924Board.org

[924inMA]

I did the unforgivable and drove my 924 18 miles with little or no coolant, not realizing the water pump had started a leak and drained all fluid during a 2 week vacation. No red light came on and I only noticed the H2O temp needle buried in the red as I turned the car off.

I replaced the pump and the fluid but noticed a significant loss of power (maybe 30%) immediately at all RPMs with a flattening of the power curve near 4000 rpm lowering the top speed noticably ( now not much over 60mph ). As I acclerate up to around 4000 I can feel a tiny bit of periodic surging such the power trembles or oscillates a little bit. By that I mean that it feel a little more powerful for maybe 1/2 second, falls off for 1/2 second, then repeats. It may also be that the car is using a little more gas but I cannot be sure since the speedometer happened to just fail also.

As a rudimentary shade tree mechanic with limited knowledge I replaced the spark plugs & wires, checked the compression (excellent 160 in all cylinders), and have been watching for foam in the oil, and changed the fuel filter for good measure (although it seemed an unlikely source of the problem after overheating. I also had an informal look at the timing and advance with a strobe, and checked the vacuum. These seemed reasonable, although the vacuum was a few psi under what the guage indicated as normal (I don't know the 924 spec.) I also metered the fuel pump output per the Haynes manual and found it to be normal.

My next thought is to look at the fuel injectors since I imagine they could be sensitive to overheating, but it occured to me that I am not particularly knowledgable and it would be best to ask those with skill and experience what likely problems from overheating might cause the loss of power. I have tried to figure out which components sense engine temperature since I suppose a sensor could have failed, but do not have clear idea of this.

So does anyone have suggestions of which components to check out next. I hope ( but am not sure ) that I was lucky and did not damage the head gasket or head itself, but would like frank advice in case it is simply likely I did major damage.

Thanks in advance.

[emoore924]

Given the symptoms you're describing, I find it surprising that the compression is still good.

Can you describe the procedure you used to check the compression? Specifically, did you remove all the plugs then check, or did you check with adjacent plugs in place?

[augidog]

maybe you melted a vacuum hose.
_________________
1978 924 95 mile daily driver.
Audi TB/POR174M/High Flow Cat/2.25" exhaust
I knew that positive thinking thing wouldn't work.

[wdb]

just because the comp readings are good doesnt mean your heads not cracked . try running the car at idle for 15 minutes , if it overheats . the heads cracked .

[924inMA]

Thanks to responders for insights.

1) The compression test was done on cylinders from 1-4 removing each plug individually as I tested but not replacing any till after all 4 tests. I think I get the drift of the question... should I retest with all plugs removed first? But note the last paragraph below to see if it sheds any light.

2) I haven't see damaged vacuum hoses but want to check further. Can you advise where best to test vacuum, what the reading(s) should be, and whether I should worry about a vacuum boost mechanism or other issue beyond just a single vacuum reading at idle?

3) The engine may get a little hotter now (not sure, it always crept up a bit at idle) but doesn't overheat significantly even if idled a long time. But I appreciate the tip that compression readings could not be the full story.

IMPORTANT ADDITIONAL INFO:
4) In my previous post I neglected a very interesting symptom, which is that the power is absolutely fine for the first 60-90 seconds I drive it from a cold start (after the 30 seconds or so it takes to get seat belts on, etc.) This has made me a bit hopeful, but I do realize that a crack or other distortion could show itself only when heat causes expansion. Still it is interesting that the power diminishes VERY quickly, well before the engine heats up. It is like something heats very quickly and immediately causes malfunction before the engine heats up generally. This is why I have wondered if the fuel injectors are suspect, because they would heat quickly.

All this advice is very helpful and more would be appreciated. Thanks.

[Sleykin]

What year/model?
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Glenn Neff
Medford, OR
87' 924S

[924inMA]

It's a 1979 924.

[924inMA]

I just retested the compression with all the spark plugs out and all readings were excellent, within a pound of 177. So the compression is good with plugs in or plugs out.

So why would it have great power for 90 seconds then lose it? Any suggesions would be appreciated.

[Smoothie]

A couple things to try -

Put the vacuum gauge on it when cold, start it and note the reading then see if it drops at the same time you notice the dropoff in power. Normal vacuum for a 924 at idle when warm should be approx. the same as for any car, so the markings on your gauge should apply to the 924 - expect something like 18"hg (+/- 2-3").

Do the compression test again with the engine hot (if that's possible without burning yourself on the exhaust header).

Try a new coil. I had one go bad on a previous non-Porsche vehicle - it started fine, idled fine, and ran decent briefly when cold, then power dropped seriously. -But it also sputtered when the power was gone and you didn't mention anything like that..
_________________
"..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

[924inMA]

Thanks for the suggested tests.

The vacuum stone cold was 18 idle then 21 warm, and 28, 31 respectively at high rpm.

I tested the compression very hot and it was still very good.

Even though I don't have the full symptoms of bad coil I plan to order one.

But see the next post for a possible solution to the puzzle.

Thanks again.