Any ideas what would make me lose power mid dyno run? Car is a 2002 Trans Am WS6 with stock 100k mile engine. Dyno tech said the car threw a P0131/P0151 for O2 circuit low voltage on both pre-O2 sensors. My post sensors are deleted with emulated caps. He also mentioned the car must have a stock tune since the cooling fans didn’t come on until 235 F so he manually turned them on via OBD2. I checked live data with an OBD2 scanner afterwards and both O2s were reading stoich at idle, lean for a second when throttle opens, then stoich, then rich when throttle closes. Seems operable?
I changed oil/filter, fuel filter (still need to check fuel pressure), recharged K&N air filter, cleaned MAF, and changed spark plugs and wires before the dyno run. Apparently the shop didn’t like my 50 ohm wires for reading spark, oops.

The 4 pulls were done at a dyno day where the shop was already behind schedule. So no data was logged nor was a wideband pulled out. All I got were the dyno sheets.. But you can see that the 4th pull dropped right back to runs 2 & 3 very systematically which leads me to think it is a controls issue. The car seems to be going into some lower power mode.

Shitty part is I interpolated the last run at a possible 331 rwhp with the 336 lb-ft which would have made my day.

I could change the O2 sensors but there goes $70 into the wind cause I can’t tell a power drop with my butt dyno. So I won’t know if any of this will fix it since I don’t plan on dynoing again until after H/C/I.

So possibilities are:
-Bad O2 sensors (sorta doubt it tho)
-Knock sensors retarding timing? (Someone mentioned my headers could be rattling against the K-member causing fake knock fault. No code was set tho...)
-Car got too hot and went into a heat protect mode?
-fuel dump for cat protect causing car to run rich?
-Vacuum leak somewhere causing unmetered air to enter intake and cause lean combustion? (Someone suggested this link to me: https://ls1tech.com/forums/general-m...kBIcF05XlgT8ls)
-Bad fuel pressure from gunked lines/tank or bad pump? (Still need to check fuel pressure)

Car does burn a little oil but passed a compression test with flying colors and makes power. I think the PCV is at fault for that issue. Someone did notice a single 1 or 2 puffs of smoke out the tailpipe during the run (odd).


Power related Mods:
-SLP Loudmouth exhaust
-Long tube headers
-SLP lid
-Cat delete


I'll report back once I check:
-Fuel pressure
-Any vacuum leaks
-Header clearance from K-member

Further background info on a possible heat issue: Car made a good 1st run, dyno tech mentioned the car was getting too hot by the 3rd run. He turned the fans on manually, turned the car off for a minute, then ran pull #4. Maybe the car started to cool down but got too hot again mid run?

 

The P0131/P0151 codes are caused by a lean AFR, causing the O2 volts to drop below .360 at WOT. Check the fuel pressure @ WOT till redline, it shouldn't drop below 50 psi. If ok, unplug the Maf and do another WOT, if the O2 volts are over .7, then try clean the Maf with a Q tip & carb cleaner. You may have to replace the Maf.

Russ Kemp

 

You need a real tune with a wideband hooked up for your mods. Headers and no real tuning allways ends up being too lean in my experience. The car just wants more fuel dude

 

I experienced this same thing on my 2000 SS. The MAF had K&N oil all over it. Cleaned it off with carb cleaner (sprayed it off). After that it didn’t fall on its face after 3000 RPM. The car was pretty new to me then, so somebody may have put too much oil on it, I don’t know. I pulled the K&N out, didn’t want to mess with that particular issue again.

 

***UPDATE***
I checked fuel pressure and all seemed fine.
-With key put into run (no ignition) pump primed to 47 psi.
-Only lost 1 psi after 10 minutes on run (no ignition) as a leakdown test.
-While engine running, car had 58-60 psi consistently.
-Steady throttle did not affect fuel pressure.
-Blip throttle would drop 5-10 psi for 1/10th a second and go right back to 58-60 psi.
-Haven't tried WOT too much yet since it was late and didn't want to wake the neighbors but it seemed similar to blipping the throttle.

Beyond that I did find that the return line after the PCV valve is in extremely bad shape and is ready to crumble any minute. You can see the large tear below!! I wonder if that leak after the MAF would suck in unmetered air at WOT. I didn't feel any suction at idle or 30% throttle (again it was late Sunday night).



Crack found in line after PCV valve going into intake manifold.

I also noticed my passenger side header has some sort of accessory pipe off of one of the runners that is maybe 1 cm away from what looks to be the engine mount? Maybe I can remove that plate and leave the tube capped to ensure it isn't knocking against anything at WOT causing false knock.


There's a little pipe that comes off of one of the header runners that is a pinky finger thickness away from contacting what I think is the engine mount.

I'll follow up once I fix the PCV valve if it fixes anything. Now might be my opportunity to install a catch can while I'm repairing the PCV line.

 

you need to check fuel pressure under WOT
Pcv doesnt do anything under WOT and that block off is just for the emissions junk that you dont need.

 

without a log with a wideband everyone is just guessing. which is basically what the internet is