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417 Posts
I have a p10. I was getting 9.77V on the Signal wire and 10.38V on the Ground wire with KeyOnEngineOff, and normal battery voltage on the Power wire. The engine could barely hold an idle, and timing seemed like it had been hugely retarded. I fouled out my spark plugs just trying to let it idle while I tested voltage. I also had the Knock Sensor code present in the ECU.
What led me eventually to the MAF sensor was I was getting crappy gas mileage and the engine did a big miss. I thought an injector was going out, but there resistances were all within spec , and the voltage on their harness was what it should be. I thought I had put the plug wires on out of order by accident.
I swapped MAF sensors with a known-good MAF sensor and got the same sort of numbers that I did with the original. The ECU is only sending 3.32V to the Knock Sensor, but it's supposed to be sending 5V, from what I have read. I was suspecting the ECU was bad. The fuel filter is new, and the fuel pressure is at spec. I cleaned the MAF sensor and changed the air filter, switched out the crankcase ventilation hose because it had a split at the top. The engine was running crappy, but rich, not lean. Nonetheless, I was thinking something was making the MAF sensor act up and the ECU throw the MAF code, so I was thinking maybe a vacuum leak, or the ECU is bad. I did not see teh Exhaust Gas Sensor circuit code on the ECU, so I did not suspect the O2 sensor.
In either case, I knew the MAF sensor was good, so that left me suspecting the ECU or the MAF wiring.
That HIGH ground voltage at KOEO suggested the ground wiring was getting voltage added onto it somewhere between the ECU and the MAF sensor. After reading a lot of threads on MAF grounding, folks' symptoms sounded very similar to mineit sounded like it could only benefit my situation. I cut the ground wire a couple of inches behind the MAF connector and spliced/soldered a new ground wire in place and ran it around the airbox to the negative battery terminal clamp.
Now, the voltage reading in Signal wire is .78V and .007 on the Ground wire. I reset the ECU (cleared the MAF code), put KOEO and checked for codes again. 55, No problem codes present. Amazing.

To secure the new ground wire to the negative battery terminal, I used a "mechanical connector", a flanged metal cube with a worm screw into the middle of it, but not out the other side. The flange has a hole through it for mounting. The ground wire goes into the cube and the worm screw is tightened down onto the wire (14 gauge) strands. I then, soldered the empty space around the worm screw. I used Ultra Grey RTV and a bit of rubber vacuum hose to give the wire reinforcement where it meets the flanged cube. I used a long, thin bolt to tighten the negative battery terminal, and the flanged cube is secured onto the bolt between a spacer and a tightening nut.
I have not cranked the engine yet but will tomorrow. That code 55 is so nice to see.
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