Varying compression - PeachParts Mercedes-Benz Forum

gawlbpd · #1 04-21-2003, 06:44 PM

I just got some bad news from my mechanic. I took it to him after my 126 was running rough at idle after warming up.

After doing a compression test, he's telling me that I have as much as 100 psi variance between cylinders. Thus, he's saying $1500.00 for a head rebuild and so forth. He's claiming 11 hours labor at $70.00 an hour.

He said an injector rebuild or replacement would neither help nor hurt initially, but if the injectors are real good at washing the lube off the cylinder walls, I'll get scuffing.

I'm sure someone has redone the head before. How bad of a job is it? Will the Haynes manual be thorough enough?

I have read somewhere that varying pressures between cylinders is hard on the bearings. True? Or should I just run it 'til it dies? Anyone have an opinion?

jhal · #2 04-21-2003, 08:58 PM

Did they check the valve clearance, or do a leakdown test? How much blowby does this engine have? I would get a second opinion, personally.

leathermang · #3 04-21-2003, 09:04 PM

The reason Jhal asked about a leak down test is that this would be the normal way to distinguish between lack of compression due to valves or rings...
Unless your mechanic did this already and you simply did not mention it... I would certainly have another mechanic look at it...
Of course , if idle is the only thing it is affecting... you might consider driving it a while ( a long while probably ) before it actually gets bad enough to warrant fixing... who knows , you might run into a deal on another car... LOL...

For the record, my 240 was tested and had more difference than that on one cylinder.. that is why I got it for a song... but I get 28 miles per gallon out on the road... have a problem tearing into the engine with it doing that... LOL

300d4me · #4 04-21-2003, 10:23 PM

leathermang, can you elaborate on the leak down test. Will this give you an idication as to weather it is a valve problem or not.

-300d4me

leathermang · #5 04-21-2003, 11:40 PM

Well, you need to do a seach because there are some great posts about it... but in a nutshell...

On a gas engine one would often put some oil into the cylinder through the spark plug hole and do the compression test... if there were much more compression with oil in it as compared to doing it without it would point to the rings.. since the oil ( like an ounce ) would temporarliy seal the rings but would not affect the valve sealing...

You don't want to do a compression test on a diesel with oil in the chamber for fear of starting it up.. diesel fuel is just lightweight oil in the first place...

So you plumb up a way to inject air into the cylinder ( with the piston at the bottom of the stroke) and time how long it takes to leak down ... lets say from 100 psi to 40 psi...
Then compare that with how long it takes it to do the same leakdown with oil in the cylinder...
If oil in the cylinder really increases the time it takes to leak the same psi distance... then it points to the rings being the cause of your lose of compression... if not then it points to the valves....this is assuming no head gasket leak....

300d4me · #6 04-22-2003, 03:28 AM

Thnaks!! I'll run a search to get some additional information. I will be bringing in my 240 and want to make sure I get a mechanic that knows what he is doing. I have found in the past that if you start asking the right questions you tend to get much better service.

gawlbpd · #7 04-22-2003, 09:57 AM

Thanks guys.

That gives me a better idea of what to do. I believe I'm going to drive it until it dies. If I were to do a head rebuild myself, any idea of what I'm going to lay out in dollar figures?

83 300 SD 171000
96 Dodge 2500 5.9 Diesel 105000

leathermang · #8 04-22-2003, 12:39 PM

Is this a trick question ? ALOT ? I don't know ,,, it depends on how well you search, how lucky you are, all sorts of things...

" Drive it till it dies".... this IS our motto ....

JimSmith · #9 04-22-2003, 01:36 PM

gawlbpd,

As leathermang noted the cost depends on the damage and how much you do. It is reasonable for you to bracket the costs by assuming a condition with lesser damage and another with more damage, needing more parts. After the chore of head removal and partial dissassembly is complete, a shop will do the rest, and putting it back together with old or new parts is about the same cost. So, make some assumptions and do the math.

You can get prices from Fastlane (click on the Fastlane tab at the top of the screen, then click on the request for the on-line catalog and the rest is obvious) for these parts, then assume a machine shop will be around $250 to $300 for just disassembly, measurement/inspection/testing, minor machining tasks (removing/pressing new valve guides and possibly seats in, assembly, and putting up with your questions about condition of parts, etc).

The parts to buy as a minimum will be a head gasket package/kit, valve stem seals, valve guides and possibly a valve or two, and also possibly a valve seat or two. I did this a long time ago on a 220D and the cost was very low as most of the trouble was leaking valve stem seals, which left thick deposits of carbonized engine oil on the valve stems and upper poppet surfaces, which ended up interfering with valve operation. All the valves cleaned up, the springs tested ok, and I had one cracked valve seating ring in the head. This was machined out and a new one was pressed in, the head was skim cut, Magnetic Particle tested for cracks, pressure tested and then it was all put back together for a whopping $123. The parts I bought (gasket kit, seals, guides, chain tensioner rails, various bits and pieces like pins, nuts bolts and washers) were about another $100. I bought some sockets for injector removal and installation, and a few other nice to have tools for another $50 to $75. I did not buy tools like prechamber pullers/installers or any other very specialized MB tools. I let the shop handle those chores as they had done it before and already had the tools. This was a while ago, so the costs have probably escalated by a factor of two. And you have at least one more cylinder.

On the high side assume you will replace all the valves, and the prechambers. If it costs more than that the shop is gouging you or your head is not worth repairing. In this case check another shop and research costs of rebuilt, or rebuildable heads.

But, make sure the problem is in the head. If it is in the cylinder to piston interface, fixing problems in the head is not of any value, overall. Good luck, Jim