2F Misfire (8 Viewers)

[chbrow10]

Often when I go through deep puddles I get an occasional misfire that eventually goes away on the interstate ride home. Sunday we went driving around town to see the flooding and high tides from Cristobal. After I drove through some puddles over the paved road at like 30-40 MPH, it started missing like it usually does. But it didn't go away.

Yesterday (Monday), it was still missing. Same with today. I'm starting to get nervous.

I am trying to figure out which cylinder is missing. You can really only get to #1 and #6 easily with the laser thermometer. I can shoot the laser thermometer through the carb heat shield to #2 and #5. #1 is quite hot, like 300+F. #2 is 280-ish, #5 is 260-ish. #6 is 150-160. No clue on #3 and $#4. Swapped out the #6 spark plug wire. No change. Swapped out the coil wire. No change. I opened the distributor cap, no signs of moisture. I have a spare cap and button, so I changed them. No change, still missing.

I suppose pulling all the plugs is next. Not sure what I am looking for besides a wet plug. Plugs were changed 4000 miles ago, along with the wires that broke when I pulled the plugs, about half. Is it normal for #6 to be colder than the others?

Any advice is appreciated.

 

[Spike Strip]

Did you remove the dissy cap and dry everything out first? Are the vent hoses still attached correctly to the distributor cap?

Is the large 'O' Ring still there?

 

[chbrow10]

Yes. I used an air compressor on the distributor, but it was dry under there. Also, valves have been adjusted in the last 500 miles.

 

[Seth S]

Try spraying all of the spark plug wires and coil wire with wd-40 and see if it goes away.

 

[Seth S]

you can also start the truck and pull off one wire at a time and see if there is a noticeable change in how the engine runs. Put that wire back on and then pull the next. Go through all 6 of them. If you have a wire that when off makes no discernible difference in running then start there. if you have a couple of wires that don't seem to make any effect then I'd do a compression check and inspect the plugs. Did you do a new rotor when you replaced the cap?

 

[chbrow10]

@Seth S ,

I did that and found that it was #6. I pulled the plug and it was WAY over gapped. With the factory spec of 0.031, it was more than 0.040. I replaced it with a 0.031 gapped plug and the misfire went away.

Assuming I gapped them correctly when I installed them 4000 miles ago, what would cause a spark plug electrode to wear that fast?

 

[OSS]

-

 

[Seth S]

@Seth S ,

I did that and found that it was #6. I pulled the plug and it was WAY over gapped. With the factory spec of 0.031, it was more than 0.040. I replaced it with a 0.031 gapped plug and the misfire went away.

Assuming I gapped them correctly when I installed them 4000 miles ago, what would cause a spark plug electrode to wear that fast?

Most likely culprit is it just got installed with too much gap and as you drove it might have worn more from having to jump the larger gap. A bigger gap results in a grater spark area but at a shorter duration unless you are running a high energy ignition that can support the spark duration and the bigger gap. Otherwise you are forcing the spark to jump a longer distance and this requires more energy which comes at the price of being a shorter duration and could also lead to an increase in wear.

 

[Seth S]

Think of it like this: you have 100,000 volts to work with. If the gap is .030" maybe it requires 25,000 volts to jump that gap to start the arc....leaving you with 75000 volts to maintain the electrical arc for a set amount of time. Now if that gap is .060" maybe you need 50,000 volts to jump the gap and now you only have 50,000 left to maintain the arc...so your duration will be shorter. Eventually the gap becomes so big that all the energy is used just to jump the gap and you have no duration.

 

[Beehanger]

Did you remove the dissy cap and dry everything out first? Are the vent hoses still attached correctly to the distributor cap?

Is the large 'O' Ring still there?

View attachment 2335162

hey spike, circling back on this, I get some misfiring-like noises on deceleration at idle. I'm wondering if this is my issue. Can you elaborate a bit more on the nuances of misfiring and an improperly vented dizzy, or no o ring etc?

Curious what you meant here,
Dan

 

[Beehanger]

hey spike, circling back on this, I get some misfiring-like noises on deceleration at idle. I'm wondering if this is my issue. Can you elaborate a bit more on the nuances of misfiring and an improperly vented dizzy, or no o ring etc?

Curious what you meant here,
Dan

also, my spark plug gap is about .34 on one and .31 on all the other ones. I lost my adjuster and was using a different one when I double-checked. Can a margin this small cause the problem?

 

[Spike Strip]

hey spike, circling back on this, I get some misfiring-like noises on deceleration at idle. I'm wondering if this is my issue. Can you elaborate a bit more on the nuances of misfiring and an improperly vented dizzy, or no o ring etc?

Curious what you meant here,
Dan

'Deceleration at idle' is contradictory: If you're talking about some popping noises when you decelerate, that will be normal if you're desmogged, as the idle-cut solenoid will no longer function during deceleration.

Difficult to detect misfire in a pre-OBD vehicle without a scope, but if you suspect a bad cylinder, at idle, you can remove spark plug lead, one at a time, and if the cylinder is good, you should notice a decrease in RPM and a stumble. If you find a bad or misfiring cylinder, there will be little or no change.

also, my spark plug gap is about .34 on one and .31 on all the other ones. I lost my adjuster and was using a different one when I double-checked. Can a margin this small cause the problem?

.031" or 8mm. Gap it correctly. These are not high-energy systems.