Topic: Oil light at idle after a warm ride

[Miob]

No doubt there are already topics about this, but the search doesn't seem to work for me.

Summer has started here and yesterday was a very warm and humid day. Fearing a closing in thunderstorm I rushed home on my CB750 K2 over the highway. The last ~7km's on the highway there was a traffic jam. Luckily lane splitting is allowed over here, but still, speeds were low, the weather was warm and I was riding between very warm cars. I could really feel the bike getting hot.

At the traffic lights at the end of the highway the oil light came on at idle, minutes later,  back home, it came on at idle again. Oil level is at maximum.

I know it could mean serous damage to my engine, but is it also possible that the oil just got a bit thin because of the high temperatures? It's due to be replaced.

[dave500]

replace the oil and see how it goes,its not uncommon for the light to flicker at idle,the sender may even be getting sus?

[Miob]

I hope so, but I'm a bit reluctant to replace the oil when there's a reasonable change that the engine is (severely) damaged. It would be 50,- euro's 'wasted'.

Extra information: The engine ran like it always ran: Smooth, no change in idle speed.

[dave500]

itll be alright,how much is fifty euros?

edit,,i just worked out thats 64 aussie dollars!so much for super duper oil,mine cost about ten bucks to change.

[Miob]

That's why

If the engine would need serious work, it is much cheaper to find a new engine, meaning I would have wasted the oil or would have to transfer it (with all the polution/possible metal) to the new one.

[dave500]

it wont need serious work.

[Rigid]

Here we go guys, your engine was designed and built pre-synthetic oil.  That means it will perform best with common petroleum base stock oils (cheap) I use rotella at $12.00 American for 4 quarts.  That is one cheap experiment and will, no doubt, cure the thinness problem that is tripping the light.

[70CB750]

What oil do you have in it? Brand/weight?

If there was serious damage to the engine, it would not get you home.

[xenoscr]

Could be a faulty pressure sensor. Had a similar issue last year, scared the crap out of me as I was 6 hours from home on my own. After pulling a few tappet covers to see if things were still slathered in oil my heart stopped racing. One new oil sensor later, I'm good to go.

XeNoSCR

[bollingball]

This came up a few days ago. I like to have a gauge on mine also.

http://forums.sohc4.net/index.php?topic=78643.0

[70CB750]

Yes, the gauge is proly the next best step. It will tell you much more than just the light.

[Spanner 1]

Regular fossil oil 20/50 weight for Summer, whatever brand you like  .... anything lighter weight may show low pressure when hot @ idle rpms.

[Miob]

Well, I have an oil temperature sensor (I have a very fancy ACEWELL speedo/tacho uni), but I never checked it for accuracy. I have the sensor in the side of the engine, just above/behind where the contact points are (I think I placed it in the place where you can connect an additional oil line to the head?).

Under normal circumstances the temperature does not read over ~72 degrees Celsius, yesterday it was 95 degrees Celsius at it's maximum. But I can't be sure that that's actual oil temperature (that's why I didn't mention it in the openings post, and I really could feel the engine getting way hotter than normal).

I don't remember the oil brand, but the weight is 15W40, considering the last reply that might be the problem...

[chewbacca5000]

replace the oil and see how it goes,its not uncommon for the light to flicker at idle,the sender may even be getting sus?

Dave what is sus?  How do I know if my sending unit is bad?

[70CB750]

I meant oil pressure gauge. The temperature is helpful but pressure is more important.

[bollingball]

replace the oil and see how it goes,its not uncommon for the light to flicker at idle,the sender may even be getting sus?

Dave what is sus?  How do I know if my sending unit is bad?

suspect maybe? I think they either work or don't. I think they trip at about 7 pounds You could put a gauge on and watch the light to see what is going on. clean and tighten the wire connection.
Ken

[MCRider]

replace the oil and see how it goes,its not uncommon for the light to flicker at idle,the sender may even be getting sus?

Dave what is sus?  How do I know if my sending unit is bad?

suspect maybe? I think they either work or don't. I think they trip at about 7 pounds You could put a gauge on and watch the light to see what is going on. clean and tighten the wire connection.
Ken

My experience and observation: 99% of the time, a red oil light, even a flickering one, on a previously working engine, with no "event", its the sender. Best way to check it is to replace it. I've heard standard auto units work fine. Even from a Honda automobile.

Good luck!

[kpier883]

I have had no issues with 15w40 in my '74 CB750.  Would not hesitate to run 20w50 if my light was coming on with 15w40 though.

[fmctm1sw]

Make sure the lead to the oil pressure switch isn't shorted somehow.  Usually that happens when the sprocket cover pinches it but you never know... 

[iron_worker]

Loads on the engine are very low at idle. I wouldn't worry too much about it.

A key to pressurized lubrication is FLOW. Flow is what draws the heat away from the parts. The pressure doesn't really tell you that much other than the pressure at that one point in the engine. You would have had a good amount of flow with the oil viscosity being low from the high heat.

That said, you do need a certain amount of pressure to keep the moving surfaces separated. A good rule of thumb that I've read is 10psi per 1000RPM.

These old engines are tough, I wouldn't be too too concerned.

IW

[Bob Wessner]

I agree. I believe the Honda service manual even mentions that the light might come on under these conditions. Whether these switches are still accurate, I believe the light is said to come on whenever the oil pressure drops below seven pounds.