990 GM Oliver Engine Question

[Richard Kinney]

The 990 I am restoring has had the engine rebuilt with new sleeves, pistons, rod bearings, main bearings, and the cylinder head rebuilt. The engine runs great. But, in reviewing the shop manual on the tractor, it indicates the oil pressure is regulated by a valve in the oil pump which should maintain the engine oil pressure at 45 lbs whether the engine is cold, hot, fast idle, or slow idle. My tractor has about 50lbs. of oil pressure cold at fast idle and maintains about 40lbs of oil pressure at fast idle when working at operating temperature. The engine only shows about 20lbs of oil pressure at slow idle after the engine has been at operating temperature. Do you think this is adequate pressure? If not, do you think the pre-set pressure valve should be replaced? Or, any other thoughts.

 

[Anonymous-0]

I think you are O. K. Now what are you using for oil. Around here there are still a fair number of 4-71engines and a lot of those operators will not use a 15-40 oil. They run 24 hours a day weather permitting. Remember at lower engine speed you are moving less oil through the same bearing clearances as you do at higher speeds. Also pressure is caused by resistance to flow. Your pressure relief is working fine. You want me to find a second opinon I can but I am out of state for about 10 days. Glad you got almost done. J.

 

[Richard Kinney]

I am using 30 weight oil. The crankshaft looked very good and standard size bearings were installed. The oil pump was checked out, and it looked good also.

I was working the tractor @ 80% power and the coolant temperature ran about 195 with the outside temperature being about 70 to 75. I am also wondering if this is too high and the radiator needs to be cleaned inside. What are your thoughts?

Also, I notice the engine indicates that the maximum high idle RPM should be 1920, while the 995 seems to be exactly the same engine and is rated @2150 high idle RPM. Are the engines exactly the same? and do you think I am running much risk to set the high idle RPM on this 990 @2150?

This tractor will only be used for parades and tractor pulls in the future, and all of the tractor engine will remain "stock".

 

[scottinwa]

You need more oil pressure. Your engine won't have any problems running 2150 RPMS. Assuming the oil pressure is what it should be.

 

[Anonymous-0]

You are fine with the oil pressure. If you have 10 psi at low idle with an oil temperature of 195F you are good. Remember a large volume of oil is sprayed to the underside of the pistons which in turn lowers the oil pressure at an idle. 2150 will be fine. They did that to compensate for the frction loss in the torque converted.

 

[Anonymous-0]

The crank "looked" good, or it "measured" good? There can be a big difference between the two. How about the cam bearings? Were they good?

 

[george md]

Richard,

Somewhere in the genuine detroit service
manual for 71 in line engines it says that oil
pressure is fine at 2 lbs per 100 rpm of engine
speed .

george

 

[3-71 Fan]

Normal operating temp is 160-185.