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Old 04-25-2006, 07:24 PM
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BBC 454 question

I did a mild rebuild on a GM 454 2 years ago and have driven it without any issues for approx. 4K miles off and on over the last two years. Powers was plentiful and it ran strong. Recently, the engine has lost a lot of power. the symptoms are below:

Under moderate - heavy throttle there is a loud "rattling" sound. It sounds like the headers are loose or something, but after I stop and look, the exhaust all looks tight.

Under the same moderate - heavy throttle, the engline lacks power, feels like its running off half the cylinders. It will still accelerate, but not with anywere near its full power.

There seems to be excessive blow-back through the valve cover filler hole when its running at any speed and at idle.

it burns through 1/2 quart or so every few weeks of oil.

any ideas appreciated..i am thinking valve seals??

Th\anks!
paul

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Old 04-25-2006, 07:34 PM
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Kinda gives the thought of a cam going bad...? Or possibly the distributor has come loose and moved the timing around...?
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Old 04-25-2006, 09:27 PM
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Sounds like it might be pre-ignition. Might be caused by oil getting by the rings into the combustion chamber. Even if just a couple of cylinders are burning oil, power will be off from the pre-ignition.
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Old 04-26-2006, 08:24 AM
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I followed break-in procedures very carefully to ensure cam, rings etc.., would set right and not become a problem later down the road. I guess I would be very surprised if those were the issues. I will check compression to see if the rings are sealing OK.

My inclination is that it is something with the heads or hyd lifters. I had one lifter a while back that apparently collapsed as its hydraulic functions no longer worked and it was functioning like a solid lifter. I replaced that one and didn't have any problems with it for some time.

I used umbrella valve seals, and the heads were "re-manufactured". Could it be an issue with the valve seals or the valves not seating properly? Would these things deteriorate over time or should I have expected them to be problematic from the start?

Thanks guys
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Old 04-26-2006, 08:26 AM
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Checked timing, it seems to be OK.

Oh, and one more thing that is probably significant. If I am in Park, not moving, I can rev the engine up and it runs perfectly. No issues in Park when not under a load, it only occurs when in Drive or 2 and under moderate-heavy acceleration.
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Old 04-26-2006, 02:15 PM
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Cams have been going flat A LOT over the last few years. Flat tappets can't handle the new oil formulations that have reduced oil additives to prevent wear in flat tappets. Since flat tappets aren't made in production engines anymore, the additives aren't needed (so they say). Furthermore, cheap Chinese lifters have entered the market and they do not have the right taper. So, a cam going flat is a real possibility. Plus the worst part is, a cam going flat will eat up engine bearings. So just changing the cam and lifters is not enough. I have been going though this with two different BBC and BBC are more prone to eat cams than most engines.

I recently had a cam go flat on a 396. I changed the cam and lowered my spring pressure and the engine still had a funny noise. I pulled the engine out, took a look, and discovered some of the bearings chewed down to the copper. The same thing happened to my 454 about a year earlier so I converted it to a roller cam to solve that problem.

The cure is to use good USA lifters and use a diesel engine oil which still has the additives in it (at least for a few more years). CompCam has an additive which can be used in the oil pan for cam break in and for each oil change. Compcam also sells a tool to groove the lifter bores to help cam oiling.

Pull the valve cover and start the engine. Watch each rocker arm move. A lot of times you can see when a rocker arm isn't moving as much as the others.
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Old 04-26-2006, 02:53 PM
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BBC 454 Question

Have you changed the spark plugs lately?Big Blocks are hard on plugs,and this might be causing your problem(s).I put at least one set in my BB and sometimes two and this is a summer driven only car.
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Old 04-26-2006, 05:15 PM
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the most common prob with bbs are that the hydraulic stock cams go flat. Ive had 7 lobes go flat and still pull 21 in vacuum. they arent any harder on plugs than a small block if they are tuned right.
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Old 04-26-2006, 05:27 PM
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BBC 454 Question

Put a solid roller cam in!
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Old 04-26-2006, 05:42 PM
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Possibly cam............or, timing chain........Was it replaced at the time of your "mild" rebuild?
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Old 04-26-2006, 05:55 PM
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Wow, it looks like a lot of people are thinking cam and/or lifters. I will take a look at the rockers this weekend with it running and see if I can tell if any are going flat. I will checkout the plugs too. Updates to follow.

Thanks!

p.s. the engine is in a 79 C10 1/2 Ton 2WD pickup
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Old 04-28-2006, 12:31 AM
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What about something simple like the centrifigal advance? If it's advancing way to much under load wouldn't you get the detonation symptoms?
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Old 04-28-2006, 11:25 AM
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Does your oil smell like gas?
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Old 05-03-2006, 07:30 PM
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Yes, it does smell like gas. I discovered a bad vacuum leak on the carb today and fixed it, seems to run the same though, which most likely would indicate more vacuum leaks somewhere. I will have to get the propane out and figure out where they are.
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Old 05-03-2006, 07:36 PM
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Also, do a compression test. Could be a gasket bad between two cylinders. I've had small blocks do this exact scenario, and the culprit was a bad head gasket.
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