Can Pre-ignition blow up my engine?

Hi,

I have been posting my findings in a different thread about converting a 95 RIV non SC to SC. However my boss and I have differing opinions on how my non SC engine blew up. I would like to hear your opinions.

Here is the scenario: 1995 RIV 3.8L VIN K, NON SC. 144,000 MI on car, recent rebuild of transmission (2000) mi.

The car was skipping badly for about 3 weeks to a month, but only on the highway or whenever it shifted into OD with a load on it. Installed plugs and wires about 200 miles ago and all was well.

Now occasionally I have put some old gas,(2 -3 gallons at most), that was laying around the shop in the tank, but drove to the gas station and put in and least 5 gallons of good stuff right away. I have never heard any prolonged pinging or had any check engine lights come on but my boss thinks that the bad gas caused the engine to ping and that has caused my piston to fail.

We took the oil pan off and in the bottom of the pan were a wrist pin and some pieces of the skirt of the #6 piston. The top of the piston looks intact. The connecting rod cracked the block on the down stroke near the oil pan rail and dug itself into the cylinder wall on the upstroke which seized up the engine. (My theory)

We have not taken anything else apart yet, (engine is still in car), but I think that the timing chain let go or jumped time somehow and the piston contacted the valve in that cylinder and once it made contact with the valve the weakest point was the area of the wrist pin, and that’s what caused the grenade. My boss says that it's from the bad gas and the pinging.

What’s you thought?

Let’s have fun with this one

i don't believe the 3800 is an interference engine (i have been wrong before though)

pop off your valve covers and see what that all looks like

Good post, should make for interesting conversation. The short answer to your question is yes, pre-ignition can blow up your engine, but first i'll explain my opinion on pinging, knock, detonation, pre-ignition, etc. I base this on an article I read a while back, written by one of the engineers who worked on the Northstar V8's development. Not everyone will agree, but it is my opinion. Take with a grain of salt.

The general term for any fuel combustion not caused by spark can be called "knock". Knock can be divided into two basic types:

1) "Ping" or "detonation" are two names for the type of knock that occurs when fuel reaches its flash point just before the spark, igniting slightly before the optimum time. This is usually caused by too much heat near TDC, a lean mixture, low-octane fuel, or over-advanced timing. Due to clashing flame fronts, or the rapid explosion of the air/fuel mix (not sure of the physics behind it), the result is a harsh detonation rather than a smooth, slow burn. Instead of the piston being pushed along, it receives a shock from the sudden pressure. In older engines, this used to make a "ping" sound. Nowadays, modern computerized PCMs can monitor for knock and attenuate it. Even when ping/detonation does occur, most engines can tolerate it for some time. The shock of detonation isn't good for pistons, but small amounts are part of normal engine operation to some extent. When you hear about a chipped piston, excessive pinging/detonation is usually the cause.

2) Pre-ignition is an entirely different phenomenon. If the air/fuel mix is somehow ignited quite a few degrees (up to 180) before TDC, the resulting burn will actually fight the upstroke of the piston, creating super high heat and pressure on the piston crown. The main difference between this type of knock and ping/detonation described above is the air/fuel isn't flashing because of heat and pressure, it's being ignited by another source, such as a hot spot in the combustion chamber. Pre-ignition is almost always preceded by detonation, and can actually be caused by it if the pinging is left uncontrolled. Pre-ignition used to occur in low-compression NA engines without problem, but in forced induction applications, it's a near-instantaneous piston killer. You can burn a hole straight through the center of a piston in just one or two revolutions.

So what this means is: if your piston has a hole in it, like this one, pre-ignition is most likely responsible:



But if your piston is cracked or chipped, it probably received too much pinging over a long period of time. the metal's integrity was lowered until pieces started breaking off.

In your case, Big Al, your engine was a pretty tough one, but had high miles. I'm sure any detonation you had was minor, as the PCM would've cut timing to compensate. Did you ever hear anything? If so, that's when you should start to worry. Even then, it doesn't mean something will break. To put into perspective, I drove an old Dodge Dart with a 225 cid I-6 for 5 years, and that engine pinged like mad every day of those 5 years, yet the engine never died up to the day I got rid of it.

It's possible the connecting rod could have broke from old age. Perhaps it had a small crack in it and finally just gave up. If the timing chain did in fact jump or break, it could very well have caused the whole thing. Our engines are interference engines from what I know.

Thanks double A,

Very informative reply.

I did hear a slight ticking sound when i started the car whenever it was cold, which sounded like a timing chain tensioner, this is why i think it was a TC related explosion.

I will be taking the blown engine out tonight. let you know what i find.

BIG-AL

AA wrote:

Our engines are interference engines from what I know.

This is what I believe too.

But can someone give me a solid defenition of interference engine? Doesnt that mean our engines can change timing and such?

NO 4 EVR wrote:

AA wrote:

Our engines are interference engines from what I know.

This is what I believe too.

But can someone give me a solid defenition of interference engine? Doesnt that mean our engines can change timing and such?

I found this info on the gates belt co web site, but it applies to timing chained vehicles also.

Timing belt-equipped car engines can be classified as either free-running or interference, depending on what occurs if piston/valve synchronization is lost due to a failed timing belt. An interference engine usually sustains damage if synchronization is lost. This could result in very expensive engine repairs.

Basically an interference engine is NOT designed to have extra room in the combustion chamber for both the valves and the piston when the piston is at or near TDC. If the timing is off then they could make contact. Free running engines have this extra room and are not usually affected by a broken chain or belt.

BIG-AL

An interference engine is one where the valves and pistons can make contact in the case of timing error, causing damage in the event of timing chain failure.

I think this is one reason the L67 has a chain instead of a belt. Many of the belt-driven OHC engines need to be the non-interference type in case the belt brakes or slips.

AA wrote:

An interference engine is one where the valves and pistons can make contact in the case of timing error, causing damage in the event of timing chain failure.

Sorry AA,

We must have been typing at the same time.