oil pressure

[Guest]

I am in danger of becoming an oil pressure bore, but am concerned about the low oil pressure on my merc om636 engine, which dates from 1979, it starts and runs ok and consumes a little oil but gives a pressure of 60-80 psi on cold start but drops to 15 psi when hot at cruising revs (2000rpm) but goes right down to almost the bottom of the scale when hot at idle (650 rpm) which is below the handbook min of 0.5 bar
assuming the gauge is ok, is this preterminal? am I looking at a new or reconditioned engine, can I carry on like this: the pressure has drifted gradually down season by season, will it seize up one day?
advice has soo far varied from "if it runs leave it" to a new engine: of course if you go to the engine stands at the show they are bound to want to sell a new engine
any advice on new vs recon? ? a chance to clean up the engine bay and service things i otherwise can`t reach?

 

[charles_reed]

It doesn't sound as though the engine owes anyone much - so deserves to enjoy its senesence.

Seriously though, the symptoms you describe are typical of an elderly engine where a combination of ample bearing clearance are exaggerated by a worn oil pump.

If you want to sort out the problem it's a strip, rebore, and rebuild with all new bearings - problem is finding someone who'll do the job with adequate competence at a reasonable price. You could always try yourself - Merc engines of that age are fairly simple.

You could try a thicker oil - say SAE30/50 and see how much that changes things.
I'd also be very suspicious of the accuracy of the pressure guage and check that out before embarking on any more ambitious rectification.

 

[HenryB]

My OM636 is the same age and maintains 40-50 psi at 1600 revs. I believe that a full range of spares are still available and you may well find that the engine can be rebuilt at considerably less than a replacement. I would be wary of running it with such a low pressure.

There is a company called Pentagon on 01202 642502 who can supply spares (they may be able to rebuild as well).

Also Westfield. They have a website http://www.west-4x4.demon.co.uk/om636/index.html
but the telephone number quoted seems to have changed. They advertsie a rebuild service.

I was also given a contact name of Andy at Mercedes on 0181 2005151

Hope this helps,

 

[HenryB]

I've found another telephone number for Westfield - try 01524 791968

 

[Col]

If fitted, check bypass valve is not sticking, or spring may be weak.

<A target="_blank" HREF=http://www.arweb.co.uk/argallery/colspics> Cols Picture Album</A>

 

[vyv_cox]

Although, as someone else has said, the engine seems to have enjoyed a long and fulfilling life, It does seem to me that the symptoms you describe are unusual. For such a high cold pressure to decline to such a low hot pressure suggests some leakage, perhaps from the pump or maybe (don't know this engine at all) it has a filter bypass valve that is lifting prematurely. Also check that any piped supplies, for example to the rockers or camshaft on top of the head, are not leaking from banjo unions internally. I don't think the gallery relief valve is the culprit but it's very easy to check on most engines. Worth investing a little time on some checks before starting to think about replacements.

 

[oldharry]

I am inclined to agree with Vyv - firstly 22 years is not a long life for a marine diesel, unless it has either been badly neglected, or has been used commercially. I also agree with Vyv that the variation in oil pressure is more than I would expect. If the bearings are so worn as to allow the pressure to drop to near zero at working temp, then you would be unlikely to get such good pressure at start up.

Careful checking on the lines suggested is called for before going any further.

Other indications of a well worn engine would also be evident - rattly 'loose' sounds due to piston slap etc, louder 'dieseling' sound on load, blue smoke in the exhaust when working hard, high crankcase pressure (disconnect the breather tube to see if it is blowing badly), valves leaking. Any one or combination of these point to a worn engine.

If you are looking for an engine rebuild and can get it to them, try Banda engineering in Fitzherbert street, Portsmouth, who do an excellent job (used by many local garages), at a very reasonable price. They checked out and set up a Perkins block, including a rebore, for me for under £500, when a recond block was being quoted between £1100 and £1300 by suppliers.

 

[Plum]

You have already had lots of good advice, although I personally would not rush into buying a new engine or getting this one overhauled yet). However, my thoughts are:
1) fit a different, calibrated, pressure gauge
2) change the oil for a thicker one to see what effect this has on pressure
3) an engine with such a low runing (not idle) oil pressure would probably exhibit other symptoms of age (smoke, noise, oil consumption, poor starting) so if you have no other symptoms then certainly do not throw the engine out as all you probably need is a new oil pump and pressure releif valve.
4) get a compression check done to determine condition of bores/rings/valves/piston to check that these parts really do need overhauling. If compression pressures are OK and you need to have the engine out to investigate further you will only need to intruct the mechanic to concentrate on oilpump and bearings.

 

[philip_stevens]

I won't dispute what has earlier been written, but,
I would not suspect the pressure relief valve, as you have good pressure at startup with cold (thick) oil.
I would not suspect rings or bore problem, as you do not say there is blue smoke from the exhaust.
Thicker grade oil will give you a higher pressure at startup, but will fall - not so much - to a lower pressure when the oil is hot and thin.

What I would suspect, as I have seen it on diesel engined container ship main engines and auxiliaries, is that your big end is worn. You would still have oil pressure when the engine is running faster, but when hot and slow, the oil would run freer from a worn big end bearing.
It would be prudent to check (or have checked) the play on the big end. It may not be much, but enough to drop oil pressure if the bearing shells (if they are fitted) or the lining of the big end is worn.
Worst scenario is that the bearing will wipe itself and sieze, doing major damage to the crankshaft.
But so long as you do have oil pressure, take things easy. And don't rush out to buy a new engine, as most things can be refurbished.

regards,
Philip

 

[johnt]

Oil pressure relief valve is my first guess too, a sticky or worn valve will show these symptoms..if you have a cartridge filter its easy to get at for changing.

