Running FULL rich, I think

[Dave 53]

Had a great weekend at Willow Springs Raceway, CA (1:38.9 best for those that know the track). Left happy and about an hour into a 5 hour drive home, my 818 started to run like ****. Kind of sputtering. Got behind a slow moving big rig and limped it home. Gas milage went to about 7 mpg, so I'm thinking for some reason the ECU went full rich. Was tough to restart after stopping and I wasn't sure I would make the last 2 miles into my garage. No check engine light or recorded OBD codes.

Bad O2 sensor? Bad or dirty mass air sensor? What's wrong?

Note: I have 1 O2 sensor. The second one was deleted and ECU was tuned accordingly. So, if the ECU normally compares the reading of the OEM 2 O2 sensors and triggers an OBD code based on that, mine doesn't. Two tuners I talked to said the second O2 sensor isn't used in the running of the motor, or something like that... So, a bad O2 sensor and no check engine light might make sense.

[lance corsi]

It’s my understanding that the second o2 sensor is only used as a comparative reference to determine the status of the cat.

[TheHelixx]

Had a great weekend at Willow Springs Raceway, CA (1:38.9 best for those that know the track). Left happy and about an hour into a 5 hour drive home, my 818 started to run like ****. Kind of sputtering. Got behind a slow moving big rig and limped it home. Gas milage went to about 7 mpg, so I'm thinking for some reason the ECU went full rich. Was tough to restart after stopping and I wasn't sure I would make the last 2 miles into my garage. No check engine light or recorded OBD codes.

Bad O2 sensor? Bad or dirty mass air sensor? What's wrong?

Note: I have 1 O2 sensor. The second one was deleted and ECU was tuned accordingly. So, if the ECU normally compares the reading of the OEM 2 O2 sensors and triggers an OBD code based on that, mine doesn't. Two tuners I talked to said the second O2 sensor isn't used in the running of the motor, or something like that... So, a bad O2 sensor and no check engine light might make sense.

Driving for an extended amount of time with it running like that should have triggered a misfire and or lean/rich fuel condition cel. You don’t have a Cat by any chance do you? That’s exactly how a plugged converter sounds. It could be the front O2 sensor, but it isn’t cheap. Might be good to rule out mechanical issues that before you try swapping it out. You can pull your plugs and see if any are fouled. And check compression while they’re out. If you don’t have one, Autozone or another local parts store should have them to rent.

Fwiw anytime I’ve had an issue the way you described on any vehicle, no cel and running that bad… it’s been plugged exhaust or fuel delivery. Fuel delivery meaning outside of engine management (pump, filter, etc..)

Brian

[Bob_n_Cincy]

I second Brian's clogged catalytic converter theory.
I had a similar experience while towing a couple of ATV behind a 07 Impreza wagon.
Bob

clog.jpg

[Dave 53]

No Cat (hence no second O2 sensor).

Tried a new O2 sensor. Didn't fix it.

Don't think fuel delivery is an issue - it was sucking down the gas for 7 mpg!

Got an email into my tuner (Snail Performance, Auburn, CA). Sent him some data logs and hoping he has some ideas.

[Bob_n_Cincy]

Don't think fuel delivery is an issue - it was sucking down the gas for 7 mpg!

Dave, Please explain this sentence you wrote.
Why don't you think it's a fuel delivery issue?
How do you know I was getting 7 MPG?
Any chance you had a fuel leak and were dumping fuel down the highway?
Bob

[Dave 53]

This happened on a 300 mile drive on straight as an arrow highway 5 between Northern and Southern California. Nothing but highway driving at only 65 mph behind a big rig. Filled up the tank, drove 70 miles, watched the gauge drop, pulled over and added 10 gallons (had to do this several times). I didn't even need to use my fingers to calculate milage.

I thought about a fuel leak, especially since I just installed a surge tank and had been messing with fuel hoses and about a dozen hose clamps. Didn't find or smell anything. I also ruled out a clogged filter or tank venting issue (fun fact - you can crack open the gas cap while driving). Plus, if it was simply a fuel leak, that wouldn't explain why it was running so poorly, but poor milage is consistent with the ECU dumping fuel into the cylinders. I guess it was a fuel delivery issue in the sense that the ECU was OVER delivering fuel.

Just replaced the mass air flow sensor. That fixed it.

Shout out to my tuner, Travis at Snail Performance in Auburn CA. He immediately replied to my request for direction, and he was right on. Wish I reached to him before buying a $200 O2 sensor.


Note: After thinking about it, perhaps a fuel leak would explain the poor performance with symptoms similar to a clogged filter. I forgot to mention the back of the car was covered in soot and a passing car signaled to me it was smoking, so I quickly ruled out a leak after the first gas stop inspection and assumed it was an ECU running too rich problem.

[lpmagruder]

Hey Dave,

Was the MAF for sure all that was needed to fix this issue or did you find anything else after the fact?

I'm getting pretty much the same issue. I've tried 2 MAFs and they both do the same, but they are both used so they're not guaranteed good, just seems unlikely that they're both wrong in the same way. Both cleaned too.

Tried rescaling the MAF and ending up with some really funky looking not-smooth curves.

[Dave 53]

Yes, a new MAF sensor fixed it.

I heard if you clean them wrong, it can mess them up. So that might explain why both failed. I don't know the correct way to clean.

I recall my research concluded to only use a Denso or OEM. And the cheap Denso on Amazon or Ebay are forgeries. Only buy from a reputable parts store.

I was having issues with the MAF on race tracks, so I switched to a Speed Density tune.