Butler Tech • Shop Stories

The New Engine That Sat: Why a Low-Mileage Pontiac Starts Leaking

A fresh Butler Pontiac in a TV-show-built ’67 Firebird convertible came back with oil on the garage floor and barely 2,500 miles on it. The owner’s question was the honest one: how does this happen on a practically new engine? Here’s the real answer — and the free thing to check before you pull anything apart.

Front and rear main seal seepage on a low-mileage engine that sits is more common than most people expect, and it usually comes down to time, not miles. This particular engine was built and tested years back and then spent most of its life parked — first waiting on a build and film schedule, then in a home garage getting driven a handful of weekends a year.

When an engine sits, the seal lips take a set against the crankshaft and lose their spring, and the film of oil that keeps them soft and pliable drains away. Fire it up after a long nap and those hardened lips just don’t seal like they used to. Engines that get driven regularly rarely seep at the mains. Engines that sit for months at a time are the ones that do.

Before anything comes apart, check the crankcase ventilation. This is the first thing we’d look at because it costs nothing. If the PCV isn’t actually pulling — the valve has gone bad, it’s plugged, it got left unplugged during installation, it’s routed to a port with no vacuum or the wrong port, or a valve-cover breather is plugged with oil — pressure builds inside the engine and pushes oil out past the seals.

In other words, the seals very often aren’t failing at all. They’re being overwhelmed. Confirm the PCV is live and the breathers are clear before you assume a seal is bad. It’s the cheapest fix on this list and one of the most common culprits.

On an engine with this little mileage, rings that never fully seated are a real possibility on their own — and there are a few ways it can happen even after a good start. An engine that spends most of its running time idling can wash fuel down the cylinder walls and effectively un-seat the rings, especially if the carburetor was never fine-tuned for the temperatures and air where the car actually lives.

Sitting makes it worse. A needle-and-seat can stick after months parked and make the engine run rich, which glazes the cylinders and gives you the same blow-by and crankcase pressure that pushes oil past the seals. While you’re in there, check fuel pressure too — it can drift over the years and end up running too high, which feeds the same rich-running problem.

Here’s the one that saves people real money: oil from the valley pan or the oil-pan rails runs down the back and sides of the engine and looks exactly like a main seal leak. We’ve seen plenty of ‘rear main leaks’ that were nothing of the sort.

If it does turn out to be the seals, don’t assume the engine has to come out of the car. The front cover seal and the rear main can often be done in place. The Pontiac rear main is offered as a two-piece seal — with the oil pan down and the crank still in the block, the upper half can be rolled out and a new one rolled in. It’s tight in a ’67 F-body, but it’s worth having your shop evaluate it that way before anyone quotes you an engine-out job. It’s a very different bill.

One more thing worth knowing, because it bites people who convert an automatic car to a manual. If an engine was built and starter-shimmed for an automatic flexplate, then later gets a flywheel installed for a manual (a Tremec swap, for example), the geometry changes. On Pontiac engines that conversion requires re-shimming the starter, and sometimes milling the starter mounting block.

Skip that step and you get intermittent starter trouble — the kind that gets blamed on the engine when it’s really an install detail from a change made years down the road. If your starter got noisy or quit engaging after a driveline change, this is almost always why.

The bigger takeaway: a freshly built engine isn’t a run-it-once-and-forget-it deal, especially if it’s going to sit. Drive it, keep the PCV honest, tune the carb for where the car lives, and confirm where a leak is really coming from before you spend money. Do that and a Butler Pontiac will run happily for a very long time.