04 4.0 P0128
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Cross-Country
Joined: Dec 2005
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04 4.0 P0128
Just bought the truck a week ago. The check engine light came on and I pulled the P0128 code. Checked coolant level, it was low added maybe 1/4 gal, erased code and it came back. Research shows probable thermostat, code is coolant temp below thermostat regulating temp. Gauge is at 1/4 for temp, which doesn't seem to be as if thermostat is sticking wide open, I'm in Michigan and temps have been 30's to 50's. I have no problem replacing the thermostat but do wonder if with the temp gauge showing at 1/4 , ( compared to a fuel gauge) if it may be the temp sensor and how to diagnose the difference? I just came out of 99 3.0 Ranger and the 4.0 is new to me so maybe it should run higher on the gauge?
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Joined: Oct 2003
Posts: 4,516
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From: SoCal
Make sure the coolant level in the overflow bottle is rising as the engine warms up & dropping as the engine cools off.
The last time one of my cars flunked this simple little test i borrowed a cooling system pressure gauge from Autozone & found a defective radiator cap was leaking & breaking the vacuum that should have pulled the coolant back into the engine as it cooled down.
Screw the gauge onto the radiator & pump it up yo 15 PSI and you will know in a couple of minutes if theres any leaks in the cooling system.
The last time one of my cars flunked this simple little test i borrowed a cooling system pressure gauge from Autozone & found a defective radiator cap was leaking & breaking the vacuum that should have pulled the coolant back into the engine as it cooled down.
Screw the gauge onto the radiator & pump it up yo 15 PSI and you will know in a couple of minutes if theres any leaks in the cooling system.
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Joined: Jan 2004
Posts: 13,851
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From: SW Va
There are two temp sensors, a single wire one for the dash gauge & two wire one for the computer engine temp sensor. Here is a link to test the engine temp sensor to make sure its PID to the computer isn't corrupt. Fuel Injection Technical Library » Engine Coolant Temperature Sensor (ECT) The trouble code & what your seeing on the dash temp gauge sounds like the thermostat is sticking open & if its OEM, they're designed to fail open, mine did. Replace with a like heat range & design thermostat, that has the air bypass valve to enable getting a good system air burp, after opening & refilling for the thermostat replacement.
^+1 on checking the radiator cap, I usually replace mine when replacing the thermostat, or radiator hoses. Let us know how it goes.
^+1 on checking the radiator cap, I usually replace mine when replacing the thermostat, or radiator hoses. Let us know how it goes.
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shelbymay
1987 - 1996 F150 & Larger F-Series Trucks
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Feb 23, 2008 08:54 PM
