05 OBXT: surging while cruising

[Taropatch]

Hey all I have been having an issue with my OBXT surging when cruising. It happens in all gears. Usually around 3k rpm its most noticeable and only if I am holding the throttle in one spot.

 

The engine is stock except DW65c pump and Tomei FPR (added after the issues began, thinking it was fuel related)

Competition clutch "stock" replacement with SMFW.

 

LGT suspension geometry conversion and full refresh of bushings and coils front to back.

 

 

Today I did some logging and during a cruise the AFR is fluctuating from 11-18 without any change in throttle input. In a 3rd gear pull its leaning out to 20.33 and now it's parked until I can figure it out. I was thinking maybe a post MAF leak but couldn't find any yet.

 

Anybody have issues with this before? I have been unable to find a thread addressing this.

 

 

 

 

 

here are the logs:

Cruising: https://docs.google.com/spreadsheets/d/1vZstxaAro40Ks2ZI1yxb_RbfNPZj7JDDdKR2l56qdic/edit?usp=sharing

 

3rd gear pull:

https://docs.google.com/spreadsheets/d/1Ct4bmL7FL9L9UlofDehYj4SR_UGC5uRUp7SHNnrCbBk/edit?usp=sharing

[awfulwaffle]

Took a look just now, those are some crazy fluctuations in your cruise log. Leanout in the 3rd gear pull is no bueno too, as you've already gathered.

 

My thoughts:

 

Without launching into a full troubleshooting/diagnostics fault tree, initial suspicion would be front O2 sensor. Reasons being:

 

Calculated load appears stable, which to me says that the MAF is probably OK.

You lean out to 20 up top in 3rd gear, but didn't report feeling like your car hit a wall so I assume it drove OK. Since you're in open loop during the pull, the front O2

doesn't contribute to fueling at all and only sends unused data. So, if you're feeling OK and not registering any knock but seeing a 20 AFR on your factory sensor, could

be a sign that the sensor's going bad or there's an exhaust leak upstream of it.

 

I'd say check fuel pressure too, but if the car felt OK up top this would be second on my list.

 

Do you have the means to log closed loop fueling target with the AP? You should be able to see from that if bad sensor input is yanking your AF target around in cruise, which should be reasonably safe if you don't get into boost/load.

 

Final thought - is that peak boost normal for a stock OBXT? I can't remember exactly what my stock boost peaked at in 3rd on my LGT, but I vaguely recall it being on the order of 13 psi.

Edited November 3, 2019 by awfulwaffle

[Taropatch]

awefulwaffle:

It didn't feel a like there was any loss of power while doing the pull. fuel pressure is great at idle and is set to factory specs so should be adequate for stock tune.

 

I have attached the stock boost table I pulled from RR and will go log a cruising run with the added parameter of CL fueling target.

 

Thanks

[Taropatch]

ok here is the log with the added parameter:

https://docs.google.com/spreadsheets/d/1v04XsFcmQAaZydyUx8unsPTz7O9CiSkVR4rYri4oj4I/edit?usp=sharing

 

I'm not sure how that relates to to the measured AFR. Is the CL target fueling the requested fuel like the final fueling base? or is it final estimated AFR after corrections in relation to the current measured AFR?

 

Thanks

[awfulwaffle]

Can't open the log or see the table at work, so will have to take a look at it when I get home.

 

For now - my understanding of the CL fueling target has always been that it's the final target AFR that the closed loop control scheme tries to hit, defined by the base plus all the stacked adjustments on top of it for things like rear O2 sensor reading, atmospheric pressure, IAT, coolant temperature, ETC.

[awfulwaffle]

Very strange. There's fluctuation in the CL fueling target, but offset 180 degrees from what you see in the actual AF values. Much smaller amplitudes as well, but could be driven by the AF sensor oscillations.

 

Not sure if anyone else will chime in having seen something like this before, as I haven't. If no other responses, I'd still try a front AF sensor if you have one on hand or know someone with a good spare.The surging is very likely fuel trims responding to the observed swing in AF values.

[Taropatch]

Ok, thanks for taking the time to look at it. I'll try swap out the front O2 sensor tomorrow when i get home from work.

[Taropatch]

I swapped the front O2 sensor out and just like that the problem is fixed. Did some logging to be sure that was the only problem. Everything looks good while cruising and no more leaning out when doing a third gear pull.

Interesting that it didn't throw a code with that front sensor malfunctioning.