Oil Leak 1986 GM 350???

[Dave1]

My fresh rebuilt 1986 GM "M" series 350 in my 4WD pick up has a bad oil leak. It appears to be leaking high on the rear of the engine near the oil pressure sending unit and is running down the drivers side of the rear of the engine and transmission bell housing. The oil pressure sending unit was replaced when the engine was installed.

I got under the hood last night and tried to see the leak but it is very conjested back there and I could not see the back side of the engine in that area very well. I could see the sending unit itself and it did not appear to be leaking. Actually there are two sending unit type things. They are screwed into a two way brass adapter that is screwed onto what appears to be a small diameter tube about 3" long that is srewed into the top of the block right behind the distributor. It was "wet" around the base of the tube where it screws into the block so maybe that is where it is leaking. I sprayed it off with parts cleaner well and tonight will explore again. If it is good and dry I will crank the engine and try to see if that spot gets wet again. It is very hard to see back there and I am not sure I can see it well enough to tell where the leak is.

Any suggestions of where/what the leak may be? Is the intake manifold gasket a single piece gasket or is it the four piece design? Could it be that intake gasket or a head gasket leaking back there?

Thanks, Dave
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[mouse]

Check the distributor gasket as well, they can leak if its old.

 

[dyeager535]

You might be on the right track, but the intake seals right there as well, so it could be either thing...sounds like you've got it narrowed down pretty well if it's wet near the oil pressure tap.

If you can get at it, you might get a pipe plug and replace the switch/sender/tube with a plug to eliminate that as a cause. I've seen those brass tubes "tear" on install, you'd see that if you removed it.

Might just need some teflon tape around the threads though too.

Intake gasket design depends on what the builder used. Could be a gasket but SHOULD be RTV, even GM tells you to throw those gaskets away in the service manuals IIRC.

 

[Dave1]

I can see part of, but not all of, the distributor base and could not see or feel any oil around it so I am fairly sure it is not the dist base that is leaking.

There is plenty of fresh oil below the sending unit tap where it is screwed into the block and over on the block below the driver side head. I just can't see back there well enough to see where it is coming from.

One of those little mirrors on a telescopic stic may help if I had one. I may have to "borrow" one of Sally's compact make up mirrors and rig something up to use.

A complicating problem I am having is because I wear bi-focal glasses and have to be exactly the right distance away from something before I can see it clearly and in focus. That is very difficult to do under a hood and back in tight places. I am either too close or too far away from it to see clearly. May have to enlist some help to find it but will try again tonight. Sure hope it is not the intake leaking.

BTW, is there another location on the engine block that is more easily accessible that the oil pressure sending units could be relocated to? The wiring harness oil presure sending unit connections has limited length so it would have to be fairly close.

Dave

 

[Dave1]

Well, I crawled all over this thing this afternoon. After spraying it down with parts cleaner last night it was fairly dry this afternoon when I got under there. I swiped a make up mirror from Sally's collection, took my flashlight and went over the back side of this engine best I could and could not find a single wet spot. Cranked the engine up and let it run continuously while I went over everything I could see. Where I thought the oil was coming from was perfectly dry.

But, what I did find was a line of fresh oil seeping out from beside the oil filter base near where the bell housing cover butts up against the engine block. That joint is right beside the base of the engine block where the oil filter recess is. It now appears the leak is coming from behind the bell housing cover. Bad news!

The only thing I can think of back there that could be leaking is the one piece rear main seal. If so, that is more than I want to tackle so it's going cost me some money. I can't believe a fresh engine with a new rear main seal would be leaking in 200 miles. Of course as sloppy a job as the garage did on this engine swap, nothing surprises me now. I refuse to take it back to them so I will have to pay out of my pocket to get it fixed. I may call the engine builder first just to see what he says but he didn't put the seal in so he is not going to waranty that. If there is anything else behind that bell housing cover that could be leaking engine oil, let me know.

What should it cost at a shop to replace the rear main seal?

Thanks, Dave

 

[dyeager535]

You are certain it's not coming FROM the oil filter adapter itself?

If you aren't familiar with the assembly, the piece the oil filter threads on to is seperate from the block, bolted to it.

