1998 c1500 4.3 misfire troubles

[straight6chevyguy]

Hello everyone I have been having trouble again with my 98 c1500 misfiring/running poorly and I'm a bit stumped.

The truck is a 1998 c1500 4.3 v6 5 speed manual 2wd. with 163K on the od

So my problem is: Engine Idles a little rough and has what seems like a misfire.
Misfiring is worse or more noticeable at what I would call lite load higher rpm conditions. Like 1800 2k-2500 rang while shifting through the gears under lite load conditions and also driving at cruising speeds.

The condition is worse in damp wet weather.

The condition is like a time I had a bad dis. cap.

I have tried Three (3) different dis. caps, one that I had been using, one almost new and one brand new cap.

rotor button looks good.

Plug wires are not very old and test good with a ohm meter

Swapped around spark plugs one cylinder at a time with a good but used spark plug.

Brand new Ignition coil (original went bad June of last year)

Pretty new ICM (Replaced a year or so ago)

I have tried a different ignition coil and Icm from anther truck.

TPS sensor tests good.

I only had trouble codes for EVAP
I drove the truck a for a MONTH like this and had NO MISFIRE CODES I checked several times and then gave up and did not check for a couple months and drove the truck like it is (mostly just to work and back) (12 miles to and 12 miles back 5 days a week)

I Recently checked again and I had a code for a misfire on cylinder no.4
I changed out cylinder no. 4 wire just because. I thought at first it may have ran better but was wrong.

In my opinion it seems like multiple cylinders misfiring but I can not be 100% sure.

Fuel pressure is close to 60psi ignition on engine not running.
With engine running its just above 50psi. If my memory is correct the pressure is good?

Other then doing a compression check I'm not really sure what next to check so I'm hoping some of you out there have suggestions!

[crsgmc]

This post is probably not going to solve your problem, but it might help. I have a 1996 GMC with the 5.7. I purchased the truck about eight years ago. It had a lot of problems, so I replaced the engine with a GM factory crate engine. When I did this I replaced a lot of other parts including new wires, distributor, cap, rotor, plugs. Truck ran great. A couple of years later the truck died and would not restart. I had it towed home, and started to look for the problem. I didn't even consider the distributor, cap, rotor or plugs because they only had 16,000 miles on them. I won't go into all the work I went through, before I finally checked the spark. There was no spark. The cap and rotor looked almost new. I replaced the cap and rotor and the truck ran fine. I know you have checked the cap and rotor by multiple parts so it appears that it is not the problem. I am wondering if you have a week spark. Check the battery, battery terminals, and the ground going to the PS firewall. Check the coil, it maybe working but weak. Not much help, I know.

[straight6chevyguy]

Thanks for the reply. I have tried another coil from another truck, that being said, that truck doesn't run right either lol. But I think between the two coils and one being brand new I'm gonna rule out the coil for right now anyway. I will be checking the battery connections as well as the grounds gonna try to do that today.

It seems to me since the problem is worse in wet damp weather it would be an ignition or electrical problem rather than a fuel or compression issue, but what do I know. I will be doing a compression check as well just to rule that out.

[straight6chevyguy]

Well I checked a couple ground connections I cleaned them up and retightened the bolts No change.

I went ahead and did a compression test here is what I found.

Cylinder
#1 155 psi
#2 140
#3 150
#4 130
#5 145
#6 120

Compression test was done with engine warm but not up to full temp.

[crsgmc]

I think you are right. Wet conditions and poor performance tend to indicate an electrical problem. I know, I sound like a broken record, but....... what about the rotor. You tested multiple caps but you are using the same rotor. There have been a lot of posts with 1996 to 1999 that end up a faulty rotor or cap. I was one of them. I never could figure out what was wrong with the cap or rotor. I tested the continuity on the cap and it was good, no arc lines and light corrosion. Maybe some one else will chime in, but it seems in the last couple of years there are less people on the board.

[straight6chevyguy]

Well I suppose it wouldn't hurt to try another rotor. I'll give it a shot tomorrow.

[straight6chevyguy]

Well I tried the rotor off of my S10
The truck seems to run a lot better still not 100% though but it's running good enough for now. I'm a bit curious as to if it was actually the rotor or moving the plug wires a bit was what actually fixed I say fixed but it's really not fixed as I already said it's still not running perfectly.
I do have a check engine light on its been on for a few weeks now I still need to check the codes. maybe this weekend.

[HO455]

The first thing would check at this point would be for cross firing between spark plug wires and or ground. There are a couple ways to do that.
One is to have the truck in a dark place then using a spray bottle mist water on the wires while the engine is running. The bad wires will arc and spark and you will be able hear and see the the flashes.
Or you can use an old school 12v test lamp with the alligator clip clamped on a good ground run the probe along the plug wires looking for sparks. This method can also detect cracks in the cap and around the coil output.

There are videos out there on both of these tests.

[1998c1500]

I had a similar problem with my 98 4.3L. It would idle rough and when cruising at highway speeds, it felt like it was surging. Replacing fuel filter seemed to help a little, but it turned out that the spider injectors needed to be replaced.