If it isnt that ......its a strip down job. Short job is standard mains ,big ends and oil pump. The full job is mains, big ends
reground crank, pistons .rings, rebore and a new oil pump.

Which it will be depends on how oval the crank is!

Ive seen these symptoms on a car , AFTER it had had a crank regrind and new shells.........they didnt check the oil pump ..it was totally worn out !

 

[vyv_cox]

I wouldn't rule out the big-end bearings entirely, but in the past I have had big-ends on cars that knocked very badly, and on inspection were completely clapped out, but the oil pressure never fell as low as this one. I once broke a crankshaft on an Austin Healey Sprite, it continued to run and the oil pressure hardly fell at all. You need to consider the dimensions of the oil supply system. Pipework from the oil pump, through the filter and into the gallery is large. Only when it enters the crankshaft drillings does it go considerably smaller. So a leak, such as a worn bearing, from the crankshaft does not release so much oil as one from upstream. I still favour something wrong in the upstream area, although it wouldn't be the first time I've pointed in the wrong direction (see previous posts on coolant leaks)!

There is clearly little wrong with the bores as consumption is low. These have no effect on oil pressure anyway but do give some idea of engine condition.

 

[duke]

I have an OM 636 about the same age and I have had the same symptoms,
exactly as you describe.
Unfortunately my problems developed further.
See a recent thread on this forum- Diesel engine Diagnosis?

I have had enough - and am in the process of purchasing a Nanni 4.193 HE ( 40 to 43 hp, depending on who you talk to). I considered a similar sized Beta- I understand they are basically the same engine-
my decision was based really on who (local stockist) I was comfortable with, fitting it.

Also it seemed to me that it was inevitable that I would need a new engine in the short to medium term - so I might as well enjoy the use of it - the boat when sold
( whenever that is ) - is going to reflect the engine either way.

Duke.

<P ID="edit"><FONT SIZE=-1>Edited by duke on 13/08/2002 15:29 (server time).</FONT></P>

 

[philip_stevens]

Vyv,
I get your point about the crankshaft drillings, they are very small. What I don't think it is, are the piston rings, as he doesn't say he is burning oil or mention any blue smoke from the exhaust.

I would also think that it is not the relief valve as the oil is then hot, and he has a lower pressure - then again, the relief valve could be stuck/jammed open slightly or the spring has got weak or broken. Perhaps, yes, it is the relief valve, but in a stuck slightly open position.

Also, if it is the valve stems worn, he would get blue smoke from the exhaust. Again, he does not mention using/topping up the oil.

It is a conundrum though, and I hope he keeps us informed (if he decides to have it stripped down) as to what the fault is/was.

regards,
Philip

 

[david_steward]

Re: You Must...

Get the oil pressure checked with an independent mechanical guage. I have just had my Volvos checked coz the oil pressure seemed to have dropped. Both guages were reading lower than the true pressure. One guage was showing 20 psi under!

After that hope it is the bypass valve, anything else will be expensive'






Please insert your smart ass pretentious tag line here...


Dave S

 

[Guest]

thanks everyone: I do have to top up the oil a little, so far a 5 l tin of oil for older engines has lasted since april, the exhaust is a little smokey which coats the transome but not worse than many other boats, does this help?

 

[chippie]

I think there the previous posters have covered the possibilities, I had similar symptoms on a car and it proved to be a blocked oil filter coupled with a worn oil pump, although I couldnt figure why the pressure was ok at startup.(cold oil perhaps.) The filter problem was easily replaced and the oilpump problem was overcome by using Moreys oil booster additive. This may be a cheaper way of postponing the eventual repair/replacement.
Good luck

<P ID="edit"><FONT SIZE=-1>Edited by chippie on 14/08/2002 00:02 (server time).</FONT></P>

 

[andy_wilson]

Insider information.

Get your oil analysed.

It is quite likely to be crankshaft bearing related but oil analysis will prety much confirm this, and possibly a multitude of other maladies.

I use the same lab the RNLI use, Finnings (Caterpillar agent) based in Leeds. Directory enquiries will sort out the number for you, but if you can't be bothered to ring 192 send me a PM and I'll dig out my last invoice.

A test kit is under a tenner, they will swap your credit card details for a sample bottle, tube and post paid jiffy bag. Draw off a hot and well stirred sample (don't suck too hard) and send it to them along with details of engine, oil brand and grade used (and yours is using a bit of oil by the sound of it) and type of cooling (raw water engines tend to run cooler than closed circuit).

They used to send results within about a week but I understand they are now posted on the web for your inspection. Progress you see, many fleet / plant operators wait to see the analysis before they decide whether an oil change is due.

The analysis shows about 20 different contaminants and interprets them in combination to suggest where the problem may lie.

The usual scheme is to have two samples. First use flushing agent (crank-case cleaner or whatever) and fill with fresh oil of the correct grade. Sample after about 10 hours. Send the second sample after about 50 hours. They log your details and relate the second analysis to the first to plot the progress of your motors demise!

 

[vyv_cox]

That's a slightly different version of your original "uses a little oil" posting. 5 litres in 5 months sounds like quite a lot to me, depends on hours run, of course. My Yanmar has used 1 litre in 100 hours this year, although it is barely run in to date and I expect this figure to reduce.

 

[Avocet]

It just sounds like a tired old engine. I've got one of them in my car! If you don't want to spend money, you could try thicker oil or a viscosity enhancer (Halfords sell all sorts of patent treacly "goop" to add to your oil to thicken it. This will help you eke out a few hundred more hours out of the poor old thing. You could also try fitting an oil cooler to stop the correct grade thinning out quite so much but sooner or later you will end up having to do a "botton end" re-build. If you're going to that length on an engine that old it would probably be as well to do the whole thing - bores, piston rings etc. Depends how much you want to keep the engine really.