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Engine explosion

What would cause an explosion while trying to start the engine after 1 revolution from a cold start? The 1996 Buick with a V-6 and 52,000 miles had not been started for 2 weeks and was setting outside. The oil filler cap was blown off and the plastic shrouds on top of the engine are broken and blown off and then it started on fire.

What first comes to mind is a heavy concentration of fuel in the oil. Has the car been repaired,any clues surface during the repair process? Vehicle in time (ignition and valve timing?)

We have not started any repairs to the car,because we are waiting for the insurance adjuster so the car has not been moved or towed. This has been a really good car with great gas mileage and it has been well maintained.

How has the dialogue be going? Is the Insurance company open to a explaniation other than tampering or neglect? Are they spending any money to investigate (or do they appear willing to?)What I am getting at is it on you to prove what happened?

I have to agree that it sounds like a misfire blew past the PCV valve and ignited volatile fumes in the crankcase. The force would have blown up through the oil return channels and blown out the filler cap…however I can’t figure how it would have blown the shrouds off.

Anyway, the PCV valve is a check valve put there to prevent this from happening. Apparently, in this case it did not.

The insurance adjuster was here this A.M. and his comment was that it does not look like vandalism although I don’t know how he could tell from a short visual inspection and the only explanation I could give them was the turing of the key and then the explosion. He did take alot of pictures of the inside of the car and of the engine, pulled the dipstick on the engine oil and the transmission but very few questions. With the car locked the hood and the gas cap cannot be opened and this car sets rather close to the ground so it would be hard to crawl under the frame and do any kind of work. No mention of any money to investigate and so far all that they have asked of me is to fill out a statement that describes the details of the event.

Sound like a real nasty back fire through the plastic intake manifold. The plastic engine shroud goes on before a plastic oil filler extension. the oil filler extension actually act as the hold down for the shroud. So possibly when the shroud blow off it pulled out the oil filler extension with it. That is if we are talking about a 3800 series 2 engine.

Aha! Now my mind can visualize it.

Thanks for the clarification.

Now, to add to the OP’s question, I wonder if leaky injectors allowed the fuel system pressure to push fuel into the cylinders when the car was shut off and sitting for two weeks allowed it to wash down past the rings through the ring gaps and become entrapped fuel vapor in the crankcase.

I would bet that somehow fuel got into the oil and ignited.

This sounds like the same thing happened to this car