Compression test results - PeachParts Mercedes-Benz Forum

uneasysunday · #1 06-04-2011, 12:37 AM

I wanted to do a compression test today because I was already going to take out my newly upgraded glowplugs to apply anti-seize. (Didn't have anti-sieze a couple weeks ago when glow plugs were upgraded).

So I drove to harbor freight, and picked up their diesel compression tester for 23.99 with a 20% off coupon.

My car is a 79 240D with about 40,000 miles on a rebuilt engine, I think...(I was told by the previous owner that the woman who owned the car about 8 years ago had the engine rebuilt by professionals). The car runs awesome, starts right up especially after pencil glowplug upgrade and valve adjustment.

My procedure for compression test was:
Edit: Test done on warm engine
1.Hook up battery charger
2.Remove all glow plugs
3.Fully Depress "stop" linkage with a piece of 2x4
4.Crank each cylinder about 8 times with tester hooked up.

My results were in this order (because I had to use the 90degree adapter for cyl.2 and cyl.3) and only had to use the straight adapter for cyl.1 and cyl.4

#1 380
#4 390
#2 395
#3 410

When I saw that the compression rise with each test I began to think I had goofed up cutting the fuel. It seemed as though the cylinders were still getting fuel creating a better seal (wet test like) and thus higher compression.

My question is, did I cut the fuel correctly?

I thought I read somewhere on this forum that was the correct way to cut fuel. I am sure I had the stop lever linkage fully depressed with my special cut piece of 2x4 and when I had a vaccume leak and the car did not stop with the key the stop lever worked perfectly.

I am pretty sure all my cylinders are about 380, not bad for a 900$ rust free car

Stretch · #2 06-04-2011, 02:42 AM

Quote:

Originally Posted by uneasysunday

...

My question is, did I cut the fuel correctly?

...

If you hadn't cut the fuel correctly as you put it then it would have started or coughed and spluttered like it was trying to start.

The effect you saw of the compression rising is how it should be - you were seeing the effect of lovely lubricating oil on the piston rings...

layback40 · #3 06-04-2011, 03:33 AM

Looks like you did it right !!!
If it was still fueling, it probably would have fired on the last couple of compressions & blown the gauge off scale.
You should be happy with those numbers.
Just drive the car & enjoy it for now !!!!!

Jditom · #4 06-04-2011, 10:07 AM

375-425 are considered excellent results.
And you have less than 10% DIFFERENCE from your lowest to your highest number.

Does not get any better than that

funola · #5 06-04-2011, 10:22 AM

There is no way in hell that engine will fire if there was fuel. With all glow plugs out, there is no compression except in the cylinder with the gauge.

barry123400 · #6 06-04-2011, 10:26 AM

Well if you had all the glow plugs out. The other cylinders would have been firing a mist out of the glow plug holes I imagine with the fuel on.

scottmcphee · #7 06-04-2011, 10:51 AM

Life lesson: I did a compression test on VW TDI w/o cutting fuel and taking all glowplugs out, just one cyl at a time. On the 2nd cyl I was doing - bang! the test gauge blows and the engine stumbles to start. Then... just pulses of pressure going back to zero readings on that cyl. It ignited and blew the schrader valve out of the tester.

Better to practice on a VW, and a cheap-o tester tool, than on my Benz.

Stretch · #8 06-04-2011, 11:26 AM

Good points barry123400 and funola - yet another wood and trees situation... but there was one cylinder that had a gauge attached to it...

leathermang · #9 06-04-2011, 12:27 PM

Quote:

Originally Posted by Army

Good points barry123400 and funola - yet another wood and trees situation... but there was one cylinder that had a gauge attached to it...

How does that compare to a ' trees and forest ' situation ? More or less dangerous ? Harder to recognize ?
One being the ' longer vision ' even though it is the example of too short vision ?

Stretch · #10 06-04-2011, 01:22 PM

Quote:

Originally Posted by leathermang

How does that compare to a ' trees and forest ' situation ? More or less dangerous ? Harder to recognize ?
One being the ' longer vision ' even though it is the example of too short vision ?

I believe the saying is "can't see the wood for the trees"

http://idioms.thefreedictionary.com/can%27t+see+the+wood+for+the+trees

It is something I find myself suffering from - sometimes I concentrate on the details and not the whole picture. This situation reminded me of one such situation - that's all.

leathermang · #11 06-04-2011, 01:44 PM

Quote:

Originally Posted by Army

I believe the saying is "can't see the wood for the trees"

http://idioms.thefreedictionary.com/can%27t+see+the+wood+for+the+trees

It is something I find myself suffering from - sometimes I concentrate on the details and not the whole picture. This situation reminded me of one such situation - that's all.

Once again you are using foreign language as your reference..note your phrase is specifically ' British '....

http://www.englishclub.com/ref/esl/Idioms/C/can_t_see_the_forest_for_the_trees_149.htm

When are you coming back to The States for remedial American studies ?

Stretch · #12 06-04-2011, 01:51 PM

Quote:

Originally Posted by leathermang

Once again you are using foreign language as your reference..note your phrase is specifically ' British '....

http://www.englishclub.com/ref/esl/Idioms/C/can_t_see_the_forest_for_the_trees_149.htm

When are you coming back to The States for remedial American studies ?

Never! I'm English - and proud of it!

Stretch · #13 06-04-2011, 01:59 PM

Quote:

Originally Posted by layback40

Looks like you did it right !!!
If it was still fueling, it probably would have fired on the last couple of compressions & blown the gauge off scale.
You should be happy with those numbers.
Just drive the car & enjoy it for now !!!!!

Hey layback40 this got me thinking...

...by how much would ignition raise the pressure?

Then I thought of that SAE paper

http://papers.sae.org/780633/

Page 5 in particular - and the graphs of cylinder pressure.

They are of terrible quality - the photocopying / scanning is really bad - but the scale for the idle example (700 rpm) shows a maximum value of 40 bar. I think the line concerned - the wavy one (not the normal distribution-like curve which I think is the injector needle motion) - seems to only reach 30 bar.

This graph suggests that ignition wouldn't make too much of a difference on a compression tester gauge. 30 bar is 435 PSI so about double the amount? Care to try it?

(I don't!)