1990 B2600i 2WD 2.6L Blowing crank seal over and over.

I have replaced the front main seal 4 timers now and it keeps blowing. I am sure that I am getting crank pressure that it pushing it out rather than blowing the seal. Either way it dumps out all the oil in the pan in around 3 minutes.
I have replaced the front main as I said. The PCV valve, the spark plugs. What is next? I have been told to try replacing the Oil pressure sending unit. That is next. Any guesses as to what might be causing the pressure and how do I resolve this issue?

Thanks +

Well if you have excessive Crankcase pressure, it is pretty easy to see - you can try pulling the oil cap (put a rag over the valve cover hole to avoid a mess) and rev the motor up.

If the seal was replaced properly - i.e. you have pressed the seal in fully and there is no oil residue on the surface ( the bore of the oil pump) where the seal presses in (this is important because the seal will slip out easily if you have oil on it) then you can be pretty sure it is the crankcase pressure that is pushing it out.

When you repeatedly replaced the seal, was the old seal actually pushed out each time?

If not then possibly the seals are just not being installed correctly?

Also keep in mind that if there is excessive blowby causing the seal to "leave" the motor - it could be that the excessive pressure is only created while the motor is under load (while driving) so in this case, you way not see much pressure out the oil cap while simply revving the motor.

If you suspect that it is building pressure under a load, then what you can do is connect a pressure gauge to the crankcase (most probably at the valve cover) and go for a drive.

Did the motor use oil before and was the original problem that the seal got pushed out?

While the truck is idling, pull the PCV valve and verify that there is good suction on the valve. See if a small piece of paper will stay "stuck" to the valve while the truck idles. Then pull the vacuum line off the PCV valve and see if the motor stalls, If it doesn't, then the line is blocked and that will allow excess pressure to build up.

How many miles are on the motor?

Check your crank shaft end play. Your thrust bearing might be toast.

Thank you for the suggestions.
the last time the seal pushed I had just replace the spark plugs and PCV valve. It was when I went to start the truck back up the seal pushed out again.

The kind of pressure required to pop a seal out isn't going to be overcome by a PCV valve. Either the surface is messed up, you have incredible amounts of blow by (need new rings), you're building ungodly amount of oil pressure, or your thrust bearing is bad.

Even reving the motor will tell you if you have excessive blow by, open the oil cap and put a rag over it (hold on to the rag) you'll be able to tell if it's too much. If you question if it's too much, then it probably isn't. If you're like HOLY HELL WHY IS MY TRUCK POSSESSED then yeah it's too much.

If you just installed the last one, you should be able to tell if it seemed to seat right, not too much oil on the surface etc.

Your thrust bearing usually will give you a thunk when you release the clutch, easy way to check that is to jack up the back end securely, put it in first and let on and off the clutch while listening to the motor. Otherwise you have to drop the trans and push the crank back and forth and measure any movement. Theres a spec for how much play you're allowed before a thrust bearing needs to be replaced. I don't have that info on these engines but it's out there somewhere.

Too much oil pressure. When was the last time you hooked up a mechanical oil pressure gauge? Sometimes pumps will go bad and there is a pressure relief valve built in to reduce the pressure to a safe level. Usually has a plug in it and is accessible off the side of the pump. If you have too much pressure you can adjust the depth of the plug till it falls within a decent tolerance, or better yet just replace the pump.