I have a 91 F150 4X4, 302 engine with 75,000 original miles. The engine is leaking oil. The valve cover gaskets were leaking bad and I put new gaskets on and that was fixed and I thought that was all my problem was. It is still leaking. The engine is dry from the top until you get to the bottom starter bolt. That is where the oil is coming from. I removed the rubber plug from the bottom of the bell housing and oil will not drip from there. When the engine idles it will not leak. It only leaks when driving with the engine reved up. Sometimes it leaks more than others. I just got back from driving to another town which is about 10 miles. When I got there there was oil everywhere underneath. When I got back and looked it was barely leaking.

This has got me stumped! If it was the rear main seal wouldn't it be leaking out the bottom hole in the bell housing? The oil seems to be coming from around the starter is mounted and drips onto the exhaust pipe.

Any thoughts on this? Is there something back there that might leak other than the rear main seal? Can the rear main seal be replaced with the engine in the vehicle?

 

You are quite correct. If there was a leak at the rear crank seal you would have oil round the bottom of the bell housing. I would bet that one of your valve cover gaskets is still leaking.

 

No, that sounds *exactly* what my truck did before I replaced the oil pan gasket. It would not leak when idling, only when driving. Drove me NUTS. I finally put it on the lift at work and revved the snot out of it, and it was obvious it was the oil pan gasket. Take a look at where the pan meets the block. If the gasket is pushed out, that's your culprit.

 

It definately is not the valve cover gaskets anymore. I checked and double checked.

I will look at the oil pan gasket again. I checked the pan bolts but they were tight.

 

It's not a question of them being tight. The gasket gets old, deteriorates, and leaks. However, if the gasket was replaced previously, they could be *too* tight. That will also cause a leak eventually.

 

How difficult is it to change the oil pan gasket on the 5.0?

 

It is a pain in the behind to change any oil pan gasket on the 80-96 trucks.

My vote is for the oil pan gasket or rear intake manifold gasket. If your rear intake manifold gasket is leaking, oil will run down the bellhousing / back of the engine. Reach between the lower intake manifold and firewall and check for oil. These are not too bad to change although it first appears to be a difficult task due to all of the emissions and EFI equipment.

 

After thinking for a while I realized that my trouble started when I had to switch to the synthetic blend oil a while back. My supplier told me the straight oils are being discontinued because the new cars require the synthetic blends.

Is there something to this theory?

 

The research I've done on this for my company suggests that there is no harm switching from dino oils to synthetics.

 

If the oil pan gasket looks good to you, when was the last time you replaced your PCV valve? If it is plugged the crankcase may be pressurising just a little bit too much pushing oil out thru a possibly weakened gasket.

 

what ^^ he said

 

I would bet on the oil pan gasket bein bad. And as far as synthenic goes. alot of the older vehicles relied on reg oil to help swell the cork gaskets to help them seal. So when you switch to synthentic it can cause leaks.

 

It was the pan gasket pushed out right above the starter on the right rear portion of the block. I loosened the pan bolts and pushed it back in. It seems to be holding for now. It has a rubber gasket. Didn't factory come with a cork gasket?

Thanks for everyones help!

 

Based on my own experience and reading the experience of others on this forum, I firmly believe that idea to be a myth. I have switched several vehicles (two with 100k+) to synthetic oil and no leaks.