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Thornycroft 90 Oil pressure a bit low


Ratkatcher

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One for the gurus...

The oil pressure on my engine only gets to around 40 psi when cold and drops to between 20 and 30 when it reaches normal operating temperature (70c when just poodling along and around 90c if sustained high speed  - about 4 mph...)

Engine revs at normal speed are only around 700 RPM, higher speeds around 1000/1100 RPM.  (I think the Hurth gearbox has a 1.5:1 ratio)

Engine oil is about 100 hours old and will be replaced (and the filter) in the very near future.

 

Can anyone suggest where I might start looking to solve this please?  The engine 'meter' shows around 8500 hours but the previous owners appear to have been meticulous about servicing so I don't believe it had been abused over time.

Thanks in advance.

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The BMC 1.8 manual: https://www.asap-supplies.com/media/faq/132000-bmc-1.8-manual.pdf

Gives 50psi 'running' and 15psi idling. Now, I'm guessing their definition of 'running' is going to be a lot faster than your engine speed when boating

Basically, your oil pressure doesn't sound low enough to worry about. 

Richard

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When I bought my first narrowboat 6 years ago it had a BMC 1.5 engine and I decided to get it serviced before I picked it up.

 

On the service report it stated that the oil pressure was low and to fix this they had to use SAE 30 oil.

 

I enquired about this on here as I was a bit concerned and was told that there is a bit of debate about this but SAE 30 is the correct engine oil for this engine.

 

So you may not have the correct engine oil which is causing the low pressures ?

 

 

Edited by Bloomsberry
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15 minutes ago, Ratkatcher said:

Thanks, Richard.

She seems to run ok, but reading about others with much higher oil pressure I worried just a little.

 

David

Ahh, the perils of engine instrumentation. When you should be cruising and enjoying the canals, there you are worrying about your oil pressure

I wonder how accurate your pressure gauge is?

Seriously, it's OK

Richard

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Trad stern, Richard - all of the guages are just the right height to attract attention :P

That and as I am still really a 'newbie' with only about 100 hours cruising so far I am still getting used to how Tich behaves.

Worry level is lower thanks to your comment and 20w50 was going in next time anyway, if the temperature is below -10c outside I'll light a tea-light under the sump -perfumed of course!

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Could be the oil pressure relief valve not seating properly, sticking or dirt under the seat. This was quite common on the BMC A and B series petrol engines as well as the diesels. Quite easy to get at.

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1 minute ago, bizzard said:

Could be the oil pressure relief valve not seating properly, sticking or dirt under the seat. This was quite common on the BMC A and B series petrol engines as well as the diesels. Quite easy to get at.

It's still not drastically low though, is it Biz?

Richard

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Just now, RLWP said:

It's still not drastically low though, is it Biz?

Richard

Oh no Richard, its pretty normal init. A relief valve not seating even by a tiny bit would reduce the oil pressure quite a lot, possibly almost to zero if its stuck well off its seat, like if it sticks soon after start up when the oils cold and the pressures high and the valve is open relieving the pressure and it sticks and stays open in that position even though the engine is up to temp and the oil warm and thinner, pressure plummets.

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I don't think you have a major problem but.....

1) The 15W/40 oil could usefully be replaced by 20W/50, as already suggested, particularly in view of the 8,500 engine hours, which may mean clearances are no longer the tightest.

2) As an aside, if it is a thermostat controlled engine with a skin tank, then maximum temperature should be the thermostat temperature, or very close to.  If that is indicated as 70 degrees C when "pootling", it is probably a 74 degree stat in there.  If so temperatures should not rise to 90 degress when you go a lot faster.  It tends to indicate your skin tank is smaller than ideal.  Probably not affecting oil pressure greatly, but if you are doing sustained work on rivers, keep an eye on it.

3) I would agree it is as likely to be the instrumentation giving a low reading, as the engine having a low oil pressure.

16 minutes ago, Mike the Boilerman said:

In my experience provided there is 'some' oil pressure, even 5psi, an engine will usually continue to run perfectly well indefinitely, or until something completely unrelated to low oil pressure kills it.

Whilst it is fairly standard to hear that about Listers, I would not want to be running a BMC at cruising revs with only 5 psi on the dial!  However I don't think OP indicated what he is getting on tick-over when hot, did he, which would be useful to know!

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3 hours ago, alan_fincher said:

 

Whilst it is fairly standard to hear that about Listers, I would not want to be running a BMC at cruising revs with only 5 psi on the dial!  However I don't think OP indicated what he is getting on tick-over when hot, did he, which would be useful to know!

 

Nor would I, but when the choice is take it out for a rebuild now, or carry on using it for another ten years, I'd choose the latter.

Provided the bypass valve being stuck open has been ruled out, I'd just carry on using it, knowing it is on borrowed time. Quite possibly a LOT of borrowed time...

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8 hours ago, Mike the Boilerman said:

 

Nor would I, but when the choice is take it out for a rebuild now, or carry on using it for another ten years, I'd choose the latter.

Provided the bypass valve being stuck open has been ruled out, I'd just carry on using it, knowing it is on borrowed time. Quite possibly a LOT of borrowed time...

I know exactly what you mean but a bypass valve stuck open will not alter the oil pressure although the blocked filter that caused it to open may cause it to rise to a small degree. Its the oil pressure relief valve that will cause the pressure to drop when stuck open

This post is really for the less knowledgeable members.

