Compression test methodology
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Senior User
Joined: May 2010
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From: Richmond, VA
Compression test methodology
Howdy folks. On this weekends agenda is a compression test. I have the tester, but have read various instructions for properly performing the test. Some places say to just disconnect the coil wire from the cap, some say to disconnect and ground the wire. Some say to pull all the plugs, some say pull them one at a time. I have a mechanical fuel pump, so I assume I have to disconnect the fuel line as well? I assume I disconnect the line connecting the pump and the tank? A step-by-step specific to our (my) truck would be most helpful.
Thanks.
Thanks.
Moderator
Joined: Jan 2001
Posts: 56,951
Likes: 2,718
From: Virginia
For the best results, you need the starter cranking fast on any engine. The slower it cranks, the lower the reading you will get. To help this you need to;
1. Warm the engine up fully to thin the oil out, and get everything to operatiing temp.
2. Make sure you have a good battery and it's fully charged for fast cranking.
3. Take all the sparkplug wires out. This will let the engine crank really fast.
4. Tie the carb open. This will let the max amount of air into the engine. If the engine doesn't suck lots of air, then it can't compress it and give you a good reading.
1. Warm the engine up fully to thin the oil out, and get everything to operatiing temp.
2. Make sure you have a good battery and it's fully charged for fast cranking.
3. Take all the sparkplug wires out. This will let the engine crank really fast.
4. Tie the carb open. This will let the max amount of air into the engine. If the engine doesn't suck lots of air, then it can't compress it and give you a good reading.
Post Fiend
Joined: Dec 2007
Posts: 8,521
Likes: 16
From: Lost
Franklin covered it pretty well, but I'll toss in a couple of my favorite tips.
Rather than unhooking or grounding the coil wire, I prefer to remove the connector from the coil. It won't try to fire without power. Just remember to hook it back up.
A brick on the throttle pedal does a pretty decent job of holding it open for the duration of the test, just don't forget to remove it before attempting to start it afterwards.
Rather than unhooking or grounding the coil wire, I prefer to remove the connector from the coil. It won't try to fire without power. Just remember to hook it back up.
A brick on the throttle pedal does a pretty decent job of holding it open for the duration of the test, just don't forget to remove it before attempting to start it afterwards.
Post Fiend
Joined: Dec 2007
Posts: 8,521
Likes: 16
From: Lost
Unless you unhook and cap off the fuel line, and run the engine till the carb is dry, it's still going to have fuel in it.
Having the throttle wide open isn't going to put much fuel into the engine as it cranks. There simply won't be enough airflow to suck the fuel thru the carb passages. Granted, it's going to squirt a good shot of fuel when you open the throttle, but that won't hurt anything. This is why you block it open for the duration of the test, only one shot of fuel goes in. Do that before pulling the plugs, and it will evaporate long before you start cranking.
Having the throttle wide open isn't going to put much fuel into the engine as it cranks. There simply won't be enough airflow to suck the fuel thru the carb passages. Granted, it's going to squirt a good shot of fuel when you open the throttle, but that won't hurt anything. This is why you block it open for the duration of the test, only one shot of fuel goes in. Do that before pulling the plugs, and it will evaporate long before you start cranking.
Moderator
Joined: Jan 2001
Posts: 56,951
Likes: 2,718
From: Virginia
For the best results, you need the starter cranking fast on any engine. The slower it cranks, the lower the reading you will get. To help this you need to;
1. Warm the engine up fully to thin the oil out, and get everything to operatiing temp.
2. Make sure you have a good battery and it's fully charged for fast cranking.
3. Take all the sparkplug wires out. This will let the engine crank really fast.
4. Tie the carb open. This will let the max amount of air into the engine. If the engine doesn't suck lots of air, then it can't compress it and give you a good reading.
1. Warm the engine up fully to thin the oil out, and get everything to operatiing temp.
2. Make sure you have a good battery and it's fully charged for fast cranking.
3. Take all the sparkplug wires out. This will let the engine crank really fast.
4. Tie the carb open. This will let the max amount of air into the engine. If the engine doesn't suck lots of air, then it can't compress it and give you a good reading.
In the above post I said "sparkplug wires". It should have said "sparkplugs". An engine with all the plugs out will have no compression on any of the cylinders, and it makes it really easy for the starter and the battery to crank the engine over.
