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  #1  
Old 04-16-2021, 04:48 PM
Broken Mercedes Enjoyer
 
Join Date: Apr 2021
Location: Kamloops BC Canada
Posts: 12
Questionable compression test results?

Hello fellow Mercedes enthusiasts! I am new to the forum and I don't really know what I am doing so call me out if I am doing something wrong, But basically I am hoping for some advice from some people with experience as that's what forums are for right?

So my W123 wagon shakes reeeealy bad at idle, like I get a real massage at red lights, but it only shakes below 1100RPM and some RPMs below 1100 it doesn't shake at all. The amount of shaking changes based on if I've just come off the highway, weather or not the engine is hot or cold, and the "frequency" of the shake changes also with these factors.

It has been sorta shaking ever since I bought the car, but I have barely any blow by and the engine starts up instantly as well as shuts off with authority, so I never thought to do a compression test (even though that's the first thing you should really be doing on a car you just bought with 550000k). I did a diesel purge (nothing happened) and even found some brand new injectors for it. The injectors actually made the shaking significantly worse which was a little annoying.

So anyways before I spent anymore money on the injection system I figured I should probably do a compression test to find out if I have a low cylinder, so I did a valve adjustment and got some "results"

Cylinder 1 was at 380 and then, well, I broke my compression tester. The rest of the cylinders all read exactly the same 320 but the gauge quickly leaked all its air out, in my opinion this would mean I can't really trust my compression tester. I needed to get my car back together so those are the results I have.

So basically at this point I am wondering where to go from here, this was a compression test on a fairly cold engine (<10C) and all I know really is that every cylinder has more than 320PSI. When I first got the car and dismantled the turbo, clearly a bolt or a nut had gone through it, so it wouldn't surprise me if one cylinder is down, but it just seems strange that one cylinder was that much higher than the rest, and I figure if a nut went through a cylinder causing the engine to shake so terribly badly wouldn't it be below 320PSI, and if it was a bad cylinder, why would the shaking and type of shaking change based on all these strange factors (temperature and etc)

Anyways I am just curious what everyone's opinions are on the matter, is my compression at the (best case) 380 or worst case (320) to low for me to bother doing anything, should I keep working on the fuel injection system, or should I just run this thing with the shake and try my best to ignore it.

Thanks!

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  #2  
Old 04-16-2021, 06:32 PM
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Join Date: Aug 2019
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Do a leakdown test they are much more reliable. Seen many worn out engines with good compression yet a leakdown test showed a fault
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  #3  
Old 04-16-2021, 08:09 PM
Diesel911's Avatar
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Join Date: Sep 2007
Location: Long Beach,CA
Posts: 51,043
Besides your noted issue. Look at your Oil Cooler Hoses. If they are seeping oil those need to be replaced. The hose under the collar shrinks and sometimes come out or cracks off there. People have lost their engines due to oil starvation when a hose leaks.

Look at your power steering belt and see if it is getting close to the Oil Cooler Hose due to a collapsed motor mount. (Collapsed motor and transmission mount can also cause the shaking.)

Not having the valves adjusted for a long time effects the compression results and it is one if those items past owners that cannot do the valve adjustment themselves won't pay to have someone do.

It is cheap and easy to change the rubber hoses down at the Fuel Supply/Lift Pump inlet to cure any air leaks there.
On the Fuel Supply/Lift Pump is a hand primer pump. The old ones that have the shiny aluminum body and a usually white hard plastic knob are a frequent cause of air leaks.
Unscrew the knob and pull it up and push it down. If it leaks fuel you know you have an issue. Not the new style ones are covered with black rubber and you simply push down and release it and it comes back up to be pushed down again.

That is some of it.
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  #4  
Old 04-16-2021, 10:15 PM
ykobayashi's Avatar
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Join Date: Jun 2005
Location: Irvine, CA
Posts: 1,261
I had a thread about my 300sd shaking a few months back. Same issue, shakes at idle, runs smooth at speed. I had a couple of cylinders in the 200s. It was strange because my car makes great power, doesn’t smoke and doesn’t burn oil. It does have awful compression on a couple of cylinders and you can hear it speed up and slow down when you crank it without glowing. A leakdown suggested it was the rings. Anyway I was able to get rid of most of the shaking by doing a number of things.

