Topic: 1978 CB750K — Post 836 OIL PRESSURE - Seems bad :(

[PeWe]

I have seen oil pressure warning blinking when idling a few minutes, bike on side stand.

Opened the oil cap and saw very low oil level, almost nothing. Pump filled a little that engine needed direct.

The angle of bike was not good for oil pump to fill tank with oil.

After that, no more warming up parked on side stand.

Happened both my CB750. 

[HondaMan]

I have seen oil pressure warning blinking when idling a few minutes, bike on side stand.

Opened the oil cap and saw very low oil level, almost nothing. Pump filled a little that engine needed direct.

The angle of bike was not good for oil pump to fill tank with oil.

After that, no more warming up parked on side stand.

Happened both my CB750. 

I've seen that happen when the seal between the 2 halves of the pump is getting hard and the engine is cold. After it warms up the oil returns faster to the tank from the sump: I'm imagining the seal gets more flexible with the heat, so the suction side of the pump gets more efficient(?). Replacing the seal(s) in the pump makes it stop doing that, which was how I pondered up this diagnosis. 

[SKTP]

Thanks guys...I will be replacing the pump very soon.

[SKTP]

Life has been busy...I have the new/rebult oil pump from Cycle-X and I am installing a 2002 Ducati Monster oil cooler...those are the Ducati lines for mock-up.

I plan on ordering the new lines next week. Need to get back on the road!

I think I can custom make a bracket that will allow the horn to still fit, turned 90 degrees

[SKTP]

Almost there!

[SKTP]

On Thursday night my friend and I swapped in the Cycle-X oil pump and tightened down all the new oil-cooler lines and started the bike for the first time since all of this oil-cooler stuff began. The bike fired right up and the oil pressure read 60lbs and stayed steady. There had been an issue with the pressure dropping and and then going back up while warming up. I was hoping that this new oil pump would solve that issue. We finished the pump install and oil-cooler installation fairly late, so we were content with is just running, seemingly really well, on the center stand. We could see the oil returning strongly through the oil tank hole and the pressure gauge read 60lbs steady.

So today I finally had time to test drive the bike. I checked the dip stick, the oil wasn't on the stick but I know that I put in 3 liters/3 x 1.05 quarts. The idea was to get it totally warmed up then add oil to account for the new capacity from the cooler and lines. I warmed it up on the side stand and the pressure gauge read 60lbs steady. I guess that I warmed it up for 2-5 minutes? No issues. I took off down the street and got a taste of how much fun this bike is as an 836cc.

I went 0.3 miles (GPS TRACKING) and the pressure light came on (red light) I looked down at the pressure gauge, and out was zero. I pulled over and turned off the bike.

I pushed it home 0.3 miles and checked the dip-stick. It had some oil on it.

The end. I am bummed.

http://www.cyclexchange.net/Oil%20System%20Comp%20Page.htm

Everything seemingly ready to go:


Pressure during warm up:


Red pressure light on:


Oil on stick after pushing/failed test drive:

[BenelliSEI]

That just sucks! If we assume there is nothing wrong with your pump or the built in oil pressure relief valve, the only thing that can cause that sudden loss of pressure is no oil?

When the bike (and you) have totally cooled down, is there any oil left in the tank? Could the return line from the scavenge side of the pump to the tank be collapsed internally or twisted and kinked? If the engine is sucking oil faster than it gets returned to the tank, this could be a cause of zero pressure…….

[PeWe]

Is the oil hose to the oil tank plugged or can collapse?
Correct amount of oil is filled?
3.5 Liter in a dry engine, 3L for an oil change.

I could get sudden low oil level when checked after 30-60 minutes ride. Less than half on dipstick. No oil lamp warning.
Returned home to see full.

I also swapped oil pump to discover the same sudden low.
Changed oil hoses and swapped pump again, back to the most restored one.
Have not seen the issue after that.

The replaced oil hoses were not old, not cheap aftermarket. Expensive aftermarket.

The newer ones from a guy in Germany that repair old oil hoses. My K2 has used a set since it was restored. No issues.

I'm happy that I bought 2 sets last time. Still have a set to a CB750 build.

Engine inside and pump screen very clean.

[MauiK3]

I hate to say it
Something sloshing around and blocking the pickup screen?

[seanbarney41]

Sounds like the pan needs to come off and pull the pump.  Last cb750 I worked on that never made oil pressure, we never figured out after trying everything.  Ended up swapping to a complete different bottom end after trying everything.  While swapping the stator cover we found where all the oil was going, there and the trans.  Still don't know why.  The faulty bottom end was also found to have one extremely worn rod bearing.
     Also worked on a cb360 with no oil to the top and was traced to a broken gear drive shaft pn the oil pump, so this needs to be checked thoroughly that the gears and shafts of your pump are not damaged.

