Compression Test Procedures?
 

I Plan to test the compression on my 86 300 SDL in a couple of weeks and I have a question.

Is a wet test valid for a diesel? I've done many gas engines and always made a point of doing dry, then wet as an indication of ring condition.

Whoops, I have another question. What is the procedure for the wet test? If its done , that is.

Thanks
Harry
86 300 SDL

What kind of problems are you having? How many miles on the old girl?

Harry,

Be VERY careful doing a wet test on a diesel. It CAN start and blow the compression guage in your face. Also, the combustion chambers are so small you can get too much oil in the cylinder leading to a bent rod or other damage.

If the initial compression check doesn't come out good, use a leakdown test to determine the origin of the compression loss. With air pressure in cylinder, you will here the leak through the oil filler hole in the valve cover if it is rings.

I also await the answer to turbodiesels questions.

Good luck,

Hi Turbodiesel

Rough idle is my only known problem and I will be pulling the injectors for a bench test. While there, I would like to have a record of compression.

107 K miles but I have some doubts because every since I got the car in September, a fuse periodically blows. That fuse controls the speedometer, tach and drivers seat at least. That's another sad story, (sigh).

Harry
86 300 SDL

Rough Idle
 

You know, I think rough idle is the one universal bane of pre-1986 MBZ diesel owners. That is the quest that we all strive for...the holy grail or golden fleece....to fix the rough idle. I have spent three years on this quest to no avail...new injectors, check injector spray pattern and break pressure, measure fuel delivery at each injector, compression tests, valve adjustments, clean boost hose, vacuum elements, new coolant temp sensor, idle rack adjuster, check EGR, new valve stem seals and springs, cetane boosters, diesel purge, motor mounts, engine shocks...etc. etc. But no matter what, when I stop at a red light, the car does the Texas two-step!!!

Oddly enough, the engine is smoothest when I first start it!?!?

I think the ONLY thing that would solve the shake problem on most of these cars is to have the injection pump rebuilt...but, in most cases, the injection pump is not that bad so as to warrant rebuilding (tough job and expensive)....and these cars will still go forever, shaky idle or not!! And at road speeds, they smooth out very nicely!!

So i think it is something we just have to live with!!

How about another poll.....how many pre-86 diesels shake at idle..I'd be willing to bet over 95% of them do.

85' 300D; Count me in for YES for a rough idle; though it was 90% improved by adjusting the idle rack bolt and replacing the motor mounts. Runs much better, though after a long drive you can still see a shake if you eye the hood ornament to a fixed object.

Oh well.

Good luck with your test.
Adam (ATLD)

P.S.; Harry; I don't know if this is what you meant, but when my car was comperssion tested at the dealership, they tested it through the glow-plug holes, not the injector holes.

The rough idle seems to be much more common with the turbo engines than normally aspirated.

Have a great day,

Wow MarkM, your remarks are not very encouraging for a new owner of an old MB, Ha, however, I take your comments seriously, thanks. I could learn to live with it only after I have gone through your exaustive list of things to try. I am not sure my wife could follow suit. Hmmm, I wonder if a divorce would be cheaper than....

If you do the poll, please include through 1987.

ATLD, thanks for your response. When I ordered a compression tester there was a choice of injector or glow plug adaptors and I selected the injector. I assume and hope the test would produce similar results but perhaps I am wrong, Anybody?

Harry
86 300 SDL

nevertheless
 

Harry, I didnt mean to sound so negative....these are GREAT cars even in light of the bad idle...kind of like a great wife who has a bad habit that bugs the hell out of you!! Either the injector or glow plug adapter will produce the same result, but you were wise to get the injector adapter because it is much easier to access injectors than glow plugs....just look where the glow plugs are!!..good choice.

Larry, your comment about rough idle most common on turbo models.....I agree absolutely....I retract my statement about the injection pump as the likely culprit. Not too long ago I was at a local used part yard/repair guy...and in his yard there was an older 123 diesel idling with the hood open, and I made the comment about the "240 over there"...and the mechanic said no, no...thats a 300....I said I had never seen a 300 idle that nicely, so assumed it was a 240....he replied that it is a non turbo, and that those idle much better than the 300TD's...this particular car had over 400,000 miles on it and was not pretty to look at, but you could have stood a glass of wine on the engine while it idled.

