Engine misfires after rain storm

[kbryant]

Hi
I have a 2011 Tahoe Q5i 4.3 MPI. Runs great at idle - no issues starting - under a load it misfires and backfires after sitting in a rain storm. I had this issue before about 2 years ago. Shop said it was a loose battery cable. Worked fine for 1 1/2 years until last week it sat in the rain. I’ve just replaced battery and fuel filter and check pickup tube. Anyone else have this issue and some ideas where to begin.

 

[kbryant]

This is an inboard motor

 

[matt167]

New distributior cap, rotor, and wires with di electric grease in the connections.. see if it comes back. It probably won’t

 

[kbryant]

Do you think it will continue to do the same problems after I change those out?

 

[Scott06]

Do you think it will continue to do the same problems after I change those out?

No that why he suggested to replace. Typical symptom of misfire when wet is bad cap rotor and wires. If you don’t know when they were replaced start there. Check your timing and spark plug condition as well.

 

[kenny nunez]

Does your engine cover allow water to land on the distributor? If so water is getting inside the distributor body through the flame arrestor screen on the side. You will have to rig up some type of shield to flow the water away.

 

[Scott Danforth]

you should have the crab cap..... those are a replace nearly every other year regardless type of thing. because the cap breaks down internally.

 

[kbryant]

Hello,
Here is an update - I replaced the Cap, rotor, plugs and wires, pickup tube and fuel filter. Boat ran great for about 10 minutes and now doing the same thing ither than it is not back firing anymore -- Just will not go on a load. I have ordered a new Ignition coil and see if that helps. Any other thoughts on what it could be? Thank you again for all the help.

 

[nola mike]

Any water entering your fuel system? Are you sure this is an electrical misfire?

 

[Scott06]

Any water entering your fuel system? Are you sure this is an electrical misfire?

might be worth running of a remote tank to see if it makes the problem go away

 

[nola mike]

might be worth running of a remote tank to see if it makes the problem go away

Or my favorite Easy Go to " throw a timing light on a coil wire test"

 

[Scott06]

Or my favorite Easy Go to " throw a timing light on a coil wire test"

i have become fond of the spark gap tester for this so you can see how strong the spark is vs just there.

ten or so years back went through an older evinrude outboard. Last thing I replaced was the coils - this was after fuel pump, carb rebuild, points and condensers... I had spark showing but it was weak as hell, ohm'd the coils and it was one of the few times I actually had a bad coil (2 in 40 years of wrenching). Both coils were way out of spec on ohm test but it still ran just not for chit when you hammered down

 

[nola mike]

i have become fond of the spark gap tester for this so you can see how strong the spark is vs just there.

ten or so years back went through an older evinrude outboard. Last thing I replaced was the coils - this was after fuel pump, carb rebuild, points and condensers... I had spark showing but it was weak as hell, ohm'd the coils and it was one of the few times I actually had a bad coil (2 in 40 years of wrenching). Both coils were way out of spec on ohm test but it still ran just not for chit when you hammered down

Yeah, ha ha actually going through that right now with my Merc. Has spark with the light but weak on a gap tester. Still a good first test I think, and if it's missing you know it. Also guessing that a weak but not absent spark is more common with a stator driven ignition, multiple places to weaken. A 12v system like a TB more likely to either have it or not.

 

[Scott06]

Yeah, ha ha actually going through that right now with my Merc. Has spark with the light but weak on a gap tester. Still a good first test I think, and if it's missing you know it. Also guessing that a weak but not absent spark is more common with a stator driven ignition, multiple places to weaken. A 12v system like a TB more likely to either have it or not.

Not sure on the stator driven set up but this 1974 Evinrude is a coil eater. I went through everything but the coils as I saw spark on the plugs, did the coils last which I may have been able to skip the carb rebuild.

When I did my repower and used the gap tester on the TBV then I saw what a really good spark actually looks like, so I learned something. I checked all my engines when running good to get a feel for what the spark should look like on that engine

 

[nola mike]

Not sure on the stator driven set up but this 1974 Evinrude is a coil eater. I went through everything but the coils as I saw spark on the plugs, did the coils last which I may have been able to skip the carb rebuild.

When I did my repower and used the gap tester on the TBV then I saw what a really good spark actually looks like, so I learned something. I checked all my engines when running good to get a feel for what the spark should look like on that engine

Yup, I also assumed carb initially and rebuilt it yet again. If you don't have one for your 'rude (and if it's a CDI type system), definitely get a DVA. Nearly impossible to troubleshoot properly without one. The merc also has a spark at the plug, but very weak. About to (hopefully) fix it with a new trigger...

 

[kbryant]

Well, another update. I finally have given up. It’s going to the shop. I can’t figure it out. This will be the last inboard motor I buy.

 

[kbryant]

Thank you everyone for your time and advice

 

[jimmbo]

High Humidity, which can happen when it rains, can wreck Havoc on Plug Wires. It can also help create conductive Paths on the Ceramic Insulator of the Spark Plug, especially if that Insulator is cracked

Outboards can be just as finicky, the newer ones, with all the Electronics, even more so