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

[markreimer]

Hi there,

Just finished rebuilding my '74 750K engine. I've taken it on a few rides, everything is excellent.

This morning it was about 3 degress Celcius (37F). Bike fired up without much trouble, and I drove it for a few blocks when the oil light came on at a stop light. I blipped the throttle and it went out by about 1,500 RPM.

A few blocks later the same thing happened. Always out by 1,500RPM. It happened three times in the first few blocks of my ride, then never again. When I arrived at work 4 miles later, it stayed off even at idle, which is around 1150 RPM.

I thought cold weather made oil pressure go up, and hot engines made the pressure drop.

When I rebuilt the motor, I removed the pressure valve and cleaned the stopper, inspected the spring, but didn't replace the o-ring as I didn't have one at the time.

I'm ordering a pressure gauge for the right side galley plug and will look around for an oil pump rebuild kit as winter is a week away.

Anything else I should look for?

[markreimer]

I'm running Shell Rotella T 15w-40 by the way

[calj737]

In my opinion thats pretty heavy oil for cooler temps. It would take some engine heat to get that oil flowing well to make appropriate pressure based upon the information I reference when selecting oils.

[markreimer]

Yeah, I was kinda thinking the same thing. This is the first time I've run Rotella in the 750, I've always used Castrol 10/40. And the first few rides I did, post-rebuild, temperatures were closer to lower 50F. I wanted to do an oil flush soon after the build so maybe I'll switch back to the 10w-40 and see what changes.

[calj737]

Personally (not trying to create an oil thread) I would not use heavier than 5w-40. The leading number is for temperature protection. Unless you plan to run in extremely cold temps, then 5w is good. And given its an air cooled engine, the temperature protection (cold weather) is less relevant and theres no water to protect from freezing.

But you may see heaps of dissenting opinions, so I'll just leave all these as my opinion only and my choices. 

[70CB750]

15W40 is what I run in winter - VA it means about 0C mornings.

Nothing wrong with your oil choice.

[markreimer]

When I did the engine work, I used a donor bottom-end as my old one had a cracked case. The bottom end was sitting for an unknown amount of time, but for sure at least five years. Perhaps the oil pump seals dried out and/or the release valve is still sticking. I had to clean it up to get initial oil pressure. I could take my old oil pump from my cracked engine and put it in the bike as it's known to be good. That would be an easy enough job for comparison too.

[markreimer]

Also while it was sitting, there was no oil in the pan/pump, so it was sitting dry

[rotortiller]

Might be thick cold congealing in the switch area would be my guess. Once the block gets warm it heats up that area.

[70CB750]

I see you are getting oil pressure, thats good.  Better to know actual psi numbers for hot and cold.  My K0 and my K3F all idle at 20psi when fully warmed.  Running highways in hot weather made the K0 idle at 10psi and even the oil light would come up  - with 20w50 oil.
Also both bikes idle 50 or even over 60 psi when cold, depends on how cold.

If you have doubts about the oil pump, get the kit from Elan (member here) and rebuild the pump when you have it out of the bike.

[Deltarider]

Nothing wrong with your oil choice. Before you dig in too far, I'd start with the easiest. You must know that the oil switches on our Honda's are known to become defect sooner or later.

[firebane]

15w40 is too heavy for cooler temps unless your bike is stored inside. At a minimum I'd be running 10w40 or even 5w40 and when your back to warm temps run a heavier oil again like 15w.

The 15w in cold temps can become almost syrup like and not flow very well. I ran into this last year when I had some oil sitting in my shop and it was getting down to the zero temps.

[markreimer]

Nothing wrong with your oil choice. Before you dig in too far, I'd start with the easiest. You must know that the oil switches on our Honda's are known to become defect sooner or later.

BOY do I EVER know that hahaha....last year when I rebuilt the top end of my bike, I had a faulty oil switch - it would flicker on and off at all times. I removed it, cleaned it, problem went away for several weeks. Then it came back, so I bought a new switch....over torqued it during installation....and cracked my engine case. That's what prompted my latest engine work!

I couldn't find a pressure gauge locally so I ordered one online this morning. Should be here within a week or so. I'm curious to finally know what numbers I'm getting.

I saw the kit from Elan, looks real good. I could pick that up and rebuilt the pump from my old motor, then just do a simple swap. Something to consider for sure. 

[Davez134]

I would bet this is what is happening... When cold the oil is much thicker. I'm sure the pump is fine, creating adequate pressure. It pumps the oil up through the engine up to the head/cam. Since the oil is cold and thick, the pump is pumping it up faster to the motor than it will flow back down passively back into the pan. Then, there is not enough oil in the pan pumped up to adequately fill the oil tank to the top of the drain tube in the bottom. The pump cavitates momentarily causing you to lose pressure. Do this, next time you start the bike cold: have a look in the oil tank with a light and see if you can see this happening. You will be able to see the top of the tube.
As the bike warms up, the oil flows faster down to the pan, and the problem goes away. (This may have happened to me, and I just let the bike warm up for a while in cold temps) BTW, I have a pressure gauge, and on startup my pressure is 80, and idles warm at about 20psi.

