GMC 454 Oil Leak Question (almost Mustang related)

Yeah, I know this isn’t a Mustang question, but considering this is an engine oil leak issue, I’m sure it’s applicable to most engines.

My 89 GMC pick-up, with 137,000 miles on it, has been leaking oil for a few years, though I’ve never been able to figure out where the leak is. I only put a few 1000 miles on it a year, so it wasn’t a big deal. I just check it every time I use it. A few weeks ago on the highway, on my way to Mustangs Unlimited to pick up new fenders, the serpentine belt slipped off the tensioner pulley. (Apparently NAPA lists the same pulley for the tensioner and the idler which I now know to be wrong. ) Not wanting to get stuck on the highway, I drove it for a few minutes to get off the exit and close to a phone. The coolant was bubbling out of the overflow tank as I stopped in the parking lot of the local fire station.

I had it towed home. Replaced the whole tensioner assembly this time, since that was the only way I could get the correct pulley. Changed the oil, filter, and coolant. No sings of coolant in the oil, nor oil in the coolant. It starts and runs like a champ, with a little more valve chatter than it had previously. Though I think overheating has caused the oil leak to get worse – at least I can now see where it’s coming from (sort of).

I saw smoke coming from around the exhaust manifold near the front right cylinder so I cleaned the area with some carb cleaner and took a closer look. The oil is visibly coming out, at that one cylinder, between the exhaust manifold and the head. It must be a pretty bad leak since I can see it ooze upward onto the head from the top of the exhaust manifold as soon as the engine is started. I’m sure new oil is helping the situation.

I could do a compression test, but am wondering if I wouldn’t just be wasting my time. I’m thinking it’s probably time to buy a rebuilt long block. What do you guys think?

Thanks

truck90278 said:

Most auto shops can put a dye in the oil, after running it a while, a black light can be used to determine where the leak is coming from

Click to expand...

Cool idea! Though that doesn't say much about the shop I've been going to, since I asked them to try to locate the leak about a year ago and they gave up.

I got a price for a rebuilt long block from PowerPro for $2,200. The mechanic quoted $1,000 for the install. I haven't made any commitments yet.

I wish it was just a leaky valve cover gasket, but they were replaced about a year ago in hopes that it would solve the problem. Also, after cleaning the area with carb cleaner and running the engine, I can clearly see the oil traveling up from the exhaust manifold seam onto the head while the area just below the valve cover at that cylinder remains clean. Thus, I'm assuming this means the oil must be leaking into the cylinder via the head or the piston rings.

Edit: I guess maybe it's time to break down and buy a leakdown tester.

Hmm, good point. Yes, it smokes. The way oil smokes when you spill it on a hot engine. Not like exhaust smoke under pressure though. I'll take a look at it again afterwork tonight.

It seems to me, considering how quickly after starting the oil appears, it almost has to be coming from the exhaust valve guide.

OK, I took a few shots of the offending area:

First cleaned:

After running 5 min.:

After running 10 min.: