Help needed with odd dyno results (2.5l time trial car)
 

I had a power loss problem with the car last season but I thought I resolved it. Instead of the complete power loss I had previously I’m seeing a huge dip in power at 5,000rpm (according to the dyno I’m not feeling it). The air/fuel ratio is a little hard to read and I’m told it wasn’t an accurate reading but all the noise aside it’s definitly leaning out. We ran 5 runs with identical results. Any ideas? It has a brand new MAF and O2 sensors. It’s only throwing codes for rear O2 sensors which are removed along with the cats. The previous shop I took it to is at a loss (not the dyno shop they haven’t looked into it yet).

https://s17.postimg.org/vfquda00f/23...D56_B46162.jpg

Link to a video of the dyno run...for science

https://youtu.be/wq97ZKzER5s

Well I'm no expert but since no one else has chimed in let me be the first then someone may come on to correct me!! :) If the AFR is even close then it's too bad you do not have the ignition curve on that plot. From a casual view, if the AFR is that lean then detonation could be causing the timing to retard which could cause some of that 50 HP loss. It might be interesting to see a run with no MAF and base map.

Why do you think that the A/F ratio is in error?

Any faults can/will cause issues. Without knowing the exact how that the system was designed to react to the faults, you can only guess as to what is going on. The downstream O2 sensors are often used in setting fuel trims, so you may be running into issues there, but that is just a guess. Personally, I wouldn't the car that way.

Have you tried flashing the DME with ROW code?

something i strange, hey? afr actuals are way off, but probably due to the 'o2 sensor stuffed up the tailpipe' routine and huffing enough ambient o2 to make it read lean.

so, ignoring afr actuals and just looking at the shape of the curve, something is still way strange. afr should run 14.7 until 4500 rpm or so, then drop to around 12.5 as the dme adds a bit more gas to keep things cool. certainly not what is happening here. so, given that it's already pretty certain that the way they are measuring afr is hack, perhaps chuck afrs out alltogether and just look at dyno.

no escaping there is an odd power drop. that drop could be causing a lag in the exhaust which is causing the o2 sensors up the tailpipe to misread even worse. or, chicken/egg, the afr fluctuation is causing power drop.

so, what's going on? that's around cam rollover rpm, so perhaps a solenoid sticking and causing the lurch which is throwing afrs off - get your durametric out and log cam position.

or resonance flap sticking? again, might cause a bump in power delivery.

perhaps some crud in the fuel system that is plugging the filter once fuel flow hits a certain rate - this will lean out your afrs and cause a bump, but not necessarily throw a code.

bad throttle body with a flat spot or a loose/sticky flapper? flat spot in the pedal actuator - you are cable throttle, yes? perhaps log throttle position, or swap in a known-good throttle body.

did you try a known good-maf? more than one maf has been bad out of the box. perhaps log your maf readings.

trk out.

Thanks for all the feedback! A few things to note

1. The car would not run in closed loop under any load at all last season, but would run great with the maf unplugged(open loop). The shop couldn’t figure it out (it was jumping between open and closed loop but they couldn’t figure out why) so I changed the maf (twice), and the O2 sensors. The car didn’t improve, I actually ran it at the track in open loop for one weekend, and afterwards tried plugging the maf back in and somehow it started running like it is now.

2. It’s a cable throttle car

3. I just reinstalled the test pipes right before I drove the car over. Could this throw it off somehow?

:cheers:

what codes were you getting when it wouldnt run closed loop?
was this recent - did you reset the dme?
were you getting any trims - rkat, fra?
the dme switches between open and closed loop based on engine speed (rpm) engine temp, and throttle position - perhaps on of these sensors is wonky (hmmm, throttle position comes up again ...).

Radium King I can’t thank you enough for your help! Here’s answers to your questions


-I forgot to mention resetting the dme. I tried this multiple times and finally letting it reset over night did the trick and allowed the car to run.
-The dme was switching back and forth at open closed loop at idle.
-I replaced the engine temp sensor during the original trouble shooting
-I haven’t replaced the TPS, could it cause a repeatable rpm related problem?
-The only codes were rear O2 sensors and random misfires(before dme reset)

well, no codes is an indicator - how can something like this happen, but fool the dme such that it doesn't throw a code? dme knows what it knows from sensors - o2, tps, temp, rpm, etc. for example, if the throttle plate is flapping around while the car is in open loop, there would be no signal back to the dme outside of a random misfire (open loop so no o2 sensor, and tps, etc., would all be reading ok; only issue would be misfire/knock from super lean condition).

dme switching between open and closed loop at idle is an odd thing too - at idle it shouldn't be in either - it should be working off the idle maps. so, for some reason the dme didn't think the car was idling - again, i'd look at sensors.

personally, i'd check throttle body, throttle cable, throttle position sensor. perhaps map them to see what signals you are getting. or some old oil crudding up the throttle plate? but really, i have no real clue - i'm just throwing ideas at you.

Max hp looks ok but those 2 dips that coincide with your mixture getting super lean look like a sensor issue to me. Wrong MAF for a modified intake tube? Wrong/aftermarket O2 sensors?

I would want to see a smooth mixture curve without this weirdness. Maybe plug-in your old aft O2 sensors and just dangle them under the car like the spec guys do sometimes. This may keep your DME from freaking out.

figure it out?

No I’m still stumped! I really need to get this fixed, I’m curious if he car does not have a map to cover the cam roll over. There seems to be very different fuel maps between my car and a test Boxster we were comparing it to (although it was an e-gas 2.7). Would could cause map problems? I wonder if it’s worth doing an ROW flash. The car feels right to me, but now that I’ve seen the dyno I can tell it’s loosing power. I mention this because I’m guessing the car has had his problem for the 8 years I’ve owned it. Sorry to dig up an old thread.