Now that I finally have some time on my hands(finished basement remodeling) I intend to begin " My " winter project of maintenance and debugging of the 1990 C4 I bought in August. Before I spend any money " foolishly " on cosmetic goodies (new wheels,etc.) I want to perform a compression test to determine where best to invest. Can anyone recommend a sequence of which plugs to use that are more accessible than others that limit the need to remove unecessary items to gain the access. The engine compartment is much more cluttered than my old 911sc, and not to mention the additional plumbing and shielding of the exhaust system. So any helpful hints that can be given before I get started would be appreciated. Also what would be some acceptable test numbers for an engine with 80k on the clock.

 

Been thinking of this myself. What's the best way to stop unnecessary fuel being squirted into the cylinders?? Disconnect injectors, or remove main DME connector? Obviously don't want the coils charging up either?

Hints?

 

My guess is that if you leave the ignition off (no key in it) and jump the starter motor with a remote switch the injectors nor the coils will be energized. At least that is how I plan on approaching it. Can anyone confirm this.

 

What about removing the DME Relay? This would stop the fuel, perhaps removing the 2 main leads from both the distributer to stop the spark?

 

You only need to remove the DME relay and this will disable the ignition and injection. When you pull the relay out, check the date and if over 5 years old replace it with a new one.
Part number 993 615 227 00.

None of the plugs on a 964 are particularly easy, I would remove the upper ones on 1-3 and the lowers on 4-6. You have to remove the rh muffler for this so check that the bracket bolted to the cylinder head is tight as the bolts break.

 

GMS is correct, you can also remove the fuse for the fuel pump, but the DME relay should cover it.

As far as compression testing goes, use the lower spark plug holes on 4-6 like GMS says...easier access.

ALSO, consider performing a "leak-down" test, run your air supply at 90psi and take your readings, listen for air escaping through the intake, exhaust and crankcase to determine where you're losing power in that particular cylinder.

If you hear air through the intake or exhaust, you may have valve issues, but first, rotate the engine a bit clockwise and counter-clockwise to see if you can eliminate the "leak" as cam advance may be a little "askew".

I think a leak-down test is a better indication of engine "health"...but your compression test is a good start.

 

Uh oh Jeff

I think you're about to start another Compression test VS leakdown test debate......

...get your nomex suit on & ready.....

My 0.02 is compression test first, if reading a little low, repeat with a little oil squirted into the cylinder (confirms whether the compression loss is blow by the rings).

Then if comprssion numbers are good, go have a beer!

 

Now that you have me thinking leak down test, I know the valves are to be closed but what position should the piston be in. Should it be at the top of the stroke valves closed or bottom of the stroke valves closed. Or do you allow the compressed air to push piston to the bottom of the stroke. Any help to clarify would be appreciated.

 

The air pressure will tend to push the piston to the bottom of it's stroke...SO LOOK OUT! <img border="0" title="" alt="[Eek!]" src="eek.gif" />

...this can be dangerous if your hands/fingers are anywhere near the belts/fan while you're doing your check.

If you hear air out of the intake or exhaust, carefully "bump" the engine via the fan pulley belt to see if you can eliminate the air leakage...again, sometimes a valve can be "cracked" open slightly if your mark is off.

As far as leakdown -VS- compression test, to each his own, I don't think there's a real argument or debate to be had here, I'm pretty much convinced it's a matter of preference.

Personally - I do BOTH!! <img border="0" alt="[cheers]" title="" src="graemlins/beerchug.gif" />