Please read, comments are welcome.

Drove engine for 15 miles (operating temperature 87C). I used a compressor tester prurchased in Sears for US 25.00. The measured pressure is in PSI.
Cylinder one is the closest to the front of the car, count increases going back. All steps from the manual were followed; removed fuel pump relay, removed the plug (green cable) from the control module of the ignition system.

Cylinder Try 1 Try 2 Try 3 Try 4

1 200 200 200 200
2 197 200 200 200
3 205 205 205 205
4 205 205 205 205
5 215 215 215 215
6 200 205 200 197

My concerns:
The book states "The compression pressure should be between 10 - 12 bar. The wear limit is reached at 8.5 bar".
"The pressure difference between the various cylinders may not exceed 1.5 bars"

1 Bar is approximately 14.5 PSI, so the values should be between
145 PSI - 174 PSI. And the maximum allowable difference among cylinders is 22 PSI.

Obviously I am in very good shape when it comes to pressure difference among cylinders .

BUT 200 to 215 PSI?

Isn't that TOO high or is the tester BAD?

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1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car
609 Certified

A compresion test is not done with a compressor. Your test is worthless.
A proper compression test is done only with a compression gauge, pressure is built-up by the engine. What you need to measure is the engine compression, not a compressor pressure.
JackD

Ok, I mis-used the words.

It is an automotive-car-engine compression tester made by actron model# CP7827 (see it at www.actron.com)

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1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car
609 Certified

Every good 103 I've ever tested measured around 160-180 PSI. I always allow six compression strokes per cylinder. Did you crank the engine until the guage stopped moving? I'm also a bit leery of cheap tools, although yours may be just fine. BTW, is there something wrong with your car?

Hey
I am no expert on these engines but resorting to basic principles.

1. Your gauge is inaccurate and needs calibrating (although the tests look repeatable there may be an index error).

2. Your 'book' may be incorrect.

3. Your head has been skimmed and an incorrect head gasket fitted raising the compression ratio (unlikely?!).

a. If we take 205psi as a representative test result then you are squeezing at 14:1 whereas your book is indicating you should be at a maximum compression ratio of 12:1.

4. I do not know if you want to continue this out of academic interest or if you really have a performance issue. Normally absolute values are less important than the spread of achieved pressures across the engine. If you do address paras 1 & 2.

Hope this helps - Good luck!

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NormanB
230 TE (W124) 1989 with 153,000 miles on the clock - hoping for at least another 100K

Thanks for your replies.

The car has an intermittent rough idle (when warm), nothing really bad just annoying. It uses a quart of oil every 500-750 miles.

During the test the car was cranked until the needle of the tester did not move any higher (it holds the highest reading).

Before I decided to change the valve stem seals I wanted to check the condition of the engine with the compression test (to see whether a complete head job was required).

I do notice a small amount of black-caked deposits in sparkplugs 5 and 6 (mostly on 5 where the compression is highest). And the threads are usually a little oily on all sparkplugs.

I bought this car over 3 years ago, good condition in and out. I have put in over 25000 miles the last year and half (NY to FL and back), no problems except for the rough idle.

It overheated once (changed thermostat - problem solved).

NormanB, besides the fact that I enjoy working on the car, I also do this things out of academic interest.
I appreciate everyone's responses and inputs.
(specially when it is free )

__________________
1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car
609 Certified

You can run diesel at those numbers.

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Jim

Carbon build up and/or oil in the cylinders can contribute to the high numbers. Compression numbers are really only good for comparison. Since the readings are consistent I wouldn't be concerned.

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Scott Diener
89 300E
93 300E
92 Volvo 740 Wagon aka "Mutt mover"

I agree with chowpit-

"Carbon build up and/or oil in the cylinders can contribute to the high numbers. Compression numbers are really only good for comparison. Since the readings are consistent I wouldn't be concerned."

Keep us posted,

Haasman

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'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)

You're over analyzing and worrying too much. So what if the gauge reads a little high. You have good compression, all holes well within 10% of each other.

Screw the plugs back in and consider the compression good. A rough idle unless it is a dead cylinder type rough idle is most always caused by something besides low compression. As a matter of fact if the compression were low on all cylinders equally, you would probably have a smoother idle.

So, consider the compression good, eliminate that from your list and move on to the rest of your troubleshooting. What plugs and gap are you using? Platinum plugs have a very thin center electrode that will sometimes offer a slightly rough idle on these engines. Also you can't gap them too wide, or you will get a slightly rough idle.

Good luck,

Thanks for the input LarryBible.
I use the Bosch copper plugs recommended for the car, they come already gapped.
I cleaned the sparkplugs and the car feels like it gained an extra 25 HP, and idles much better. Still my concern is the plug at #5, it had plenty of caked-black deposit on it. The others were "clean" compared to that one.
I am starting to think that injector is "dripping" rather than "spraying".
I will be driving to NYC next weekend, and will try the "2 bottles of injector cleaner on one full tank - drive at 85 MPH for 7 hours treatment". Followed by an oil change.

__________________
1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car
609 Certified

Hi pesuazo, from your description of the #5 spark plug it may be time for new valve stem seals. Depending on the amount of oil you're using, the catalytic converter(s) may wind up clogged if this condition is not rectified. Good luck