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So I had this gremlin in my wife's BMW 330i for at least 4 years. The car runs great but once in a while you would start it in the morning and the engine would sound rough with some shaking. After few minutes it would go back to normal and I wouldn't see this problem for months.

Few weeks ago we had a snow storm and while snow blowing the driveway I had to move the car. After moving it, the next day I got the same misfire and it got me thinking that a lot of times when I had that misfire, I did something else to the car, like drive in/out of the garage (very short operating time as well).

So what would cause misfire like that, several hours (12+) later after very short run?

In the past, I've simply ignored this quirk, but lately been trying to see if I can understand it. When it happened once back in the fall, I pulled the engine cover off and disconnected ignition coils one at a time until I discovered that cylinder #4 was the one that was misfiring. When it happened this time, it was 5 am and I was going to the airport, so didn't have time to see if it was the same cylinder.

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2 Answers

Have you checked your spark plugs? I had a similar periodic issue. Turned out I had oil in one of my spark plug holes, which messed up the cable. I replaced my valve cover gasket, all plugs and cables, and was good to go.

Also, a misfire can happen and be inaudible. I cool trick I've learned is to hold up a dollar bill to your tailpipe with the engine running. The bill should not get sucked into the tail pipe, as that would indicate a misfire. If you have an assistant, have them give it a little gas, to get the car out of idle, while your run this test

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Interesting you should mention the oil. In May of last year I replaced the valve cover gasket because it was leaking oil. I had oil pool in all spark plug holes, then drip out onto exhaust manifold and smoke. When I did that job, I also replaced all spark plugs and attempted to do my best to clean any oil from electrical connectors (using q-tip with rubbing alcohol). Problem is that this car has ignition coils sitting right on top of the spark plugs and there's no standard spark plug wires that I can easily replace (engine wiring harness goes straight to the coils and is non-serviceable)... –  DXM Mar 5 '12 at 4:21
    
... there could definitely still be oil somewhere in the connectors (either bottom part where it goes onto spark plugs or top where harness connects to the coils). But even if I did something in May of last year, seems this behavior persisted through all that completely unchanged. Is there a good explanation why oil would cause the whole thing with 15 sec run time? Otherwise, I can run this car for half a year without a single misfire. –  DXM Mar 5 '12 at 4:23
    
I would guess that any oil on the cables would eventually seep through the protective covering, and cause a short on the wires. At first startup, this would cause the wires to heat up and eventually burn off whatever minute amount of oil was causing the short. After this, the car would run normal. After letting the car sit for a bit, however, the oil has a chance to seep back through the plastic covering and cause a misfire again. I should emphasize that this is purely speculative, however. Replace those cables! –  wesanyer Mar 6 '12 at 1:44
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Do you notice any smoke when this misfire happens?It is possible that you have a leaking injector. The injector may not be holding fuel pressure after the engine is shut off.When this occurrs with a hot engine the heat of the piston vaporizes the fuel.When the engine is cold the fuel sits in the cylinder and causes a misfire on start up until the cylinder clears its self.This might explain why it is not setting a fault code.

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Can't say if there was or wasn't any smoke. I was suspecting leaky injectors and I've had them in the past, but it seems an injector wouldn't care if the car ran for 15 sec or for 2 hours. With my last car, injector problem caused misfire regardless of how long I drove the car for. In this case, the problem is extremely rare and generally crops up if only when I do something to the car outside of normal driving. I guess hooking up a fuel pressure gauge would take the guessing part out of this question. –  DXM Mar 5 '12 at 4:16
    
I had this problem with my Toyota MR2. If I didn't run it for more than 5 minutes, it would struggle to start and misfire next time. Took a couple years to find the culprit, but it was a leaking cold start injector. –  Brian Knoblauch Mar 5 '12 at 13:52
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