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Discussion Starter #1
Hello all, long time reader, newbie member here.

I have an unusual P0521 code issue that does not seem to match any of the other posts on the forum - I’m hoping someone can help...

First issue:
I’ve recently gotten a P0521 code. I realize this is not a pressure code but rather a signal code indicating too many/too few signal variations, however, I did check the pressure with a gauge. At operating temperature PSI at idle is >15 pounds and at WOT/6k RPM’s the pressure is ~70 PSI. The pressure is steady at all RPM’s with no significant gauge fluctuations indicating a smooth pump and/or relieve valve operation. As a result, I changed the PSI switch and filter screen under the switch. Within 30 miles of replacing the switch, the light came back on. After further testing with the mechanical gauge and days of running with it installed, I have not seen a single issue with the pressure or pressure stability that concerns me. On the forum I’ve seen filters replaced, assumptions that the DOD/AFM turning on all have an impact on the code - so, I’ve changed the filter and I updated my tune to a DOD delete version to ensure it never activates and impacts the pressure. I cleared the code, drove ~ 30 miles and the code is back.....

Second issue:
Since I’ve been chasing this fault, I now have another interesting issue. Within moments of the CEL coming on, I now get a tire pressure system fault indicator and all 4 tires have the dash vs. the PSI. They seem linked because I get one then the other - seemingly the CEL always followed by the TPS system fault.

Third issue:
Of all things, when the prior 2 faults rear their heads, my brake performance changes drastically!?!? When those faults appear, the point where my brakes engage in the pedal travel drops and it feels like only/mostly the front brakes are operating and the car takes longer to stop and nose dives significantly. The obviousness of this is the dangerous factor here which makes me VERY uncomfortable driving the car.

When I clear the code everything returns to normal for about 30 miles and then the cycle repeats itself....

About a year ago I had a headlight issue which was related to a corroded ground on the passenger frame rail in the engine compartment. I rebuilt the ground, and now with the recent issues, I’ve removed every ground cable, cleaned the surface, chased the threads reinstalled the ground and then used liquid electrical tape to seal the ground point. I used an OHM meter to verify the integrity of every ground and all check perfect (this did REDUCE but did not eliminate the damn stereo whine when using the AUX cable... which indicates I still have a ground loop somewhere...).

Because of the seemingly unrelated faults and the history of a ground issue - I still assume this is also a ground related issue, so my asks are:

- Has anyone ever had such issues?
- Does anyone have a wiring schematic for the engine control systems?
- Does anyone know if the engine control module, body control module and electronic brake control module share a common ground and where it is?
- Does anyone have other suggestions?

Thanks in advance!
 

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Are you using a plug in scanner to clear the codes, or a device that is constantly installed in the DLC ? I only say this due to some of the owners that have had a device plugged in full time, have developed a major interference problem that shows up on the data line. Unbelievable things started going south on the car because of them. Your TPMS dropout is similar to mine. But mine happens between Monday thru Friday, an only after sundown at 2 specific locations. It always recovers by the morning, or many minutes later, once far enough way from those troubled areas.
It says the ground for the ECM is located on the front of the passenger side cylinder head. For the EBCM, there are 2 grounds. It says that both wires are grounded on the inside of the right frame rail (approximately below the fuse box). The BCM has 3 grounds. 2 of them are grounded under the Instrument panel on one of its support braces, an the other is all the way back by the trunk latch actuator.
Having a direct reading oil pressure gauge setup is nice, but if the 3 wire circuit to the oil pressure sensor has a problem (even intermittent), its logic will say one thing to the ECM while the gauge says another. Are you sure that the pin/socket combo taking place at the oil pressure sensor connector is rock solid ? Any one of the 3 that has lost its tension could cause the sensors output signal to get lost from time to time.
 

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Discussion Starter #3
Hello all, long time reader, newbie member here.

I have an unusual P0521 code issue that does not seem to match any of the other posts on the forum - I’m hoping someone can help...

First issue:
I’ve recently gotten a P0521 code. I realize this is not a pressure code but rather a signal code indicating too many/too few signal variations, however, I did check the pressure with a gauge. At operating temperature PSI at idle is >15 pounds and at WOT/6k RPM’s the pressure is ~70 PSI. The pressure is steady at all RPM’s with no significant gauge fluctuations indicating a smooth pump and/or relieve valve operation. As a result, I changed the PSI switch and filter screen under the switch. Within 30 miles of replacing the switch, the light came back on. After further testing with the mechanical gauge and days of running with it installed, I have not seen a single issue with the pressure or pressure stability that concerns me. On the forum I’ve seen filters replaced, assumptions that the DOD/AFM turning on all have an impact on the code - so, I’ve changed the filter and I updated my tune to a DOD delete version to ensure it never activates and impacts the pressure. I cleared the code, drove ~ 30 miles and the code is back.....

