Replacing exhaust manifold slowed down oil consumption somehow?

Apr 9, 2008
Central NY
This is a strange one. I'm sure there has to be some correlation but I don't understand how/why. This is in my truck - 01 F350 5.4 with the engine out of (I believe) a 1997 F150. The engine is up to almost 238K miles now.

It had used a little oil. I played around with the 640:1 mix of TCW3 in the gas and that seemed to slow consumption a bit. The right head gasket was seeping a bit as do many early build 5.4s.. I had to add a quart every 800-1200 miles. Not terrible.

Last summer the passenger side exhaust manifold blew out (again). I let it go because I didn't think the truck was worth it. Shortly thereafter (probably immediately - within a few weeks) I had to start adding oil every 125-200 miles. If I was towing, it was a top off the gas fill the oil situation. I made a 500 mile round trip and went through 4 quarts of oil. Made a couple of 160 mile round trips towing the Jeep and had to add a bit more than a quart. Consistently , the round trip to my parents (130 miles depending on the route) was a whole quart of oil. All leaking out of the passenger side head gasket. There was always a cloud of blue smoke coming out of the passenger side wheel well because the blown exhaust manifold / gasket was causing the leaked oil to smolder.

While I didn't end up running used oil in the engine , I haven't changed it either. Just changed the filter and kept running the same supertech high mileage 5w-30 as before.

I had the exhaust manifold done very recently. I didn't think the truck was worth putting the work into, but it was really confusing the computer --- the right bank would run +30% fuel trims! Taking off in the morning I would make it right to the state road and it would go closed loop and immediately run on 4 cylinders.

Haven't been driving the truck after it was fixed due to family things going on. I've managed to finally put about 800 miles on it and it was finally 1/2 way down on the dip stick. 1.5 pts brought it back to full. That would put me right at the oil consumption I was at early last year.

The head gasket is still seeping. But it doesn't immediately soak the right side of the engine and starter like it had been.

I don't understand it. but I'll take it. Anyone have any idea on what was happening here?
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well, . . . the mechanic who did the job topped off the crankcase, checked the tire pressure & probably fitted new wiper blades too. : l
it happens. he put an extra quart & a 1/2 in there too ! yipee!
Let's see...
Running rich due to the extra oxygen getting into the exhaust and playing havoc with the fuel trim. Check.
Poor combustion in the rich cylinder resulting in low pressures and misfires, kinda like cylinder deactivation (Caddy 8-6-4 without the 6). Check.
Oil getting sucked past the rings 'cause there's little combustion pressure to periodically push it back, much like cylinder deactivation. Check.

With proper combustion that head gets hotter; that might make the head gasket leak less.
Interesting theory! It's also possible that even when it was firing on all 8, would the extra fuel keep the combustion chambers on that head much cooler leading to a head gasket that won't seal as well?

Even at it's worst, there were no combustion gasses in the coolant and it wasn't mixing coolant and oil. That's why I never bothered to replace it -- it's a huge pain on this engine.

My theory was that the extra gas was diluting the oil enough that it was able to leak out of the head gasket easier?

It went from 6-8 MPG to 14 or so.