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I'm sure it's been covered before but I can't find it. My 86 computer controlled F250 has a 300 and it;s running rich. The O2 sensor is reading mostly around .9 volts and I can hear the mixture control solenoid clicking. TPS is in spec as I recall.
I suspected the MAP sensor is bad so I tested it. 5.0 reference voltage, good ground but 2.54 volts while running. I have 20 inches of vacuum at the sensor. Voltage is the same when it's not running. I understand running voltage should be around 1.0 volts. I'm thinking bad sensor...
Have you checked for codes? A MAP sensor does not send out a varying voltage. It sends out a varying frequency. You need a frequency counter. The guy in the book says you can use a tachometer for a go no go test. He says put the tach on 4 cyl operation, At sea level with the tach hooked to the center wire, engine off, with the tach powered up the MAP sensor should read about 4800 rpm. When you hook a vacuum pump to the sensor, the rpm/frequency should change.
The MAP sensor can be tested on the bench or in the truck. On the bench see pic) I use a motorcycle battery and a 5v converter. The test wiring is made from junk yard salvage. The digital multimeter with a hertz setting is handy to have. In the pictures you can see that the MAP output is 2.5v @157 Hz at atmospheric pressure. At 20" of vacuum it will be 106 Hz. Also, if using a tachometer you will see 1400 rpm on the 8 cylinder scale. You can use a Mighty Vac vacuum hand pump to test output at idle pressure.
While the test setup I use is handy you don't need it. Back probing the MAP sensor connector works fine. You should see the same readings on the truck. However, if the truck is running, the frequency readings may be all over the map (pun). I don't know why this happens. I suspect it has to do with the quality of 5v reference. With my meter set to DCV I can see voltage spikes on the graph. Or it maybe it's the quality of the inexpensive graphing meter?
Anyway, the bench or on the truck, not running test should confirm you have a good MAP sensor.
The O2 sensor should read 0.45v on an analog voltmeter. A digital should show readings bouncing between 0.1v and 0.9v with an average of 0.45v. Stuck at 0.9v is a problem and will throw a code.
I have no experience with carbed EEC IV trucks but would suspect common rich issues like flooding bowl or stuck choke.
I have no experience with carbed EEC IV trucks but would suspect common rich issues like flooding bowl or stuck choke.
I drove my truck (300 six) with a push button to control the mixture solenoid mounted to the side of the carb. While monitoring the O2 sensor with a fluke DVM. The normal reading of the O2 was about .7 volts, with the solenoid not energized.
I was trying to see if a different carb might improve my gas mileage. Over a month of testing, the gas mileage didn't change, so I kept running my feedback carb without a computer.
My point is the computer can't make your truck run as rich as it currently is. It mainly is used to lean the mixture, for emissions issues.
So I agree... Your problem is something else.
Try running with 12 volts and a ground applied to the mixture solenoid. Does it drive the O2 reading lower.
Try wiring the choke open.
Can you wiggle the top of the carb, it can come loose from the throttle plate.
Good luck, Jim
The 300 six is a strange beast when it comes to MPG readings.
I ran my truck without over drive and then with over drive and got the same MPG over the same roads to / from work.
I avg about 15 MPG but have gotten a high of 17 MPG and a low of 13 MPG and have no clue what changes?
I also have a AFR gauge and with the ebay knock off it was really lean when first installed.
I now have it adjusted to about 15 AFR and still get 15 MPG
Dave ----
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