Low cylinder compression
01-03-2004, 06:24 PM
#1
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Low cylinder compression
I just finished an intake rebuild. While I was changing plugs and wires I decided to check compression. I had 180 PSI on the 1st cylinder I checked 70PSI on the second 30 PSI on the 3rd.............. the numbers were all over the place. I didn't do a leak down test. I put the car back together. Starts fine, Idles great and runs good................ Any ideas why my compressions showed bad?
01-03-2004, 07:07 PM
#3
Nordschleife Master
Because your valve and rings are leaking?
I suggest doing it again and making sure to follow the same procedure on each. Also look for a pattern as cylinders done later tend to register a lower pressure. Do the first one again at the end as a check.
I suggest doing it again and making sure to follow the same procedure on each. Also look for a pattern as cylinders done later tend to register a lower pressure. Do the first one again at the end as a check.
01-03-2004, 07:15 PM
#4
Nordschleife Master
Sorry, this just occured to me.
When you removed and reinstalled the intake, is it possible some sort of crap feel down some of the valves, preventing them from closeing all the way? I would guess so. Maybe run the car for a while, and throw some detergent in the fuel for good measure, get the car good and warm, and then try it again?
When you removed and reinstalled the intake, is it possible some sort of crap feel down some of the valves, preventing them from closeing all the way? I would guess so. Maybe run the car for a while, and throw some detergent in the fuel for good measure, get the car good and warm, and then try it again?
01-03-2004, 09:20 PM
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Only two reasonable possibilities that I can think of:
1) Bits of crud on the valves prevented the exhaust valves from seating. It's not too unusual for a high-mileage engine (which includes most of us!) to have some deposit build-up in the combustion chambers. When you turn the plugs, small bits of the deposits around the plugs flake off. When you do a compression check, the low-speed airflow thru the cylinders is enough to carry the deposits to the exhaust valves and seats, but not fast enough to really blow them out of the engine. This is common enough that the standard procedure on a compression test used to include breaking the plugs loose in all cylinders, then cranking the engine and revving it for a few seconds to clear the deposits.
The deposits will be crushed and blown out in a few seconds of running - no cause for concern.
2) The intake valves somtimes stick at certain temperature levels. (This is not at all likely to have been your problem, as it rarely occurs on a cold engine.) On many engines (especially back in the days of poorer-quality oils) the intake valve stems ran just hot enough to coke the oil, but not hot enough to burn it off, as the exhaust valves did.
The key to detecting this is to watch the needle on the compression gauge. Each jump in pressure from the first four strokes should result in a repeatable pattern - big jump, somewhat smaller jump, smaller jump, much smaller jump. If the pattern doesn't happen on a particular cylinder - if one compression stroke doesn't result in any increase or results in a very small increase, yet the next stroke results in the normal increase, the cause is usually a sticking intake valve.
This was common enough that back then, the first step in a tune-up was to clean the intake valve stems by running a strong solvent (e.g., butyl cellosolve) thru the carb on a hot idling engine.
If your engine truly had 30 psig on one or more cylinders, it would NOT idle smoothly. I wouldn't worry about it.
1) Bits of crud on the valves prevented the exhaust valves from seating. It's not too unusual for a high-mileage engine (which includes most of us!) to have some deposit build-up in the combustion chambers. When you turn the plugs, small bits of the deposits around the plugs flake off. When you do a compression check, the low-speed airflow thru the cylinders is enough to carry the deposits to the exhaust valves and seats, but not fast enough to really blow them out of the engine. This is common enough that the standard procedure on a compression test used to include breaking the plugs loose in all cylinders, then cranking the engine and revving it for a few seconds to clear the deposits.
The deposits will be crushed and blown out in a few seconds of running - no cause for concern.
2) The intake valves somtimes stick at certain temperature levels. (This is not at all likely to have been your problem, as it rarely occurs on a cold engine.) On many engines (especially back in the days of poorer-quality oils) the intake valve stems ran just hot enough to coke the oil, but not hot enough to burn it off, as the exhaust valves did.
The key to detecting this is to watch the needle on the compression gauge. Each jump in pressure from the first four strokes should result in a repeatable pattern - big jump, somewhat smaller jump, smaller jump, much smaller jump. If the pattern doesn't happen on a particular cylinder - if one compression stroke doesn't result in any increase or results in a very small increase, yet the next stroke results in the normal increase, the cause is usually a sticking intake valve.
This was common enough that back then, the first step in a tune-up was to clean the intake valve stems by running a strong solvent (e.g., butyl cellosolve) thru the carb on a hot idling engine.
If your engine truly had 30 psig on one or more cylinders, it would NOT idle smoothly. I wouldn't worry about it.