Big service on the horizon? - PeachParts Mercedes-Benz Forum

clarknova · #1 02-20-2006, 07:02 PM

the great orange beast ('79 300D) has become increasingly more difficult to start and is burning more oil when she does decide to run...
when I got her in '03 the PO said the #3 had low compression but didn't seem to hamper her, until recently.
now she is a bear to start, sometimes I'll have to recharge the battery twice before she'll kick. so I have a sinking feeling that she needs the topend overhauled. some things trouble me, for instance, when adjusting the valves i've noticed that some of the rockers and/or cam lobes are worn funny. while passing a feeler between them it will be a little loose on one side and impassably tight on the other...
an unhealthy amount of blowby. I performed the oil cap test and it performed a smoky little dance that almost landed it on the ground. incidentally, the cap only danced intermittently, as if only one or two cylinders had significant blowby...
I am worried I need rings or a valve job and I am definitely not equipped to go yanking the head off if it comes to that. a friend is a local MB mechanic and I would certainly trust him to do what is proper and necessary but would like the intelligent feedback of this forum before throwing $$ around. thanx.

Hatterasguy · #2 02-20-2006, 09:25 PM

Have a compression and leak down test down to determine what is worn.

If the bottom end is shot it is much more simple and cheaper to find a good used engine. Figure ball park for a rebuild about $4k.

t walgamuth · #3 02-20-2006, 10:13 PM

is not good. you will prob need a cam and rocker arms too. uneven blowby will not be cause of valve problems. bottom end.

good luck.

tom w

clarknova · #4 02-21-2006, 12:25 AM

what should I be braced for?
the engine only has 150k original on it. I am hoping this isn't going to necessitate a new engine or a full rebuild. what can I expect to determine from compression and leak down? never had to get into big service before.

Al Magaloff · #5 02-21-2006, 05:13 AM

The number of miles on the clock really have no bearing on the condition of the motor. How it was taken care of, is the main factor. If the cam and rockers are worn as you say, that motor will "need" some major work. Re-do your valve adjustment, finding the tightest spot on the cam lobe, to see if that effects a positive change.

Pete Burton · #6 02-21-2006, 06:47 AM

Somewhere around 80, MB started to harden their cam lobes a different way, due to unacceptable wear. A better glow plug system was also introduced. I think a used engine from a slightly newer donor would be your best bet. Paying someone to rebuild that engine properly will almost certainly cost more than it's worth. I rebuilt mine a couple years back, cost me 2K, did over 95% of the work myself. Oh, and I'm nuts.

Hatterasguy · #7 02-21-2006, 10:57 AM

Its a 28 year old engine you have time working against you. All the seals are shot, thats a given.

A leak down test will tell you if compression is leaking past the pistons or valves. Also it will tell you if just one cylinder is low or all of them are low, ect.

If its the valves you just need a valve job. I don't know what one of those will cost on a 617, probably $1k+/-.

If the rings are shot you are looking at a complete rebuild or replacement.

clarknova · #8 02-21-2006, 02:41 PM

is it possible that the uneven wear on the cams and rockers has made for erroneous valve adjustments and loss of compression? would a maladjusted valve cause the smoky dance of the oil cap or is that strictly blowby from the rings? can I swapp out the cam and rockers for a later model with the different hardening?
ideally I would like to do as much as possible myself once I have ascertained precisely what needs to be done.
hopefully willl have leak down and comp data this weekend and figure out how to proceed.
incidentally, on the prospect that I may be in the market for an engine swap, what model years will match?

Al Magaloff · #9 02-21-2006, 06:30 PM

Yes, the uneven wear has made for erroneous adjustments, and possibly a loss of compression, if the poor adjustment is holding a valve or the valves open. That's why I would do as I said above. Find the tight spot, and adjust from there. A loose or tight valve adjustment won't have much effect on the dancing oil cap, in a healthy motor. If the valve guides were whipped out, then that may be the case. That being said, piston rings don't achieve their best seal, unless the motor is running well. So, for the moment, forget the cam wear, and do as best a valve adjustment as you can.

clarknova · #10 02-25-2006, 04:18 PM

what is involved in the leak down test? can it be done in the home garage like compression testing?

Diesel Giant · #11 02-25-2006, 04:37 PM

Check the rings and valves and go from there. The engine does not really have many miles, but may not have had a good service history.

clarknova · #12 03-01-2006, 10:06 PM

so tried to get in for a leak down and comp test, but can't get started.
she'll crank and crank and fire a little, but just when she's about to start up the starter disengages from the flywheel and just spins...
haven't had time to tinker but hopefully this weekend i'll adjust the valves and yank the starter to see what's going on with that.
had the #1 glo-plug burn out about a month ago (series type) and thinking that now would be a good time to swap them out for the pencil style. not sure if the relay is malfunctioning, might just bypass it with a manual switch for the new plugs

clarknova · #13 03-04-2006, 02:28 PM

can a leak down test be performed by the DIY mechanic?
what exactly is involved?

any ideas why my 300D just won't run? (please refer to previous post)

Al Magaloff · #14 03-04-2006, 06:11 PM

Yes. A tool know as a leak down tester/gauge set and an air compressor, and the proper fitting to mate the leakl down tester to the glow plug threads.

clarknova · #15 03-10-2006, 04:31 PM

and still can't get her to run...
the battery has just about had it (three year old Diehard), so I'll be getting a new one. probably get an Interstate, I've had problems with warranty service at Sears for this Diehard.
I don't think the gloplugs are up to snuff either, so I'll be swapping out for the new style.