Compression Test Results - PeachParts Mercedes-Benz Forum

biostudent · #1 07-15-2010, 01:14 AM

My compression test came out a bit strange and was wondering if someone might be able to offer their advice. I haven't done a valve adjustment and not sure when it was last done. Any tips on why the compression is so low? I did the test using a compressor I bought off ebay on my 300D. The injectors(All 5) were removed and each cylinder one checked in order. I did 3 runs so I could try to have an average.

Run1
190
210
260
225
225

Run2
250
225
265
210
210

Run3
250
210
265
200
200

turbobenz · #2 07-15-2010, 01:25 AM

PSI? Does the motor even run? This is almost like gasoline engine compression. How fast is your motor cranking? Btw, this isnt because your valves are off, that will barely make a difference in your numbers. If your able to start the car then there must be something wrong with your compression tester.

graydonblair · #3 07-15-2010, 01:41 AM

How many times did you crank the engine over on each cylinder?
I crank the engine at least 8 times and quit cranking when the PSI doesn't go up any more.

That looks REALLY low though. If it's starting and running and idling well, I agree with turbobenz, somethings up with your compression tester.

-Graydon

biostudent · #4 07-15-2010, 01:42 AM

The cars been running fine. The engine makes some noise but it may just be my ears not being used to these engines. I am use to gas. I checked the blow by and it was fairly strong which is why I bought the compression tester.

This is a similar one I bought

I hope I am doing something wrong. Since I don't want to replace the engine.
Normal is in the 400 range?

I screwed the fitting into the injector hole and it fit snug.
I turned the engine over the way the directions say to. I put my foot all the way down on the gas and cranked the engine over 8 times.

Any idea on what I did wrong? Or can this be my correct reading?

jkubica · #5 07-15-2010, 01:17 PM

Quote:

Originally Posted by biostudent

The cars been running fine. The engine makes some noise but it may just be my ears not being used to these engines. I am use to gas. I checked the blow by and it was fairly strong which is why I bought the compression tester.

This is a similar one I bought

I hope I am doing something wrong. Since I don't want to replace the engine.
Normal is in the 400 range?

I screwed the fitting into the injector hole and it fit snug.
I turned the engine over the way the directions say to. I put my foot all the way down on the gas and cranked the engine over 8 times.

Any idea on what I did wrong? Or can this be my correct reading?

Hi
Are those readings from a hot engine? Engine temp should be at 85C. before pulling all the injectors & testing. On my 1985 300td, hot compression is about 100psi higher than cold compression.

No point in putting you foot all the way down on the gas - that doesn't do anything. You can press the stop lever under the hood while testing to cut off the fuel supply so fuel doesn't spray out from the IP.

Use a remote starter switch ($8) attached to the wiring block on the passenger side to spin the engine while testing compression.

While I adjust valves, I rotate the engine with a 22MM wrench on the power steeling pump. With the injectors out, it turns easy. This procedure is recommended in the Valve Adjustment DIY pictorial on Dieselgiant.com. But others warn against doing this. You can make your own bent and thinned valve adjustment wrench from a cheap 14MM wrench from HF or other.

My Harbor Freight diesel compression tester works fine at the injector holes - there is no pressure difference between the injector and glow plug holes. But I do have a problem of the pressure release button sometimes sticking and have to tap it with something for it to release. But for $26 I'm happy with it.

The noise could be nailing from a bad injector. You can locate the offending cylinder by cracking the injector lines one by one until you hear the noise change.

Have you done the "tea kettle" test? Let the oil fill cap rest loose in the hole with the engine at idle. Its OK for it to dance around - but it shouldn't fly off.

PS: I just remembered that you said you bought your compression tester on ebay. Please be advised that there is a liquidator named TADD on ebay that sells Harbor Freight stuff that has been returned to the store as not working or broken. If you buy from them, you stand a high chance of ending up with something that doesn't work right. Just a Heads Up!
Good Luck,
Joseph

graydonblair · #6 07-15-2010, 02:01 AM

Normal is usually 375 to 450; depending on the engine.
What year is your car?

