Compresion Check Questions
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From: Florida
Compresion Check Questions
I am going to do a compresion check/leak down check on the truck to try and figure out a problem I have.
What is the correct way to cut the fuel? Pull the fuel pump fuse?
Also, on the compresion check; should I pull all the plugs, or just the one I am checking?
And, on doing a leakdown test; do you have to pull the valve cover, or is there another way to tell which cylinder is on top dead center?
What is the correct way to cut the fuel? Pull the fuel pump fuse?
Also, on the compresion check; should I pull all the plugs, or just the one I am checking?
And, on doing a leakdown test; do you have to pull the valve cover, or is there another way to tell which cylinder is on top dead center?
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Joined: Apr 2004
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From: Bella Vista, AR
Well, I will try to answer part of your question.
You should to the compression check on all cylinders because you need to see the difference between the highest and lowest (which shouldn't be more that 15lbs I believe). Cutting the fuel can be done by either eliminating power to the pump or connection to the injectors.
Unfortunately your I can't answer your TDC question ...
You should to the compression check on all cylinders because you need to see the difference between the highest and lowest (which shouldn't be more that 15lbs I believe). Cutting the fuel can be done by either eliminating power to the pump or connection to the injectors.
Unfortunately your I can't answer your TDC question ...
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From: Richmond, VA, USA
Re: Compresion Check Questions
Originally posted by Casey02L
I am going to do a compresion check/leak down check on the truck to try and figure out a problem I have.
What is the correct way to cut the fuel? Pull the fuel pump fuse?
Also, on the compresion check; should I pull all the plugs, or just the one I am checking?
And, on doing a leakdown test; do you have to pull the valve cover, or is there another way to tell which cylinder is on top dead center?
I am going to do a compresion check/leak down check on the truck to try and figure out a problem I have.
What is the correct way to cut the fuel? Pull the fuel pump fuse?
Also, on the compresion check; should I pull all the plugs, or just the one I am checking?
And, on doing a leakdown test; do you have to pull the valve cover, or is there another way to tell which cylinder is on top dead center?
for the compression check, at first I thought you meant that you wanted to know if you should only check one or two cylinders (suspect cylinders) or should you check all 8. I believe that is what ben_g99 thought as well. but after reading it again, I see that you meant should you have ALL 8 plugs removed when checking each individual cylinder. in reality it probably doesn't matter much, but the answer to your question is YES
here are the instructions from Ford:
Compression Test—Compression Gauge Check
1. Make sure the oil in the crankcase is of the correct viscosity and at the correct level and that the battery (10655) is correctly charged. Operate the vehicle until the engine is at normal operating temperature. Turn the ignition switch to the OFF position, then remove all the spark plugs (12405).
2. Set the throttle plates in the wide-open position.
3. Install a compression gauge such as the Compression Tester in the No. 1 cylinder.
4. Install an auxiliary starter switch in the starting circuit. With the ignition switch in the OFF position, and using the auxiliary starter switch, crank the engine a minimum of five compression strokes and record the highest reading. Note the approximate number of compression strokes required to obtain the highest reading.
5. Repeat the test on each cylinder, cranking the engine approximately the same number of compression strokes.
Compression Test—Test Results
The indicated compression pressures are considered within specification if the lowest reading cylinder is at least 75 percent of the highest reading. Refer to the Compression Pressure Limit Chart
1. Make sure the oil in the crankcase is of the correct viscosity and at the correct level and that the battery (10655) is correctly charged. Operate the vehicle until the engine is at normal operating temperature. Turn the ignition switch to the OFF position, then remove all the spark plugs (12405).
2. Set the throttle plates in the wide-open position.
3. Install a compression gauge such as the Compression Tester in the No. 1 cylinder.
4. Install an auxiliary starter switch in the starting circuit. With the ignition switch in the OFF position, and using the auxiliary starter switch, crank the engine a minimum of five compression strokes and record the highest reading. Note the approximate number of compression strokes required to obtain the highest reading.
5. Repeat the test on each cylinder, cranking the engine approximately the same number of compression strokes.
Compression Test—Test Results
The indicated compression pressures are considered within specification if the lowest reading cylinder is at least 75 percent of the highest reading. Refer to the Compression Pressure Limit Chart
as for the leakdown test, with the compression guage extension hose threaded into the plug hole, you can relatively easily determine when each cylinder is on the compression stroke, when air stops blowing out you know you're at TDC. also you could stick something down the hole (please something large enough that it can't be dropped into the cylinder such as a long 1/4" drive extension or a bendable rod like a coat hanger or something) to locate top dead center on each one if you REALLY don't want to pull the valve covers. but ideally, yes the valve covers should be removed.
good luck.
later,
chris
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From: RogersAr
Sorry to differ with you Superford but to do an accurate leak down the cyl MUST be at TDC,what you are trying to determine is the status of the cyl on the compression strok.If it is down at the bottom of the strok you will not be checking the rings correctly since the cyl wears on a taper.Also unplug the pcv valve and remove the rad cap. that way you can check for cyl leakage into the crank case seperately from the leakage into the intake system.by removing the rad cap you are ck'ing for a coolant problem NOT necessary a head gasket.STan
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From: Colorado,
If it was me I would follow the advice given but I would establish cylinder # 1 top dead center with a long peace of brass welding rod through the spark plug hole and then mark the crankshaft pulley with magic marker or whatever.
From then on each 180 degree turn of the crank will bring you to the next fireing cylinder. You will have to go in the firing order.
You might want to use a chrome plated welding rod!
You might also be more patient when making posts in the middle of the day on a work day, as one or two of us have jobs that require our full attention and can't help others until after we get home and kick the dog and pay the bills
From then on each 180 degree turn of the crank will bring you to the next fireing cylinder. You will have to go in the firing order.
You might want to use a chrome plated welding rod!
You might also be more patient when making posts in the middle of the day on a work day, as one or two of us have jobs that require our full attention and can't help others until after we get home and kick the dog and pay the bills
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From: Richmond, VA, USA
Originally posted by Ruslow
Sorry to differ with you Superford but to do an accurate leak down the cyl MUST be at TDC,what you are trying to determine is the status of the cyl on the compression strok.If it is down at the bottom of the strok you will not be checking the rings correctly
STan
Sorry to differ with you Superford but to do an accurate leak down the cyl MUST be at TDC,what you are trying to determine is the status of the cyl on the compression strok.If it is down at the bottom of the strok you will not be checking the rings correctly
STan
but in this case, I think it's just a simple misscommunication. I agree that the piston needs to be at TDC on compression stroke for the leak down test, so I'm not sure where we differ on this one?
later,
chris
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From: RogersAr
Where we miscommunicated is I thought that when you remove the valve cover you might be wanting to make sure valves would be made inactive some how [so they would remain closed].As for using a rod and roll the motor over till it stops moving might work if you were lucky.But with the rod ratio in the 5.4 motors figure on about 12 degree of rock over[no movement of the piston].And unless you have the piston EXACTLY at TDC the piston will move as you put the air to it.Stan