bad compression - PeachParts Mercedes-Benz Forum

Sierra240d · #1 10-05-2006, 09:08 PM

74 240d w115
Ran a compression test via injectors today and it came out not so good.
#1- ~200psi
#2~300psi
#3~300psi
#4~300psi
I adusted the valves and it made no difference. glow plugs tight.
What are my options here? Is it hope less or is there a possibel not so expensive option?

All the other cylinders are in great shape... but I did find fine metal particles in the oil. I pulled the oil pan previously and couldn't find anyting a miss, no movable parts.

Thanks
Steve

Matt L · #2 10-05-2006, 09:11 PM

The next step is a leak-down test.

t walgamuth · #3 10-05-2006, 10:51 PM

i would suspect a bad valve. but did you find any tight when you adjusted them? if one was tight it might need to be driven a bit to re seat itself. (this is a guess).

tom w

Sierra240d · #4 10-06-2006, 02:17 AM

Nothing wierd re: the valves... they were actually a bit loose, not the nuts but the clearence. I adjusted and got the same compression.

I understand the concept of a leak down test but have never done one. Should just turn the engine till the valves on No-1 are closed and hook up the compressor and listen... is that it? How will I tell if air is escaping from the bottom end?

How likely is it that this could just be the head gasket? or am I just dreaming.

Thanks
Steve

t walgamuth · #5 10-06-2006, 08:40 AM

i believe you use air plus a guage to measure how fast air escapes. it seems unlikely to me to be the head gasket but that is a possibility i suppose.

tom w

Sierra240d · #6 10-06-2006, 01:31 PM

I don't understand the need for a guage... aren't I just seeing where the air comes out?

What is the best and worst case scenario here?

Thaks all
Steve

rrgrassi · #7 10-06-2006, 01:39 PM

If you have x-ray vision and can squirt a mild soapy water mixture on the rings, head gasket,valves, pre chamber seat, injector seat, glow plug seat, valves, and valve guides, then you wont need a guage.

You can time how long it takes the guage to measure 300 psi down to 200, down to 100, and so on.

Unless it is a really fast leak, you will not be able to hear air escaping.

Worst case, cracked head, block, or piston. Medium case, bad rings, burned valve, blown head gasket, bad valve guides. Better case, need new crush washers on prechambers, injectors or glow plugs. Best case, nothing wrong.

Sierra240d · #8 10-06-2006, 01:48 PM

mabye I'm missing something, and sorry for my ignorant questions but...

What would measuring the speed air escaping tell me?

Can I just use the built in guage on the compressor?

Should I go all the way to 300psi?

Judging from the compression guage going down, it goes pretty fast. Be honest guys is the a lost cause? Am I looking at a lot of money to fix this one? because I dont have it

rocketman93116 · #9 10-06-2006, 02:52 PM

try searching threads use - leakdown test

Leak down v. compression test

Sierra240d · #10 10-06-2006, 03:01 PM

Thanks I've s been still searching compressiopn test

Although after reading that thread, I think I'm jsut going to put 100psi in the the cylinder and see where the ait comes out. I really don;t care how much or what pecentage, because All I want to do is acertain where/what the problem is and if it is worth the time/money.

how easy is it to replace a ring on one of these>

thanks
Steve

rrgrassi · #11 10-06-2006, 03:17 PM

No, you can't use the gauge on the compressor. Separate guage is needed, the compressor gauge measures tank pressure. The tank has a much bigger volume capacity than the cylinder does, so your reading would not mean anything.

Timing the leakdown gives you an indication just how big the leak is. People well versed in the leakdown test (I am not one of those) can tell you what the likely culprit is by the timing and pressure values.

Changing piston rings is not that bad if you are mechanically inclined. You do need to remove the head and check for scoring on the cylinder sleeve, and if there is a ridge, you will have to ream it out, so you can getthe piston out. You will also need to remove the oil pan so you can access the rod cap bolts so that the piston and rod can be tapped out by using a 2x2 piece of lumber. Some engines you can remove the piston from the bottom, but you will have to reinstall through the top, because of the need for using a ring compressor.

Matt L · #12 10-06-2006, 08:35 PM

I have to disagree. Putting 100psi into the low cylinder without the proper tool may indeed be helpful. He knows that it's not developing proper compression, and maybe there will be a leak that he can hear.

The piston for that cylinder must be exactly at TDC. Also be sure that you won't get caught in any moving parts if it's not.

Sierra240d · #13 10-06-2006, 10:07 PM

Quote:

Originally Posted by Matt L

Also be sure that you won't get caught in any moving parts if it's not.

I don' t understand

thanks for the comment Matt, I agree... I think I will be able to hear it. I was going to try today but got rained out. Is tdc for number 1 the same position that you check the timing at on a w116... I have yet to get a manuall for this car.

Oh... I think I just got it... do you mean if it is not a TDC it will ast like the cylinder is running and force it self to TDC once air is induced?

Matt L · #14 10-06-2006, 10:33 PM

Quote:

Originally Posted by Sierra240d

I don' t understand

thanks for the comment Matt, I agree... I think I will be able to hear it. I was going to try today but got rained out. Is tdc for number 1 the same position that you check the timing at on a w116... I have yet to get a manuall for this car.

Oh... I think I just got it... do you mean if it is not a TDC it will ast like the cylinder is running and force it self to TDC once air is induced?

If it's not *exactly* at TDC when you introduce compressed air, the force will turn the crank, pushing the piston toward BDC.

If there's a TDC mark in your balancer, that should be for piston #1 (and presumably #4), assuming that the mark is accurate. You'll know pretty quickly if it's not, so stay out of the way.

t walgamuth · #15 10-06-2006, 11:15 PM

i would think the noise of the compressor will make it tough to hear where the leak is coming from.

maybe not. i havent tried it either way, just specualting.

tom w