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The car (Renault Megane 1998 1.6e hatchback) lost power (about 500rpm in a bursty rhythm) after I re-ignited on a traffic light, the engine started shaking, and I could smell liquid fuel. There was no obvious place where the fuel could have dripped from. I left the car and checked for any marks on the ground later. About two hours later the car started and ran OK. Then after a few more hours of rest it ran OK again for a few miles, but after a brief stop when I ignited while the engine was hot it started shaking and smelling like fuel again. My first guess would be spark plugs, but I'm afraid of something more serious. What else could be the reason?

UPDATE: Checked the spark plugs -- not them. I have some suspicion about bad fuel as I filled about 5L of petrol the day before.

UPDATE: I tried pulling the the ignition leads from the engine side during the faulty running of the engine. I didn't dare pull them all the way out, but an inch was just about enough to hear the distinctive sound of a spark jumping to the plug. Two of them (1 and 4) failed to produce that sound. I've ordered ignition leads, so I'll soon find out of they were the problem.

I also had another experience I tried driving the car with the engine shaking and about 500 down the road after I stepped on the gas quite hard, it started working properly again.

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How did you check the plugs? Did you see them producing sparks? –  R.. Jun 13 '13 at 12:51
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2 Answers

up vote 4 down vote accepted

Firstly, what car is it? (Make, model, year).

Is the fuel smell more pronounced around the engine bay (suggesting a leak) or around the exhaust (suggesting unburnt fuel)?

Shaking and low power suggests, as you suspect, that it isn't running on all cylinders (which would result in unburnt fuel coming out the exhaust). If it is only doing so when hot, my first thought would be either the HT leads or coil(s).

If it is an older car with a single coil, a distributor, and a separate lead to each spark plug, then, when it is playing up, disconnect each in turn and see what happens. If one or two are failing, it will run just the same with the failing ones disconnected, but worse or not at all with the others disconnected. In this case, replace the whole set. Note that they carry a very high voltage, so DON'T play with them with the engine running! (or wear insulating gloves if you do) - As they get old, the insulation degrades, and they can give a nasty shock.

If it's a newer car with a separate coil pack for each cylinder, then again it could be a failure of one or more coil packs, and I believe a similar technique can be used to test them, although I've not tried that...

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OP has added make, model and year. –  Bob Cross Jun 11 '13 at 15:59
    
It was one of the coils. Car works fine after I changed it. Thanks! –  Aleksandar Savkov Jun 24 '13 at 20:30
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The Megane Mk1 has a coil pack per spark plug - they sit in the engine block just above the plugs themselves. One of the coil packs may be faulty causing intermittent spark at the plug.

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