OK guys, got a real brain twister here and it's driving me nuts. Any help/suggestions would be greatly appreciated. I have a '98 F150 with a Eaton M-90 supercharger that had tuned by a certified SCT dealer who is well respected in the area. He did an awesome tune, airflow is 11.4-1 and truck makes plenty of power/torque. However, the only way he could get the truck to run right is by turning off the rear 02 sensors. If he uses the same program but turns the sensors back on under hard acceleration the truck starts to run pig rich, airflow drops to 8-1 and truck begins to hesitate, stumble. At first we thought the sensors could possibly be bad so we had them turned on in the tune but unplugged them. Same thing happens, runs rich under heavy acceleration and begins to stumble, etc. So obviously the sensors aren't the problem. Cats are brand new Magnaflow high flow, and the front sensors have been changed out. It wouldn't be an issue except that here in NJ for inspection they plug into the OBDII port and read the puter to make sure sensors are on and no codes have been thrown. Thus I need to get them turned back on. Any ideas would be welcomed !!!
TIA

 

I'm not an expert on your MY truck (or any truck for that matter). :o But it seems the O2 sensors are signaling an overly lean mixture to the PCM so it's trying to balance things.

I would measure the voltage across those sensors. That's the way they transmit data to the PCM.

Is it possible (it sounds like it is) that the tune that was written will not allow an input from the O2 sensors? Since you say it runs fine with them "off", the PCM is receiving a zero voltage signal in that state. Suddenly, (running the same tune?) it sees voltage when they're connected. I'd be confused in that situation and it sounds like your PCM is too.

- Jack

 

Thanks for the reply J&J. From what I was told when he datalogged it, the voltage went from .8 and continued to fall. Not sure where/what the voltage reading was from but I'm assuming it's from the 02's.
To answer your other question, yep same tune only difference is with them turned off, then turned on.

 

AND, I can't believe a single tune would work properly in both cases. I know that in open loop, (high acceleration) the feedback is negated so that extra fuel can be delivered, but, with a turbo, you are going to get extra air even in closed loop, and I think that's the problem.

I think your tuner needs to account for this, with the O2 sensors online.

- Jack

 

I'd look into those cats.

 

Cats are brand new, and the airflow is a constant 11.4-1 with the 02's turned off measured on a dyno. Thus I can't see how it's a tune problem. But hey, I'm far from an expert and am willing to listen to all suggestions.

Almost getting ready to put it back to "stock" except for the supercharger (putting back the stock MAF, injectors, Throttle body, etc) and running the factory tune on the PCM. But I'd really like to find an answer.

 

I put a link in the exhaust section also. Seems like there is something thats called a gotts02mod that might work.

I'm no expert, just know what I read here.

 

This should work...click link>>>https://www.f150online.com/forums/ex...-mod-pics.html

 

The rear sensors do not control A:F. They only detect working cats. Are you sure yours are wired correctly? Maybe front and rear got swapped when exhaust was installed?

 

Thanks for all the suggestions guys and will check out that exhaust mod.

The 02's are in the correct position because the harness from the front's would never reach the rear ones, and I only swapped out the front 02's , the back ones are still factory. They only have 44,000 miles on them so I'm going to assume they're still good. Besides, being that when the harness is disconnected from the rears it still goes rich.

I agree that the rears only check to see if the Cats are working properly, but if the detect a rich or lean condition by monitoring the amount of Oxygen in the exhaust it sends a voltage signal ( 1.0-0) to the PCM to add fuel or lean it out. At least that's what I read