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Discussion Starter #1 (Edited)
Has anyone experienced a misfire mainly when the engine is cold?

I am getting P0303 and it's misfiring quite bad. Interestingly, once engine warms up the misfire is either completely gone or significantly reduced (99 G20, Auto, 232K).

Things already done (~1.5 years/15,000 miles ago):

Replaced plugs
Replaced plug wires
Replaced distributor cap and rotor
Replaced fuel filter
Replaced air filter
Replaced PCV valve
Cleaned MAF sensor and a throttle body
Cleaned IACV
Gaskets for a throttle body, intake plenum and IACV replaced
Checked that the EGR valve diaphragm wasn't stuck
Compression check - all good
No sound of a vacuum leak
Vacuum hoses all good

Earlier I had a misfire on cylinder #1 (more 'standard' misfire, regardless of the engine temperature) and the above repairs/replacements were done that time. Eventually, it appeared to be a fuel injector. All 4 of them were replaced with the aftermarket ones (STANDARD MOTOR PRODUCTS, FJ285). That fixed a problem for a while, than the new injector failed (again #1 cylinder). Now it's number #3 cylinder and the cold misfire. Do you think it's an injector again? The replacement is not cheap, so I wanted to know what's your experience and if there is anything I may be overlooking.

Thanks.
 

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1. What spark plug wires are you using
2. Did you check timing after replacing the dizzy
3. What gas are you using
 

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Discussion Starter #3
1. wires - currently I have PRESTOLITE, but I have tested the old wires (STANDARD MOTOR PRODUCTS) but no difference.
2. I have not checked the timing. If it's off can only one cylinder be affected? Is it sensitive to a engine temperature?
3. I tried various brands (Co-op, Husky, Shell, Petro-Canada, Costco) and 87, 89, 91, and 94 grades - no difference.
 

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Discussion Starter #5
Thanks for the suggestion, I will try to have the timing checked.
Another thing that comes to mind is a Camshaft sensor.

Yet, why is the misfire for cyl #3 only? Can wrong timing/bad Cam sensor be limited to a single cylinder?
 

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I don't think timing would affect just one cylinder or only affect the engine when it's cold.

I would think it is something unique to that cylinder and something that goes away after running for a while. A leaking fuel injector might do that. Does it puff a little black smoke on start? Does the oil smell like gas?
 

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Discussion Starter #7
No black smoke on start. Oil definitely seemed gas-diluted (and changed it while I am chasing the problem).
 

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My money is on a leaking fuel injector on that cylinder.

The fuel drains through the injector into the cylinder, past the rings and ends up mostly in the oil. The remaining fuel takes a while to burn off causing a cold misfire. Once it's warm the fuel is gone and all is well. When you shut it off the cycle repeats.

You might pull the spark plug on that cylinder and take a look and a sniff. May not tell you anything for sure, but worth a quick look.
 

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Discussion Starter #9
The old spark plug was all black and fouled. It fouled a new one in minutes. It does smell of gas.

I am replacing the injector tomorrow. Will report back...
 

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Meant to ask, which plugs are you using? I've been trying different NGK plugs...after reading a lot of old posts here and on the dash I'm going with NGK BKR7E next.
 

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Discussion Starter #11
I have tried various, no big difference, I plan to stick to BKR5EGP (G-power, 7090). On NGK website 'NGK BKR7E' is not listed as compatible with 99 G20. Which year is your G20?
 

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Discussion Starter #12
The injector replacement fixed the problem. No more misfires, the car runs like a dream. The cost of labor was for 3+ hrs. Besides the injector the part required was an intake plenum gasket.
Lesson learned, use OEM injectors.
 

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Glad you figured it out.

See you're in Alberta. I did design work on a power plant in Calgary a few years back. Beautiful country out to the west.
 

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Discussion Starter #14
It is, however there are practically no G20's here :). I've seen only two in the past 5 years. I imported mine from the US when moved.
 
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