Engine clatter when started

I have a 1997 Pontiac Firebird, 3.8 liter V6 engine and 93K miles. For the past several months I’ve been hearing a noise when initially starting the engine from cold which I have been attributing to sticking valve lifters. Now if its full of oil the noise will occur on a regular basis and as it looses oil… Yes it has oil leaks at the pan, rear main as well as the lower intake manifold… the noise will occur less frequently. In fact the other day I went to start the engine and the “low oil” light came on and I noted that there was absolutely no noise from the engine upon starting. I’m stumped… Any help would be appreciated.

Your engine is probably damaged from to many “low oil level” operation cycles.

Nope, I don’t agree as this is the first time in years that I have experienced a “low oil” light.

Your “low oil” light is not meant to indicate the level of oil in the motor. It indicates the oil pressure. So a lit “low oil” light means no oil pressure and that means no oil circulating, and that means no lubrication on a cold start. Not good.

Where the noise is coming from is hard to answer, lots of stuff can make a clatter. Since you’re willing to live with several oil leaks and not repair them it seems you are just driving the motor into the ground anyway. So, why worry about noise? Add oil, keep the oil level up to the marks on the dipstick, keep a case of oil in the trunk and keep dripping away.

While the leaking oil doesn’t bother you it is a pollutant. You are creating a hazard for motorcyclist as oil on the road is very slick in the rain.

I am surprised that a '97 has a ‘low oil’ as opposed to just an oil pressure warning light. Pontiac was ahead of their time. Low oil light sensors tend to get gunked up and fail, so having one illuminiate on a 12-year-old car might not even indicate a low oil level.

I cannot explain why your car would make less startup noise as the oil level goes down. If the noise happened when it was warm, I would suspect that the dipstick was wrong and the car was initially overfilled with oil. That would cause the oil to foam, which would result in noise, and would explain the leaks. 97k miles is very young for a rear main seal leak on a Pontiac. However, high oil level should not cause noise immediately after startup.

The low oil light is telling you that your oil pressure is low.

You have signs of a worn out engine.

The way the oil pressure develops is by the oil pump pushing oil through the small spaces between the bearings and their respective wear surfaces. When the spaces get too big from wear, the oil goes through too easily and the pump has a hard time maintaining pressure at low RPMs. It’s sort of like trying to keep a balloon blown up that has a leak.

In addition, oil leaking out of several spots, especially the mains and the pan gasket, are indications that you have excess crankcase pressure. This happens when wear on the rings and in the cylinders allows too much of the combustion pressure to “blow by” the pistons and pressurize the crankcase. The pressure pushes oil past worn seals. This can also be caused by a stuck PCV valve. The PCV valve allows some ressure relief of the crankcase.

The noise you hear when starting the engine could be from wear in a number of places, and may even be “piston slap” (google the subject and see if your engine is one prone to this". If the cylinders are worn, engines prone to piston slap will be more prone to exhibit it.

First, I suggest checking your PCV valve.

Second, you may want to try a heavier base-weight oil to see if it helps.

If you’d rather start with diagnostics (yes!) then start with a compression check. Post the results.

Since this has occurred in the past couple of months, I have to ask if the start of these problems coincided with your last oil change? You could have a defective or improperly installed oil filter. Could also be the wrong grade of oil, but the filter is more likely.