Nevetheless, my basic conclusion holds...we just have to live with the rough idle on these turbodiesels...maybe there should be a support group out there for us.

How do you make a MBZ turbodiesel owner slowly go bonkers??? Keep telling him its real easy to fix the rough idle!!

;)

Idle is a strange condition for diesels -- they are much happier at higher rpms!

So little fuel is injected that you can feel it -- a bump and then the thump from the next cylinder coming on a compression stroke. All mine (and my brothers) have an odd vibration at idle that isn't really a harmonic of the rotation, more a constant buzz. Goes away as soon as you touch the throttle, and I've come to think of this as a normal engine condition. Anything else makes me wonder what is wrong.

Raising the idle speed a bit will help -- most of mine seem to idle below 700 rpm.

Unfortunately, noisy idle is characteristic of most diesel engines, and the #1 reason people don't like them. A bit annoying for me, but more than made up for by the longevity and fuel economy!

I just fixed a severe idle roughness by replacing the pressure valve holder seals on the 300D -- I'd suggest this for anyone with an intractable idle problem with no other discernable source. I'ts not difficult or expensive (watch out for dirt getting into the pump, though!). Worth a try, anyway, as no one ever touches these except to replace leaking 0-rings on them. I think that was done on my car, and they weren't torqued correctly -- one screwed out by hand! The others weren't anywhere near 30 ft/lbs.

Peter

Sorry LarryBible, I missed your post the other day when you gave me my answer about the wet test. Thanks, I will not do a wet test, never even thought about combustion instead of compression.

For some reason I went back through the responses and discovered your first reply. I posted the same question on another list and Doktor Booth gave me the same answer.

psfred, thanks for your tip for helping idle. Are these the seals on the IP, under the injector line connections? Are they o-rings?

Thanks,

Harry
86 300 SDL

Harry:

The seals I replaced are the copper washer between the pressure valve holder and pressure valve seat in the IP. There is also an o-ring in there that seals the holder to the pump body.

There are no separate seals on the injection lines -- they are flare fittings.

If you ever remove the injection lines, when you put them back on, pull the nut back on the line a little and insert the line fully into the injector or pump and hold it there while you screw the nut down. This ensures that the line is properly seated. If you just screw the nut down and draw the line in with it, you can get it crocked and ruin both line and fitting!

Another note -- I read in a diesel manual that some engines are very sensitive to variations in opening pressure between injectors -- make sure that all the injectors are set to the same opening pressure, not just within specs. May make a big difference in idle. If the shop doing your work isn't privy to this info, you can get a wide variation in opening pressure and have them tell you they are "fine" -- which they are, just not all the same!

Peter

That's good advice on reinstalling the injector lines, makes a lot of since. I'll do those things you listed when I pull the injectors. My list is getting longer. Motor mounts have been added also.

Thanks Peter.

Harry
86 300 SDL

What are we all defining as "bad idle" and "shake?" I always wonder about the idle of my car but I wouldn't go so far as to say that it shakes. It's more of a buzz, or vibration. Probably a little less violent than a pager. If anything, it's the clanking that's bothersome. Maybe a newer exhaust and better hoodpad will fix that. I don't know if this is what you guys are trying to eliminate. Is your ultimate goal to make these idle as well as gasoline cars?

Alex

Alex:

Considering that my 220D used to shake so hard at idle that people would point when they didn't thinkg I would see, the "normal" buzz isn't too bad!

Some vibration is inevitable on a diesel due to the short fuel burn at idle. I believe the "buzz" or normal relatively high frequency vibration isn't the complaint. A shake is different -- if you can see the hood ornament moving in relation to a fixed object, for instance, or can see waves in your coffee.

Really severe cases are like the 300 before fix or the 220 -- car visibly bounces up and down and engine thumps on the mounts. Very annoying, and the drum from the tranny mount grates on the nerves, too.

My 300 now has some low frequency "drumming" at idle, but this is getting smoother -- only about 40 miles since the fix -- and I anticipate that it will be much smaller after my trip to Tennessee next week. My brother's 300 SDL has just a little "buzz" at idle that vanishes before 5 mph (so does the 'diesel' noise!). The 220 is louder due to failed exhaust pipe, but now has the same "buzz", as does my brother's 75 300D.

Peter