I have a '95 Pontiac Grand Am GT with about 174,000 miles, 3.1L, that ran great until recently. Never had any problems with the engine, went to start it, it ran rough for a couple seconds, stalled, tried to restart it and it was locked up, starter couldn’t turn it over. Got it started the next day and it ran like it had almost no compression or was out of time. Pulled the heads, found chunks of aluminum in the intake, then severe damage to four of the pistons (the four nearest the timing chain). The other two looked good. The valves were not bent or damaged in any way, but the pistons and combustion chambers were destroyed. I don’t intend to fix it, I just want to know what happened.
No lubrication is my theory. What is the oil change history? It there evidence of sludge build up? What was the oil level on the dipstick? The motor either ran “dry” (no oil in the pan) or the oil pump failed, or the pick screen was clogged. For some reason no oil was flowing and friction destroyed your motor.
That’s a good theory, but how do you explain the aluminum in the intake?
By “intake” do you mean the intake manifold? If so, is there anything broken upstream from the manifold, or is the manifold itself broken? The aluminum had to come from somewhere.
Is it possible that aluminum from the block, head, or pistons could be shedding into a cylinder and then get pushed up into the intake?
Are you sure it is all aluminum? Or might you be looking at a mixture, some of which is a valve seat or two that dropped and then flew around chunking up other things. Look up in the head to see if all of the seats are present and accounted for.
Perhaps through the EGR valve?
The aluminum pieces were from the pistons and combustion chambers and had reached all the way to the plenum and lower intake manifold. There was no damage inside the plenum, throttle body, or lower intake, just the chunks of piston. Cylinder #2’s piston was so badly damaged it had a hole in it. The cylinder walls were smooth, no scoring, bluing, or damage of any kind, not even much of a lip (surprising given the mileage). I changed the oil every 3000 miles during the two years I had the car, even after a road trip from central IL to Denver, CO four months before this happened. Oil pressure was always 45 psi on the dash gauge and it never burned oil or made noise. It even had good oil pressure the last time I ran it before I pulled the heads.
There can be a number of things behind damaged or holed pistons.
Question first. My memory is real fuzzy since this era of car is in the transition period as far as diagnostics.
Does this car use a funky distributor or is it strictly a coil pack setup?
If it has a distributor, has there been any work involving that distributor, timing chain, etc, etc.?
Reasons for holed/damaged pistons are:
Ignition timing too far advanced. (see above about distributor)
Air leak or coolant leak into the combustion chambers. (head or intake manifold gasket)
Severe pre-ignition rattle caused by timing, inferior grade of gasoline, faulty EGR system, etc. IF the rattle is severe enough.
To damage pistons this bad the rattle would have to be chronic and severe.
If the piston tops are pretty clean, or even shiny in spots, then I would suspect a coolant leak into the combustion chambers. Hope some of that helps.
How about intake manifold gasket failure, dumped coolant into the intake manifold, then into the combustion chamber(s) where it hydrolocked the engine and blew chunks back into the intake manifold and down into the oil pan and probably into the exhaust as well.
Thanks to all for the replies and ideas. I am actually an ASE certified tech and have been working on cars for nearly 10 years. I have seen a lot, but nothing like this, ever. This engine does use DIS, the same setup used from the late 80’s thru about 04 or 05 on all 2.8/3.1/3.4 engines. Preignition or valve timing crossed my mind, but I would think that would have damaged all the pistons and likely the valves, too. The combustion chambers are polyspherical, so the pistons would almost certainly hit the valves if it jumped time, bending them to the point where they would not close. The strangest things in my mind are that only four of the cylinders showed any damage, all the damage is on the pistons and combustion chambers in places the valves could not possibly touch, the spark plugs were in perfect condition, and the valves were flawless. I don’t think it hydrolocked because it never lost coolant and I was in the habit of intentionally looking for evidence of lower intake gasket leaks every time I had the hood open. The pistons were also not ‘steam cleaned,’ so I don’t think it was burning coolant.
