OM 606 Compression Check

[208benz]

I have been searching for a tutorial for a compression check on my 99 E300D and can't find one. Anyone out there have any tips?
I have a compression test kit, but do I need a special adapter to perform the test at the injector?
Should I disconnect the fuel shutoff plug at the IP?
Cold engine or hot engine?
What numbers am I looking for?

I have 2 issues with this engine, one issue is when the engine is cold during the winter months it starts fine then approximately 3 minutes later it idles very rough then goes away after a minute or 2. It only happens at the first start of the day and only at idle. This problem does not happen during the warmer months. This problem got a little worse this winter.

The other problem is after the engine has been running and up to temp, then shut off the engine, during the next start the engine will crank for 3-4 seconds and sputter before it starts, then it runs fine.

Thanks,

Mike
99 E300D 220K

[Diesel911]

This only goes up to 1995 but except for the shutoff the compression test should be similar.
http://w124-zone.com/downloads/MB%20CD/W124/w124CD1/Program/Engine/605_606/01-1200HA.pdf

Mercedes Benz Model 124 - OM606 Maintenance Manuals

You definately don't want Fuel getting injected into the Cylinders during a compression test.

[jake12tech]

Sounds like you have air in the fuel. Compression, hard to start, lack of power etc. if I don't have leaking rings at 500k, I doubt you have a compression issue..

[208benz]

Thanks Diesel 911!
That's a nice service manual, do you know where I can find one for the W210? I have an online manual subscription but its not as user friendly as the one you sent.

[Diesel911]

Originally Posted by 208benz

Thanks Diesel 911!
That's a nice service manual, do you know where I can find one for the W210? I have an online manual subscription but its not as user friendly as the one you sent.

The Manual you see is parts of the Mercedes Service Manual on CD or DVD.
I don't have a W210 site in my notes but do a google search for: Free Mercedes W210 Service Manual and see what comes up.

This is for a W201 but it used to be you could go through startekinfo.com and access all of the service manuals for free. But that was a few years ago. Note that I don't know or have much info or notes on other vehicles past 1985.
https://www.startekinfo.com/StarTek/outside/11832/?requestedDocId=11832

[jay_bob]

For the W210 you’ll need to get one of the WIS manual sets from eBay. The Startek info site only has free access for the 123, 124, 126, 201, and 107.

Find an old computer you don’t care about and put it on your router’s guest network to install these, since some of these disks may harbor viruses.

[RunningTooHot]

You guys will think I'm nuts, but I'm going to take this in a whole 'nother direction. Don't bother doing a compression check; you'll likely find that you're barking up the wrong tree.

Years ago on my N/A 606, I experienced the same behavior: Only when quite cold (winter) and only on the first start of the day - the car starts fine, then after 2-3 minutes it starts running rough like it has fuel starvation/air infiltration. Then after it's fully warmed up, it's running like a sewing machine again. Weird. Really weird.

I tracked it down to a leaky / faulty fuel thermostat. It appears that as the thermostat transitions between the two phases (pre-heating vs. bypass) it allowed air to get sucked into the fuel loop. But you say it's not leaking? Remember, it's not experiencing pressurized fuel to cause it to leak out. The thermostat is under vacuum; the fuel is being drawn into it from the tank via suction.

This perhaps could also explain the second part of your issue, but this part is shear speculation on my part: Since the fuel thermostat is leaking, when the car is parked the (air) leak would allow air to enter the fuel loop, allowing the fuel to drain back into the tank (instead of holding it in the lines) on the T-stat end. The air entering the lines could also allow the fuel on the other side of the loop to act as a siphon, pulling more air into the loop on the filter & injection pump side.

This was a really tricky one to diagnose when I had the issue...

EDIT & P.S. - Please come back to this thread when you solve the problem. That could really help someone in the future from tearing their hair out of their head pursuing a ton of dead-ends trying to track it down.

[t walgamuth]

Originally Posted by RunningTooHot

You guys will think I'm nuts, but I'm going to take this in a whole 'nother direction. Don't bother doing a compression check; you'll likely find that you're barking up the wrong tree.

Years ago on my N/A 606, I experienced the same behavior: Only when quite cold (winter) and only on the first start of the day - the car starts fine, then after 2-3 minutes it starts running rough like it has fuel starvation/air infiltration. Then after it's fully warmed up, it's running like a sewing machine again. Weird. Really weird.

I tracked it down to a leaky / faulty fuel thermostat. It appears that as the thermostat transitions between the two phases (pre-heating vs. bypass) it allowed air to get sucked into the fuel loop. But you say it's not leaking? Remember, it's not experiencing pressurized fuel to cause it to leak out. The thermostat is under vacuum; the fuel is being drawn into it from the tank via suction.

This perhaps could also explain the second part of your issue, but this part is shear speculation on my part: Since the fuel thermostat is leaking, when the car is parked the (air) leak would allow air to enter the fuel loop, allowing the fuel to drain back into the tank (instead of holding it in the lines) on the T-stat end. The air entering the lines could also allow the fuel on the other side of the loop to act as a siphon, pulling more air into the loop on the filter & injection pump side.

This was a really tricky one to diagnose when I had the issue...

EDIT & P.S. - Please come back to this thread when you solve the problem. That could really help someone in the future from tearing their hair out of their head pursuing a ton of dead-ends trying to track it down.

I agree that its not likely to be a compression problem. The rest sounds complelling too.

[jay_bob]

Yes I totally agree with the others. The OM606 is a plumber’s nightmare on the low pressure fuel side. There are many opportunities for leaks that will cause issues with the engine running smoothly.

