[V8] O2 sensor grounding, round II

[Tony and Lillie]

Fellow V8'ers,

While we are on the subject of grounding the O2 sensor, I want to share something else of personal experience. It may coincide with a lot of you who have a "great" running V8, other than bad gas milage.

My car is a 91 stock 3.6, with the check engine hooked up. I have a '90 ECU in the car. I have had my car for about three years and 75000 miles so far. It currently shows 328K. And, it currently runs like crap, If I had it to do all over again, Shiv's old car would be in my driveway, but I digress.

When I bought my car it had a couple of little problems, as well as a bad cold start problem. However, it got 24 MPG (my calc) on the highway. The ECU was the fix for the cold start. The car would also "surge", which I describe as accelerating in 50 RPM increments. And, the trans would occasionally limp, more and more frequently. The trans problem only magnified the surging, of course, as it would accelerate from a stop in third gear.

So, after a lengthy discussion with Keith, I decided to get the 4-wire O2, and the matching Ford plug to splice into my car. After that, the surge was gone, and at the same time I got a used MF trans switch from Bruce, to fix the limp problem. The car shifted fine, and the surging was gone!! 

The gas milage was at about 25-26 MPG, with a high of 27 coming home from Las Vegas. After about a year, the milage started slipping. It ended up at about 18 highway. Oh, I forgot to mention I've had a permenant check engine light since about the six month point of ownership. Anyhow, the code was always the same. "bad ground to O2". Well, with a 4-wire, it shouldn't be. So, I cleaned the connection for my 4th wire (the Rad ground on the dr shock tower) and added a ground (4-ga) wire from there to the engine. I also took the factory ground off on the drivers side and cleaned and inspected it and the connections. Still bag MPG!! So, I replaced the O2 again. Still bad MPG!!! Then I started thinking about the engine harness grounds. I removed the engine wiring harness from the car and replaced the ground wires which were hard and somewhat corroded. Still bad MPG!!!! 

As you can imagine, by now I'm getting pretty frusterated with this car. The only thing on the car left would be the wiring harness from the Ford plug to the factory plug by the drivers side distributor. I pulled and inspected it, and it looked fine. I built a new one anyhow, and installed it. Just like that, the milage was better. The first trip (after that fix) with the car was towing my 4000Q home, it averaged 22 MPG. The next road trip, it got 25 again. <SIGH> Problem fixed.

So, for you who might not have thought about that little piece of wire, you might check it. Oh, and pull codes. That at least kept me on the right path.

Hope this helps someone,
Tony Hoffman, with 50+ hours troubleshooting and fixing bad MPG