diesel knock

[ladster1]

77 300D
Can anyone explain to me why my diesel knocks when first started on a cold morning and continues to knock when it is really cold out, even after water temp is up? I have had the fuel pump timing set and that helped considerably.
In the hot summer in Michigan, there is almost no knocking and pinging.
As soon as weather starts to cool, I am hearing what seems to be preignition knocks and pings.
Is this harmful to the engine and can it be stopped?
What do you guys think of adding a quart of the old style ATF trans fluid to fuel to keep diesel healthy?

Thanks,
Lad Guest

[psfred]

Knock cold is a characteristic of these engines, made worse by wear and consequent low compression.

I use some RedLine diesel treatment in the winter to protect against fuel gelling and to give some relief from the knocking. MB diesels are sensitive to fuel quality too -- shop around for the next few tanks and try different sources of fuel if you can. Some work better than others -- I cannot use Phillips diesel in any of my cars -- poor milage, lousy acceleration, and excessive smoke. Amocco or Shell is fine, as is Busler, the local cheap stuff.

The knock is a result of late fuel ignition -- a compression ignition engine cannot pre-ignite since there is no fuel to ignite early. What happens is that the fuel ignites poorly until there is a bunch of vapor in the prechamber, then goes all at once.

At the age and milage, if it starts OK cold, you probably have tired injectors. Take them out and have them checked, they most likely don't have a good spray pattern and are opening at too low a pressure. Poor spray pattern is the classic -- MB injectors will spray a solid stream off to one side or the other rather than a cone of fine fog when worn. Makes them rattle much more when cold.

Knock won't hurt the engine as long as the injection timing isn't too fast.

Never, never add ATF to the fuel, or lubrication oil, either -- use a diesel additive. The additives in the oil are NOT good for the rings -- they make ash, which grinds them up.

Peter

[ladster1]

Peter,
thanks for the information
does "diesel purge" help the injectors, or do you recommend replacement?
I have also noticed a difference in a given tank of fuel.
thanks again.
Lad

[psfred]

Lad:

Diesel Purge will clean the injectors up, but if the problem comes back right away, they need to be checked. Ditto for low compression -- clean injectors will knock less, but will carbon up again right away from poor combustion.

My 220D rattles like crazy for the first quarter mile if I push it. Stops mostly after that, but will crack once in a while on cold days. I don't know what it would be like in really cold weather, as it won't start reliably below freezing without the block heater, and I don't want to be stranded.

Peter

[ladster1]

Thanks again,

MY car runs very well otherwise, seems like good power, compression,
starts in cold weather after a couple warmups on the glow plugs.
getting 20 to 23 mpg.
200,000miles on vehicle but maybe 40,000 on rebuilt engine.
(I bought it after rebuild, don't know why it was rebuilt)
some blue smoke after taking off from a couple minutes of idling.
otherwise, no blue smoke at driving speeds.

I will try the diesel purge.
Thanks for your advice,
Lad

[psfred]

Lad:

Blue smoke on takeoff after idling means oil is leaking past the valve guide seals. On a freshly rebuilt engine, I'd suspect a poor valve job, especially if the rebuilder wasn't familiar with MB diesels. Off idle, these engines can comsume vast quantites of oil with no smoke at all -- just fuel, after all, only a little thicker than diesel fuel!

If it takes a couple glow plug cycles to start in cold weather, your compression is low, most likely due to tight valve clearance. Get your valves adjusted, or do it yourself -- intakes are 0.005" and exhaust is 0.016". Instructions are available in several threads on this group. MB diesels are very sensitive to inadequate valve clearance, and it always goes down! They need to be checked and/or adjusted every 15,000 miles without fail.

20-23 mpg is rotten for this car -- should be getting upwards of 28, as high as 32. Cold weather is hell on the injector overflow lines -- black fabric covered hoses between the injectors. There is a plug in a short one on one end, the other goes to the fuel filter houseing overflow line and back to the tank. If these lines leak, you can spill a terrific amount of fuel -- you can usually smell it. Don't use anything but MB overflow line -- ordinary vacuum hose will swell, soften, and leak. It's kinda expensive, but I've not found anything else that works well. You won't see fuel on the ground -- it evaporates off the engine.