There are 6 fuel hoses with o-rings on the connnectors. They are supposed to be almost transparent when new. If yours are old enough they are opaque replace them. Collectively these hoses are about $100.

Note there is one connection to the back of the IP (the rack return connection) that is still made with an old school banjo bolt with crush washers.

Also replace the o-ring on the back of the fuel shutoff valve (remove the 2 screws and pull the valve out of the IP).

And while you have it all apart replace your primary and secondary filters, be sure to replace the o-ring on the primary filter, and fill your secondary filter with clean fuel or diesel purge when you reinstall it.

In short rule out all causes of low side air in fuel before going down the compression test road. These engines are quite tough and compression issues are rare with these.

[208benz]

Fortunately for me this problem has been a nuisance rather than a real problem such as engine won't start, loss of power, always runs rough, excessive smoke, etc. So I just deal with it and go through the motions of replacing what makes sense when I have the spare time and without spending too much money.

I think last winter I replaced all of the clear fuel lines with viton o-rings attached from the OEM. Also replaced the o-ring on the shutoff valve. It didn't fix the problems. By the way this is inexpensive and easy to do for anyone not familiar with the job, and this should be considered preventative maintenance in my opinion.

I replace the fuel filter, pre-filter, and air filter every other oil change. I change oil (Mobil 1) & filter every 10K miles and consistently need to add 1 pint of oil after 5K miles. That's not bad.

Last spring I decided to have the injectors rebuilt because I consider this preventative maintenance (car has 220K) and mostly was hoping the problem would go away, but it did not go away as I expected.

The fuel tank screen was replaced 5 or 6 years ago, I'm thinking about replacing that again for preventative maintenance, but again I don't expect that to fix the problem.

I replaced the crankshaft sensor 2 years ago. Someone had a theory that this sensor could cause a delayed start if the sensor was heat soaked, and I agree with that theory because I have seen heat soaked components cause issues, so I tried that but it didn't work. And of course the connector was so dried out that it broke when I removed it so I had to replace that harness. That was a fun time.

I tracked it down to a leaky / faulty fuel thermostat. It appears that as the thermostat transitions between the two phases (pre-heating vs. bypass) it allowed air to get sucked into the fuel loop. But you say it's not leaking? Remember, it's not experiencing pressurized fuel to cause it to leak out. The thermostat is under vacuum; the fuel is being drawn into it from the tank via suction.

[208benz]

Sorry, this was a post from RunningTooHot (I accidentally hit send on my last post):

"I tracked it down to a leaky / faulty fuel thermostat. It appears that as the thermostat transitions between the two phases (pre-heating vs. bypass) it allowed air to get sucked into the fuel loop. But you say it's not leaking? Remember, it's not experiencing pressurized fuel to cause it to leak out. The thermostat is under vacuum; the fuel is being drawn into it from the tank via suction."


I'm thinking about trying this. I haven't pulled the IM in a while and I like to see how things are looking in there. Maybe I can verify a leak on the fuel thermostat.

I will keep you posted and sorry again for the paste error.

Mike

[RunningTooHot]

Just like for you, this was a minor annoyance - not a significant obstacle to normal operation. Although it would act fuel starved for about 30-45 seconds if driving while it hit that magical transition temperature. It was more of a curiosity to figure it out than a need to fix it.

Keep in mind that you may not necessarily see a fuel leak at the fuel thermostat since that part of the fuel loop is under suction; it may be a "one way" leak - pulling air in, but not leaking fuel out externally.

Instead of throwing money at it speculatively, try to simply bypass it temporarily. When I changed out my clear lines, I kept the old ones to butcher them for an easy way to run an occasional can of diesel purge. So if by chance you have your old lines, just cut the two appropriate ones and clamp a piece of fuel hose on them for a bypass. Pop those on the car, and I'll bet you'll find the problem has disappeared.

[208benz]

Here is a picture of the fuel thermostat bypassed. I have a good video with the engine running in this configuration (rags removed) but the forum is rejecting it for some reason. I will run the engine tomorrow morning and hope its cold enough to reproduce the problem. If not cold enough I would like to put the IM back on and drive the car for a while with the fuel thermostat in bypass. Is it ok to run the car like this?

[208benz]

Bypassing the fuel thermostat did not fix the problem. The outside temperature was 42 deg which is borderline for this problem, but the engine did skip twice. If it was 30 deg I would expect the engine to run rough for a minute or 2.

I did notice while running the engine with the IM removed that it is easier to see the fuel running through the clear fuel lines. There are air bubbles passing through the lines when I advance the throttle potentiometer to approximately 2000-3000 RPM. The only lines that do not have air bubbles is the line going to the fuel thermostat and the rack return connection line with the banjo fitting. The other 5 lines all have very noticeable air bubbles but they disappear at idle.

Last year I changed all the clear fuel lines, the shutoff valve o-ring, and the o-ring at the fuel filter bolt, but it's possible I could have cut an o-ring when I installed them. I think I'm gonna replace all these o-rings again.

Does anyone know how to attach a video? I have a 7 second video I can share, but having a hard time posting it.

[Diseasel300]

I think you're on the right track with the O-rings. Your air leak will likely be somewhere near where you see the bubbles show up when you're revving the engine. Air in the system will definitely cause rough starting, especially when the engine sits and all the air accumulates in the filter and hard lines.

The forum doesn't allow video attachments. You can upload to a hosting site like YouTube and paste the link back in this thread.