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Oil leaking 85 720

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Hi all, need some help. I Noticed oil leaking from around the area where the oil filter is located. I know it’s not the oil sending unit because I could fit my finger behind the switch and everything is dry. The oil trails back and down the passenger side of the engine block towards the oil pan and then down towards the lowest part of the trans bell housing. I also know it’s not leaking from the oil pan or bell housing. Any suggestions? I read somewhere about the oil check ball and spring  and I’m thinking that has something to do with the leak but I’m not sure. I also made sure the oil filter was centered and tight, almost too tight. Any help would be greatly appreciated. For what it’s worth I’m using a KN filter.

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The check valve is mounted on the block behind the oil filter. It's not that.


I would suspect the valve cover leaking and dripping down onto the oil filter. Rinse the oil off with carburetor cleaner and watch for the next sign of oil.


Check that the oil filter is on firmly. Don't over tighten, just see if it will turn.


Replace the oil filter but spray the area with carburetor cleaner and wash all the oil away first. This way, if it continues to leak you'll see it. Check the old one. What can happen is the previous rubber ring gasket can stick to the block and then you screw on the new filter and there are two of them, causing a leak.

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I highly doubt it’s from the valve cover, everything is bone dry halfway up the block from the bottom with the exception of around the oil filter or just behind it. It’s Gona be a tight space but I’ll try and change the filter out for the third time and see if that helps once more. Btw is there suppose to be a spring behind that check valve ball? On mine, the ball just sits in the hole. I tried pushing it in but I’ll don’t feel it giving, unless I’m wrong. Thanks for your response datzenmike. Also I just recently changed the valve cover gasket with an OEM and everything is dried around it, I don’t see any signs of a leak coming from that high up on the block.

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The ball has a spring behind it and is firm. It's a by-pass in the extremely unlikely event that the oil filter should plug up. The pressure increase would push the ball inward and allow unfiltered oil to go around the filter and still lubricate the engine. If you were racing this would be removed and plugged. Most owners change their filters and having it plug up is almost impossibly rare.


Depends how accurate the gauge is but the oil pump by-pass limits the oil pressure to 50-60 PSI when revved up. At idle the oil bleeds past old worn bearings faster than it can be pumped and 20 PSI hot is normal. As the engine revs up more oil is pumped and the pressure quickly rises.

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Right, replace the low hanging fruit here is the filter. Replace it and clean the engine and keep an eye on it.


Does this filter thread directly to the block, or is there an aluminum adapter between the filter and the block?


How about the fuel pump? Mechanical fuel pumps have a weep hole that can leak if the diaphragm goes bad.

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Sorry boss, the threaded pipe that sticks out from the center of the oil filter hosing is not original/stock to the truck? That’s what I was referring to. The pipe is threaded at the lower part of the piping which the oil filter screws onto.

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Only Datsun I know of with an adapter is the 280zx turbo automatic and that was for the oil cooler. I have it on my L20B but am going to move the cooler behind the rad as it blocks too much air flow.

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I’ll try and posts photos of the issue and update you guys. 

8 hours ago, Stoffregen Motorsports said:

I was wondering if it was one of the Japanese imports that could have had a stock oil filter adapter.


Like this -


image.png.4eed900d62c9eafc71fc35e05fb28995.pngI’m for sure my truck does not have this.

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18 hours ago, datzenmike said:

Only Datsun I know of with an adapter is the 280zx turbo automatic and that was for the oil cooler. I have it on my L20B but am going to move the cooler behind the rad as it blocks too much air flow.

Back when I was a kid, tearing down engine cores, a lot of the later model engines (Z20/Z22/Z24) came with an adapter bolted on or at least a gasket stuck to the block where there had been an adapter. Lots of cores came from Japan as used engines.

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