P0090 After Changing Thermostats

[aaro9991]

Hey everyone,

This morning I changed my thermostats in the truck because one was stuck open. I changed it by removing serp belt, ac compressor, removing the two main harness plugs that rest above the hardline to the upper rad hose. No problem changing them out but I put everything back together and I immediately get a P0090 code when I turned the truck back on. Is there something that I maybe wiggled lose or is there something on those two main plugs that could have been damaged? I am stumped because I never have had any type of issues with rail pressure or anything relating to fuel so I must have done something to trip that code.

Currently, I have the batteries unplugged because maybe it'll reset?? but I am stumped as to why I would have P0090 after swapping out thermostats. Again I did nothing out of the ordinary and I wasn't anywhere near my CP3 or fuel filter, Just changing thermostats and now this. Any help is appreciated

Thanks guys

 

[aaro9991]

Also something to note, I am monitoring rail pressure and desired pressure on my edge monitor and it doesn’t seem to be lower than the desired pressure so I am assuming that there isn’t anything wrong with the pressure regulator. The truck is in limp mode so I can’t go past 2k but when idling and pressing the pedal to 2K, the rail pressure is keeping up with the desired pressure. Although right now I’m idling at ~11.5k when before I was in the typical 4.5 range. Attached is a video showing the pressure and even when I press the accelerator pedal, it only increases ~1k.

Sorry to blow up this portion of the forum but I am under a real time crunch because I am about to go on a 1200mi trip from ND back to my home in CO.

thanks guys,

Source: https://youtu.be/2ae9E4_m4jg

 

[2004LB7]

What does it do if you reset the code? Does it come back?

 

[aaro9991]

What does it do if you reset the code? Does it come back?

Hey LB7 if I reset the code, it immediately comes back. I tried resetting the code while running and just key on. I tried unhooking batteries for a couple hours and hooking them back up and cranking and it comes back instantly.

Tomorrow morning I will check all the wiring from the pressure regulator all the way back. I’m hoping it’s a rub or something but one thing that does concern me a lot...prior to this, my desired FRP at idle is typically around 4K and now all of a sudden desired is 11k?? All I did was replace thermostats and in doing so disconnected the two main harnesses to get to the hardline and now I’m going crazy trying to diagnose this. I am hoping that because I disconnected those two main harnesses that something is just grounded but I am stumped about that desired fuel rail pressure now too


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[dndj]

P0090 diag runs when the key is on and the commanded FRP regulator current is between 400-1500mA. The DTC sets if the FRP current is not between 50-1600mA. A wire may have been broken during your repair, or possibly smashed under something (grounding out).

Regarding the high DFRP at 11k, if the FRP is not working correctly, perhaps the ECM defaults to this in limp mode. Just guessing here.

 

[2004LB7]

Maybe the fuel pressure regulator is not plugged in all the way?

 

[aaro9991]

P0090 diag runs when the key is on and the commanded FRP regulator current is between 400-1500mA. The DTC sets if the FRP current is not between 50-1600mA. A wire may have been broken during your repair, or possibly smashed under something (grounding out).

Regarding the high DFRP at 11k, if the FRP is not working correctly, perhaps the ECM defaults to this in limp mode. Just guessing here.

Thanks a bunch for that. In regards to diagnosing this, should I take readings with my multimeter at the connector closest to the regulator or on the regulator itself? Reason I’m asking is I noticed that one connector has three wire going in and two coming out to the FPR. This does appear to be from the factory that way tho.

And I never thought about DFRP being higher due to limp. I bet you’re right I was real worried about that for a minute.


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[aaro9991]

Maybe the fuel pressure regulator is not plugged in all the way?

My second go around taking everything apart I verified that the connector to the regulator itself was plugged in snugly and the second connector that rests near the resonator (not the main harness) was tight and secure as well. It’s looking like I will have to remove all the loom and electrical tape from the regulator all the way back to the main harness connectors but first I will check everything with a multimeter to see it’ll tell me anything.

