XJS ( X27 ) 1975 - 1996 3.6 4.0 5.3 6.0

1993 XJS 4.0 Low-Speed Stumbling/Misfire

Old 07-09-2019, 01:41 PM
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Default 1993 XJS 4.0 Low-Speed Stumbling/Misfire

I've had my 5-speed manual 'box 4.0 XJS a few months now. It has 125M on it. It passed California smog no problem. No check engine lights. Starts perfectly, idles smoothly. But driving along at low speeds (such as in a parking lot or small surface streets) at a consistent speed, it appears to have a misfire and slight stumbling, enough to make progress slightly jerky. Passengers don't notice it but I do. Same cold or hot. It accelerates smoothly, and I do not notice this at high speeds. I've run injector cleaner through a tank of gas, and replaced the spark plugs (not the coil-topped type). No difference. Any input?

~ Dave
Old 07-11-2019, 06:17 AM
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Given no injection harness quality concerns, I think it’s worth having a professional injector cleaning service clean your injectors and return you a balanced set.

At larger throttle openings the injector is remaining open longer. Any variation in the opening and closing speed of the injector becomes less and less of a factor in the total amount of fuel delivered as a larger percentage of its time ( milliseconds ) is spent at full flow. At idle, any variation here is a much large factor.

‘But I like starting cheep and easy and working my way up to expensive and difficult only if necessary. A full can of Seafoam in 10 gallons of fuel. Burn it up. Repeat several times. Once ain’t gonna do it.
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