My pickup has 225000 miles on it. I've always kept the rpm's down and changed oil often.
Other day it was really cold so I left it idling so I could get in and warm my fingers occasionally.
It ran for a couple hours, then it started missing on a cylinder.
Drove it home 40 miles......still missing. Changed plugs the next day as I thought maybe one was fouled. It seemed to run good, but I didn't let it run long.
Next morning it was -17 and I had a bugger of a time getting it started. When it did, it's mis-firing again, possibly on more than one cylinder.
Checked distributor cap and rotor, they look good.
I'm thinking either the timing chain/gear has give out, or the plugs got wet again because it didn't start right away the next morning.
Thoughts???

Mine was missing a little a few weeks ago and I ran a tank of premium gas no ethonal threw it and it hasn't missed since. Might try seafoam and also check to see if your low on antifreeze.

that many miles you might want to do a compression check. if that's good you know the problem is external. check to see if each plug is firing. if one isn't you need to find out why. cap, rotor, plug wire or plug. if compression is good and all plugs are firing its in your fuel injection someplace.

Had one do the same thing 98 with a 7.4 liter and it was the injector

When were the plug wires last replaced? Could also possibly be an injector issue.

Replace the wires, cap, and rotor; they're cheap and a good first starting point. If you think it's a timing issue, beg/borrow/steal a timing light and check timing. Does it have regular fuel injectors or is it that goofy CFI system with the "spider" under the intake manifold? Those CFI systems are terrible and known to leak or otherwise go bad, causing all manner of issues. Your best bet is to find out which cylinder is missing by pulling off spark plug wires from the cap or the plug with the engine running, which ever is easier. Use a long pair of needle nose pliers to pull them off, and ground your pliers by wrapping a wire around your pliers and the other end at the negative battery cable; that'll prevent you from getting shocked. Pull one wire off, and if the engine runs worse, that's not your missing cylinder; if it runs the same, you've found your missing cylinder. If it's got regular fuel injectors, you can unplug the injectors one at a time and get around the whole 40,000 volts through your arm issue Oncew you've found the missing cylinder, start swapping components until the miss moves: for example, say cylinder 1 is missing. Move the fuel injector to cylinder 2 and see if the miss moves to cylinder 2. If it does, then you've just proved that fuel injector to be the cause of the miss. If the miss stays on cylinder 1, try moving the spark plug, or the wire. You get the idea. This assumes that it's a failed component causing the miss on one cylinder. IF it's got the CFI system, obviously you won't be able to move the injector, you'll have to just eliminate everything else first. You could also have a leaking intake gasket, burned/leaking valve, worn cam lobe, loss of compression, etc etc.

Cap. Check again.

Osky

you do a compression check without spending a dime. if compression is bad there is no point buying stuff just to see if it will work. do some trouble shooting and you can buy what you need to fix it, whether its a rebuilt crate engine or a spark plug

Mine did the same last year at this time. It was the pick up module behind the distributor.
It's a 91 350 TBI.
I checked timing slack first by rocking damper while watching rotor.
My timing chain could be next.

Sometimes if compression is off you can hear it in the cranking. Pull the coil wire off the cap and crank it. You'll be able to hear an imbalance in the compression of the cylinders. A good engine goes ruh ruh ruh ruh ruh ruh ruh ruh, an engine with a loss of compression in one cylinder causes an imbalance of the sound in cranking...ruh ruh ruh RUH ruh ruh ruh RUH ruh ruh ruh RUH ruh...(those are official engine sounds found in every chevy workshop manual)

Could be the Ignition Module .. Check it out I've had the same issue on a Small block chevy setup.

Plug wires was my first thought. You can either check the resistance on each one to see if they are okay, or if it's been awhile just replace them.
I'll bet it's a simple problem like plug wires.
You could also run it by O'Reilly's and have them plug it into their diagnostic tester. Is the check engine light on?

THIS

They had a bad habit of blowing the headgasket between two clys. In fact, we're replacing one right now that did it.
First things first. make sure your working with good compression, then go down the line.

Thanks for the replies. Yes, check engine light is on. I went to tech school for auto mechanics in 1974 but engines have changed slightly since then. I will check compression, then try to get an analyzer on it.

I had one that the water pressure ate a little groove in the block under the head gasket from a water port to a cylinder. Was blowing white smoke of course. Thought it would be a head gasket change b engine change.but ended up being an engine change

Start with the simple stuff. Put some SeaFoam in it.

Also see if it occurs after a rain,that might indicate bad wires possibly.

But...don't jump into complex stuff, many times its the simple things first.

Start it up in the dark. Look for arcing . Lots of time the cap may have a crack but don't see it unless you check it after dark.

Check the fuel pump pressure.

Read the codes first. Much simpler than everything else, including the compression test.

My 97 that had the 350 Vortec was famous for cracking the distributor cap. I had to replace it once a year. Something about a screen in the cap or some such thing. I personally would start there and buy the lifetime warranty one from O'Riely's . This fixed my problem every time. It would start missing when it needed changed. I replaced the rotor at the same time. Just an idea