I rebuilt my injectors and balanced them. I changed motor mounts. Two valve adjustments. I soaked my piston tops in the environmentally safe version of Berrymans overnight. Diesel purge. Seafoam purge. I changed to synthetic oil and ran a cup of marvel mystery oil (I don’t believe in this stuff but it may have helped). I also put an oil separator on the breather. My theory was the cylinders were getting gummed up with too much oil coming in the intake and I wanted to give them a chance to burn some of it off...slow the rate in and maintain the rate out. Let’s see what else did I throw at it, new engine shocks. New engine shock bushings. Replaced filters, rack damper bolt. Adjusted rack damper bolt. Turned down idle to 700rpm. I also drive it a lot harder than I used to.

And after all that...drumroll...it is smoother! Not quite as good as my 300d but it is pretty smooth. Quite tolerable. Before the valve cover would swing back and forth an inch in each direction. Now I can balance a bottle of beer on it while it runs.

I never did check compression again. I think it isn’t good. I can hear the loping as I crank it indicating a weak cylinder. You can hear the crank speed increase when it hits the weak cylinder.

But it starts. Not with a bang like my 300d but it does start without too much cranking. So I’m happy and I just drive it.

I’m not really sure what smoothed it out. My wife agrees I did something dramatic to the car and it is remarkably smoother now.

Your compression doesn’t sound bad at all. I’d kill for those numbers. I’d start with easy stuff first like rack damper bolt replacement and adjustment. Valve adjustment. Injector pop test.
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  #5  
Old 04-16-2021, 10:51 PM
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Might start by checking for air getting into the fuel system. Replacing the hose from the return line on the injection pump with a clear one allows you to examine for air in the fuel.

Actually a valve adjustment check should be first. Mercedes recommended every fifteen thousand. On many of these cars the owners did not do it or assumed it had hydralic lifters.

If you find them tight. Adjust them and drive a few hundred miles and check them again. '

Also when you have the valve pan off check for the amount of chain stretch present. This is an easy check with information in the archives. The same with the valve adjustment check.

A timing chain stretched out will not cause your problem. Yet if stretched too much needs attention. Can cost the engine if it lets go. Also if you see no air in the clear plastic return line. Close it off temporarily. To see if that changes anything.
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  #6  
Old 04-17-2021, 12:14 PM
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Location: Sacramento, CA
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It is easy to get a bad compression reading, especially on a diesel. The adapter must exactly replace the same volume of the part removed (injector or glow plug), the check-valve at the tip of the tester must open easy and seal well. Those are special Schraeder valves with a weak spring (white collar to denote). If someone got the bright idea to replace w/ a regular tire Schraeder valve, it will read low. You need to crank over at least 5 compression strokes before recording the final reading. That is to compress the air inside the long hose and account for pressure loss as the compressed air cools.

If your engine starts easy on a 40 F morning, it is likely fine. If not, first verify all glow plugs are good. Also check the "valve cap dance" while idling (youtubes) to judge ring blow-by. A poor-man's compression test is to turn the engine over by hand and feel for each cylinder resisting you (requires 2 crank revolutions). If each fights you hard and you must wait >2 sec for it to leak-down to continue, the compression is likely fine. Anybody who had kicked over a motorcycle engine or started an older lawnmower (new ones are easy-pull) knows what I'm talking about. The power steering pump has a convenient nut on the pulley for a wrench (19 mm?) to turn over the engine without getting grubby and scraping knuckles. Turn it clockwise, viewed from the front, as for most engines (old Lincolns rotated ccw and a few others).

I don't know that a worn engine will shake more at idle. The usual suspect is the rubber motor mounts. These engines are very sensitive to them. The cheap India mounts (Anchor, ...) seem to cause more shaking and sag sooner. Many posts which show the best ones to buy. I don't think the engine dampers affect idle much and people say they are just to limit "shutdown shake". There is also a spring damper at the end of the injection pump which is said to help, something like the "gold one from a 1985" (forget). I've fooled with that but noticed no big difference.