I'm not even sure there is a factory gasket or o-ring on non-oil cooling applications, but on my oil cooler adapter (replaces the stock filter adapter) there is an o-ring that could conceivably leak.

 

[mouse]

Another thing to consider-if its been leaking for more than a day or two, the oil has probably made it under the bell housing. The built up oil (not cleaned out) would show up when you run the engine. Sometimes it takes a while to narrow down a leak - I know!

 

[Dave1]

Here's what I did. I took parts cleaner and sprayed all around the oil filter, the base of the filter, flywheel cover, and the areas around it, wiped it all as clean and dry as I could get it, then cranked the engine and got under there with a flashlight. I was directly under the oil filter looking straight up at it to be sure the filter itself was not leaking. It was dry around the filter base itself up in the recess where the filter screws in so I was sure the filter was not leaking. I laid there for probably 10-15 minutes looking all around with the flashlight with the engine running then I noticed a tiny bit of fresh engine oil seeping/running along the bottom of the engine block. The line of oil was coming from where the fly wheel cover butts up against the engine block and transmission case which is only a couple inches or so from the oil filter. If you put your finger on the bottom of the engine block that surrounds the oil filter and run your finger away from the filter toward the transmission, where you touch the flywheel cover is where the oil is coming from.

My truck does not have an engine oil cooler on it so it does not have an adapter between the oil filter and the engine block itself. The filter screws right to the engine block.

I was wondering; if the leak is behind the flywheel cover, like if the rear main seal is leaking, why wouldn't the oil be leaking out the small hole that is in the very bottom of the flywheel cover.

Something else that I was wondering about is the engine builder installed a high volume oil pump in this engine. The oil pressure idles at about 45psi and as soon as you touch the gas the oil pressure gauge needle goes past 60psi so I really don't know how much oil pressure this thing is running at. Is it possible the pressure is so high that it is essentially blowing the oil by the rear main seal or whatever else is behind the flywheel cover? If the crankcase ventilation system is not working properly could/would that contribute to the problem? I am just trying to think of all angles hoping it will require a fix other than replacing the rear main seal.

Dave

 

[Robert79K5]

I had a 350 that I rebuilt do exactly the same thing that your describing. Well you know how every head is tapped with threaded holes on one end for the accessory brackets so you always end up with threaded holes to mount the brackets and on the other head that will be turned around will be threaded holes on the back of the engine that aren't getting used for anything. It turned out to be one of the threaded holes in the back of the head. I guess it was drilled a little too far and got into the oil passage. I put some gasket sealer on a bolt and threaded it in and no more oil leak.
It was a really hard leak to find because it followed the crack between the bellhousing and the block and it sounds exactly like the leak your describing.

Good luck.

 

[dyeager535]

Heh, I thought I was the only one that would ever have this problem. My $200 (new) pair of Vortec heads must have been factory rejects. Luckily I saw daylight through the holes while I was building it on the engine stand. Bolts/thread sealer in every accessory hole now.

These aren't pressurized, they would just seep, from oil splashing in the rocker arm area. Easy enough to reach behind the head and see if your finger comes up wet with oil.

 

[Dave1]

There was no oil dripping or running off the back of the heads that I could see. The fresh engine oil I saw was deffinitely coming from behind the flywheel cover. I haven't removed that cover yet to look. That may reveal the real story.

Dave

 

[mouse]

Fresh oil is sometimes hard to see, especially on a clean engine. Even worse if its chevy orange. Behind the block it could be rear main seal or the cam oil galleys which are plugged. There is also a large "freeze plug" at the end behind the cam shaft. If not totally sealed these could leak. None are easy to access, and would require dropping the tranny (my preference) or pulling the engine to get at.

Drop the dust cover on the transmission and point your flashlight up inside. You should be able to see everything I mentioned using a combination of moving around and looking through the holes in the flywheel. You might have to rotate the engine.

Good luck.

 

[Dave1]

I haven't pulled the flywheel cover off yet but will do that this weekend and look around in there and see what I can find.

I called a local garage here yesterday and they quoted me $350 to replace the rear main seal (if that is what it is). I sure will be glad to get this over with.

Dave