I tried verifying that all fuses relating to ECM were good to go and they all appear to be good with no fuses needing to be replaced.


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[2004LB7]

Have you checked the pins in the two large Bail connectors? Make sure none are bent or pushed back?

 

[aaro9991]

Have you checked the pins in the two large Bail connectors? Make sure none are bent or pushed back?

Yes sir they all look good too. None are folded over and the female ends aren’t pushed in on either of them.


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[aaro9991]

Does anyone have a wiring diagram/schematic for an LBZ? This is driving me insane and I am trying to see the pin outs for the C2 connector. The best I can tell right now is that the physical wires from the regulator to the main bail connector is good.

Or recommend a thorough manual that would have the pins called out.


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[2004LB7]

Looks like pin 24 is the supply voltage and 72 is the grounded control

 

[aaro9991]

Thank you for that. When I was tracing the wire back I noticed the harness reduced from a 3 wire off the main harness to a two wire at the regulator. What is the purpose of reducing from the 3 wire to the 2 wire? Is there some sort of resistor or capacitor or fuse in that little harness? I’m referencing GM #97327685. And how do I test that that harness is working properly? I only found continuity on one wire but if there’s a fuse or some sort of capacitor, that would explain why I couldn’t find continuity on the other wiring


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[dndj]

About the harness. At the FRP end, you should have a yellow(regulator control) and a purple/white (regulator supply voltage). Schematic shows this to be a shielded harness, so perhaps that's what the 3rd wire is on the other end, just a connection to ground.

 

[2004LB7]

Looks like one terminal is just a shielding connection

 

[aaro9991]

You guys are very helpful thank you.

Okay if the black wire is just a shielding connection of some sort, then I should have been able to get continuity from yellow to yellow and from purple/white to purple/white correct? If so then the harness checks out but if not, then I will replace the harness.

This harness is ballpark near the thermostat housing so I could have pulled something loose inside this harness is what I’m thinking.


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[dndj]

Correct. Yellow and purple/white should have continuity from the FRP regulator all the way to the pins on the ECM. You should also unplug both ends and verify no shorts to ground or power either.

 

[aaro9991]

UPDATE: after looking around the marketplace I was able to find someone with a harness close by. I will still keep you guys posted but for now I am not going to be splicing any harnesses.

Okay, I checked for continuity across the yellow and it checked out; good to go.

The purple/white did not have continuity on the harness and so I’m really hoping that is the culprit here. Out of curiosity, I exposed copper on the purple/white wire directly behind the female connector (I was suspicious of this one because it was closest to the thermostat housing) and tested for continuity and to my surprise, it checked out.

Therefore, the actual female connector is what’s hindering continuity. I will check for a complete harness at the local Chevy dealership but because I’m under a major time crunch (moving from ND to CO this weekend), I may overnight just the female connector and splice it until I’m back home and can find a replacement harness much easier (it seems that without paying an arm and a leg the fastest I can get it is Friday). Being that this harness has that grounding shield around it, do you think there would be an issue using a water/weather proof splice for this female connector only although it’s shielded?

Additionally my friend and I checked continuity from the harness to the main bail connectors and everything seemed to check out.

I appreciate the help a lot. Attached are photos of the continuity for the yellow wire, discontinuity for purple/white (when checking connectors) and continuity from purple/white copper to connector.

Thanks again guys

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[dndj]

Thanks for the update - looks like you got it. Very helpful for people with similar issues years down the road reading this thread.

Sometimes open connections like these can change depending on how you flex the wire, those are even tougher to find because everything looks great until the wire is flexed in just the wrong direction. My Edge CTS2 monitor had a bum cable that was doing this. Took me quite awhile to figure that out.

 

[aaro9991]

Absolutely. I appreciate the help from both of you guys as well. I am picking up a used harness tomorrow (will verify continuity before purchasing) and will let you know if this wipes out the code.


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