Compression Results - 350;350;350;100;350 - PeachParts Mercedes-Benz Forum

dazsa · #1 06-17-2007, 04:41 PM

I just completed a compression test on the 79-300cd and the reuslts were:

#1 - 350-375
#2 - 350-375
#3 - 375-400
#4 - 100
#5 - 375-400

What are my options with these results?

Thanks, Don

Hatterasguy · #2 06-17-2007, 04:42 PM

Did you do a leak down?

Next I'd yank the head and see whats up with number 4, you may get lucky and just need a valve job.

kerry · #3 06-17-2007, 04:44 PM

Quote:

Originally Posted by Hatterasguy

you may get lucky and just need a valve job.

Or maybe even just a valve adjustment. Have the checked the valve clearances?

Hatterasguy · #4 06-17-2007, 04:46 PM

Quote:

Originally Posted by kerry edwards

Or maybe even just a valve adjustment. Have the checked the valve clearances?

One would assume that was done before he broke out the compression testor. I don't think one valve would be that far out of adjustment, to cause a dead cylinder.

Biodiesel300TD · #5 06-17-2007, 04:49 PM

Did you check the #4 compression a couple of times to make sure the reading was correct. It seems really odd that one cylinder is that far off. I would first suspect some misadjusted valve.

dazsa · #6 06-17-2007, 04:49 PM

It was NOT a leak down test; but I used a new Harbor Freight diesel tester. The valves were adjusted last weekend and I have not driven it since then.

I know this is a stupid question, but I have never done a valve job. Is it DIY or not. How much to have a mech do it?

Thanks, Don

kerry · #7 06-17-2007, 04:53 PM

There was at least one recent instance here where a reading of that low was caused by maladjusted valves. I'd double check the clearances on that cylinder before proceeding further. Were there any symptoms leading up to the adjustment and compression check? How long since the last valve adjustment. A too tight exhaust valve could have been burned. What was the initial clearance on #4 exhaust before you adjusted it?

It's a DIY as far as removing the head and reinstalling. Valve job itself will involve a machine shop. I don't think you should be jumping to the valve job conclusion just yet.

Hatterasguy · #8 06-17-2007, 04:55 PM

A valve job is not a DIY thing, well it depends on how experianced you are and if you have a machine shop. I'd yank the head, see if cylinder number 4 looked good, IE no scoring, and measured true, if so I'd ship the head out to Metric Motors. They seem to be one of the few competent places that can work on these motors. Ask Brian he just had the head on his 617 done, I'd guess about $1,500 maybe a bit more.

Cost depends on what you do, its and old car, and it depends if you are going to keep it. If it were mine and I was keeping it I'd have them replace the intake and exhuast valves, along with the cam and pre chambers. They would also check the head for cracks and make sure it's flat. Also while its off I'd do the timing chain and rails, along with having the injectors tested, etc, etc.

lietuviai · #9 06-17-2007, 05:06 PM

$1500 for just a valve job?

I'd just look for another engine at that cost or just keep driving as it is as long as you can.

Hatterasguy · #10 06-17-2007, 05:10 PM

Internal engine work on Mercedes is very expensive, our engines are actualy the least expensive in that regard. The V8's and V12's cost a lot more...

Personaly I'd never bother with a junk yard engine, just someone elses problems. Like I said it depends what you want out of the car. Are you looking to restore it, or just drive it for a few more years.

lietuviai · #11 06-17-2007, 05:20 PM

I guess with the expense involved with these cars is why they end up in junk yards. I'm trying to keep my 300D from becoming another carcass for the junk yard vultures. I haven't priced what it'll cost to fix either one of my engines but I hoping to stay under $500 to do it. If for some reason I don't, I know I can get my $200 back that I spent on it by parting it out.

dazsa · #12 06-17-2007, 05:54 PM

I will check the valve clearance again before doing anything else and retest the compression.

The car has been using 1 quart of oil per tank, idling rough, blowby, and some smoke. I adjusted the valves, changed the filters, and ran diesel purge through it.

I do have a possible replacement motor I will be looking at tomorrow, if it is needed.

I will report the finding soon.

Don

kerry · #13 06-17-2007, 06:04 PM

Quote:

Originally Posted by dazsa

I will check the valve clearance again before doing anything else and retest the compression.

The car has been using 1 quart of oil per tank, idling rough, blowby, and some smoke. I adjusted the valves, changed the filters, and ran diesel purge through it.

I do have a possible replacement motor I will be looking at tomorrow, if it is needed.

I will report the finding soon.

Don

Sounds more like a bad piston.

dazsa · #14 06-17-2007, 06:38 PM

I rechecked the valves and they are fine. I did the compression test again and only got 100.

So, would a replacement motor be the best choice?

I want to keep the car because it is a CD and the body is rust free and most everything else works. I only paid $400 for the car.

What say the experts?

Thanks as always, Don

barry123400 · #15 06-17-2007, 06:43 PM

When you adjusted your valves do you remember if that low cylinder had any extra tight valve clearances at the time? Thinking carbon buildup. If not with the excellent compression the other cylinders have I would talk to any good automotive machine shop and see what they think.
So much depends on What they think and how much you want to really spend. But first a leakdown test or air injection test to see where the leakge is. If you have any difficulty just compare with any other of your good cylinders.
If it is just a poor sealing valve in that cylinder you can spend from very little to as much as you want. Since the rest of the engine is so good I would think long and hard about changing it out unless your cylinder wall is torn up on that cylinder. Even then I might consider repairing what I have. You do not have to totally rebuild something if you do not wish and it does not to be seem indicated in your case as required anyways.
One way or another you cannot live with that cylinder as it's too far down. Not an expert by the way just from my past experience. Try to avoid the clip joints. Just asking around should find out a fair and skilled machine shop. This is not rocket science.
Some of todays charges for certain jobs border on the insane for what is required or they just operate on the premise of what the market will bear.