Cyl. 3 & 6 misfire all of a sudden?

So on my way home from work yesterday my truck out of the blue started running absolutely horrible. At an intersection the engine started sputtering extremely bad, and after accelerating and RPMs rising above 1500 the sputtering went away but I was extremely short on power, with the truck struggling to make hills even at full throttle. Not soon after my old friend CEL came on. When I got home the codes were cylinder 3 and cylinder 6 misfire. Nothing weird showing up on any gauges before, during or after.

Today after work I did some investigating under the hood. Put an inline spark tester on the supposed trouble cylinders and spark looks strong. Did a quickie compression test (only pulled one plug at a time, and by the time I got around to it the engine had cooled way below operating temp) and came up with these numbers:

1: 118 2: 111 3: 130 4: (Did not check, PITA to get to.) 5: 120 6: 120

So these numbers are way below what I was expecting, but they are all for the most part within 10% of each other. I didn't have someone to look at how the gauge pumped up while I was cranking, but these are fairly decent numbers right? So this should rule out a bad valve or gasket or rings? I threw a pair of JY heads with only some lapping of the valves on about 7 months ago because cylinder three had a burnt exhaust valve. Before it became apparent cyl 3 valve was shot (roughly 14 months ago), I had changed the coil pack, cyl 3 fuel injector, all plugs and wires, and the fuel filter. Up until now the truck has been running great, I don't really understand what happened. The only other thing I think I should mention is that when I began reassembling the truck I was on my 18th hour, and can't remember if I put the head gaskets on correctly. I may have put the blocked coolant port side in the back, but I can't remember. If this was the case, wouldn't it have been a problem right away? I have made several long roadtrips and pulled some heavy trailer loads with not so much as a hiccup.

Probably also worth mentioning is that the cam sensor is still the original, with the truck having about 145K on it but the engine only about 75k. I planned on having it changed soon, but it never gave any warning signs of going bad.

I noticed a bit of a burning rubber smell before pulling into the garage, though this could have been coincidence. It was cold out today (about 15F), but I noticed no vapor or condensation from the exhaust. No funky smell either.


So any ideas? If I can't find something soon I might just take it to a shop, but I'd rather find and fix the issue myself. This is my DD and work machine, so it needs to be dependable.

First question. What vehicle type, engine type and what model year?

Yeah, I realized when I woke up this morning I forgot to put that. It's a 98 Ranger with the Vulcan 3.0.

Yes, starter motor can't generate enough RPMs for compression test unless all spark plugs are removed.
Engine doesn't need to be hot for the info gained to be valid.

I would test fuel pressure, the '98 should show 65psi at the rail.
I believe 3 and 6 injectors are each at the end of the rail on their sides, could be the common denominator.
When was fuel filter last changed?

CPS needs to be changed, just as general maintenance.
CPS is used for injector timing and advanced spark timing, I don't think it could cause your current issue though.

Yeah I was being lazy cause my feet were wet from work. But these numbers would be better with all plugs pulled, not worse right?

I will have to get a fuel pressure gauge. Low FP occurred to me but my tire gauge was all i had to check and that didn't work, haha.

Filter was changed maybe 18 months ago.

I have also some kind of back fire every once in a while. Almost like a fart out of the exhaust, but not extremely loud. I bought a FP gauge but it was $50, so I'm not sure I want to open it or just return it to the store (I can't see using this thing too much after this). My cat doesn't look like it's getting crazy hot either, but I didn't point my thermometer at it and I don't know what it normally runs at either.

If I can't figure something out tomorrow morning I'm taking it to my shop. I'm afraid of being stranded someplace with $4000 worth of tools and equipment in my truck and I have nothing else to drive.

Yes, all rings and valves leak pressure, metal to metal seals just don't hold air very long.
So faster RPM with only 1 compression stroke every 2 RPMs means you should read compression of about 170psi per cylinder.

But the only real valve of a compression test is if one cylinder is way way off the others.
Your numbers are fine for that test, if you had one with 80 or less I would follow that up, but you don't so in that respect all is well.

You can use any pressure gauge as long as it goes up to 100psi.
But you can attach it to schrader valve NOT just hold it on.

Have seen people bleed system pressure then remove center on schrader valve, exactly like removing valve stem from a tire, same tool.
Then use a hose clamp to attach gauge to schrader valve.

^That sounds right up my alley, haha. Definitely have that stuff laying around, and I did end up returning the FP gauge to the store.

I also took my truck to the shop, which turned out to be a good idea (possibly not monetarily). He did a code check and in addition to 6 and 3 2 was also misfiring. There was also system too lean something or other. Fuel pressure was exactly 65. He also checked the spark and it looked good as well. At this point I had to go so I left the truck there for a few hours.

As it turns out, despite the plugs only being about a year old and looking good when glanced over, a closer inspection revealed that they were cracked and damaged. The mech replaced them (only th trouble ones though, not real happy about that) and voila! truck seems better now. I threw in some Lucas fuel system cleaner anyway since I haven't been doing that. CEL is out and I can drive around again. For this plus a quart of tranny fluid $103, also not entirely happy about that but it is what it is. I'll have to pick up some side work.

Thanks for the update

I had a plug fail after about a year, but it die outright while I was driving along at about 60mph, very odd thing, all of a sudden a steady miss.
Plug looked fine when I pulled it, so checked if cylinder was getting fuel, it was.
So tested spark plug........deader than a door nail, thing wouldn't spark even if it was on Ben Franklin's kite.

I've heard of several instances on that happening, as well as times where the plugs were bad out of the box. It just seems really odd that all of a sudden 2 and then another would go within such a short time. And like I said they looked okay (though I'll admit I was looking more at fouling and electrode condition than the insulator) and with an inline spark tester everything looked good as well as with a timing light. I even ran the truck in pitch blackness and looked for spark jumping and saw nothing. I guess the insulator was damaged and the spark was jumping to the block giving the appearance of everything working.

So today I walked out to my truck to discover a puddle of coolant under it. Antifreeze was shooting out of the weep hole on the water pump. Yay! I love how you have to take 3/4 of the belt driven accessories off to remove one bolt on the pump housing. Tomorrow I reassemble.

I guess you have to expect these things with an older vehicle. Well some of it is still old...