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I have a Impala SS 2004. I've been chasing a misfire at idle on number 3 cylinder . I've changed coil pack, spark plug, plug wire, swap fuel injector, finally ran a compression check. I got 70 PSI on number three and 90 on number five. This is a running compression check. I didn't block the throttle plate open and pull all the plugs. I just cranked it up and let it run on the other five cylinders. Do you guys think it's valves leaking ?the car doesn't use any oil doesn't smoke, I don't think it knocks from loose rod bearings or anything. Engine is fairly quiet and doesn't smoke. I bought it a month or two ago, it shows 135,000 on the odometer? So, what is the most likely cause of low compression on my 3.8? Thanks in advance
 

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You have done many of the things to chase down a cylinder-specific issue. Any OBD codes or other diagnostics to give a clue?

You might have some carbon build-up on the valves in that one cylinder. Since the vehicle seems to be running fine otherwise, you might want to try some fuel injection cleaner like Gunout Regane. There are other brands that also work well. It can clean carbon deposits throughout the system. Might be worth a try.....
 

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2002 Impala 3.4L V6 LT FWD 242k miles
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@Foosmadman : assuming those PSI numbers can be trusted, you're below the rule-of-thumb 10% variation between the highest and lowest compression readings. You seem to have eliminated the electrical possibilities ... and if you're not losing (or burning) any coolant or burning any oil (more than, say, a 1/2 quart over 5,000 miles), I guess I would suspect a valve not closing tightly.

Another consideration (maybe) ... something commonly overlooked because they don't necessarily throw codes: O2 Sensors? What are your short-term and long-term trim readings? What do your fuel economy numbers look like? I'm not sure if an O2 sensor could give such bad information to the ECM that it could result in a mis-fire, but I would think it's possible. Just another thing to check before you start tearing into that Head to get at those Valves ...
 
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