I’ve been having some rough idle on my 82 300SD. Over the last few months I’ve really been enjoying my 85 300D and I’ve noticed it doesn’t shake a bit at idle. It is so smooth, I noticed the engine shock on he driver’s side has broken out of its rubber isolator and it really doesn’t need the shock. It’s that smooth. Very different from my 300SD that shakes at idle and stammers to a start in the morning.

So that put a spotlight on the SD. The rough running is a rocking like rack damper pin rocking. I changed the bolt back in 2010 and I started by buying another this month thinking the old one was worn out. No luck. Still rocks at idle. I changed filters. Changed o-ring in filter. New cigar hose. Adjusted valves twice (actually once but checked it after a few hundred miles). I put in a new set of monark injectors that I pop tested at 1950 psi. This actually made the shake worse. My old injectors were squirting, leaking and popping at low pressures.

New engine mounts. New engine shocks. Checked for air leaks. Sprayed the goo out of turbo. Adjusted idle. Rebuilt the lift pump. Twiddled with the rack damper pin dozens of times. I got little improvements on the way but the engine still rocks. Especially when hot.

Oddly the car is smooth over 1000 rpm and after all my work this month it makes amazing power coming up the 10% grade to my home. I mean it has really improved and it makes good smooth power. No smoke. But it rocks at idle. Almost makes me seasick at red lights. Hit the pedal and at 1000rpm it mellows out.

I have not adjusted timing ever on this car. The reason is when I got it in 2010 I tried and rounded off the bolt on the back of the pump. Since it ran well I never touched it. At some point I’d planned to take off the oil filter and properly extract the bolt. I even have the filter and pump gaskets sitting on the shelf...for a few years. Just never felt I needed it. Till now.

So that is the background. I started getting a little flustered and realized I may have a tired pump. As I read more posts here I realized that maybe one of my cylinders may be weak too and that no tweaking would bring back compression. Before I attacked the pump I thought I’d check compression.

So I decided to bite the bullet and get out my compression tester. I bought the cheap one at Harbor Freight in 2010 and I never was too happy with the way it would leak off pressure if I wiggled the quick connect. I’m still on the fence whether I should trust this thing. I took multiple measurements to get to these numbers. Some of the low pistons started out at 150 psi and came up to 200s after cracking a lot. Not sure if this is the gauge or car but I think it’s the car.

Anyway here are the results. Warm engine.
#1 390 psi
#2 390 psi
#3 280 psi
#4 220 psi
#5 230 psi

I don’t think I’m ever going to get a glassy smooth idle like my 300d with these numbers. I’m really bummed out. I guess the next step is so a leakdown and try to see where I’m losing pressure. And tackle the injection timing.

My teenage son shook some sense into me and said “is it really broken? It drives great.” He has a point and maybe I’ll just let him drive it.

Opinions? Start looking for a new engine or car? Keep driving and forget about it? I have really put a lot of work into this car and it is really nice save the idle.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles

might be worth adding a can of "Engine Restore". might bring up the compression numbers a bit. Can't hurt.

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1977 240D auto w/OM616.916 engine
2005 Matrix 5spd
2006 Prius

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I'm on the fence about doing a compression test because of results like this.

My car start fine, runs okay but I have no history or experience to compare it to.

It's slow but I went from 285hp pickup to a 300d, it's gonna seem slow.

I even bought the test kit.

If it starts misbehaving of course I will test it but until then no news is good news.

On your results, the discrepancy between the cylinders would bother me
and most likely have me starting to search for an engine or donor car.

IMO...do the leakdown test. If it is leaky valves do a valve job. If it is in the rings/pistons do a chemical soak/cleaning of the rings, run it hard then retest if you want or just drive it.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Ok guys. Thanks for the tips.

I went out to the garage and plumbed up my injector adapters for a quick and dirty leakdown. I just put an air quick connect on my injector adapter from the compression checker and ran it to my air source. Put it on #4.

I almost forgot how to find TDC on any cylinder other than #1 when the valve cover is on. Can’t stick a screwdriver down the injector port.

Cranked the engine around with a balloon (finger of a glove) on my adapter to find TDC compression. Locked a wrench on my power steering pump to the firewall (yep I said quick and dirty).

Slowly cranked up the air and listened intently for the hiss. The crank didn’t even turn with 60psi on the piston. (Drumroll).

Oil filler cap and crank case breather.

At least I know. #4 is gone. No sense in checking #3 and #5.

I kind of knew over the years because I saw the blowby steadily increase. Starting got a little more stubborn. It never would compression start and wouldn’t start if the glow plugs weren’t in perfect order. Also if I pulled the glowplug harness and cranked it, the cranking would sound uneven. I have to keep telling myself that this didn’t just happen yesterday...it has been happening for a long time.

