Topic: Low oil pressure when cold, good when hot??

[markreimer]

The plot thickens. I started the bike this morning, with temperatures just above freezing.

The pressure initially spiked, and then quickly fell low. Very low. The oil light came on for a second. Blipping the throttle returned pressure to normal startup highs. Then it would slowly sink down again.


Since I had normal pressure last night when it was a fair bit warmer, I have to conclude this 15w-40 oil is too thick for near freezing temperatures.

You can see in the video that the pump is trying to build pressure. Near the end when I blip the throttle the pressure follows. But at idle it just can't suck the oil through properly. I'm guessing it's getting caught up at the relief valve when it's really thick and cold. I dropped the oil pan last night to make sure the screen was clean and replaced the oil and filter too. I recently removed and cleaned the relief valve in the pump as well.

I guess I'll change back to 10w-40 and see if it changes.




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[70CB750]

Refresh my memory, did you rebuild your oil pump already?

I ride in freezing temperature - my winter oil is 15w40, I never use thinner oil than that - and it never behaves like this.

[markreimer]

No, not really, but I did rebuild my engine using a donor bottom end. My main oil galley got a crack in my motor so I took my almost new top end and plunked it onto a used bottom end. I was having trouble priming my oil pump during initial startup, so I removed that relief valve (the plate with two screws, with a long spring and small piston behind it), cleaned it up (thought it wasn't dirty) put some oil in the pan and bolted it all back up. I immediately had pressure and never had an oil light issue till it got cold.


So it could presumably be the pump as I don't know anything about it's history. I could remove my original pump and bolt it into my donor bottom end as a comparison as well. But the fact that my oil pressure readings seem perfect during only slightly warmer temps makes it seem like it's working fine?

I've also never used Rotella 15W-40 in my 750 before. I've always used Castrol 10W-40. So I have two 'new' factors at play here...new oil, new pump.

[70CB750]

Do you mean that you replaced the top half of the engine?

That is generally bad idea, since all the holes for bearings are machined with top and bottom clamped together.

[markreimer]

No, not the top half of the bottom end, I mean I took only my cylinders and head (which had just been redone) and put them onto a complete bottom end. I know you can't mix and match top/bottom cases for exactly the reason you specified  Sorry I probably wasn't that clear.

[Davez134]

I don't think its a problem with your pump. Mine started behaving the same way when cooler. I did swap pumps, the last one being a reconditioned one offered by cycleX with no changes. My pressure is 80psi on startup, then around 20 when warm. However when cold it will momentarily drop, come back up, then the needle will flutter. I will take a video so you don't feel alone!

[70CB750]

No, not the top half of the bottom end, I mean I took only my cylinders and head (which had just been redone) and put them onto a complete bottom end. I know you can't mix and match top/bottom cases for exactly the reason you specified  Sorry I probably wasn't that clear.

My bad. Here i thought we are onto something

[strynboen]

is it the over pressure/safty valve ..vho opens and bypass the oil filter!!!!!!!!..and give a dramatic pressure lost for some few sekunds

[rotortiller]

I'd look at the relief valve in the oil pump. The oil filter bypass feature should not cause the issue you are having. There could be a burr or something that sticks the oil pump relief valve piston from returning immediately when cold and contracted. I'd do the cold check and make sure the engine rpm is kept low enough below 40-50 psi to see if it happens again. If it does not happen you have your answer.

[markreimer]

I'd look at the relief valve in the oil pump. The oil filter bypass feature should not cause the issue you are having. There could be a burr or something that sticks the oil pump relief valve piston from returning immediately when cold and contracted. I'd do the cold check and make sure the engine rpm is kept low enough below 40-50 psi to see if it happens again. If it does not happen you have your answer.

Not sure I understand how to do the cold check you referred to? Could you explain again?