Check the valve stem clearance while replacing the stem seals (or having it done) -- try to rock the valve sidways while lowering it off the seat a bit -- perceptible motion means the valve guide is shot. A terrible problem with american machine shops -- they always want to replace the bronze exhaust valve guide with a steel one because it is "better" -- it's not, of course, because these are sodium filled valves and get MUCH hotter in operation that a simple steel valve -- the bronze guide expands fast enough that the valve won't get bigger than the hole. Most shops also want to ream the guides out to give 2-3 thousandths clearance, which will allow vast quantities of oil down into the cylinders. If the valve goes in, oiled, and can be foreced up and down, it has adequate clearance. It isn't usually necessary to ream MB guides at all 00 the fit from the factory. If the valve stem clearance is too high, you need a valve job -- guide replacement and new seals, maybe valves ground, possibly new exhaust valves -- people tend to "save" money by not replacing them and selling the car -- new exhaust valves are $45 each. They won't last, however, so it isn't saving money to have to do the job over again after 30,000 miles!

Peter

[ladster1]

Peter,

I did replace the overflow lines a year ago, and with the MB tubing.

A couple things I am not clear on from your last note.

If compression is low from improper valve adjustment, would this cause low MPG?

also, what only lasts 30,000 miles..... grinding the valves and that is why you recommend replacement? I wasn't sure.

Thanks for your time.
Lad

[psfred]

Lad:

If the exhaust valves are too thin at the edges when ground, they will overheat and wear even faster. I've seen them with a 1 mm deep groove in them. They will not seal properly (the sealing surface must be flat, hence grinding....) and will give low compression. They won't last long, maybe 30,000 miles or so. It will only be possible to assess condition by removing them -- i.e. a valve jog. I'd have the seals replaced by a reputable mechanic if you don't want to tackle it yourself, and have him check the valve guide clearnce at the same time.

Yes, low compression from over-tight valves will give low milage. So will dragging brakes and slipping transmission, though.

Rebuild quality is very highly variable -- completely dependant upon the person doing the work. If done to MB spec, usually as good as a new engine. If done by a backyard mechanic, probably not much better than beforehand! I've seen some pretty lousy work.

How much black smoke do you get at full throttle at speed?

Peter

[ladster1]

Peter,
at highway speeds, with the throttle open all the way.
I get just very faint black smoke you can see in daylight.
In other words, not too much. only happens when motor is laboring to try to gain more speed in high gear.

I do notice I can see it in the headlights behind me at night, in the rearview mirror.

I have had my brakes worked on by a good mechanic. rebuilt as necessary.
I am wondering now about my trans. doesn't seem to slip going thru the gears, but I would say the top speed of this car is about 80. And, it seems to turn a lot of rpms to get there (no tach). have never noticed any intermittent slipping and grabbing, always a steady pull through the trans.

any other info, I am interested
thanks,
Lad

[psfred]

Lad:

Smoke is correct, seems engine is probably running OK. Get the valves checked, though, or do it yourself, and check starter rpm (should be 100 rpm or better cold -- much faster than a gasoline engine).

Top speed on a 300D is about 95 -- there is a mark on the speedo. If you can't get there, you may have dirty fuel filters -- will restict fuel flow at high speed enough to prevent full power.

And watch your odometer -- you may be getting better milage than you think, since the VDO odometer usually starts to slip by now -- NEVER set the trip meter while moving -- it binds the gearset in the speedo head and will cause the drive gear to slip on the shaft, leaving you without an odometer. Easy fix, but you must be very careful to document the milage reading before and after to avoid getting accused of odometer fraud. Not a problem for me, as the 220D's didn't work for five years anyway, and I would never claim the odometer to be accurate, but I did "lose" 20000 miles when I fixed it -- forgot to line the numbers up properly and didn't want to take it apart again. I'm just happy to have an odometer now!

Peter

[ladster1]

thanks for continueing tips.

I will get the valves checked. and the injectors. and log the mileage on the odometer.
I will let you know the results.

Lad

[jcd]

Peter,

I echo your comments on Fuel Filters. I changed them this weekend as part of winter time "get ready" maintenance. I was amazed by the positive impact it had on starting as well as speed.

JCD