Actually, I have seen engines that suffered pre-ignition caused piston damage and it did not affect all of the pistons.
Many years ago I bought a very slick low miles Subaru in which the owner had taken it into one of those “quick tune” places. They replaced the plugs and checked the timing. The timing was way off according to them and they reset it.
Unfortunately, they did not reset it according to the underhood sticker and allowed several vacuum lines to remain in place instead of plugging them.
This error caused the engine to run with close to 10 degrees more advance on the timing.
Around town that week everything was ducky. The next week the owner headed to WY from Ok and less than a 100 miles down the pike the engine gave up. At highway speeds it was running at full advance and then some.
One piston disentegrated completely (nothing in the hole but a rod and wrist pin) one piston was coming apart on one edge, and the remaining two were fine.
Seems like there was another case too but the details are fuzzy. Maybe a VW Golf that suffered the same thing? It’s been a few years. 
With DIS I don’t know how excessive advance could occur though and any pre-ignition rattle this severe should have been noticeable for quite a while before the aluminum starts to give up.
A strange case you have here.
One of the things that bugs me about this one is the lack of warning and symptoms you would think would be present in a case like this. The last time I drove the car was a few blocks to the gas station, filled it up, then drove it back. Next morning, I turned the key and made an instant boat anchor out of the formerly reliable 3.1. I suppose it could have jumped time on startup and burned up the pistons with the faulty timing during the few minutes of runtime it got after the incident, but I never pulled the timing chain cover since I don’t have a harmonic balancer puller, but given the design of the valve train, I would think it would have bent the valves. Plus, the engine locked up for some reason. Does any of this make sense???
This is a pretty strange failure alright. Maybe you will find the reason due to something happening to the crank. Perhaps the crank seperated at the position for the last two piston rods. In fact, from the damage you describe and the way it happened I don’t see how this could happen without the crank breaking.
Good thought about the crankshaft, since that would explain why two cylinders were not affected. Unfortunately, that was not the case. That’s what I checked first when I noticed only four damaged pistons, but all six pistons reciprocate normally. I was almost convinced for a while that the engine computer or ignition module went berserk and chewed up the four pistons with preignition, but I don’t think I ran it long enough to do the kind of damage I saw. This would also not explain why the engine stalled and was locked up. I got it to turn over the next day by simply ‘tapping’ the key a few times, then the initial cranking sounded like the cam timing was off - it was spinning over rather freely. I continued to crank it and got a heck of a backfire into the crankcase (blew the dipstick halfway out of its tube), which tells me at that point the hole was already in the piston. This would make a good cartalk puzzler if there were some explanation or answer…
This one is about as strange as it gets. Even with a severe pre-ignition problem pistons should not disentegrate that quick; especially with low speed driving.
It usually requires some steady highway driving, etc.
I’ll think about this a bit more but at the moment it’s very puzzling.
When I got the car to start after it locked up, it would not run unless the gas pedal was about 3/4 to the floor, then it ran very rough and would stall if I let the engine speed get below 2000 rpm, which was all it could muster. I tried putting it in drive and it just stalled right away, not enough power to pull itself. I probably ran it for a total of 2-3 minutes under these conditions, spread over a few different times of starting and running it (to show my father and brother to see what they thought. Everyone thought it had jumped time and bent the valves). If it had jumped time, could that much damage be done that quickly?
Have you checked to see if there is any water in the oil pan?
I never drained the oil, but I did check the condition of the oil while I was trying to figure out what happened. I pulled the dipstick right after shutting it down the first time I managed to get it started and the oil looked good. It was clean and did not have the familiar ‘chocolate milk’ look of water or coolant dilution.
If there is water in the pan it would not have had enough time to mix with the oil. I think Keith may have got it right about the hydrolocking. You should find some water in the pan if that is correct.