compression test or leak down? BTW my intake gaskets had less than 5K miles on them when it started leaking. I wrote it off to a bad cam (rotten machine shop) so I didn't discover the real problem until I replaced the head.

Other areas you might check are the intake boots, EEC valve and charcoal cannister.

Dennis
_________________
81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

[ideola]

I'm going to go with the theory that the O2 sensor is OK, and that there's a vacuum leak somewhere. I have not run a compression or leak down test (don't have the equipment), but I don't see that being the problem given the recent topend rebuild and the otherwise nice running engine.

I'm going to try the carb cleaner technique around the injectors and intake manifold just to verify there's nothing going on there. I replaced the injectors and seals, but didn't replace the inserts...I suppose one or more of them could be shot. Also, I didn't put clamps on the charcoal canister when I replaced the vac lines because they fit so tight, I figured they'd seal on their own...but to eliminate everything, I'll chase those down as well.

What is EEC valve???
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erstwhile owner of just about every 924 variant ever made

[dpw928]

The EEC valve is the vacuum control valve for the charcoal cannister. It's up by the brake booster, discus shaped and has about 3 vacuum lines running to it. The O2 sensor should be compensating for your lean running. Odds are it is coked and not working.

One other area to check is your valves. If one or more intake valves are too tight it would cause a loss of vacuum.

Dennis
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81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

[Mike924]

Sorry to butt in, Guys, but I'd like to do a vacuum test on my car.

I've bought a gauge which is marked 0 to -1 bar. I think it's also marked in mmHg. So when you guys say 17-18 HG is that inches of mercury? So to get the reading on my gauge I just need to convert to mm, is that right? Taking the mid-point (17.5 HG), I make this about 450 mmHg, or around -0.6 bar. Am I about there?

Also, what reading should I expect to get at the manifold (I was thinking of tapping in at the brake servo hose connector)?
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1985 Porsche 924 'Lux', Kalahari Beige (my ex)
1993 Porsche 968 Coupe, Midnight Blue, 6 spd

'There is no substitute for a little grease under your fingernails.' - Chrenan, 924board.org

[dpw928]

Since you are at or near sea level your reading should be 17-18 inches of mercury. I've never seen a conversion but since it is only a measurement of distance, your conversion should be right. You should hook it as near as possible to the intake on the head side of the throttle body. Think I would stay away from the brake booster line as any leaks could affect your braking.

Dennis
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81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

[Mike924]

Great, thanks for that, Dennis.

I should have explained, this is a test gauge, not one that 's going to be fitted all the time, but thanks for the advise.
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1985 Porsche 924 'Lux', Kalahari Beige (my ex)
1993 Porsche 968 Coupe, Midnight Blue, 6 spd

'There is no substitute for a little grease under your fingernails.' - Chrenan, 924board.org