I have a 94 4.0 ohv that burns a quart of oil every 250 miles. The drivers side, at firewall, plug fouls and has to be replaced often from oil residue. My question is, can the intake gaskets cause this much oil loss, or am I seeing a cracked cylinder head issue?

 

Start with a compression test to determine if your piston rings are worn or not. Hows the condition of the coolant? any smoke coming out of the exhaust? How many miles on the truck?

If your truck is eating plugs, then it could be bad rings, worn valve seals, a bad PCV valve, etc.

 

Miles: Bought it used, previous owner said it had only 100k miles, I tend to believe him, seats and pedals not worn out. It now has 105k miles. The first four thousand I used it, did not have oil issues, it started using oil suddenly.

PCV: I have replaced it and the vent tubing is clean, no oil residue anywhere in tubing.

Coolant: No problems with the coolant, no foaming, no loss, and temps are fine.

Smoke: None that I can see, I believe the cat is doing it's job cleaning up the exhaust.

Compression: I will run the test and report back, but I believe that it will be well, engine runs smooth with new plug installed.

Valve seals: Don't know how to test for seals without being able to see smoke from the exhaust. Will a bad valve seal seal be able to flow a quart of oil into cylinder every 250 miles?

 

You will see that in the results of the compression test. Start looking after you get the results I suppose. More then likely it's gonna be short of spec.

 

Crack in head or bad gasket won't cause oil fouled spark plugs, it will cause other problems but not that problem.

Compression test will tell you if valve timing, rings and valve seats are OK, valve guide seals are not tested, as they are outside of the cylinder.

And a compression test result isn't very useful unless several cylinders are tested, you need a baseline.

A vacuum gauge is a nice, and inexpensive tool, to have.

A fast flutter of vacuum gauge needle at warm idle would indicate worn valve guide seals.
Would one worn seal burn a quart of oil in 250 miles, no probably not, but I have never seen one worn seal, lol, you just may have one that is leaking more oil, but all would be leaking some oil.
And because of the physics of motion the oil in the valve covers tends to pool at the rear of the head, where #3 and #6 valves are located, so higher possible oil flow into those seals.

Usually valve guide seals will crack if allowed to dry out, this happens if engine sits for long periods.
100k on a '94 would mean it was sitting for long periods.
Low mile vehicles are a pain in this respect, seals dry out and crack, rear main is the bad one.

There are products like Lucas oil treatment that swell rubber seals and soften them, you could try some and see if oil consumption goes down, this would indicate worn seals, since rings would not be effected.

Valve guide seals can be changed without removing the heads.