Quick puzzler for the gearheads...

Quick puzzler for the gearheads...

Doc Nickel
Doc Nickel

June 4th, 2012, 12:50 pm #1

Here's our scenario:

Big-Block Chevy (454 cid) carbureted (Quadrajet) and electronic ignition (later HEI.)

Engine starts and runs, and actually runs reasonably well, but not smoothly. But at anything above about 1000 rpm, it "barks" or cracks out of the carb- not a backfire, but a popping sound. At any speed- more noticble at lower RPM- some fuel is seen spitting out of the choke horn. As it warms, a ticking noise becomes apparent sounding as if it's coming from a valve cover (but of course it could be coming from elsewhere.)

Compression test shows zero in cylinders 7 and 8- the rearmost one of each bank. That's zero as in none. Doesn't even bump the needle. All the others show between 125 and 180 psi. (All plugs removed, cranked for 3-5 strokes per test. The dead two were tested twice.)

Engine was running fine when last used last year, save for a broken mechanical fuel pump which has since been fixed. There were signs that some quantity of water infiltrated over the winter, but plugs and vapor blown out of plug holes during cranking show little or no signs of rusting. Quantity of water might have been enough to hydraulic the motor on one cylinder, but if so, it was while just starting to crank.

So what's wrong?

Clearly the zero compression indicates the valves aren't closing. Most likely the exhaust valves, and the popping coming through the carb is due to exhaust gas coming through the exhaust port while the intake valve is open. But no actual backfiring because that cylinder can't compress and ignite the charge.

Yes? No? Options?

Now, why?

I could see water getting in and rusting the valve stems to the point they stick open, but the expelled water (mostly spray and vapor, not like a stream or fountain) and the removed plugs showed no appreciable signs of rust. That much rust- enough to stick the valves- would probably rust the bore enough that we'd have needed to basically "break it loose", and that wasn't necessary.

I could see a broken valvespring not letting the valve close- but the same failure on two separate cylinders?

Am I overlooking something?

No, we haven't yet pulled the valvecovers or done anything more intrusive than the compression test. That's next, but I'm trying to cover bases as we go.

Doc.
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Bill
Bill

June 4th, 2012, 1:25 pm #2

The valves can be stuck for any number of reasons. One set was stuck by what looked like jelly. Turned out it was a sludge ball form the oil pan that got through the passages, then set up while engine was not used for two months. That said remember that the oil is pushed back by forward driving in engines. So some gunk, and it don't take much could have collected around the two rear valves. And was car stored pointing slightly up hill, even a few degrees? If so, sludge shift. Hope this helps.
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Johnny Logan
Johnny Logan

June 4th, 2012, 2:40 pm #3

Here's our scenario:

Big-Block Chevy (454 cid) carbureted (Quadrajet) and electronic ignition (later HEI.)

Engine starts and runs, and actually runs reasonably well, but not smoothly. But at anything above about 1000 rpm, it "barks" or cracks out of the carb- not a backfire, but a popping sound. At any speed- more noticble at lower RPM- some fuel is seen spitting out of the choke horn. As it warms, a ticking noise becomes apparent sounding as if it's coming from a valve cover (but of course it could be coming from elsewhere.)

Compression test shows zero in cylinders 7 and 8- the rearmost one of each bank. That's zero as in none. Doesn't even bump the needle. All the others show between 125 and 180 psi. (All plugs removed, cranked for 3-5 strokes per test. The dead two were tested twice.)

Engine was running fine when last used last year, save for a broken mechanical fuel pump which has since been fixed. There were signs that some quantity of water infiltrated over the winter, but plugs and vapor blown out of plug holes during cranking show little or no signs of rusting. Quantity of water might have been enough to hydraulic the motor on one cylinder, but if so, it was while just starting to crank.

So what's wrong?

Clearly the zero compression indicates the valves aren't closing. Most likely the exhaust valves, and the popping coming through the carb is due to exhaust gas coming through the exhaust port while the intake valve is open. But no actual backfiring because that cylinder can't compress and ignite the charge.

Yes? No? Options?

Now, why?

I could see water getting in and rusting the valve stems to the point they stick open, but the expelled water (mostly spray and vapor, not like a stream or fountain) and the removed plugs showed no appreciable signs of rust. That much rust- enough to stick the valves- would probably rust the bore enough that we'd have needed to basically "break it loose", and that wasn't necessary.

I could see a broken valvespring not letting the valve close- but the same failure on two separate cylinders?

Am I overlooking something?

No, we haven't yet pulled the valvecovers or done anything more intrusive than the compression test. That's next, but I'm trying to cover bases as we go.

Doc.
OK (cracks knuckles)...

The only way you get zero on a compression gauge is if you have a really big hole or the pistons ain't moving. So, this leaves two things: connecting rods or cam. Since it is HIGHLY unlikely you threw two rods at the same time without any noise, (this only happens in Chrysler 2.2l turbos...) then we need to look at the valves.

Since a valve, probably at least one intake valve, is open, we can assume that a) you have two really bent valves (unlikely), b) two wildly bent pushrods (still unlikely as bent pushrods tend to NOT open valves) c) broken camshaft (more likely given the noises) or d) stuck lifters (very likely, given the same thing happened to me not long ago).
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timberwolf
timberwolf

June 4th, 2012, 2:57 pm #4

my guess is the cam snapped between the next to last journal and the first lobe for cylinder 7, or even between the cyl 7 lobes.

