Misfire Mystery and Herky Jerky's Solved!

As some of you may recall, I have been fighting with multiple misfire codes, CEL's and severe herky jerkies. Well, problem(s) solved.

Ok, remember that the conditions during which the misfire codes would show up were 1. Less than 3/4 tank of gas and 2. end of a long (13 mile)downhill coast, as power was re-applied. Same conditions every time on my daily 244 mile commute up and down the Grapevine in So. California.

Looking at this, I was thinking that the fuel tank pressure was dropping off during the downhill (throttle off, low manifold pressure) phase. I think it was so slow that the evap sensors did not see a "gas cap off" situation, and by the time it would have, I had re-applied throttle and the tank started pressurizing again. (This is a pure guess.) However, the loss of fuel tank pressure caused a quick, across the board lean burp and subsequent misfires due to momentary fuel starvation/instability when the throttle was applied. I informed Honda of these conditions and suggested a leak down test on the tank. They said that the car does that and it couldn't cause a misfire anyway.

When I left Honda on Thursday last week, I bought a new gas cap (Based on my gut feeling about the above.) I drove from Santa Monica up the 405, after they had cleared the codes and with the old gas cap still on. While coasting down the Sepulveda Pass with 1/4 tank, the misfire codes popped up again. I stopped in Santa Clarita, cleared the codes, filled up and installed the new gas cap. From that time on, no codes.

I have driven 3 commute cycles (732 miles) so far, no codes, no light, same conditions exactly. And, the best part, the 'herky jerkies" have completely vanished.

So, for what it's worth ($30.00), it was evidently a poorly sealing gas cap after all.