Why is my car surging?

Question:

I have a 1979 Pontiac Trans Am with a 400 cid engine and 4-speed transmission. The car has 52,000 original miles.

I performed a cosmetic engine restoration, down to the valley pan including a rebuild on the carburetor and installation of a new fuel pump, new coil, wiring etc. After the engine has been running for 25 minutes, at any throttle position, the engine surges and will not run smoothly, but it does idle nicely. I have bypassed the fuel system, tried an electric fuel pump, tried different carburetors, but

I still get the same result. Once the engine runs for 25 minutes or more I get a repeatable surge at any accelerator position except idle.

While performing the cosmetic update on the engine I noticed a significant amount of carbon in the intake. I glass beaded and removed the carbon I could see in the intake but did not hot-tank the intake. The heads did not look that bad with carbon. There is no indication of pre-ignition or pinging, and no unburned fuel in the exhaust either. The engine sounds like it is starving for fuel or in a vapor lock condition but since I have bypassed the fuel system the last thought is: was there enough carbon still in the intake that the fuel is being vaporized when the carbon is heated to a significant point? Before I take the intake back off and hot tank it I am hoping to see if you think I have overlooked anything.

Answer:

Though it doesn’t answer your question, I want to offer a comment about you glass beading the intake manifold. The glass bead material can easily get lodged in crevices and other hard-to-reach areas of the intake and come out over time after it’s installed on the engine. As you’d expect, glass bead will do some serious damage to an engine’s internal parts.

If you do glass bead an intake, it should be hot tanked or at least scrubbed inside and out with a solvent.

That aside, I don’t think that a buildup of carbon in the intake is causing the surging condition you’re experiencing. The most common causes of engine surging are vacuum leaks and a fuel delivery issue. You say that you bypassed the fuel system and the problem persisted but you don’t say exactly what you did to bypass it. One item that is often overlooked is the fuel filter “sock” that’s on the pickup inside the tank. Make sure your pickup filter is in good shape.

Something else to look at is the exhaust system. If there’s an obstruction in the exhaust it can cause surging. Make sure the exhaust gases are flowing freely through the entire system, including the catalytic converter and mufflers.