What is a Leak Down Test?
What is a Leak Down Test? - PeachParts Mercedes ShopForum

help with compression test
help with compression test - PeachParts Mercedes ShopForum


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1985 300SD
1983 300D
2003 Volvo V70

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So there's been a bit of down time, and a bit of "progress."

It seems that the skipping cylinder is #2. A second cracking of the injector lines revealed consistently that when number 2 was loosened there was no change in idle, and the smoke was greatly reduced. Tighten back up, idle stays the same, smoke comes back. All other injectors caused the idle to worsen when loosened.

Driving at higway speed, the heavy grey smoke disappears, but anything other than that the car is a public nuisance.

I tried to beg, borrow or steal a compression adapter with no luck- noone seems to have one for the 603, so I ordered one from Baum tool, should be here tomorrow. Based on the fact the car starts perfectly each time, and occasionaly seems to regain power on all 6, I don't think (and hope I won't!) I will find a compression issue, so that puts me back to fuel. I'll know more tomorrow.

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Chief

1991 350SD

Is there coolant consumption or indication of oil in the coolant? Is the engine back together to where you can do a radiator squeeze test? If you're taking a poll, chalk another vote for pulling the head. IMO, you should have done that before pulling the IP.

Sixto
87 300D

No coolant/oil exchange. Overflow tank is clean, no pressure in the hose. The engine exhibited these exact symptoms before the brand new (not used) #20 head was installed. This seems to exonerate the head or gasket as causal factors.

The lack of engine oil consumption also has kept me from worrying too much about #2 being ovaled. At the time of the rebuild, it was found NOT to have a bent rod- numbers 3 and 4 did have bent rods, which were replaced. The rebuilder attributed the problem to "cylinder glazing, and improperly seated rings."

Once the IP is re-installed, I'll run and evaluate the change, if any. Obviously I'm hoping that #2 will wake up. If not, I'll do a leak down test, before I pull the head. I'm pretty satisfied that it will not be the head, I already know it's not the injectors, and won't be the IP. That would leave the cylinder and piston/rings. I would tend to think a piston issue would have been obvious during the rebuild. If I have to go down this road, I will very carefully measure the bore, piston and rings.

I understand that early on, MB said the OM603.971 was not suitable for cylinder sleeves. Later, in August of '94, they issued TSB 01/32 that said it was ok to install sleeves in this block. I have not read this myself, so I am looking for a copy of this TSB to verify.

Anyway, one step at a time. I hope to have the IP installed in the next couple of days.

__________________
Chief

1991 350SD

* The injection pump WAS damaged.
There where at least two major delivery valve issues found, now repaired and calibrated.

My advice when we spoke on the phone when he picked it up from the injection shop was:

* Install the injection pump and see if the issue is gone, before any further diagnosis or repair.

Until he reports back with some results, there is IMO no logic putting more time, labor, and/or money forward.

********************************************

Reviewing:

The engine was rebuilt at 40k miles:
* #2 cylinder issue has been there since then, and may have been there to some degree before.

The #14 Cylinder head was replaced with a #20:
* #2 cylinder issue was there before and after.

* The engine consumes no lubricating oil or coolant.
* Coolant system is not showing indication of combustion pressure.
* The engine started smooth when cold.
* The engine ran progressively worse as it warmed up to full operating temperature.

Engine running; His mechanic "cracked" the injector lines loose one at a time.
* Only 3 cylinders where significantly affected during this test.
* The other 3 made NO difference.

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I understand the your logic and Roy's logic. Prefaced with IMO, any compression irregularity tells me to pull the head. I agree with not putting more time and effort without understanding the situation which is precisely why I'd have the head off immediately after the compression test results. I'm happy for the IP results but it remains an IP without a worthwhile engine to fuel. I'm not challenging the process since I know about 2% of what you know of the situation. Just, again, IMO.

And a fact check - the original head should have been a #17, not a #14. Do you have a .970 or .971? It's an irrelevant detail but a 91 SD would have a .970 engine.

Some shots in the dark to check without pulling the head:

- inspection of #2 cam lobes; visual and actual measurement of lift vs other lobes.
- do service records indicate that valves and lifters were reused on the new head?
- do service records indicate that prechambers were reused on the new head?
- can you check for a compression leak through the prechambers and lock rings?

I don't know whether resleeving is possible but Performance Products sells .97 cylinder liners for $99 each. I'm not sending you somewhere other than Phil/Pelican. Just pointing out that they exist if Phil or even MB doesn't carry them.

Sixto
87 300D

Can't tell you guys how much I appreciate all the the thought that has gone into this so far!
As Roy said, Once the IP goes back in, I'll go from there.

Lastly, thanks for the fact check. It's a 603.970. Never saw the original head. WHen the new head was installed, there are also reciepts for new exhaust valves and 2 new injectors, but not for anything else.

Early on I pulled the cam cover to inspect the cam, etc. All cam lobes look absolutely new, and everything in the cam chamber is clean, bright and shiny, just like new.

__________________
Chief

1991 350SD

The injector pump installation is now complete. No joy.

Leak down test is next, then off with its head.

__________________
Chief

1991 350SD

So I spent some time at my mechanic's shop today, and helped pull the head ( a #22 by the way, now that I could see the casting mark clearly). First things noticed- The head gasket was perfect, no sign of any problems there. Proceed to looking at the piston crowns and bores. Since #1 is already at TDC, I saw a very light, uniform carbon soot deposited over its surface. Number 2, the real problem cylinder- dead clean! Number 3 was also pretty clean, while 4, 5 and 6 were nicely carboned like #1.

All cylinder bores look fine. No ridges, scoring etc. Just what a good bore should look like, so we rotate the crank to look a little deeper. As #1 descends, I watch #2 come up to TDC and guess what? It is at least 1mm shy of where it should be! A friggin bent rod! Number 3 appears to crown slightly off kilter, but crests the bore. 1, 4, 5 and 6 all fine.

There it is. I'm assuming the fact that there is so much head space between the piston crown and the head explains the low compression and lack of ignition. In anticipation of a fatal flaw, I found an OM603.96 with 140k miles. I'm going to listen to that engine tomorrow, and if it is as described I'll acquire that engine. My plan is to freshen it up with a set of rings, swap over all the good bits like the head and injection pump from the 3.5 and end up with a solid car. I will be searching the site for threads on this swap, but please, any tips as to what I should be looking for, what changes I need to accomodate etc. would be SO welcome!

There is nothng like diving into mess like this to really learn a lot. While it IS pretty inconvenient, I know that when I'm done I will have a 126 that I can have real confidence in.

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Chief

1991 350SD

Jim/engatwork is going through that right now - Pulling the head on a 1991 350SDL

Sixto
87 300D

Thanks- I missed that thread.

My 3.5 has lots of new parts, like the oil pump and of course my newly overhauled injection pump. As I undestand it, the injection pump and head will swap right on to the 3.0 without any problems. Since I am going into it this far, I want to use the best of what I have, like the new oil pump, the later timing cover from the 3.5, check out the vacuum pump, etc. I appreciate you all helping me avoid the pifalls that can accompany this sort of thing!

__________________
Chief

1991 350SD

I checked out the 3.0 OM603 engine today. It is currently in a Chevy Tahoe, which is why it is for available. The installation is a bit crude, and it broke the flex plate so the owner who acquired the truck this way is going to repower it with a GM engine.

Engine started right up from cold, running on all 6 cylinders. It sounded good, no funny noises other than a squeaky idler and a noisey K&N air filter. Revved up and held rpms smoothly too. There is a thin wisp of steady blue-grey smoke, and some apparent blow-by with the oil filler cap off. The owner tells me the engine is supposed to have 140k miles, and by his testimony used 1.5 quarts of oil over the 4k miles he has actually used the vehicle. It has good oil pressure, and runs cool with no overheating. The price of the engine, complete is $700, with a money back guarrantee should it have any issues once I get into it.

My plan is to re-ring the engine, and transfer all the best bits from my 3.5 to this engine, selling off what remains. I'll keep everyone posted, and as always welcome the wisdom of the board!

__________________
Chief

1991 350SD

Were you able to drive the Tahoe with 603 power?

Sixto
87 300D

It was offered, but I declined. This vehicle is out in the puckerbrush, and with its cracked flex plate, I just listened to it run.

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Chief

1991 350SD

How can you tell it doesn't overheat if you're just listening to it idle in February?

Can you do a radiator hose pinch test?

Is there a chance for compression test before it's yanked from the Tahoe? Is there time to send an oil sample for analysis before you decide?

Sixto
87 300D