[markreimer]

Well I tried your suggestion, started the bike and looked in the tank. I could see it slowly turning around. Keep in mind it's now 11C vs 3C this morning. The oil light shut off immediately after starting, but came back on a minute later around 1100 rpm. Disappeared with a blip of the throttle and never came on again. I let the bike idle for two min to warm up. Then rode the short distance home in stop and go traffic with lots of idling. Never came on again. So it sure seems like it's temperature related.

Hard to say much more about this until the pressure gauge arrives or I try a lower weight oil


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[pjlogue]

In addition to the above mentioned inherent oil pressure switch problems the switch is grounded via the threads where it screws into the case.  If these get corroded it can cause problems.  Be sure the threads are clean so it has a good ground.

-P.

[70CB750]

15w40 is too heavy for cooler temps unless your bike is stored inside. At a minimum I'd be running 10w40 or even 5w40 and when your back to warm temps run a heavier oil again like 15w.

The 15w in cold temps can become almost syrup like and not flow very well. I ran into this last year when I had some oil sitting in my shop and it was getting down to the zero temps.

He mentioned 0C thats 32 F. Far from 0F.  10w40  or 5w40 is too light for cb750 no matter what temperature.

[Dunk]

By all means could be a flakey switch, so check with a mechanical gauge. Thicker oil can give lower pressure when cold, but I've only this on very cold days (single digit F*) on bigger engines. Where the engine is question would normally peg on the relief on a cold start it took a few seconds to get pressure then rose slowly and not as high as normal. When revving the engine much above idle with no load oil pressure would drop off. As it warmed it hit the relief (75 psi or so) then began dropping as more temperature got in it and acted normal when I drove off from there. I kept the engine warm all day and changed to 5w40 that night. I doubt that's happening with your 15w oil at above freezing temps, and if it's cold enough that it's really that thick pressure would drop with RPM rather than drop only at idle.

Either way, let the thing idle enough to get some temp in the head and oil before riding off when it's cold out. The oil restrictors to the head for the cam are very small and even if oil is flowing to the mains and rods it may not be getting up top until there's some temperature in it. FWIW I've been running 5w40 in my bikes lately. I notice it doesn't drag the RPM down so much at idle on a cold start in cold weather and the oil light goes out faster.

[firebane]

15w40 is too heavy for cooler temps unless your bike is stored inside. At a minimum I'd be running 10w40 or even 5w40 and when your back to warm temps run a heavier oil again like 15w.

The 15w in cold temps can become almost syrup like and not flow very well. I ran into this last year when I had some oil sitting in my shop and it was getting down to the zero temps.

He mentioned 0C thats 32 F. Far from 0F.  10w40  or 5w40 is too light for cb750 no matter what temperature.

They said 3 degrees celsius. Being Canadian myself 3 degrees celsius is not exactly warm

[turkey4me]

I have an extra pressure switch if you need one.

[markreimer]

Thanks for the advice and input everyone. I rode the bike four times yesterday throughout the day:

1) first thing in the morning about 3C, oil light came on three times

2) 5pm, 11C, oil light came on for a split second shortly after it went out after starting

3)6:45pm, oil light never came on while riding. Possibly the oil was still warm from my earlier ride

4)8:30pm, oil light never came on.

I checked the wiring, everything seems clean and snug and operational.

Oil pressure gauge on the way...will report some numbers at that time.

[Davez134]

Mine only does it first thing in the morning when cold too. Stops when warm, or even after sitting for 8-10 hours between. Just curious, what is your oil level in the tank after a long ride?

[70CB750]

Oil pressure sender is ~ 7psi, really curious what your numbers will be.

Thanks for the advice and input everyone. I rode the bike four times yesterday throughout the day:

1) first thing in the morning about 3C, oil light came on three times

2) 5pm, 11C, oil light came on for a split second shortly after it went out after starting

3)6:45pm, oil light never came on while riding. Possibly the oil was still warm from my earlier ride

4)8:30pm, oil light never came on.

I checked the wiring, everything seems clean and snug and operational.

Oil pressure gauge on the way...will report some numbers at that time.

[PeWe]

My oil lamp can also flicker at 1100 rpm in warmer ambient temp  like +15-20C
Increase idle a little to 1200 or until oil lamp will be not lit. Higher is better and ensure oil to the head.

Edit: My CB750 K6 engine had new o-rings all the way to ensure the pressure, Elan oil pump refurbish kit too. Next improvement is new oil pump rotors?
I have no problem with idle around 1200-1400 rpm.

I let my bike warm up 1-3 minutes around 2000 rpm, then down to 1200-1400...

[markreimer]

Thanks for the tip. Looks like the gauge will be here on Monday...




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