Why would a 401 spontaneously fail? Also, I need a new 401.

[Nikkormat]

Earlier today my 401 decided to throw something out of the bottom of the oil pan. I BABY my baby, I wont even drive her up 1-70 because I don't want to be mean to her. But as I take my foot of the gas to slow down for my exit I hear a loud BANG!!! My oil pressure gauge bottoms and my temp gauge skyrockets. I panic, turn it off, and roll of onto the shoulder. A fire truck was behind me and thought that I caught on fire! All five quarts dumped onto the hot exhaust and left a smoke cloud that would have made a run away diesel look like an easy ordeal.

But in the aftermath of this incident I can only be left to wonder... What could have caused a happy, low miles, and meticulously maintained 401 suddenly self destruct?

[Mrpatatomoto]

Do you know what it threw? That would help to narrow it down.

[Stuka]

If it put a hole in the oil pan, I am sure it was a rod.

How many miles were on it? AMC V8's do have a known oiling issue, which most people fix when rebuilding them. It typically effects the rear cam and main bearings.

Its ultimately going to require you to pull it apart and take a look inside.

PS: An AMC 304 or 360 will bolt right in place in your 401, if you need an engine ASAP. 360's are by far the easiest to find.

[Nikkormat]

It's parked at a friends shop because it was cheaper to tow there and because I don't have the hoist, stand, or level ground to remove an engine right now. So a diagnosis on what large piece of 401 is now missing will have to wait. The engine had been well cared for all of it's 110k mile 40 year life. The damn thing didn't even leak! Oil pressure was fantastic on the gauge in the cab and when checked with an external mechanical gauge it also showed spot on oil pressures. I was dead on the full mark 30 minutes prior to catastrophic failure so I know that low oil wasn't the culprit (I'm not ruling out pressure as the cause though). When I first bought the Jeep I discovered a slight tick, however I isolated that as being a leaky collector gasket (when the collector was wrapped with a rag to test my theory the tick was gone, uncovering the flange made the tick return instantly so I assumed: leaky collector.) But a few seconds (and were talking seconds here) there was a very loud tick and then a metal on metal noise before the BANG which signaled the end of my 401. Also the exhaust leak was on the drivers side flange.

The hole in the oil pan makes it appear as though whatever came out of my block came out of the passengers side head. The holes in the oil pan face the number six cylinder at a 90 degree angle. Could it have been a dropped valve? Would a rod shoot on a straight trajectory like that?

I realize that a 304 or 360 would bolt into place of my 401 but it seems silly to waste my time and money re installing a smaller motor I will just have to take back out later. Plus my carburetor is tuned well enough that I'm getting 13 mpg out of my 401 with stock ignition so it would be a real pain in the ass to start my carb tuning all over again.

[FSJ Guy]

It's not a 401, but I have a tired old 360 if you need something to keep you running.

A valve wouldn't break the head or the block, I don't think.

A broken rod can do interesting things to an engine block, however.

If your crank is OK, maybe you can take my 360 and make a stroker? Not sure you're interesting in the time and $$$ that would entail, however.

[fulsizjeep]

That's a bummer. For a valve to drop through, the piston would have to be busted up. My guess is rod cap ripped through the oil pan.

There is a complete 401 for sale in CO Springs at FN Jeep.

https://www.facebook.com/17076587627640 ... =1&theater

[jaber]

I have a 401 that threw a rod cap before I got it. Mine chewed up the crank and will need $300 in crank repair. I think mine threw #6, but been a while since I looked at it, so dont remember.

Good luck...

[FSJunkie]

401's can take a thrashing. The police 401's were specifically designed to sustain speeds in excess of 100 MPH for 4 hours straight with no resultant engine damage, and the only difference on them was they ran 6 quarts of engine oil, had a double roller timing chain, and sodium filled valves.

It's likely the manifestation of a seed that was planted when the engine was built. Perhaps the rebuilder didn't get a rod bearing aligned quite right and it just now decided to go. Maybe a chunk of sludge broke loose, clogged the oil to a rod bearing and that bearing spun. Maybe there was a hairline crack from a casting defect that finally fatigued and went...

it was certainly some kind of fluke, metal fatigue, or defect.

[Tatsadasayago]

I discovered why my 401 decided to do something similar while driving along at 55Mph...

Back then I hadn't learned about the distributor drive gear issue with AMC V-8s and compounded it by using a too-thick distributor base gasket AND leaving the hold-down clamp loose enough so I could adjust the timing with my hand. Mistake number one and two.
What this did was place even more pressure on the drive gears and slowly grinding them down to a knife edge.
While out putting around in the hills, the gears finally sheared off killing the engine. I replaced the distributor drive gear but not the one on the camshaft. That was mistake number three.
I also didn't pull the oil pan and clean out the gear fragments. Mistake number four.

So while I was heading home driving along all mellow, with a Nevada Highway Patrolman right behind me...the poor engine did it's grenade imitation.


After tearing down the corpse I found that the oil pump had been jammed with the gear fragments causing it's drive shaft to break. I don't know how far I drove it after that until it exploded, but am guessing around 18 miles. I had good oil pressure when I glanced at the gauge as part of my usual instrument visual sweep.

I will mourn with you over the loss of your 401 brother!

Jim

[Nikkormat]

I guess things like this happen when you put as many miles on an engine in a months as it has seen every year since 1989. Every one I have talked to about it who has suffered similar catastrophes said they had an oiling issue. Either filters collapsing, pumps seizing, or sludge breaking free and clogging the passages. They all also had failures on the number six cylinder ironically. So go outside and hug your motor; it may be the last time...

[tgreese]

Just an editorial comment ...

Many things can happen to engines. However, a seasoned engine should have very little chance of catastrophic failure if you keep an eye on the vitals, ie oil pressure and coolant temperature. Not saying that the OP's engine could have been saved by paying attention to the vitals, but IMO many catastrophic failures could be prevented that way.

Now, new engines are a different story. If there is a problem in assembly or with one or more of the parts used, the engine will fail pretty soon after installation. And they do, on a mile-by-mile comparison with seasoned engines.

So if you have a good running engine with strong vitals, keep an eye on the oil and water and it should serve you well for many miles.

[tgreese]

Another thought -

I like mechanical oil pressure gauges because the lag between the pressure and the reading is basically zero. But you have to watch the gauge. Noises should be a tip-off, if you're attuned to the normal sound and you aren't blasting the radio or otherwise distracted.

There are plenty of kits (and DIYs) available to sound an audible or visual alarm if your vitals get out of range. You could take that a little further and data-log your vitals, along with speed, load (throttle position) and ambient temperature. From this data, you could program the logger to sound an alarm if anything unexpected happens.

I wonder if there are any automotive computers that already do this ...

[haminawag]

In my experience engines that were poorly maintained throughout their lives tend to die from cumulative causes, no oil changes = low oil press. + worn rings = constantly fouled plugs, engine won't run. Or no cooling system maintennance + hard driving = routine overheating and eventually a blown head gasket and all the other problems that go along with that. But I've only seen two causes for an engine letting go, poor rebuild quality + high RPM, and poor parts quality + high RPM. My guess would be that whoever the PO of your Wag was thrashed and abused it at high RPM, and this smells to me of teenagers at play, I may be wrong but I've seen this same scenario play out many times, I won't touch a used car that has been driven by a teenager unless I'm willing to fully rebuild it.