1999 225XL EFI 3.0 V-6 port side center cylinder not firing at low RPM

[wma1972]

I have a 1999 225XL EFI 3.0 V-6 0G803689 with a Mercury Mirage SS 17p prop, pushing a Sport-Craft 231 walkaround. The port center cylinder (#4?) isn't firing at idle and the boat will barely plane. There is spark through the new Champion QL77CC plugs and new Mercury 8mm wires, the original stainless reeds are in great shape, I have good fuel lines, the stator bobbins ohmed out within 10% of 1100, RPM WOT is 5200rpm. I have removed, cleaned, and tested fuel injectors, 34 psi fuel pressure. I switched coils #2 and #4 and the problem stayed with the cylinder. Have cleaned several grounds (some minor corrosion). Where to now?

[Savage]

I would have started with a compression test.

[wma1972]

All cylinders tested between 93-95 lbs after warm up, plugs out, throttle open. Started there, forgot to include.

[lpugh]

Better check gauge accuracy and perform a leakdown test

[wma1972]

Will do and post results. Thanks!

[wma1972]

The problem stayed with the cylinder when I switched the coils because the #4 coil plug (from ecm/stator) was packed full of waterproof grease AND corrosion. I didn't notice corrosion at first but I noticed all the grease. I guess previous owner just smeared the plug with grease without cleaning corrosion. Cleaned out the connections on all 6 coils, only #2 and #4 had corrosion and only #4 was intermittent, but now it seems to be running on 6 cylinders at all RPM ranges. Will lake test tomorrow.

[EuropeanAM]

Good catch!

[wma1972]

Bad Wires.jpgSo after I cleaned the corrosion off the plug-ins, I lake tested the boat and the engine ran flawlessly... until it didn't. Once again, it started missing at low RPM and struggling to plane. Removed cowl and checked for spark on #4. It was again weak and/or intermittent. Pulled coil plug and could smell a burnt smell. Back at the shop, I disconnected the #4 coil plug and pulled the pins out. This picture is what I found. While reading back through the Mercury troubleshooting guide, I ran across this passage: "ENGINE MISSES @ LOW RPM, BUT RUNS SMOOTH @ HIGH RPM:1. Check the Harness for loose connections between ECU and Ignition Modules." Oh, sure. Now they tell me! Anyway, fixed those burnt connections (took a closer look at #2 wires, also ) and tested again. Runs flawlessly across the RPM range, idles like an angry kitten, and planes with ease. Thank y'all very much for your input and suggestions. Lesson learned, again; Troubleshoot, troubleshoot, troubleshoot.

[JOE54]

+ 100 dats right

[EuropeanAM]

Good job! That's a nasty looking pair of pins!