68 GTO 400 general engine Q's - Pontiac GTO Forum
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post #1 of 3 (permalink) Old 07-10-2018, 09:51 PM Thread Starter
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68 GTO 400 general engine Q's

Hopefully not repeating questions, but my 400 has declined rapidly and I am questioning a car that has been together for a long while but is having multiple simultaneous probs. The probs became apparent when I drove the car a few thousand miles, in differing terrain. Around town the car seemed fine.

I have a substantial oil leak that I believe is at the rear main, but I am not positive. It seems to build up somewhere on teh highway, (bell housing), and dumps out when you stop.
I have excessive blowby out of the valve cover filters.
I am having overheating at highway speed and a huge loss of power trying to go up hills.
Hideaways which were working fine, now open and won't close.

The rebuild and addition of a mild cam and headers was done about 20 years ago, and has maybe 14k miles nothing too demanding. Obviously, it sits for much of the time in the garage. It is a 389 bored 60 over. If the blowby is caused by rings, this engine is done I assume. I've had two rear main seals put in over the years, one neoprene BOP and one rope and could never stop the leaking for long, but it never leaked so substantially, mostly noticed after highway cruising.
Radiator is original and while it is good for around town, I will almost certainly upgrade to a modern aluminum replacement that fits stock. Fan clutch is new.
Carb is basic small Holley four barrel

Some or all may be related or maybe not. I'm worried that the PCV system is may have a fault and would like to know how that system is intended to work on my car.
Timing is also a concern, on the lack of power and heating up. I checked today and do not have any vacuum advance, however the mechanical is kicking in. It's just an older stock HEI pontiac distributor. I would likely replace with something aftermarket. What is the significance of vacuum advance? The hose to the distributor runs off a Holley and seems to be providing vacuum only at rpm, but it does not affect the timing. The other port on the carb is running the headlights and appears to have good vacuum always on. I'm not sure if the vacuum routing is correct or not.

If anyone could point me in a direction that I am able to handle or at least understand I would be most thankful. I need to know if this motor/trans combo is just past its time. I think I would consider an LS swap, but I think it does not seem right. If pictures would help, let me know what specifically I can show.
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post #2 of 3 (permalink) Old 07-11-2018, 08:33 AM
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"...I think I would consider an LS swap..."

PLEASE don't do that !!!

With the Pontiac stuff available now, you can easily make 400-600 very streetable horsepower.

My advice to anyone thinkin about an LS swap into an old Pontiac is to sell the car to a Pontiac lover, & buy a later model car that came with an LS engine.

Yes, I know that is not a popular attitude these days, especially with the younger crowd. It reminds me of the trend towards socialism, by our younger generation.
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post #3 of 3 (permalink) Old 07-11-2018, 05:51 PM
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The first thing to do so you don't go chasing ghosts is to get a mechanical baseline on the engine. You need to perform a compression test, and most importantly, a cylinder leakdown test. With visible blowby, I suspect worn or broken rings, but the tester will tell you what is leaking where. If you have more than 25% leakdown, you will need to rebuild or replace the engine. A tired/hurt engine will run hot, use oil, and be low on power. So, first things first: baseline the mechanical condition of your engine.
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