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Old 12-27-2010, 10:53 PM
danielwd danielwd is offline
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Join Date: Dec 2010
Posts: 4
Default compression test questions

I replaced the clutch in my 94 Ranger, but after test driving for 15 minutes, although the clutch worked great, the engine was running very, very rough. The upper radiator was swelling and I thought it was going to explode. This would possibly indicate a blown head gasket or a cracked head. So I stripped my truck of its drive belt and outer timing belt cover just to check on the timing belt which showed to be about 2 or 2 1/2 teeth off. But anyways, before I change out the timing belt, I wanted to rule out a blown head gasket or cracked head by doing a compression check.

I am confused about a couple of things.

First, to disenable the plug wire coil packs, I guess you just unplug the electrical from each coil pack. I have the 1994 V4 2.3 and the coil packs are right by each other on the passenger side of the engine. The electical inputs are simple to see and simple to unplug.

Second, I think I am supposed to disable the fuel system to make sure it doesn't flow. I think just taking out the fuel pump relay fuse in the power distribution box should do this. Can anyone confirm this?

Third, there is always airflow and to check compression, I think it should be important that air cant leak out secondary places, like open spark plug compartments. So can anyone confirm that I need all 7 spark plugs in and tightened? Obviously the 8th spark plug compartment would hold the pressure gauge?

Finally, to check compression, I need to crank the truck with my one foot on the clutch and one on the gas pedal, probably all the way to the floor to open the throttle. Can anyone confirm that I would need to hit the gas pedal to the floor when attempting to crank?

Because I am in the middle of changing my timing belt and thermostat, my drive belt is off and the upper radiator hose, along with the thermostat housing, are disconnected. I'm thinking about at least reconnecting the thermostat housing and the upper radiator hose to ensure the coolant in the engine doesn't end up all over my garage wall when I crank the truck.

Any suggestions or answers to my questions would be greatly appreciated.
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Old 12-28-2010, 07:12 AM
satan satan is offline
Ford Ranger Owner
Join Date: Nov 2010
Posts: 62
Default Re: compression test questions

Compression is measured for each cylinder individually -- remove one spark-plug per cylinder before beginning and use the "empty" plug hole to check the compression for that cylinder. Leaving the other 3 cylinders vented (open plug) will increase your cranking speed and give you solid readings on the cylinder you're testing.

Remember - with your timing belt off by a few teeth - you may have odd compression values (your cam will be opening or closing valves early or late), so the data you find may seem odd, but your values should be consistent across all 4 cylinders (each of the 4 should be equally impacted by the cam timing error)

The fuel pump - yes, pull the fuse
The coil pack - yes, pull the plug
Throttle - yes, must be WIDE OPEN to allow max air through the intake
Clutch - yes, to the floor to get it to crank (optionally you could bypass/hotwire the starting circuit so that you could crank from under the hood, but...)
'01 XLT Ext Cab 4-dr / 4.0 / 5R55e / 265-75x16 / K&N FPIK / & many unknowns
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