Second issue:
Since I’ve been chasing this fault, I now have another interesting issue. Within moments of the CEL coming on, I now get a tire pressure system fault indicator and all 4 tires have the dash vs. the PSI. They seem linked because I get one then the other - seemingly the CEL always followed by the TPS system fault.

Third issue:
Of all things, when the prior 2 faults rear their heads, my brake performance changes drastically!?!? When those faults appear, the point where my brakes engage in the pedal travel drops and it feels like only/mostly the front brakes are operating and the car takes longer to stop and nose dives significantly. The obviousness of this is the dangerous factor here which makes me VERY uncomfortable driving the car.

When I clear the code everything returns to normal for about 30 miles and then the cycle repeats itself....

About a year ago I had a headlight issue which was related to a corroded ground on the passenger frame rail in the engine compartment. I rebuilt the ground, and now with the recent issues, I’ve removed every ground cable, cleaned the surface, chased the threads reinstalled the ground and then used liquid electrical tape to seal the ground point. I used an OHM meter to verify the integrity of every ground and all check perfect (this did REDUCE but did not eliminate the damn stereo whine when using the AUX cable... which indicates I still have a ground loop somewhere...).

Because of the seemingly unrelated faults and the history of a ground issue - I still assume this is also a ground related issue, so my asks are:

- Has anyone ever had such issues?
- Does anyone have a wiring schematic for the engine control systems?
- Does anyone know if the engine control module, body control module and electronic brake control module share a common ground and where it is?
- Does anyone have other suggestions?

Thanks in advance!
Hi GOPHERIT.

Thanks for the reply. I was unaware of the the BCM grounds so they will need to be checked. As for the EBCM, that ground location was completely rebuilt by me using new wire, soldered connections and then a hydraulic crimp connector which I then soldered on for extra security. Using an ohm meter I did check continuity across each wire and got excellent readings - all consistent but I also know that was not a load check. Al other grounds were cleaned and the threads were chased to clean/improve contact.

If the BMS grounds don’t fix the isssue, my thought now shifts to a signaling issue. I can confirm the connection on the pressure sensor but I cannot confirm the quality of the signal. The interesting and unusual issue is that I get the CEL, TPS system fault and also the brake engagement changes (pedal position and highly front biased). Do you know what the range is for the signal variations on the sensor?

Thanks
 

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I have nothing useful to add other than that I'm also getting an occasional 0521 code as well. I do have a Bluetooth OBD reader plugged in all the time though, which I will now go and unplug after reading this thread...
 

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The oil pressure sensor is supplied a 5 volt reference power from the ECM pin #41 of the X2 connector. It shows up at pin #2 of the oil pressure sensor. The ground return goes from pin #3 of the sensor, to pin #34 of the X2 connector at the ECM. The signal voltage from the oil pressure sensor, goes from pin #1 to pin #50 of the X2 connector at the ECM. The signal voltage to the ECM *must* remain in the range of approx 1 to 4.8 volts to avoid a fault trip condition. Anytime the the sensor thinks it sees a pressure reading below 1.16 psi, or above 127.6 psi (for about 4 seconds or more), it will set that P0521 code. If the variable resistance value goes haywire for a long enough period, you get the code. This could just be me, but I think every now an then, these sensors develop an internal leak. An that causes the internal variable resistor to go nuts/loose its way at times.
The reference voltage reading should be the same at the ECM an sensor connectors. That would show zero breakdown of that wire. The ground return to the ECM from the sensor should be rock solid at the zero ohms value. What ever signal value sensed at the sensor, had better be the same at the ECM also.

The TPMS is a crap shoot. Ive never figured out how to solve my random problem yet. The brake pedal situation is something that I have never heard of. Doubt that I could ever help you at all on that one.
 

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Discussion Starter #6
Hi GOPHERIT,

I appreciate your information - as soon as I can get to it this week/weekend I'll check the signal and see what it looks like. I've ordered a Service manual that is supposed to have all of the wiring schematics and pin outs for the car so that I can check signals, grounds, and ..... I still think I have a common signal or ground issue because of all three items happening within moments of one another. It seems to "store the condition in the modules and as soon as the battery is disconnected or the codes deleted - the car returns to normal operation - including the brake grab point on the pedal travel. Too weird....

Thanks for everything - I'll update this post as I find things/solve it.
 

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Just fixed my p0521 by using the ac Delco updated filter. The one with the R at the end. I had replaced the sensor and screen to no effect. After oil change and filter no code after weeks, when it would come back a day later previously. Good luck.
 

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Perchance are you referencing the UPF48R Ultraguard Gold oil filter?
 

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That's it! There was a tsb awhile back for probably every ls based engine around that time that mentioned to stop using the original filter. For some reason it did not mention the G8 in it though. Other forum members have had success with it as well. Or it seems going to any quality filter other than the stock has helped as well.
 
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