Check Engine Light - Code 51
 

Hey,

Over the past two days I have had the check engine light in my truck come on and off and noticed the idle on the truck being a little bit variable. I should first state that I have a 1989 Ford F-150 with a 4.9L I6 that turned over 144k miles earlier today. The first time the check engine light came on was yesterday after about 20 minutes of driving down the highway. I drove for about another 20 minutes with the check engine light on until I got home and then proceeded to see what code was corresponding to the check engine light. Doing a KOEO test it pulled code 51 which corresponds to "ECT sensor signal is greater than the Self-Test maximum of 4.6 volts." Due to it being very cold out I chose not to do much troubleshooting and just went to the local NAPA and replaced the ECT. This seemed to work at first until about 20 minutes into my drive today when the check engine light came back on. I was about 5 minutes away from work at that point so I just continued on. Four hours later I went to start my truck for the ride home and the check engine light came on right away. About 5 minutes into my commute home the check engine light went off and the truck seemed to run fine. I pulled the code when I got home tonight and again I got code 51. So my question is what would be the next step in diagnosing this problem?

Summary ...
1989 Ford F-150 4.9L I6 144k miles
1 - Check engine light comes on after 20 minutes of driving, code 51
2 - Replaced ECT with new unit from NAPA
3 - Check engine light comes on after 20 minutes of driving
4 - Check engine light comes on right after starting.
5 - Pulled engine code, code 51.
6 - What now?

Thanks in advance.

Additional information that may be help:

The first time the check engine light came on the outside air temperature was -10 degrees F. Today the weather is around -5 degrees F.

I don't know if a water pump can cause the code 51 to be displayed but there seems to be quite a bit of noise coming from the front of the engine. The water pump does not appear to be leaking though.

Ever changed the thermostat, and is the coolant at the correct level?? check your oil cap,and check ur oil for yellow color...justt make sure the coolant isn't getting into crankcase..uh-oh .
pull the wire off the ECT and does the CE light stay on/?

is your cab warming up OK?? are you getting any action from the temp gauge on the dash?
and I'm not sure if the ECT provides both the computer and gauge with the info..there may be anothe sensor for the gauge alone.

In the couple years I have owned the truck I never changed out the thermostat. Would a bad thermostat possibly cause the check engine light to go on and throw out a code 51?

The coolant appears to be at the correct level and I don't recall ever having to top it off so I don't suspect it leaking.

I just started the truck up and it started fine and the check engine light did not come on so I could not try the pulling the wires off the ECT do see if the check engine light would stay on.

The cab seems to heat up pretty well, generally within 5-10 minutes the heat is pumping out pretty hot. The temp gauge on the dash works, it shows the system going up to the N on Normal. I am pretty positive the gauge gets its information from another sensor though and does not interact with the computer.

More information:

I finally got around to running the KOER testing (I used the method from fordfuelinjection.com) to see if that shed anymore light on anything. Fortunately (or unfortunately depending on how you look at it) the only code I got was 11, which is System Pass. There is still quite a bit of chattering sound from the front of the engine but I cannot really isolate where the noise is coming from.

bad alt bearing? or a tensioner pully?

I am a student at a vo tech in Missouri and i had to work on a teachers f 150 that the check engine light came on after driving about 20 minutes it turned out to be the egr valve was not getting vacuum at cruising speed when it opened due to a carbon build up in the intake where the egr bolts on. again i do not know what a code 51 is but it may not be the problem what if that code was just there and never got erased and a new problem is causing the light to come on and the computer just does not store the new problem code, like this truck.

deeznuts822 - I did the old visual check of the tensioner pulley and I have been meaning to put a new alternator in for a while so I may just do that this weekend.

Ford_Hord360 - Thanks for the suggestion. I don't think it is being caused by the EGR due to my erasing the stored codes after the check engine light came on and then getting the same code again which pointed at the ECT. I replaced the EGR Sensor not too long ago and the EGR system seemed to be working, but I guess you never know.

At this point I guess I plan on changing the thermostat tomorrow with a Motorcraft stock one. If anyone has any other suggestions before I do that feel free to let me know.

Thanks in advance.

have you had the serpintine belt off for any reason if the water pump is spinning the wrong way it will make one hell of a chirping sound. just thought I would add that.

Well in case anyone ever searches out this thread in the future it should be known that replacing the thermostat appears to have fixed the problem. I replaced the old one with a Motorcraft 192 F degree one and everything appears to be working correctly.

Thanks for the suggestions from everyone and if this ends up not fixing the problems I will make sure to post my next step, hopefully it will help someone else in the future.