Bobcat 773 engine oil pressure low

[jambalini]

So, the Bobcat I have just bought has done 5500hrs so in my opinion could be getting ready for an engine re-build, but it runs and starts great with no smoking of horrible rattles etc etc. however, last summer the computer gave a warning that the oil pressure was low after it had been running for a while (nice and hot), and eventually shut itself down due to low oils pressure. I installed a pressure gauge to the oil line and found that it had great pressure when cold, but sure enough went down to about 20psi when hot. Now its cooled down here in the UK, I'm not having a problem with it, not even a warning. So, my question is, do you think this could be just due to a worn oil pump, or that the engine is ready for new big end and mains shells. I know you're going to say while your in there, do the lot, but I was wondering if I could test the pump if I got it out as that is the expensive bit. It would be very annoying to fit new shells and find out it was the pump all along. any advice would be great, thanks Andy

 

[foton]

I have read that around 6000 hrs is what one should expect of design life out of these engines, I do not know that as a fact just what I have read, but it does sound reasonable. Maybe you should go to a straight weight 40 sae engine oil in the summer ,it might put off the rebuild a bit. I do not think normally oil pumps wear out much, it does happen just not much.

 

[jambalini]

I have read that around 6000 hrs is what one should expect of design life out of these engines, I do not know that as a fact just what I have read, but it does sound reasonable. Maybe you should go to a straight weight 40 sae engine oil in the summer ,it might put off the rebuild a bit. I do not think normally oil pumps wear out much, it does happen just not much.

What oil pressure would you usually expect when the engine was hot? My 773 only has a sensor as standard and the computer sorts it out, and I know the first warning goes off at 14 PSI, and shuts the engine down automatically if it drops below this. It only happen once but it was a very hot day. I don't mind re-building this engine but it seems such a shame to do it right now as it runs so well. The straight 40 oil would be an answer as you say to put it off a while. Maybe I'll wait until something else goes wrong!

 

[foton]

What oil pressure would you usually expect when the engine was hot? My 773 only has a sensor as standard and the computer sorts it out, and I know the first warning goes off at 14 PSI, and shuts the engine down automatically if it drops below this. It only happen once but it was a very hot day. I don't mind re-building this engine but it seems such a shame to do it right now as it runs so well. The straight 40 oil would be an answer as you say to put it off a while. Maybe I'll wait until something else goes wrong!

I have 4 different diesel engines and they all run over 50 psi hot at normal operating speed (rpm). Now they do drop at a hot idle to various pressures above 25 psi. You know I was thinking of this and why don't you do a compression check and see where that falls in the suggested pressures. You might just need to do the mains and rod bearings and leave the sleeves and rings alone.

 

[jambalini]

I have 4 different diesel engines and they all run over 50 psi hot at normal operating speed (rpm). Now they do drop at a hot idle to various pressures above 25 psi. You know I was thinking of this and why don't you do a compression check and see where that falls in the suggested pressures. You might just need to do the mains and rod bearings and leave the sleeves and rings alone.

Thats a good idea Foton. I forgot to say that these low oil pressures I am getting are at tick over, but it still shuts the machine down. They do increase when reved up, but when hot, still not as high as your 50psi I don't think. What sort of compression figures should I be looking at for each cylinder?

 

[Tazza]

Thats a good idea Foton. I forgot to say that these low oil pressures I am getting are at tick over, but it still shuts the machine down. They do increase when reved up, but when hot, still not as high as your 50psi I don't think. What sort of compression figures should I be looking at for each cylinder?

Personally, i'd start by changing the sender, it could be reading the wrong value and the computer thinks the pressure is low when it's really not.
50PSI sounds quite high for engine oil pressure, if the pressure really is low, i agree with the idea of using heavier oil to try and increase the pressure from worn bearings.
As it starts and runs well, i'd like to think the bearings shouldn't be worn enough to drop the pressure enough to be an issue. Normally your cylinders wear out before the bearings do, causing low oil pressure.

 

[jambalini]

Personally, i'd start by changing the sender, it could be reading the wrong value and the computer thinks the pressure is low when it's really not.
50PSI sounds quite high for engine oil pressure, if the pressure really is low, i agree with the idea of using heavier oil to try and increase the pressure from worn bearings.
As it starts and runs well, i'd like to think the bearings shouldn't be worn enough to drop the pressure enough to be an issue. Normally your cylinders wear out before the bearings do, causing low oil pressure.

Hi Tazza, I have already changed the sender unit at great cost (£120) as that was my very first thought, and it made no difference. I'll compression check it next, and if that is all OK, then plan to change the shells out. Does anyone know of the compression figures I should be looking for please? thanks

 

[foton]

Hi Tazza, I have already changed the sender unit at great cost (£120) as that was my very first thought, and it made no difference. I'll compression check it next, and if that is all OK, then plan to change the shells out. Does anyone know of the compression figures I should be looking for please? thanks

Web says it looks like 300 psi is a minimum but I think even at that number is to low, but that will give you a idea. But if it starts easily with normal starting procedures (no either) you should be well above that.

 

[Tazza]

Web says it looks like 300 psi is a minimum but I think even at that number is to low, but that will give you a idea. But if it starts easily with normal starting procedures (no either) you should be well above that.

I thought spec for a diesel is 400PSI or more, 300PSI is low enough for it to be hard to start, if it was that low, i'd think you'd know as it would smoke while cranking, trying to start and more than likely smoke due to burning oil when running.