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pfbz

Poor running after spark plug replacement?

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2004 C2, 57K miles.

Just did some service work on my C2 and replaced my spark plugs. Car was running great, but did it as part of 60K service.

[*]No history of CEL's prior to change.

[*]Plugs removed were BERU Ultra R6-336.

[*]All looked fine with good color and minimal electrode wear.

[*]All coil packs/wires/connectors looked great and were reused.

[*]Spark plugs were replaced with Bosch FGR5KQE0. Gapped as set from factory, not mis-handled or dropped.

[*]Plugs were torqued to spec.

Since I removed the mufflers for improved access, I took them to a local shop while they were out and had a gundo/fister type 2 modification done on them, essentially connecting the input and output pipe with a new piece of 2" tubing. This shop has done many of these before...

Car is back together and running... I've had it out for a few drives since the work, and to me it feels rougher, particularly during warmup and maybe even less power. Maybe even detecting a minor misfire when cold? I'm not getting any CEL lights. Rechecked connectors, all are tight.

Some thoughts and questions:

[*]Obviously it could be the exhaust, and perhaps the ECU not yet adjusted to a slightly increased airflow or just feeling the roughness of more exhaust resonation?

[*]How quickly should the ECU remap to any changes to fuel mixture due to exhaust and/or plugs? Can I speed up the process by removing the battery terminal?

[*]If I am getting misfires, how tolerant is the ECU before it will trigger a CEL? Can i assume that if I'm not getting a CEL, the engine isn't misfiring and fuel trims are fine?

[*]Is there any way to log any ECU data short of buying a durametric?

[*]Could the Bosch plugs be making the difference? I'm sure I could do the job a second time much faster, but cylinder 4 seems like a major PITA even now that I know the procedure!

[*]Any other thoughts? I'm always hyper-sensitive to how a vehicle is running after I work on it, and perhaps it's all in my head...

Also, I'm not sure I love the louder exhaust.... I really miss hearing the intake howl (had previously done the silicone tubing intake od) and it makes me feel a bit like a tool now when I open her up on an entrance ramp or something. Am the only person in the world that doesn't like the Fister exhaust??

Edited by pfbz

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Just my two cents... I replaced the plugs on my Boxster (Beru) with Bosch last fall. Had the same impression of less response, low idle burble, etc. No codes shown. Just replaced them with Denso Iridium and there seems to be a noticeable difference!

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I doubt the exhaust mod would be causing the problems. I would double check that the plugs are all seated properly and all of the coil connections are solid. I had a CEL after doing my plugs and I would have bet money that all of my connections were good. I went back in and sure enough, one of the coil wires came off when I tugged on it. I used the OEM Beru plugs and I see a noticable improvement after the change. The old plugs were Bosch but I can't say whether the improvement was due to the brand or just the age of the old plugs. My guess is the latter.

I have the Fister mod too, but I had it done by Fister. I love the sound and I have a Cab. I guess it depends on how faithful your local shop was to the Fister mod. Maybe you just need to get used to it. I've had mine for 2 1/2 years and I still get a kick out of them.

Edited by wyovino

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I would have bet money that there would be no noticeable performance difference between different brands of quality spark plugs that are of proper spec and not defective... but I guess I'll replace the plugs and see if it makes a difference.

I'm sure the job will be much easier the second time, but honestly I'm not looking forward to dealing with Cylinder #4 again. I picked up some new tools that should make it a bit easier, a ratcheting bit wrench for the coil pack torx bolts and ratcheting external torx wrench for the covers, but those muffler mounts make the plugs and coil packs a bit hard to get to, with or without the mufflers on!

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Did you verify the spark plug gap prior to installation? "Gapped as set from [bosch] factory" might not be the factory specs or might have variances from plug to plug, despite them not being mishandled or dropped.

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I didn't think the quad-ground type plugs were really meant for any adjustment... It is also much more difficult to measure than a standard plug.

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