Misfire when humidity is high

C-ya

Original poster
Member
Aug 24, 2012
1,098
I'm not sure if any of you saw my post in the full-size forum about a GMC C1500 4.3 I sold that had misfire issues when the humidity was high or not, but my TB has been doing it, too. It isn't always the same cylinder. The first time I noticed, Torque showed Cylinder 2 not playing well with the others. The next time it was either 2 or 4, and this time (we had a 3 day rainy period) it was #4 that was basically not firing at all. In a 3 mile trip, there were almost 10,000 misfires. It will set the code just about every time I drive it on these short trips to work or family visits. Today, it is less humid, so I should have no issue driving it (haven't even started it, but basing it on past experience).

Since these have individual coils, I'm planning on starting with a plug/wire change and use dielectric grease in everything when I put it back together and see what happens. On the C1500, the fix was to replace the coil. Since this one is moving, I don't think it is one particular item, but maybe more of a sealing fix. We'll see...
 
Oct 29, 2013
43
C-ya said:
I'm not sure if any of you saw my post in the full-size forum about a GMC C1500 4.3 I sold that had misfire issues when the humidity was high or not, but my TB has been doing it, too. It isn't always the same cylinder. The first time I noticed, Torque showed Cylinder 2 not playing well with the others. The next time it was either 2 or 4, and this time (we had a 3 day rainy period) it was #4 that was basically not firing at all. In a 3 mile trip, there were almost 10,000 misfires. It will set the code just about every time I drive it on these short trips to work or family visits. Today, it is less humid, so I should have no issue driving it (haven't even started it, but basing it on past experience).

Since these have individual coils, I'm planning on starting with a plug/wire change and use dielectric grease in everything when I put it back together and see what happens. On the C1500, the fix was to replace the coil. Since this one is moving, I don't think it is one particular item, but maybe more of a sealing fix. We'll see...

How many miles? I agree with your order of replacement so far (going cheap to more costly). I have also seen similar issues with a bad CPS.
 

C-ya

Original poster
Member
Aug 24, 2012
1,098
The truck has 150K on it. I got it with 140K and have been catching up the maintenace as I could. First was transfer case, an air filter at the first oil change, and the diffs. Next will be the tranny fluid and plugs/wires.

OK, so I drove it about 2 miles and it idled in the drive-through at the credit union, then I went across the street and shut it off. When I got back in, the CEL illuminated. I had Torque running before I shut it off and saw that it had accumulated over 3,500 misfires on #4. So, maybe it really is #4. It is very dry today and about 20*F when I went out. In the past, as soon as it was dry, the misfires cleared up.
 

gmcman

Member
Dec 12, 2011
4,656
Number 4 sits right under the hood seal opening, small half-moon size void in the sheet metal and allows water to drip right down on the coil. Remove the coil and look for condensation.
 

RayVoy

Member
Nov 20, 2011
939
I had that problem (water dripping from the cowl) on #4. I found, that the coil was still good, but the high voltage rubber cover (the boot), from the coil to the plug, was burned on the inside. My GM dealer had the boot in stock and a new boot fixed the problem. I added a rubber strip to the hood seal and the problem went away.
 

C-ya

Original poster
Member
Aug 24, 2012
1,098
I'll check that out. I did notice the bracket holding the #4 coil was more rusty than the others. Maybe a strip of rubber is in order. So far, with the new plugs and wires, no misfire whatsoever. I'd like to keep it that way!
 

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