The vast majority of our engines use what is known as a full flow filter that means all the oil flowing to the engine goes through the filter. This is good for engine life UNTIL the filter clogs so to prevent very fast wear and damage all the oil filters or their housings have a bypass or pressure differential valve built in. This opens when the filter clogs to allow unfiltered oil to flow around the engine. You will get no indication of this happening so it is vital to observe the correct oil & filter change intervals.

Spin on filters also normally have another valve built in that prevents then draining themselves into the sump when the engine stops, otherwise for  a few seconds after starting the lubrication will be compromised.

The quality of these valve in spin on filters is important and is why one should not try to save money by buying cheap filters from unknown sources.

 

Edited by Tony Brooks
corrected a lot of mistypes & missing words
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Thanks Tony. I was being lazy using the term 'bypass valve' instead of typing out 'oil pressure relief valve', having seen the two terms used interchangeably on here in the past.

I hadn't actually realised there was a genuine bypass valve inside the oil filter housing or I'd have typed 'oil pressure relief valve'. It's obvious now you say so that there must be one!

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Thanks very much for the suggestions and advice, they are much appreciated.

When I go back to the boat (only had a couple of days there last week) I'll take note of the idle oil pressure when the engine is hot and post it.

Thanks for the information about coolant temperature - Tich has 'keel cooling'  so this may not be working at its most efficient, likely to be compromised further when she is overplated later this year I imagine! I shall ask Martin Kedion for advice at that time.

Tony, there is a Crossland oil filter fitted and I have a replacement of the same type waiting to be spun on when I change the engine oil.

Where might I find the relief valve?  The service manual for BMC 1500 shows a picture of 2 nuts indicating which is the one to undo but as the illustration give no indiocation where to look for this it is a little less than helpful.  (maybe I shold just crawl around in the engine 'ole until I notice something similar!)

Again thanks for everything so far to everyone.

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3 hours ago, Ratkatcher said:

Thanks very much for the suggestions and advice, they are much appreciated.

When I go back to the boat (only had a couple of days there last week) I'll take note of the idle oil pressure when the engine is hot and post it.

Thanks for the information about coolant temperature - Tich has 'keel cooling'  so this may not be working at its most efficient, likely to be compromised further when she is overplated later this year I imagine! I shall ask Martin Kedion for advice at that time.

Tony, there is a Crossland oil filter fitted and I have a replacement of the same type waiting to be spun on when I change the engine oil.

Where might I find the relief valve?  The service manual for BMC 1500 shows a picture of 2 nuts indicating which is the one to undo but as the illustration give no indiocation where to look for this it is a little less than helpful.  (maybe I shold just crawl around in the engine 'ole until I notice something similar!)

Again thanks for everything so far to everyone.

 

Personally I would now avoid Crossland filter after the takeover but still use the new one you have.

I THINK the relief valve is fairly low down on the bock on the right hand (looking forwards) side. Possibly just in front of the back plate. I have not got a copy of the manual on the boat.

 

 

 

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  • 3 weeks later...

As promised...

Oil pressure when hot and at idle is around 15 PSI.

When cruising at around 900 RPM it is around 20 PSI when hot.

It is an electrical pressure gauge.

 

Oh, and I lied about the new oil filter - it isn't Crossland, a mild senior moment as there is a Crossland sticker stuck onto the control console which I have yet to scrape off, sorry!

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  • 3 months later...

Just for closure, after all of the help previously.

I did a service before taking the boat to Kedian for overplating etc. and used 20w50 Comma oil instead of 5w40.

Pressure was still variable on the outward trip as coolant temperature varied on occasion :o

Since the re-launch with a new skin tank (and getting the air out from the calorifier) coolant temperature has been nailed in the mid 70s and oil pressure sitting mid guage (about 35 psi) while running, at tickover while warm it drops to around 25 psi so all appears to be fine.

The return journey was much more enjoyable then the previous outings as it got to the stage where only occasional glimpses at the guages were given as they didn't seem to move much.  Most excitement coming back to the mooring was provided by copious amounts of leaves lurking in the water with a desperate urge to wrap themselved around the propellor...

That and the biting insects... (I have a very sick dog who was bitten on the neck last Monday and has scratched so much the bite was egg sized when we got back Thrusday - a trip to the vet in the evening meant a couple of injections and antibiotic tablets for her, another trip tonight as she is still pretty poorly!)

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On 10/23/2017 at 15:05, Ratkatcher said:

Since the re-launch with a new skin tank (and getting the air out from the calorifier) coolant temperature has been nailed in the mid 70s and oil pressure sitting mid guage (about 35 psi) while running, at tickover while warm it drops to around 25 psi so all appears to be fine.

I would have expected a bigger difference between tickover and a much higher running speed. 25psi at tickover on a hot engine is pretty good.

I would be tempted to look at the pressure relief valve. However, while it is easy to get out, it's not so easy to put back in.

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I had some pretty erratic oil pressures on my BMC 1.8 last year. After going down the new sender followed by new gauge path without success I borrowed a mechanical gauge and confirmed the fluctuations. Pulled the pressure relief valve and spring out and replaced both. Problem solved.

 

Frank 

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