-Graydon

biostudent · #7 07-15-2010, 02:02 AM

1983 300D

biostudent · #8 07-15-2010, 03:20 AM

I have a leak in my valve cover gasket. Could that be messing with my compression results by that much?

turbobenz · #9 07-15-2010, 03:42 AM

no a valve gasket leak wouldn't effect anything as long as you keep putting oil in. Are you using a diesel compression tester or one made for a gasoline engine, because im not sure how high a gas engine compression tester reads but its probably somewhere around 280psi mac. Maybe hes pegging the gauge

spamman450 · #10 07-15-2010, 04:29 AM

When you are checking the compression, did you take out all of the injectors first? The engine will crank over much easier without them.

I just looked at the pic of your compression tester. I have the same one and sometimes it will bleed off pressure when you stop cranking the car. I don't completely trust it anymore.

#11 07-15-2010, 07:58 AM

These numbers just indicate a high mileage engine. The good news about driving a diesel, when it gets tired, as long as it has enough compression to start and run, and you can carry enough oil with you to get where you're going, you can still drive it.

With a gas engine OTOH, when they get tired to a point of sucking oil, then they foul spark plugs such that you can no longer drive them.

If these were the compression numbers on a medium mileage or recently rebuilt engine, they would be alarming. In the case of an old diesel, they just come with the territory.

My $0.02,

biostudent · #12 07-15-2010, 08:06 AM

Hmm, would running biodiesel be ok? I was going to use vegetable oil but that would surely kill this engine from leakage.

It hasn't been using up oil. Last I checked it was fine. Ignoring the leak I have from my valve gasket.

I tried it again with my girlfriend starting it this time so I could watch the meter while the engine cranked. There was no loss of pressure. The meter is reading it correctly. I suppose its possible the meter is defective, however, the blow-by I was getting was pretty severe. It was popping the oil cap all around when loosened.

Couldn't this also be symptoms of a leaky head gasket?

layback40 · #13 07-15-2010, 08:07 AM

Do a valve adjustment & try again. It is possible that every cylinder has a tight valve & so its leaking a bit. If the motor breaths (blow by) like a steam train when the oil cap is off, you have a motor that may need some attention. When they are this bad, WVO will give you plenty of grief. Has it been running previously on WVO. If it has, you may be able to revive it a little by unsticking the rings.

biostudent · #14 07-15-2010, 08:17 AM

Quote:

Originally Posted by layback40

Do a valve adjustment & try again. It is possible that every cylinder has a tight valve & so its leaking a bit. If the motor breaths (blow by) like a steam train when the oil cap is off, you have a motor that may need some attention. When they are this bad, WVO will give you plenty of grief. Has it been running previously on WVO. If it has, you may be able to revive it a little by unsticking the rings.

I'll do the valves. Not sure about running on WVO previously. As far as I can tell it hasn't but who knows if someone was just pouring it in the gas tank.

This is unrelated but related to doing the valves. I was going to do the valves but then I couldn't get my wrench position figured out. How can I turn the engine with a wrench? Its the bottom pulley below the fan(not the fan pulley) where you need a long socket and extension to get in there? If so, how do you manage to turn it? Do you do it under the car or while above?

funola · #15 07-15-2010, 11:05 AM

Quote:

Originally Posted by biostudent

I'll do the valves. Not sure about running on WVO previously. As far as I can tell it hasn't but who knows if someone was just pouring it in the gas tank.

This is unrelated but related to doing the valves. I was going to do the valves but then I couldn't get my wrench position figured out. How can I turn the engine with a wrench? Its the bottom pulley below the fan(not the fan pulley) where you need a long socket and extension to get in there? If so, how do you manage to turn it? Do you do it under the car or while above?

The easiest way I have found to turn the engine to adj the valves is take 2 lengths of 10 gauge wire 3 ft long and connect them to the terminal block on the passenger side wheel well. The trick is do NOT wire up a push button switch because you will not get the same fine control as just touching the bare ends of the wires together! Using this method, I never over shoot the cam lobes.