My 1984 300D currently idles very smooth, is so quiet you can hardly tell it is a diesel, and is smooth as it comes off idle. It had ~320-350 psig compression when I checked maybe 4 years ago. My 1985 300D also idles fairly quiet but shakes slightly more. More noticeable is that it gives a slight shake when coming off idle, so feels a bit quirkier driving in parking lots. It had >400 psig compression in all cyl when I checked maybe 6 years ago. I'd have to check each book to see where I am in motor mount explorations. Glitchy response might also be due to the rpm control inside the injection pump. Your "fuel pedal" input is really an rpm setpoint to the I.P. When you replace the oil cooler hoses, cut the upper hose 1" longer to keep it further from that evil power steering V-belt. M-B should have done that. Sagging motor mounts can let the belt cut into the hose with bad results.
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  #7  
Old 04-17-2021, 12:40 PM
t walgamuth's Avatar
dieselarchitect
 
Join Date: Apr 2005
Location: Lafayette Indiana
Posts: 38,613
sounds like motor mounts to me. If it is it should sound like a jack hammer is working on your frame.
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..I also have a 427 Cobra replica with an aluminum chassis.
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  #8  
Old 04-18-2021, 08:03 PM
Broken Mercedes Enjoyer
 
Join Date: Apr 2021
Location: Kamloops BC Canada
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Wow thanks everyone the help is very much appreciated!

I just thought I would add an update and a little more information. So the motor mounts are basically brand new, I think the previous owner replaced them trying to fix the shaking but to no avail. The cap dance test is quite positive and an oil catch can is definitely on my list of things to do to the car. I replaced all the glow plugs when I got the car, and the very good diesel injection shop I bought my injectors from calibrated and tuned the (brand new) injectors for me before giving them to me.

The first thing I did when I bought the car was a valve adjustment as good ol' ************** has pretty much made me believe it is the miracle cure, and they were right the car had way more power as every single valve was waaaay out. I then I re-checked the valve clearances right before my compression test and about 500km after my last valve adjustment to find they are still perfect.

One thing I noticed is that my glow plugs and injectors have what seems to be a lot of carbon buildup on them since I put them in about 500kms ago, once again this could maaaaybe be fixed by a catch can, but i'm not sure what else it would be as the car makes no black smoke at all.

anyways I really should've said this at the very start but when I did my injectors, valve adjustment, and glow plugs I also changed every single fuel line in the engine bay with clear hose because I had air sitting in the prefilter (highest point before the pump). Since I did that I never really looked at that prefilter again thinking there's no way air could be getting in, I had just changed every single fuel line and copper washer, but sure enough, there is still clearly air in the fuel lines, and no matter how much I pump the primer it won't go away, or if I do successfully get rid of it the bubble is back after a few minutes of idling.

I am very mad at myself for so closely admiring every single detail on my benz but somehow completely missing the giant air bubble that I allready knew was there...

So now I ask all the very helpful people above, how do you find a leak in your fuel system, I just can't figure out where this air is coming from???
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  #9  
Old 04-18-2021, 10:53 PM
t walgamuth's Avatar
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Location: Lafayette Indiana
Posts: 38,613
I believe a bubble is normal isn't it?
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..I also have a 427 Cobra replica with an aluminum chassis.
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  #10  
Old 04-19-2021, 12:33 AM
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Yes retention in the pre flter of an air bubble is normal. A stream of bubbles passing through it is not.

The acid test for air is the clear line from the relief overflow valve.
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  #11  
Old 04-19-2021, 10:40 AM
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I always see a small air bubble in the clear pre-filter of both my 300D cars. The air doesn't pass to the lift pump, so doesn't hurt anything. Many people here have fretted for years over that air bubble, but seems to be normal. Sounds like you used clear nylon fuel hose. Another choice is 5/16" hose rated for bio-diesel. NAPA carries it (Goodyear Barricade I recall). I think it has a Viton liner, which is the best plastic. A few people have used PEX tubing. But, neither is clear.
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  #12  
Old 04-20-2021, 12:36 AM
vwnate1's Avatar
Diesel Dandy
 
Join Date: Nov 2005
Location: Sunny So. Cal. !
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Thumbs up Newbie ~

WELCOME NEWBIE ! .

You came to the right place, the collective brain trust here is amazing, there's very little you cannot ash that hasn't been experienced before so ASK AWAY ! .

As mentioned, the BRAND of motor mounts -can- make a large difference .

If it doesn't smoke I see no reason to add a breather oil catch can .

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I did then what I knew how to do ~ now that I know better I do better
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