72' Caprice 402 engine - miss, low power, low compression on one cylinder....

[Tennesseestorm]

Sorry, its not a Pontiac engine, but at least its a GM and my questions is about engines in general.... and no luck on the Chevy sites....

Well, upon everyones recommendation I did a compression check on my 72' Caprice wagon with the optional 402 engine. (Remind me to never buy a BB Chevy again LOL)

Backing up some... it runs good and smooth, but it has a miss and low power. Did a compression check and all cylinders are perfect on compression except the 3rd one back on the passenger side. Before it got dark I managed to remove the driverside valve cover (I did this before I checked the compression).... then I checked the compression and everything checked out OK on the driverside. I am going to remove the other valve cover tomorrow evening and see whats going on that side.

On that cylinder there is only 60 lbs of pressure on the first crank, whereas the others are much higher. On that weak cylinder, after the cranking stops, it holds the pressure and does not leak down.

I am not that "great" on engines, so I am not sure if its a worn camshaft, bent rocker arm, stuck valve etc, etc. As a side note, the car had been setting in a garage for many years. My old 69' Caprice with a 396 behaved the same way, but it ended up being a worn cam, but it also had a knock with the miss.... this 72' does not have the knock.

I hope its not the cam.... Does anyone have any ideas?

Thanks.

[455Grandville]

Some stock BBC's have weak valvesprings, my 71 402 did the same thing: 402s are good engines.
Im told some 72s had hardened valve seats & some did not: it could have a valve issue.

[Tennesseestorm]

Thanks. I will report back with more info after I get the valve cover off tomorrow evening if I get a chance to do it.

[77 Canamman]

402s are good engines..

I agree. I have had excellent performance and service with the 402 in my '72 truck.

[BVR421]

Nice looking wagon. Just tune it up and drive it. It will probably go another 20 years. LOL
For engines that were parked a long time, Ive had good results running a can of RISLONE for an oil change. Its especially good for cleaning up sticky lifters.

[sleepy]

It could be lobe wipe but usually they wipe the front cylinder lobes. Low compression with no leak down? the rings and valves are good. Look for weak valve springs and low lift on the valves. Is there any noise or tick?

[455Grandville]

If it does need a cam, throw in a 350 horse 396 Cam, yours is the same as my 71 : it had the 396/325 horse cam. I threw in the 350 horse model and it had more upper end and the low end was the same.

[mike nixon]

Very rarely will a cam lobe affect compression unless the cyl is being washed with fuel.

I'd suspect a burnt / corroded valve or seat or broken valve spring, or stuck rings, before a bad cam.


If it's rings dump rislone in it and drive it, they may free up.

Valve springs can be changed on the car.


Before you go digging too deep be sure the ignition is firing correctly on that cyl as an ign miss can wash the cyl down with fuel and lower compression.


The 396/402 was one of the best v-8's chevy built.

Mike


Mike

[Tin Injun Tom]

I agree with sleepy, on the valves and rings being good. It should fail the leakdown test if the valves are burned.
Are you 100% sure of the leakdown? The reason I ask is this:
Alot of compression testers, including the kind AutoZone sells, have a release valve on the gauge manifold. Until you press it, the gauge will "hold" the final reading. I think there is a one way check valve in there that lets the pressure build up, but doesn't allow it to escape. It fooled me into believing the cylinder was holding pressure until I thought about it.:p I think it was when I unthreaded the hose from the spark plug hole and the gauge still read ***lbs that I realized it wasn't going to work for leak down testing.
Disregard if you know you did a proper leakdown.

A bad intake lobe on the cam for #6 cylinder could very easily cause low compression.
You didn't say what the pressure actually peaked at though. Did it take more cranks than the others to get to peak psi?