The funny thing is she made and still makes crazy good power. When the turbo spins up she can climb hills like crazy. But the leakdown is like a screen door. Good sized leak at the cylinder wall. I guess it is a static vs. dynamic issue. This is probably why it idles like crap and purrs at 1000 rpm.

I tried re-ringing my 300TD when I was young and foolish. It didn’t work of course. I’m going to have to either replace the engine or sell the car off as junk. Perhaps store it at my vacant home in Pasadena and scavenge it for parts. I owe it to my neighbors up there.

Man what a bummer.

I did see that YouTube video where the guy used a can of engine restore on his diesel tractor and bumped up the compression. Unbelievable. I may buy a can tomorrow when I’m out at Walmart. Go CSL! I have nothing to lose at this point.

There’s also setting my idle at 1000rpm.

Man what a bummer. Oh I just said that.

Any tips on locating and testing a donor?

How do you verify it’s a good engine when you’re buying a pull? How do you find one other than Craigslist or eBay? (I am totally green on this).

I’m thinking it might be good to buy some beater 300D that runs okay, compression test it then pull the engine. My wife is going to kill me. She is jumping up and down right now that I may possibly buy a new car.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles

Wait I forgot the tip about the ring soak.

Anyone know a good brand of ring cleaner to put in overnight? I recall from my Saturn days that GM had a specific formula they used at the dealership.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles

Instead of using an iffy salvage motor, why not just have a machine shop deck the block, and fit up some new rings, seals, and bearings? Definitely work over the head too.

If it were me, i'd run it til she blows, then rebuild the whole thing, that way you know it's perfect before dropping it in. Even if it runs, that doesn't mean it's mechanically proper. My factory replacement motor with less than 150k mi was running OK, but still had a slightly warped head with leaking valves/seats. These things run decent when not mechanically proper

I need to find a shop who can do this. I don’t know where to start. I know a machine shop up in Anaheim that does heads. Not sure they’ll bore and hone diesel cylinders.

I started thinking about the high rpm and low rpm difference on this engine. When I’m running it slow there is time for the compression to leak away. When I run it fast there may not be enough time for the compression to leak down since the piston is moving faster.

This would be consistent with the ability for the car to register a non zero compression pressure while leaking down during a leakdown test.

I’m going to try some mechanic in a bottle as a last ditch effort. Soak the piston tops overnight then later a can of engine restore.

I’m on the fence on whether to fix or move on when it comes to rebuilding or swapping. The car really isn’t worth that much. It’s kind of a sentimental thing after all these years of wrenching.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles

My experience with rocky idle and smooth out when revving is stuck rings. That is the reason for low compression. It does not have complete burn at low rev but is better at high rev ( compression higher ). It happened to my OM603 engine. There is NO engine restore snake oil that can fix it. You need to take the pistons out and clean out the grooves. It costs nothing but a royal pain to remove the piston/engine. What you can try is to soak the cylinders with marvel mystery oil for a week or longer. It may loosen the ring or it may not. Unfortunately carbon does not dissolve easily. Good luck.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

Thanks for sharing that.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles

GM Top Engine Cleaner 1050002...sound familiar? Whatever solvent you choose let it sit for days, not just overnight. After soaking and warmed up push it very hard. Go full throttle uphill and let it coast down hill to work the rings.

What solvent??? I would consider a carb cleaner such as 2 + 2 or SeaFoam.

Please keep us posted and good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Yeah, I had a Saturn with oil consumption issues. The parts guy mentioned they had a specific solvent they used to treat the pistons overnight. That must be the stuff.

Interesting that stuff is for the oil control rings. Where the theory here is that the top compression ring is dirty. It may be worth a shot. I’ve never looked in my intake downstream of the turbo but given the amount of blowby I bet it is pretty gooey in there.

I guess if it’s wear I’m out of luck. The car has never been starved of oil during my ownership. I forgot to mention it doesn’t burn oil either. I may add a cup between oil changes. Not like my Saturn at 1 quart every 1000 miles!

Worth a try. It’s going to be interesting pouring that stuff past the prechambers. I guess if it doesn’t go in I’ll figure out how to pull them.

Guys thanks for all the tips.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles

For a ring soak check out Kreen by Kano Laboratories. I have read good reports on it, and did a treatment myself.
Pour it into the prechambers, soak at least overnight, turn engine over by hand to expel any remaining in combustion chambers, then use starter to finish blowing out.
Change oil/filter afterwards.

I had great luck with MMO, but not when poured in the cylinder. I topped up the engine oil with it and ran it for two oil change intervals. Really helped, especially with cold starting that used to be iffy below 40 unless it was plugged in overnight. Now I don't plug it in unless it's going to be under 20, and then really only to be kind to the engine it its old age.

Kano labs does make some great products. They make KROIL penetrating oil and it is amazing.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer