Mercedes OM636 rev fluctuations

[jnaiken]

Hi all.

I was entering Milford Haven yesterday in terrible visibility after sailing from the Scillies and the Mercedes Om636 in my Tradewind 33 began to run irregularly. It didnt stop but it did not inspire confidence! At that point it had been running for about an hour. Im now sitting in Dale YC having taken some gauge readings this morning and wondered if anyone might have experienced this before. I imagine in a year of ownership I have used the engine a little over three hours in total, and never for more than twenty minutes.

Ive read some other forums and suspect lift pump, regulator or something to do with the alternator and fully charged batteries.

This morning it started first time and ran smooth.

Oil pressure 40 at idle 110 with 1500 revs

After 20 minutes this dropped to 20 at idle and 40 with 1500 revs.

The temperature rises to 70C/160F and is maintained there throughout.

After 30 minutes the revs begin to fluctuate. They drop and then rise to what they should be, most notably in the neutral position, where the gauge indicates they almost drop to 0.

When revved past 1700 the fluctuations slowly stop over about ten seconds and the engine runs smoothly.

Im planning to cruise north from here, Scotland and perhaps further, mostly sailing single handed and am looking to get everything to a point of reliability.

Has anyone any knowledge or experience of this? Does is sound like something that will deteriorate or is just an idiosyncrasy of the engine?

Any thoughts much appreciated.

James

 

[PaulRainbow]

It won't be anything to do with the alternator, that will not pull the RPM down to almost zero on a 42HP diesel, even if it seized up lol

It could be the pump, it could be fuel or air problems. Take a really good look at all the fuel lines, connections, filter housing etc. Don't miss the small rubber pipes between the injectors. Any sign of even a slight damp patch needs looking at. Clean the air filter (least likely but still worth doing). Check the breather to the fuel tank is clear, if you can't be 100% certain open the fuel filler while the engine is hunting. I would then change the fuel filters, along with all of the seals.

EDIT : The oil and temp gauges are behaving normally. The oil pressure will drop as the temperature goes up, nothing to worry about.

 

[KAL]

When you've been running in gear for a while then go into neutral, do the engine revs die altogether?

There's a leather diaphragm inside the fuel pump which can go, leading to revs issues. Cheap and easy to fix yourself.

Give Manfred at Westfield 4x4 in Lancx a call. There's nothing he doesn't know about these engines and he has the parts readily available.

 

[bristoljim]

+1 for hole in diaphragm & or sticking fuel rack attached to diaphragm,also check pipes from diaphragm housing to ventury on inlet manifold.
Jim

 

[jnaiken]

Thanks all, I just spoke with Manfred, what a guy! He agreed with your prognosis, lift pump as a first check.

Many thanks again!

 

[catmandoo]

Thanks all, I just spoke with Manfred, what a guy! He agreed with your prognosis, lift pump as a first check.

Many thanks again!

i assume you meant injection pump not lift pump as lift pump is a metal piston with a stainless stiff spring and the diaphragm in question is on the end of the injection pump . By the way there is a small oil reservoir at that end with dip stick , when did you last check it ?
also lift pump spring can fail if engine is old . worth checking for cracks and or debris

 

[PaulRainbow]

By the way there is a small oil reservoir at that end with dip stick , when did you last check it ?

My OM636 doesn't have a dipstick. According to my manual they stopped fitting them in 1962

 

[KAL]

My OM636 doesn't have a dipstick. According to my manual they stopped fitting them in 1962

Mine is a 1981 boat and the injection pump definitely has the dipstick! It doesn't look like a dipstick, but it is.

I would look again to make sure.

 

[PaulRainbow]

Mine is a 1981 boat and the injection pump definitely has the dipstick! It doesn't look like a dipstick, but it is.

I would look again to make sure.

Mine is also a 1981 boat. My pump has a large screw in red cap on top of the pump, with "OEL" on the top. There is no dipstick. I've got the small A5 ish manual and a large (about A4) manual about an inch thick. The large manual does say that after October 1962 they did away with the dipstick. It goes on to say that on models without dipsticks there is a "level plug", i'm supposed to undo the level plug and fill with oil through the red top until it comes out of the level plug. That's how a lot of gearboxes are filled in cars. This has to be done at every engine oil change. It also mentions some pumps being lubricated from the engine oil.

It seems there are a few pump variations too, some have diaphragm governors, as discussed here, (i think the boat engines are all diaphragm) some have centrifugal governors. Although there is no mention of it in any of the books, not all engines are fitted with Bosch pumps.

The book also refers to the need to fill the water pump with oil, using fill and level plugs.

 

[KAL]

Mine is also a 1981 boat. My pump has a large screw in red cap on top of the pump, with "OEL" on the top. There is no dipstick. I've got the small A5 ish manual and a large (about A4) manual about an inch thick. The large manual does say that after October 1962 they did away with the dipstick. It goes on to say that on models without dipsticks there is a "level plug", i'm supposed to undo the level plug and fill with oil through the red top until it comes out of the level plug. That's how a lot of gearboxes are filled in cars. This has to be done at every engine oil change. It also mentions some pumps being lubricated from the engine oil.

Ditto for mine, but there's a dipstick that you wouldn't know is a dipstick, unless you knew! I will try to post a picture if I can figure out how to do so. As you will know, the injector pump is a sod to get to, but I need to do an oil change, anyway, this week, so will need to get in there to clean the SS oil filter. IIRC, the injector pump takes about 20ml of oil. Any excess pours out through a 'level' hole. I've never succeeded in being able to read a level on the dipstick!

 

[PaulRainbow]

Ditto for mine, but there's a dipstick that you wouldn't know is a dipstick, unless you knew! I will try to post a picture if I can figure out how to do so. As you will know, the injector pump is a sod to get to, but I need to do an oil change, anyway, this week, so will need to get in there to clean the SS oil filter. IIRC, the injector pump takes about 20ml of oil. Any excess pours out through a 'level' hole. I've never succeeded in being able to read a level on the dipstick!

Thanks for that, i'll take another look. My manual definitely says the dipstick was removed in 1962, but i'm not doubting what you say. There is also no mention in my book about TDZ pumps, which mine is fitted with. Yesterday, i found a letter to a former owner from Mercedes, this explains that the TDZ pump was made in Spain under license from Bosch. It goes on to say that all parts are interchangeable with the Bosch pump. I see from an old thread that you started that you also have the TDZ pump.

You mention that the pump is hard to get to, i was talking to a W33 ketch owner yesterday, he said the same. I'd have thought yours access is the same as mine, removable panel in the side of the engine box, beneath the sink ? I didn't think that was too bad, perhaps i'm used to working in tight spaces.

I'm going to service mine soon, so will treble check that dipstick. Not sure if mine still has the SS filter or if it's been changed for an element. What oil are you using ?

 

[GTom]

I have rev problems with mine too: after 8 hours of engine run I couldn't switch gears because it was too fast. The local mechaninc set it slower. Now, after a few weeks sitting (no engine use at all for almost a month) it is too slow, I can't get it running: it dies before swithching to gear. Hope it's not the injector, its an old rusty TDZ unit, afraid the only way to fix that one is to replace it...

Fuel system and oil filter are clean.

 

[Sea-Fever]

Thanks for that, i'll take another look. My manual definitely says the dipstick was removed in 1962, but i'm not doubting what you say. There is also no mention in my book about TDZ pumps, which mine is fitted with. Yesterday, i found a letter to a former owner from Mercedes, this explains that the TDZ pump was made in Spain under license from Bosch. It goes on to say that all parts are interchangeable with the Bosch pump. I see from an old thread that you started that you also have the TDZ pump.

You mention that the pump is hard to get to, i was talking to a W33 ketch owner yesterday, he said the same. I'd have thought yours access is the same as mine, removable panel in the side of the engine box, beneath the sink ? I didn't think that was too bad, perhaps i'm used to working in tight spaces.

I'm going to service mine soon, so will treble check that dipstick. Not sure if mine still has the SS filter or if it's been changed for an element. What oil are you using ?

Paul, does it say in the manual what oil to use for the injector pump?

 

[david_bagshaw]

good quality engine oil.

 

[ffiill]

My 40 year old omc636 starts at about 120 psi which is right for this engine and runs at about 70 again right for an engine that has done about 60 k miles car equivalent.
However suggest you check it out with Westfield 4x4 Lancaster as the engine whether marinised or for old Unimogs as he is a renowned expert and quite happy to help omc636 owners when he can.

 

[ffiill]

Oil-there is a top filler cap and side level plug as I recall but again check with Westfield 4x4

 

[PaulRainbow]

Paul, does it say in the manual what oil to use for the injector pump?

good quality engine oil.

Yes, same oil as the engine.

 

[KAL]

Yes, same oil as the engine.

But only about 20ml or so. It helps to use a hypodermic syringe if you can get one.

 

[NajadRubin]

i assume you meant injection pump not lift pump as lift pump is a metal piston with a stainless stiff spring and the diaphragm in question is on the end of the injection pump . By the way there is a small oil reservoir at that end with dip stick , when did you last check it ?
also lift pump spring can fail if engine is old . worth checking for cracks and or debris

What end of the pump? I am having rev flucs too right now and would love to get to that membrane to see if its torn. Thank you

 

[yanay]

@jnaiken wonder if you have any insights as to the diaphragm replacement on the TDZ pump. My OM636 is revving up when going from gear to neutral. My diaphragm should arrive today. Any tips as to how to remove the old one and install the new one? How do you actually pull the thing out? Any springs, shims, o rings etc I should worry about? Did you add oil to the pump? I never knew there’s a filler neck on the pump.
Many thanks