JD 750 blowby

double hunter · Jul 17, 2019

Just bought a JD 750,,tractor actually runs pretty good,,has a little puffs of blue smoke when first cranked but clears up,,but it has blow by out of the crank case vent tube big time,,not smoking,,just blow by and some oil coming out,,oil looks really thick goopy,,any ideas what would cause this and what can I do to correct,,any info appeciated

jeff9366 · Jul 17, 2019

Warm oil should not be "thick and goopy". I would immediately replace whatever oil is in there, and filters, with whatever spec oil Deere/Yanmar recommends. Oil is cheap relative to tractor repairs.

My first tractor was a JD 750 diesel. Is yours diesel?

double hunter · Jul 17, 2019

The oil in the crank case looks fine,,i was trying to say im getting a little oil out the vent tube,,

Evets Tsorf · Jul 18, 2019

Glad to hear oil is not thick and goopy, that could be a sign of someone trying to mask engine issues wth thick oil, agree an oil change in order, before changing oil I may suggest running a quart of Marvel Mystery oil in engine to help clean out engine before change. DO NOT over service oil level. If the engine starts well compression likely OK. excessive blow by usually caused by collapsed or gummed up oil control rings or worn valve guides. The Marvel Mystery oil may help clean up an free oil control rings. It will not correct worn valve guides.

As to over servicing oil, that can be a hazard on any diesel. I purchased a Yanmar 3 cylinder marine engine to overhaul, the previous owner was replacing it in an insurance claim after over servicing the oil. He had just lubed the sail track with silicone and went down and checked the oil, pulled out the dipstick, sipped it down with the rag with silicone on it. Put the dip stick in and pulled it out and it showed no oil. He added a quart and rechecked, still no oil showing. Added another quart and had the same result. ( oil does not stick to silicone).The engine only held 3.5 quarts. He decided to fire it up and the engine ran away on him as it pumped oil into the intake and a diesel will run on oil as well as diesel fuel. He got the engine to shut down by plugging intake with rags. The engine ran afterwards but smoked and lacked power. When I tore it down I found two cracked pistons.

If the engine sat for any long period the smoke could be from corrosion in cylinder bores, if is corrosion is light this may self correct wth use.

Good luck.

Edited for spelin, grammoore, and more thinkin.

Agvg · Jul 18, 2019

Put new oil in it and let it work, and by work I mean pull as hard as it can for an hour or two.

Lot of tractors are tortured by ligh use or owners that think they are fragile things, afraid of both hard work and RPM.

double hunter · Jul 18, 2019

Well guys I did all that this morning,,put some Marvel oil in it,,ran it a while and then changed the oil and filter,,no help,,still getting blowby with new oil spraying out,,tractor has good power,,i put a 4' bush hog behind and put it in some tall grass and really worked it with no help,,beats me the engine runs clean with no smoke after initial startt up,,I believe the valve guides are worn out but I dont know

Evets Tsorf · Jul 19, 2019

Hunter X2,

It occurred to me if it is valve guides try removing the oil cap and see if what comes out the breather diminishes. Blow by is usually caused by excess case pressure from combustion gases building up in the case and venting overboard. There is always some oil mist in the lower engine case and the excessive pressure will carry this out the breather. It can be caused by stuck or broken rings, cracked piston, corrosion or scored cylinder barrels or worn valve guides. As Agvg suggest working it hard may correct the issue if it glazed, corroded cylinder or stuck ring. If it does not get better you will be entering the engine at some point, do not be intimidated by this task it is not that tough and parts are readily available. Any good machine shop can rebuild the and resurface the head. If you can get the oil sump off and unbolt the connecting rods and push the piston and rod out the top with the head off, new pistons. rings, hone or replace the cylinder barrels, reworked head and some new rod bearings and some seals and gaskets will get you and engine that will likely take you to the point of being on soft food, the main bearings are very robust and seldom an issue.