Dirty plugs

[Erik Timmerman]

Let’s diagnose my truck on a ski forum. Here are two of my spark plugs. It’s a 1979 F250 351M engine. Not running great. These plugs were new in August and they are the correct plugs per the book at NAPA. There is probably less than 1000 miles since installing them.

 

[scott43]

The left one ain't bad. The right one is brutal. Looks like electrode erosion in addition to soot contamination and is the insulator busted as well?? Left one might be a bit lean but no evidence of detonation. Might want to do a compression test and check the valves on the right one..too rich maybe. Do you have other symptoms? How do the rest of the plugs look? Left one might be vacuum leak.. Right one, dunno. Sometimes those trucks get carbon up behind the valve. Is it missing?

 

[crgildart]

Right one's got earmarks of a bad cylinder.. Rings or valves or both. Is there a tale tell puff of smoke out of the exhaust when one of the cylinders fires more than the others?

 

[Erik Timmerman]

 

[Erik Timmerman]

I wouldn't say it's smoky as much as it's sooty. The picture above is after a couple of minutes of idle. It ran pretty good last summer with new plugs, wires, cap and rotor. I'm kinda leaning toward just putting some new plugs in since I don't drive it much. What I am wondering is if I should use plugs that burn hotter? Also, maybe the carb is too rich? It does use some oil, but not tons. Also, I think there is a problem with the automatic choke. Some days, the truck seems like it is starving, other days the idle won't drop below 2K RPM, but those problems are both pretty intermittent.

 

[François Pugh]

Time for a wet and dry compression test on each cylinder.

 

[RiderRay]

I would check the rest of the plugs, see if you can get it running. If it starts, try to get it warmed up. Then try adjusting the carb. Cheapest way to start.

 

[Brad J]

Time for a wet and dry compression test on each cylinder.

The compression test is the first step, if that cylinder is low it may be several things , most require more mechanical ability than I have.

 

[scott43]

I would check the rest of the plugs, see if you can get it running. If it starts, try to get it warmed up. Then try adjusting the carb. Cheapest way to start.

Yeah this is the cheapest easiest way. If it's just rich in one cylinder it's almost certainly an ignition issue..either bad plug (more common than you might think..), bad wire or bad distributer. You can adjust the carb but keep in mind it's a shared plenum so more than one cylinder should be rich. After ignition can be ruled out I'd suspect a miss caused by carbon build-up in the cylinder head port. Then I'd move to compression test. It's possible the valve oil seal is leaking as well and pulling oil in. Do you notice blue oil smoke on startup or on acceleration? You could swap a wire with another cylinder and see if the wire is the issue. Cracked or bad distributor is harder..WD 40 is an easy spray on test for cracked distributor.
Choke would again affect all cylinders unless that particular one is weak for some other reason like bad ignition.

 

[François Pugh]

The compression test is the first step, if that cylinder is low it may be several things , most require more mechanical ability than I have.

Lots of videos on youtube to help simplify things. e.g.
If you are lucky you can find one for F250s.
Bad dry compression but good wet suspect rings. Good dry compression rings and valves good. Maybe ignition problem or fowling from sucking up oil through PCV - did they have PCV valves back in '79? I think so.
How are the other plugs?

 

[oldschoolskier]

While these may be the correct plug and same number from an auto parts store, occasionally the manufacturers product sold by the parts department is slightly different, enough so that you have nightmare issues. I ran into this with a 06 Dodge Caravan. Didn't believe online it till I experienced it first hand.

With your truck check if this makes a difference, do a search online.

 

[Andy Mink]

View attachment 101081

I didn't know diesels have spark plugs.

 

[François Pugh]

I do think that you are running a little rich. Worn rings, worn valve seals, and running too rich are not mutually exclusive. Hopefully your vehicle is old enough not to have to drill out the set in place mixture screws so you can make adjustments. Your floats might have also sprung a leak (happened to me once), so they sink and the carb doesn't know the bowl is full. When was the last time the carb was rebuilt, if ever?

 

[Erik Timmerman]

Supposedly the carb was rebuilt last summer, and it's actually not that old of a carb (receipt was in the glove box when I bought the truck).

I'll try and get all the plugs out on Friday and see how they look.

 

[johnnyvw]

Looks like an ignition issue, if one plug is clean and the other is covered with soot. A lot of spark plug wires from the "big box" auto parts stores are junk these days...I've even had issues with Bosch wires. Especially with the exhaust residue on the snow.
If the truck isn't driven much, you could be getting condensation under the distributor cap. Take a look before the next time you start it, and wipe it out with a little alcohol on a clean rag (make sure you let it dry thoroughly before you start it....)

 

[Erik Timmerman]

btw - last summer I took it to the Ford dealer and they were like "nobody here knows how to work on that".

 

[scott43]

Haha. Not surprised. Not a knock really just cars are different.

 

[graham418]

I would hazard a guess that its a bad valve. Worn valve, bad seat, bad spring. Something like that

 

[scott43]

I would hazard a guess that its a bad valve. Worn valve, bad seat, bad spring. Something like that

I was kinda thinking that too but that's an expensive fix. Hoping it's the ignition.

 

[crgildart]

Dirty Plugs sounds like a cool band name..