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Misfire Mystery and Herky Jerky's Solved!

23080 Views 10 Replies 9 Participants Last post by  Insightful Trekker
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.
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I've noticed the herky jerky appears when the engine is cold and you aren't driving at a sufficient rpm. It seems like the engine wants to go into lean burn in 5th in the 35-40 mph range and it just can't handle it when not at temperature. Downshifting into 4th eliminates the problem. I've also noticed that even when the engine is warm, it will continue to jerk until you bring the engine over 2500 rpm or so and then it will immediately stop jerking when you go into a low rpm, high mileage cruise. Wierd, huh?

I'm almost completely convinced it's due to the lean running nature of the engine. All engines will do this when they run lean but the Insight is able to cope with it (mostly). Running up to a 22:1 A/F ratio with a normal engine will destroy it.
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