I fired it up again when I got home, about 10C out. I didn't touch the throttle, just had t idling around 1800 to warm up. This time the pressure jumped to about 70 at startup as usual, then waved sporadically between 40-60 for maybe 30 seconds, then slowly dropped down to near zero for a few seconds, then jumped up and stabilized at 60. It seems once the engine has run for about 1.5 - 2 minutes the pressure stabilizes at around 60-70 and doesn't change unless rpm increases or I ride long enough to heat the oil and reduce pressure at idle.

I did remove the two-bolt plate on the oil pump, pull out the long spring and the piston behind it, it seemed very clean at the time. Easy enough to check again though. Is that the thing you're talking about?


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[markreimer]

I see from this diagram i didn't take the relief valve (4) off to clean, but the leak stopper valve (3). I'll drop the pan ad check the relief valve!




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[seanbarney41]

seems like you have discovered something new and you are teaching us

[markreimer]

So I had plans to drop the pump and have a look at the relief valve today, but we're having a freak November heat wave this weekend. 18C!! Everyone is out riding, it's not really a day for wrenching... I went to fire up the bike and the pressure jumped to 70 and stayed there. No wavering, no dripping down to zero, nothing. It worked exactly as it should. That says to me that it likely is not a dirty relief piston, as that would hold up pressure no matter what, right?

I'll still remove and clean it just to be safe but I thought this was interesting, the biggest pressure difference yet due to temperature.




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[markreimer]

Been a while but I finally had a chance to rebuild my oil pump. I bought Elan's kit and his screen. It's all top quality stuff.

Before I drained the oil I checked the level in the tank. To my surprise most of it had drained down into the pan. Never had that happen before. Perhaps that is a sign the seals were on the way out.

Getting the pump apart was a little challenging but nothing my impact driver couldn't handle.

Inside I didn't find any obvious signs of blockage, but the seals were hard and dry. I replaced all the seals and springs, then went to the rotors

My rotors have some bad scarring on the faces. Seems like many of these pumps look like this. I measured the clearance and they seem to still be in spec so I'm gonna reuse them. My pressure gauge tells me I'm still getting normal results at the engine so...

The shaft seal was a PAIN to get out, and my pick slipped, scarring the pump housing. Hopefully that doesn't cause problems...??

Anyway, I primed it and put it back in and fired the bike up. It was 9C inside the shop. I had 65psi of pressure right away and zero drops like I had in fall. So far it seems to be a success. I'll have to wait to try it in colder temperatures as it was mostly an issue around 5C and colder.











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[PeWe]

No problem if all oil is in the engine, very little in the oil tank. My bike had such oil pump from the day I bought it with 17000 km, 2 years old. I guess that my oil pump was like that from the very beginning from factory.
I was shocked one day first week owning it with almost all oil gone, called a friend and told me that's typical CB750, just start it and check when warm.

I replaced the pump with a used one with half milage (only 45.000km) last winter at just over 100.000km. The new pump got Elans kit. With that pump, oil will stay in tank.
I see new "problems" with that. I had to remember to fill oil direct in the engine so it could suck oil from pan at first start-up after rebuild.
When I changed oil first time, I got a severe leak when the tank had over 1.5L, my bottle flowed over and oil on the floor.

I have new procedure changing oil with warm engine. Remove bottom plug and let engine run for a few seconds to let all oil from tank go out.
EDIT: I had never oil pressure isssues with the old pump. Oil pressure lamp could flicker a little when engine was really warm and idle around 1000rpm or slightly lower.

[PeWe]

Another pump might be better to use with Elans kit? The rotors look bad on the photos, my old pump had not such ugly rotors.
Better to be safe since oil pressure issues will cost a lot.

[markreimer]

Another pump might be better to use with Elans kit? The rotors look bad on the photos, my old pump had not such ugly rotors.
Better to be safe since oil pressure issues will cost a lot.

Yeah probably right. I have a second pump on hand actually, but haven't taken it apart yet. Next time i change the oil I'll be sure to have looked at the second pump, since that would be a real easy point to switch them around. The rest of this pump is good, so I could just drop the other rotors into this pump body.


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