With the bark coming out of the carb, my guess it you have an intake valve stuck open. When you crank for a compression check, open the throttle blades and check for air coming back through the carb. You can also check the tail pipe as well. you may feel (at either location) alternating vacuum and pressure.
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Hans
Hans

June 4th, 2012, 3:44 pm #5

Here's our scenario:

Big-Block Chevy (454 cid) carbureted (Quadrajet) and electronic ignition (later HEI.)

Engine starts and runs, and actually runs reasonably well, but not smoothly. But at anything above about 1000 rpm, it "barks" or cracks out of the carb- not a backfire, but a popping sound. At any speed- more noticble at lower RPM- some fuel is seen spitting out of the choke horn. As it warms, a ticking noise becomes apparent sounding as if it's coming from a valve cover (but of course it could be coming from elsewhere.)

Compression test shows zero in cylinders 7 and 8- the rearmost one of each bank. That's zero as in none. Doesn't even bump the needle. All the others show between 125 and 180 psi. (All plugs removed, cranked for 3-5 strokes per test. The dead two were tested twice.)

Engine was running fine when last used last year, save for a broken mechanical fuel pump which has since been fixed. There were signs that some quantity of water infiltrated over the winter, but plugs and vapor blown out of plug holes during cranking show little or no signs of rusting. Quantity of water might have been enough to hydraulic the motor on one cylinder, but if so, it was while just starting to crank.

So what's wrong?

Clearly the zero compression indicates the valves aren't closing. Most likely the exhaust valves, and the popping coming through the carb is due to exhaust gas coming through the exhaust port while the intake valve is open. But no actual backfiring because that cylinder can't compress and ignite the charge.

Yes? No? Options?

Now, why?

I could see water getting in and rusting the valve stems to the point they stick open, but the expelled water (mostly spray and vapor, not like a stream or fountain) and the removed plugs showed no appreciable signs of rust. That much rust- enough to stick the valves- would probably rust the bore enough that we'd have needed to basically "break it loose", and that wasn't necessary.

I could see a broken valvespring not letting the valve close- but the same failure on two separate cylinders?

Am I overlooking something?

No, we haven't yet pulled the valvecovers or done anything more intrusive than the compression test. That's next, but I'm trying to cover bases as we go.

Doc.
Enough that a good freeze might have expanded and cracked the cylinders? Unlikely, but always possible since you did mention water and winter.

I'd try blowing air down into the spark plug hole, see where it comes out. If it just comes out the carb or exhaust, you're not dealing with anything huge, but still a valve problem somewhere. But if it's coming out anything venting from the crankcase, you're going to have to borescope it or pull the heads.

-Hans
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Someone
Someone

June 4th, 2012, 4:08 pm #6

my guess is the cam snapped between the next to last journal and the first lobe for cylinder 7, or even between the cyl 7 lobes.

With the bark coming out of the carb, my guess it you have an intake valve stuck open. When you crank for a compression check, open the throttle blades and check for air coming back through the carb. You can also check the tail pipe as well. you may feel (at either location) alternating vacuum and pressure.
It's not a broken camshaft. The problem, as Doc put it, was with the rear cylinder of each bank. The likelyhood of two broken camshafts, on opposite banks, at the same time is unlikely to say the least.

The arrangement is:

Front
1 2
3 4
5 6
7 8

It's probably a lifter that got stuck and gummed over the winter. Pull the valve covers and check for valve movement. That should take maybe 30 minutes max. I would guess it's the intake valves.

Any sort of crack on the sidewall or head should result in some movement of the compression gauge needle. It would need a gaping hole to have no needle movement. That usually means valves.
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Hans
Hans

June 4th, 2012, 5:04 pm #7

A 454 has a single camshaft in it, above the crankshaft, like pretty much all traditional American pushrod V-8's.

It's not an OHC engine with a camshaft on top of each cylinder bank.

-Hans
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Nighthawke
Nighthawke

June 4th, 2012, 5:27 pm #8

OK (cracks knuckles)...

The only way you get zero on a compression gauge is if you have a really big hole or the pistons ain't moving. So, this leaves two things: connecting rods or cam. Since it is HIGHLY unlikely you threw two rods at the same time without any noise, (this only happens in Chrysler 2.2l turbos...) then we need to look at the valves.

Since a valve, probably at least one intake valve, is open, we can assume that a) you have two really bent valves (unlikely), b) two wildly bent pushrods (still unlikely as bent pushrods tend to NOT open valves) c) broken camshaft (more likely given the noises) or d) stuck lifters (very likely, given the same thing happened to me not long ago).
I suspect best case, broken or stuck lifters or bent rods. Worst case, bent valves or destroyed valve ports.
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Pneuman
Pneuman

June 4th, 2012, 6:06 pm #9

my guess is the cam snapped between the next to last journal and the first lobe for cylinder 7, or even between the cyl 7 lobes.

With the bark coming out of the carb, my guess it you have an intake valve stuck open. When you crank for a compression check, open the throttle blades and check for air coming back through the carb. You can also check the tail pipe as well. you may feel (at either location) alternating vacuum and pressure.
The drive for the oil pump is at the back end of the camshaft. If it broke there, we'd have zero oil pressure as well.

I sure hope that ain't it!

Pneuman
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doh!
doh!

June 4th, 2012, 6:08 pm #10

bert was here
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