Newly rebuilt engine is leaking more oil than ever.

Dear Car Talk

Dear Car Talk | Nov 01, 1993

Dear Tom and Ray:

I own a 1975 Dodge half-ton pick up with a 360 cubic inch engine that has served me well for 170,000 miles. This past summer it started using oil in town (one quart every 50-75 miles), so I had the engine rebuilt supposedly by the best shop in town. The heads were shaved; the manifold was shaved; all cylinders were bored and honed. All new valves and guides were installed. And now the engine uses more oil than before I had the engine rebuilt! It is now using a quart every 30-40 miles on short trips in town, and a quart every 800 miles or so on the highway. The "leak-down test" shows 100 on six cylinders; 96 on the other two. There are no visible oil leaks from the motor, and the engine has plenty of power. The shop has spent considerable time trying to find the cause of this "leak," but they have been unable to stop it. I've spent $2,500 already, and would appreciate your input on this problem. Can you help me?

TOM: Gee, John, you gotta watch all that technical jargon. It really confuses me.

RAY: Yeah. Words like "engine" and "cylinder" throw him for a loop.

TOM: Actually, John, we can take an educated guess, but it involves making a few assumptions. We have to assume that the shop didn't screw up the rebuild. Let's also assume that you broke the engine in correctly.

RAY: And if we make those assumptions, I'd say that the new oil leak has nothing to do with the old oil leak. And my guess is that your oil is now getting diluted with gasoline.

TOM: When you dilute the oil with gasoline, you make it much easier for this new mixture to sneak past the rings, and get burned in the combustion chambers. That's because the gas and oil mixture is not only thinner, but also more flammable.

RAY: And there are several ways gasoline could be getting into the oil. The most likely is a carburetor problem; either the carburetor is flooding at low speeds, or the choke is opening too slowly. And if the carburetor is pouring in extra gas, the problem will be much worse around town. On the highway, that extra gas gets burned up in the cylinders, so the problem is much less noticable.

TOM: The other possibility is that a faulty fuel pump is dumping gasoline directly into the crankcase at the point where it mounts to the block.

RAY: Ask the shop to do an exhaust gas analysis. That'll tell them if the carburetor is at fault. And there are other clues they can look for. They can see how much oil comes out of the crankcase when they drain it. If it's more than they put in, that's a strong hint that something is being added to it. They can also sniff the oil and see if it smells of gasoline (suggest they put out their Tiparillos before doing this).

TOM: And if they still can't find out where the oil is going, then they're going to have to assume that they made a mistake, and they'll have to take the engine apart and try again. And if they really are the best shop in town, they'll keep working on it until they fix it. Good luck, John.

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