I have a 98 QC with the 5.9 When starting on a morning after a rain or if it is really damp, the truck seems to miss-fire until it has run for about 2 to 3 minutes. Check engine light has come on and shows a Map sensor issue. It has been replaced and still no luck. Coil, plugs, plug wires and the map sensor are all less than 2 months old. My last thought would be a cap and rotor replacement, but they look a little tough to get at, so I thought I would ask here first. Any insight and ideas would be much appreciated.

Scott

 

My truck was doing the same thing after a good rain. But after a good tune-up the problem stopped. You've replaced everything except for the cap and rotor so I bet that's what's causing the problem. Like you said replacing the cap and rotor is difficult. But you can do it if you take your time and be patient. I did mine myself. How I did it was to find a position you can work from and tape a mirror to the firewall and do everthing backwards. It will aggrivate you a lot having to work this way, but it can be done. Oh, and make sure that you put the rotor back on in the same exact position as the original one was. Plus make sure that the right plug wires are going to the right places on the cap. Good luck and I hope that this will solve the problem.

 

If anything it is the plug wires. The humidity in the air works into worn wires and creates too much resistance. My brother had about a zillion miles on his old Volaire when it wouldn't run when it rained or there was extreme humidity.

All this tune-up BS is nothing more than throwing un-needed parts at the problem. I'd bet lunch it is the plug wires.

 

+1 on the cap and rotor...somehow condensation is getting in the cap(cracked or misaligned or just really old)... it takes a minute for the heat in the capto dissapate the water that is why it won't do it again till the condensation has built backup(rain/humidity)..you have done everything else might well do the whole tune up...If you still have the same problem then try the new wires..(which cost more new wires or cap and rotor)

 

Well it is obviously a moisture problem and usually that indicates plug wires or distributor cap. Since your wires are fairly new it would indicate the cap. Was the problem present prior to replacing the other items 2 months ago or did it start after that?

 

I agree, replace the cap and rotor. The cap might have a crack or a whole in it. At any rate let us know what fixes it.

 

I've seen before where a crack in the cap is the problem, and when it heats up enough, the plastic swells that lillt bit and seals the crack...+1 on the cap and rotor.

 

ok....cap and rotor will be the next step. thanks for all the great info. I am not sure, but I am willing to bet it is the original cap and rotor (1998 w/139000 and counting.) I will be sure to let everyone know if it fixes the problem. Wish I had done it last night because it has rained this morning.

Scott

 

I had brand new cap, rotor, wires, and plugs and had the same problem this past winter after a blizzard. Got the truck in the garage and used de-electric compound on all of the connections and on every electrical connection i could find. No more miss even when driving through a couple of miserable snowstorms. That stuff is better than canned beer. (I drink from bottles).

 

Ok, got a new brass cap and rotor on. No news on whether this has fixed the problem, still waiting on rain or a very humid morning. I will give another update once the weather has happened.