P0121 mystery...

[xpbr2000]

Hello, I've been scouring the web for information to help me resolve my issue. I found the "curse of the p0121" thread here so I thought I would post a question as to how or If i'm able to work around it.

I have a 96 Camaro 3.8
GT2 Cam
90# springs
Headers etc
Mail order tune
pretty much everything else is stock as far as the engine.

I continue to get the p0121 code. I've replaced the Maf, tps, iac, & map. From what I've found about this code's drive cycle is between 55-70 kpa and it compares the rpm and tp. and If it doesn't match what it thinks it should be it will throw this code. normally I wouldn't care about the code unless it caused a problem with the running of the car. It just so happens that the code itself must throw the car into a open circuit mode which causes the idle to rise to between 1500-2500 rpm. I can clear the code and the idle returns to normal immediately. I can recreate the code by simply running the ac at idle and waiting 10 sec. other than that i never get this code. below is what I've found.

idle kpa without ac 50-52 kpa
idle kpa with ac 60-64 kpa

the idle speed is about 100 rpm higher with the ac on as well.

So i know why the code is thrown but how do I fix it? I don't have Hptuners but had planned to for a future SC build. I'm afraid a retune may be the only solution. Can Hptuners fix this or am I SOL?

-Thanks

[JerryPH]

A code P0121 may mean that one or more of the following has happened:
•TPS has intermittent open or short internally
•Harness is rubbing and causing an open or short in the wiring
• Bad connection at the TPS
• Bad PCM (less likely)
• Water or corrosion in connector or sensor

It is all TPS "centralized", so looking elsewhere is not going to solve the issue.

[xpbr2000]

A code P0121 may mean that one or more of the following has happened:
•TPS has intermittent open or short internally
•Harness is rubbing and causing an open or short in the wiring
• Bad connection at the TPS
• Bad PCM (less likely)
• Water or corrosion in connector or sensor

It is all TPS "centralized", so looking elsewhere is not going to solve the issue.

I've already went through all the basic diagnostics which has brought me to "elsewhere". The tps is steady .5-4.5v all the way through. i tested the tps wires at the pcm a smooth .5-4.5v. the PCM reads a steady 0%-100% on my scanner. I've exhausted testing on the "may mean one of the following:" on these obd code sites. The TPS isn't the issue nor is the map. The old TPS and new TPS tested the same. both old and new map sensors give me the same readings.

Like i said I replaced each component one at a time to eliminate possible causes and nothing has resolved this code from appearing. After understanding what the code is caused by I've learned that cams can change the map readings quite a bit. stock 3.8 cams read ~35kpa. which is no where near the threshold of the p0121 code. with my GT2 my kpa runs 50-60 which is right on the threshold. As i mentioned in the OP this code only occurs at idle with the load of the ac. i can also use the clutch to provide the same load and it will push the kpa from 50 to 60 which is inside the p0121 drive cycle.

Also the p0121 isn't only tps related. If the map sensor is off it can throw the code as well.

P0121 conditions
-No MAP or TP sensor codes set
-ECT= >158*
-MAP= <55kpa + >70kpa and steady
-all conditions met for 10 sec

[Russ K]

Your correct about the high idle kpa setting the code. On the V8 cars, there is a P0121 error max map & min map test that can be changed when a cam with high idle kpa is used. And Hp Tuners only supports 1997+ V6 vehicles. I looked at a 1997 V6 Camaro file with Hp Tuners, and there isn't a P0121 error test table.

Tuner Cat supports the 1996 V6, but they only will sell it to an existing customer, if not you will have to purchase the tuner form Jet performance, yet on their site they only list 1998+ V6 Camaros. And then you still need a real scanner (Hp Tuners). I would look at swapping in a 1997 V6 Camaro PCM & use Hp Tuners to tune it. I'm sure all you need is to increase the idle spark timing and or raise the idle speed to lower the idle kpa below the P0121 threshold.

Russ Kemp

[xpbr2000]

Your correct about the high idle kpa setting the code. On the V8 cars, there is a P0121 error max map & min map test that can be changed when a cam with high idle kpa is used. And Hp Tuners only supports 1997+ V6 vehicles. I looked at a 1997 V6 Camaro file with Hp Tuners, and there isn't a P0121 error test table.

Tuner Cat supports the 1996 V6, but they only will sell it to an existing customer, if not you will have to purchase the tuner form Jet performance, yet on their site they only list 1998+ V6 Camaros. And then you still need a real scanner (Hp Tuners). I would look at swapping in a 1997 V6 Camaro PCM & use Hp Tuners to tune it. I'm sure all you need is to increase the idle spark timing and or raise the idle speed to lower the idle kpa below the P0121 threshold.

Russ Kemp

I fail mention that i am running a 97 pcm.

so i would need to raise the idle? its already 950 (ac on) and its supposed to be 850 normally which i find odd. which without running the ac its at 850 with no codes.

[Russ K]

Ok, looking at a stock 1997 Camaro V6 5sp tune, the idle speed tables are the same for A/C or off. The stock idle speed is 750 rpm at 194*F engine coolant temp. On the V8 Camaros, there are separate idle speeds for A/C & off.

But the stock idle timing is only 15*, I would use the scanner control to increase the timing at idle until the kpa lowers below the P0121 threshold. Also try a higher idle speed.

Russ Kemp

[xpbr2000]

Nice thanks for the input Russ.

I've seemed to confound my results with the stock pcm numbers. they are a higher kpa than the tune from 57-70 with and without the ac. And the biggest concern is that it never threw the p0121. according the numbers with the code it should have. also my obd2 adapter and my pcm didn't like each other made the pcm power flicker. not sure if thats because its a 96 pcm or what.

on to the good news after that i swapped back to the tuned pcm and immediately it threw the code. and consistently every 10 sec after clearing the code it would return. I pulled the obd2 adapter. and the code never returned. I'm guessing the issue is with my adapter or the software i'm using to scan with.

can an ELM327 obd2 adapter cause the pcm to throw codes? from what i've read the vcm scanner can control the fan and idle and such.

[xpbr2000]

Well that was a false alarm. it finally threw the code without the adapter in. so i'm back to square one.

[xpbr2000]

Ok, looking at a stock 1997 Camaro V6 5sp tune, the idle speed tables are the same for A/C or off. The stock idle speed is 750 rpm at 194*F engine coolant temp. On the V8 Camaros, there are separate idle speeds for A/C & off.

But the stock idle timing is only 15*, I would use the scanner control to increase the timing at idle until the kpa lowers below the P0121 threshold. Also try a higher idle speed.

Russ Kemp

Thanks russ

Look like i'll have to retune. Thanks for the help.