What would cause a melted piston

[lil_Blue_Ford]

So my 1992 Ranger (4.0 OHV) was running fine, no complaints at all until it blew all of the the oil out on the highway. Would still start and run (badly and loudly). Pulled it out and tore the heads off to find a piston melted.

Pretty sure the 4.0 OHV in my choptop has suffered the same fate, but I haven’t yet pulled it or tore it down yet, but it has the same issues (blew out the oil, runs badly and loudly).

So what is the culprit? And are these motors completely destroyed or can I just replace the bad piston?

 

[19Walt93]

Please don't just throw one piston in it, go through the whole thing so you don't have to do it over. I've seen pistons like that in engines that had severe spark knock and a deaf driver. 351's would do that if the plug wires for cylinders 5 & 6 were too close together and it caused induction crossfire.

 

[lil_Blue_Ford]

Please don't just throw one piston in it, go through the whole thing so you don't have to do it over. I've seen pistons like that in engines that had severe spark knock and a deaf driver. 351's would do that if the plug wires for cylinders 5 & 6 were too close together and it caused induction crossfire.

I’m only half deaf and the radio wasn’t all that great in it so I rarely turned it on. Thing ran smoothly an quiet up until it melted the piston. I put new plugs and wires on it when I had put it on the road and it was less than a year later that this happened. The choptop had some valve clatter before it blew and the motor was a bit worn.

 

[ronclark]

My old 2.9L blew a hole in the piston, when I tore it apart I found the injector on the same piston was crammed full of rust. my guess from the steel fuel rail.
Not 100% sure but my guess is that caused that cylinder to become lean and the piston failed.

 

[dvdswan]

Pinging can cause that. Running lean.

 

[rubydist]

I agree, the most common reason for the piston melting is running lean.

 

[lil_Blue_Ford]

Interesting… I’ll have to go back and check, but I don’t think I saw any indications of lean. For sure didn’t sound lean, don’t ever recall hearing it ping at all and it was on the thirsty side, 12-14 mpg.

 

[Uncle Gump]

9 times out of 10... you see a hole melted through a piston... Pre-Ignition.

You don't hear pre-ignition... unlike detonation. Yes... they are different.

Pre-ignition lites the air fuel mixture before the timed spark event. Detonation occurs after the timed spark event.

Most likely causes of pre-ignition is to hot of a spark plug... carbon or any other deposits or deformation on the piston top getting hot enough to glow and lite the air fuel mixture prior to the timed spark event.

Lean mixtures will cause increased combustion chamber temps and could be ultimately what caused the hot spot but I don't believe a little lean will cause melt through without it causing a hot spot prior.

EDIT... Detonation in extreme cases will look as if someone beat the piston with a hammer until broken.

 

[Shran]

Well a lean condition will result in a hotter combustion temperature...rich is cooler. Not sure how you would hear it running lean or rich? Bad vacuum leak on that cylinder maybe?

4.0s were somewhat known for spark knock from excessive carbon buildup, your pistons are pretty clean though. Anything on the heads?

 

[lil_Blue_Ford]

Well a lean condition will result in a hotter combustion temperature...rich is cooler. Not sure how you would hear it running lean or rich? Bad vacuum leak on that cylinder maybe?

4.0s were somewhat known for spark knock from excessive carbon buildup, your pistons are pretty clean though. Anything on the heads?

I’m saying I didn’t hear it pinging or anything like it could be running Rich/lean. Didn’t notice any likely vacuum leaks, I had resolved a few early on where vacuum lines were broken or disconnected. Don’t have any pictures but the head looked ok to me, I’ll have to pull it out and get a pic.

Head gasket looked fine, lower intake gasket looked fine, I really didn’t see anything that I noted as out of ordinary other than some piston chunks in the intake and of course the melted piston. Want to say I put the recommended Motorcraft plugs in as far as heat range, probably used double platinums. If it wasn’t Motorcraft then I would have used Autolite. Same kind of plugs I’ve used for years in multiple trucks. Aside from being a little thirsty, I didn’t really notice any issues that hadn’t been resolved by the time it melted down.

I dunno what exactly went wrong with the motor in the choptop, but I’m suspecting a similar piston blowout. That I know the engine was worn and I know it had valve chatter. I also know there shouldn’t have been any vacuum leaks because I had done head gaskets and everything with Fel-Pro. So that one had been apart and inspected.

 

[dvdswan]

Then, it was Col. Mustard in the Garage with the Lead pipe that did it.

 

[gaz]

How many miles/km were on this engine/vehicle; was it the original engine...never rebuilt?

I recommend a good hard look (inspection) of the valve guides, valve seats and rocker arms. If the guides were worn, it was using oil. If it was using oil, it was always some state of low; if the oil was always low the guides were sucking air instead of oil...lean.

 

[lil_Blue_Ford]

The 4.0 in the choptop was unknown mileage. It was definitely pretty worn, I didn’t really expect it to last super long but I needed a motor in it for the 20th anniversary ride and it’s what I had. The cylinders had a noticeable lip at the top where the rings topped out at. So I’m kinda bummed but it was also kinda expected to be near the end of its life.

The 4.0 in the red 92, well that’s a horse of a different color. I’d have to look for the exact mileage but it was around 150k. Original engine, never rebuilt as far as I know. Want to say you could still see crosshatch in the cylinders. No wear lip from the rings. Everything in the casual visual inspection when I tore it down looked normal other than the piston pieces and melted piston. It did go through oil, a lot of oil, but it also had leaks. Mostly valve cover. Left drips everywhere it got parked. One of those things I never got to resolving before it blew up. So I don’t know if it was using oil or just leaking it out. I was getting oil $10/5 quarts at the time so I figured I’d deal with stopping up the leaks once I got the green Ranger fixed. I carried a jug with me and tried to make sure it never got really any below the markings on the dipstick. Sometimes that did happen. Truck had a low oil light and once in awhile it would light up so I would top it off then.