[SKTP]

So my mechanic thought about the symptoms and thought that the only reason this could be happening was because something going wrong inside the oil tank. So we swapped out the oil tank and removed the oil cooler from the equation for now. We then started the bike with the new oil tank.

The old pressure built up quickly and reach 80psi when cold. The bike warmed up and there was no weird pressure drop.

I rode the bike about 1.5miles and had zero issues.

Once warm the pressure was at 20psi at idle.

I will do more test drives close to home before re-connecting the cooler. Hopefully this was the issue!

I will call a Ducati mechanic and ask if there's any chance a 2002 Ducati Monster oil cooler is unidirectional in flow? Seems unlikely from how the cooler looks.

[HondaMan]

If the cooler has a temperature device (thermostat) like some of the old-timey ones did, it runs in bypass mode until the engine warmed up. If plumbed backward, though, it does what you've bee describing.

The oil cooler looks upside-down to me: if it has such a bypass, it would block the oil when hot. Normally the hoses are very short and come straight up into the cooler from the bypass plate on the filter housing.

..just my 2 cents...

[SKTP]

So it turns out the oil level might have been the problem. I drove 5 miless with no issues yesterday.

Oil gauge read 60-65ish psi under load.

Once everything was totally warm my oil level is actually too high now, about 1/8" over the hash-marks on the stick...removing some oil and trying again. I have a nasty Tach seal leak...ordered a new OEM seal directly from Honda today

[BenelliSEI]

Good news!

[Alan F.]

This morning i finished re-reading this thread, what an experience you've had with that bike. It's good to hear you might have this latest oil level issue identified.

Has anyone recommended the tach seal from the DOHC bikes that followed after the K8? I remember something about it being fatter and maybe leaking less.
https://www.cmsnl.com/honda-cb750c-1980-a-canada_model50288/oil-seal_91256425003/

[Really?]

It has taken me most of the week to get through this thread.  Wow, what a rush!  Really?  Yes, Really!

[SKTP]

Thank you for the seal suggestion...I will try it if the OEM Honda one still leaks...

Turns out, the oil cooler is not upside-down?! Stumbled upon this Monster in the wild.

I have been WAY TOO busy...hopefully get some riding in this weekend

[SKTP]

Well it happened again. No oil pressure. I think I am going to pay a person to fix this issue and sell this bike.

[Don R]

 Forgive me for not reading the entire thread and possibly beating a dead horse.
  I'm assuming you have checked the plugs at the ends of the oil galleys? There's one behind the shift mechanism, one behind the dyno cover, and I think in the ends of the cam towers although they would be after the top end jets. And that leads me to ask, are the top end oil jets in place? 

[BenelliSEI]

Well it happened again. No oil pressure. I think I am going to pay a person to fix this issue and sell this bike.

Are you running with the oil cooler and lines removed?

[SKTP]

Forgive me for not reading the entire thread and possibly beating a dead horse.
  I'm assuming you have checked the plugs at the ends of the oil galleys? There's one behind the shift mechanism, one behind the dyno cover, and I think in the ends of the cam towers although they would be after the top end jets. And that leads me to ask, are the top end oil jets in place? 

I have not checked these...this is a common blockage point I am guessing? Yes. Oil jets are in place.

[SKTP]

Well it happened again. No oil pressure. I think I am going to pay a person to fix this issue and sell this bike.

Are you running with the oil cooler and lines removed?

This time was with the cooler and lines. I put 32 miles on it with the cooler and line in place before this happening.

I have been trying to get the oil level correct on the stick. When I just warm it up (when it was working) It reads in the range...but on longer rides, it reads high when I check. I do have an oil pressure gauge and it did have pressure while I warmed it up with choke for about 3 minutes or so, then I drove 0.3 miles and lost pressure.

[BenelliSEI]

Frustrating….. I’d take off that sandwich plate, lines and cooler. Makes no sense, but since it is the only real modification to the oiling system, I’d want to test without it, before ripping into anything else?

[SKTP]

Frustrating….. I’d take off that sandwich plate, lines and cooler. Makes no sense, but since it is the only real modification to the oiling system, I’d want to test without it, before ripping into anything else?

I did do that after the first time this happened...it ran fine (slightly higher oil pressure too). then I put it back on and it ran fine again....now after stilling for 2 months, pressure failure again.

[HondaMan]

By chance: does the oil cooler have a thermostat in it? Many did in the old days. Then they became direction-sensitive, and if plumbed backward or the cooler upside-down (i.e., with the entry-exit hoses on top instead of the bottom) they could pneumatically "lock" sometimes. I saw it mostly in the pits when guys would hurriedly add an oil cooler to their racebike without understanding how it worked (yep, saw them duct-taped on, too, in some non-supervised races!).