low oil pressure Bukh DV20

[neil_s]

I have owned my Seal 28 with it's DV20 for nearly 24 years, now. When I got it, the engine ran well enough, but was obviously needing some attention. Lots of fuel leaks and a build up of yellow goo - emulsified oil - in the rocker box. After several years putting up with it, I took off the cylinder head and miked up the bores - within spec. I pulled out the pistons and found the compression rings well worn, so I replaced all the rings and also fitted new big end shells. I also found the oil way to the rocker box was blocked so fixed that too. Reassembled, the engine ran well, with no more goo in the rocker box. Over the years, I've also fixed the fuel leaks so all is well - except the oil pressure warning light comes on when the engine is idling. I accepted this, being used to old BMC cars! In my spares box I had a small pressure gauge from one of those Coventry Victor twin petrol engines so this year I decided to fit it to the DV20 to see what the actual oil pressure was. Bukh specify a minimum of 14 psi and a running range of 28 - 57 psi. I found that, when cold, my motor idles with a little less than 20 psi and runs somewhere between 30 and 40 psi. When hot, the idling pressure is about 5 psi and running pressure 20 psi. There is no knocking or other unwanted noises from the engine. In a way I wish I hadn't fitted the oil pressure gauge! Any ideas? The oil pump is easy to get at on the DV20 - would it be worth replacing it? I'm using Smith and Allen mixed fleet ultra 15W-40 oil. Would straight SAE30 (as specified by Bukh) help?

 

[Beneteau381]

I have owned my Seal 28 with it's DV20 for nearly 24 years, now. When I got it, the engine ran well enough, but was obviously needing some attention. Lots of fuel leaks and a build up of yellow goo - emulsified oil - in the rocker box. After several years putting up with it, I took off the cylinder head and miked up the bores - within spec. I pulled out the pistons and found the compression rings well worn, so I replaced all the rings and also fitted new big end shells. I also found the oil way to the rocker box was blocked so fixed that too. Reassembled, the engine ran well, with no more goo in the rocker box. Over the years, I've also fixed the fuel leaks so all is well - except the oil pressure warning light comes on when the engine is idling. I accepted this, being used to old BMC cars! In my spares box I had a small pressure gauge from one of those Coventry Victor twin petrol engines so this year I decided to fit it to the DV20 to see what the actual oil pressure was. Bukh specify a minimum of 14 psi and a running range of 28 - 57 psi. I found that, when cold, my motor idles with a little less than 20 psi and runs somewhere between 30 and 40 psi. When hot, the idling pressure is about 5 psi and running pressure 20 psi. There is no knocking or other unwanted noises from the engine. In a way I wish I hadn't fitted the oil pressure gauge! Any ideas? The oil pump is easy to get at on the DV20 - would it be worth replacing it? I'm using Smith and Allen mixed fleet ultra 15W-40 oil. Would straight SAE30 (as specified by Bukh) help?

Sounds as if there is a bit of wear but not bad. I doubt changing the pump will do much. When you change the multigrade, does the oil pressure go up? If so then it’s oil related. Straight 30 I suspect will give a larger difference in pressures from cold to hot.

 

[rotrax]

Relief valve spring become weak, or stuck?

Probably more likely than a worn pump.

 

[Beneteau381]

Used to happen on old mini engines

 

[neil_s]

Thanks for that tip, rotrax - an easy check. Maybe the plunger is not seating properly.

 

[neil_s]

Used to happen on old mini engines

Mine was a Morris Oxford!

 

[Poignard]

 

[neil_s]

Well done. Poignard, thank you very much for that detail drawing. It's not in my manual. Off to the boat on Tuesday!

 

[Poignard]

Well done. Poignard, thank you very much for that detail drawing. It's not in my manual. Off to the boat on Tuesday!

I have a PDF of the workshop manual. If you want, I can send you a copy.

 

[neil_s]

I have a PDF of the workshop manual. If you want, I can send you a copy.

Thank you! I have found an updated version of the manual but I suspect my motor is an earlier version. It is 1977 vintage with the BW6 gearbox. I will check tomorrow. There is an oil pressure relief valve in the position shown so I will open it up and give it a clean out and stretch the spring a little.

 

[Boater Sam]

Don't stretch the spring if its free length is correct.

 

[neil_s]

This turned out more of a job than I expected. I had to take off the alternator and starter motor before I could get at the pressure relief valve. The spring was 44 mm long. Bukh specify 46 +/- 1. I stretched it to 47. The cold run pressure is now 50 psi, up from 35 but hot run is still 20 or less.

 

[earlybird]

Are you confident that the pressure gauge that you fitted is reasonably accurate?

 

[neil_s]

Are you confident that the pressure gauge that you fitted is reasonably accurate?

It is a very old 0 - 100 psi bourdon tube gauge so not the best for measuring low pressures. I let the motor idle and slowly increased the revs until the oil pressure warning light went out. This happened with the pressure gauge indicating somewhere between 10 and 15 psi. This agrees with Bukh's spec. for the minimum allowable oil pressure. So I think my readings are in the right ball park!

 

[rotrax]

What oil are you using? On older, low tech engines viscosity can make a substantial difference.

On a delivery trip with a 2,000 hour Yanmar 3 cylinder engine that ran perfectly, we used 6 litres of oil in 20 hours.

We used the oil supplied, Mercruiser 10/30 stuff. I changed it to Morris 20/50, oil pressure improved when hot, barely used any subsequently.

Kept that boat 8 years, never had any need to top up between changes using the 20/50.

 

[neil_s]

What oil are you using? On older, low tech engines viscosity can make a substantial difference.

On a delivery trip with a 2,000 hour Yanmar 3 cylinder engine that ran perfectly, we used 6 litres of oil in 20 hours.

We used the oil supplied, Mercruiser 10/30 stuff. I changed it to Morris 20/50, oil pressure improved when hot, barely used any subsequently.

Kept that boat 8 years, never had any need to top up between changes using the 20/50.

I'm using Smith and Allen Ultra15W-40 mixed fleet. API CG-4/CF-4/SJ ACEA A3/B4 (2012). The motor doesn't appear to use it up, but I sail whenever I can so the motor doesn't get used very much.