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Old 01-05-2015, 04:38 PM   #1
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Default 04 Bonneville gxp random misfire

I seem to have a ransom misfire even after replacing plugs and coils. Happens consistently at highway speeds, generally when I peak 80mph. Ses will flash for a moment, go off, them come on solid. Car never feels off, but I'm wondering what could cause this?
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Old 01-05-2015, 05:09 PM   #2
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If the SES light is coming on, there should be a stored code.
Plug in a code reader or have it read, & post back the code or multiple codes please.

Last edited by Soft Ride; 01-05-2015 at 05:13 PM. Reason: multiple codes added
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Old 01-06-2015, 10:33 AM   #3
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You are on the right track with replacing coils and plugs, but it also could be a spark plug wire or fuel related. Go to Autozone and they will read your diagnostic trouble codes for free. The DTC will tell you if the problem is a random misfire or a misfire on a specific cylinder. Considering you already changed the coils and plugs a misfire on a specific cylinder is probably a bad plug wire. A random misfire could be a weak fuel pump, and can be further diagnosed with a fuel pressure test. Post the DTC and we can help further.

Just for *&G'*, pull the vacuum hose off the fuel pressure regulator and smell for gasoline in the hose. If you smell gas in the vac hose the diaphram in the FPR has failed, and the FPR needs to be replaced.

The SES flashes when the misfire is actually happening, and stays lit to indicate a DTC has been stored. If it flashes back right off the gas. If it flashes for more than a moment don't drive the car (while it is flashing). You are dumping raw fuel into the exhaust and the result could be quite nasty.
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