Mechanics

KNOCK SENSOR VOLTAGE

1986 Ford F-150 • 5.0L V8 2WD Automatic • 164,000 miles

My truck is running erratically (no power, jerking, running very rich, etc.). I thought it may be the knock sensor, however when I checked the voltage to the knock sensor with the key on (engine not running) it was 1.9 volts. The Haynes manual states that it should be about 5 volts. Could this be a short or maybe a bad power control module?

Any help would be most appreciated!
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RoyF
August 10, 2014.




Check fue lpressure first if fuel injected, autoparts rent gauge then check for a vacuum leak and yes if the voltage is not high enough it could be a bunch of things like a bad ecm an dor a shorted wire. But you should scan for codes first if you can
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Hmac300
Aug 11, 2014.
I thought about fuel pump, however the truck is running extremely rich. I scanned for the codes with an OBD and it showed the following: 22, 23, 25, 34, 72 and 73. I replaced the Throttle Position Sensor, the MAP sensor, the EGR Sensor and the O2 Sensor. I looked for a knock sensor, but could not find one. I retried the OBD and came up with: 22, 23, 31, 42, 72 and 73. The truck still performs very badly with a lot of backfiring into the intake if you try to accelerate too fast. I check the timing several times and it is right on the money. Any suggestions

RoyF

Tiny
RoyF
Aug 11, 2014.
If there is a corroded connection in the PCM, will it show an error code for that, or will it only show an error code for the pins that are corroded? Should I go ahead and take the PCM off and check for corrosion? Also is there a way for me to check for vacuum leaks that is easier, I am deaf in one ear and are unable to detect leaks by sound. Thanks again!

Roy F

Tiny
RoyF
Aug 11, 2014.
A few of those codes we don'thave in our manualsbut we do have a few so the others may be false codes. In your Haynes manual you neeed to do test e for egr, g for map sensor which could be a bad hose and collapsing and I for tps. Tehegr cold be partially clogged and staying open leaving a gad/rough idle or clogged passages or the solenoid for it. And yes corroded pins can caue a lot of problems. But be very careful and try not to bend any. When you put it back together put some dielctric compound in there to help alleviate that. Matbe for the vacuum test is hook up a tach and spray choke cleaner if the tach goes up you found it.

Hmac300
Aug 11, 2014.
I checked the egr valve and it was plugged solid. Picked up new one and put it on, I will reset timing to see how it performs. Thanks for your help!

Tiny
RoyF
Aug 12, 2014.
Well, I replaced the EGR Valve and put it on the OBD and it came up with these codes 22, 23, 31, 42, 63, 72 and 73.
22-MAP Sensor voltage out of spec
23-Throttle Position Sensor voltage out of spec
31-EGR Valve Position Sensor voltage out of spec
42-O2 Sensor always rich does not switch
63-Throttle Position Sensor voltage too low
72 and 73 have to do with not enough change with vacuum and throttle during Dynamic Response portion of test. I have tested the fuel pump and pressure is fine. In addition, although timing is perfect when you accelerate you get backfiring through the intake, like it is out of time.
Any suggestions?

Tiny
RoyF
Aug 13, 2014.
Check for a broken/bent pushrod and or a broken valve spring along with a worn camshaft. If the camis worn the rocker won't go up and down as much as other ones. If it backfires through intake like you can actually see it then it's an exhaust valve not opening.

Hmac300
Aug 13, 2014.
I understand, but how would that account for the air/fuel mixture being so rich. I really mean rich, the gasoline smell and black smoke out of the exhaust seems to be from more than one cylinder or valve. Acceleration is jerky as well sometimes, like the fuel is being restricted.

Tiny
RoyF
Aug 13, 2014.
Because if you aren't opening the valves it creates a miss so it gets to much fuel. Also I have suggested testing fue pressure with a gauge and have not received an answer if your regulator is screwed up it will do that as well. I have no further comments or suggestions.

Hmac300
Aug 14, 2014.
I am sorry, I did check the fuel pressure with motor off and on demand and it performed according to specifications. I have noticed that the motor runs somewhat better when cold, it is when it heats up that I get the surging and hesitation.

Tiny
RoyF
Aug 14, 2014.

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