Need help! 98 z71 5.7 egr power problem

asracing · October 22, 2015

I have a 98 Silverado z71 with 5.7 350. The check engine light came on giving p0405 code. (low egr voltage). I have replaced the egr and nothing changed. I tested the wiring with the egr unplugged and it tests fine, but if I plug the egr in I lose the 12 volts. If I unplug it I have 12 volt. Now that's with my ohm meter. If I put a load on it, ie hook a headlight to it, it wont power it. I have traced all of the wires back to the ecm. They all are fine. I even unplugged the wiring from ecm and put 12volts from the battery thru the 12volt wire going to the egr and the wire is fine. A friend who is a mechanic by trade said it could be the ecm. I replaced the ecm today.(not a brand new one. one from a used parts/salvage yard). I got the truck to run and the check engine light came on within a few minutes. Same as before, with the ohm meter I have 12 volts going to the egr with the plug unplugged, as soon as I plug it in it goes to 0.03 volts. I even bought another egr thinking it could possible be a bad/shorted one. I'm at a total loss. any help would be great. Thank you!!

Backyard Restorer · October 23, 2015

I've worked on these trucks for many years now and I can honestly say I've never see this but with that said I think if your getting 12 volts to the EGR when the harness is unplugged you have a short to voltage somewhere. You should only get about 5 volts when read with the voltmeter, however I'm not 100% sure on this. You might want to read this article, it is the only thing I can find that somewhat explains how to go about tracking it down. I'll keep looking to see what else I can find but I hope this helps.

http://www.obd-codes.com/p0405

Duelin · October 23, 2015

Check this site. It might be of some help for you.

http://easyautodiagnostics.com/gm/4.3L-5.0L-5.7L/egr-valve-tests-1

asracing · November 2, 2015

Check this site. It might be of some help for you.

http://easyautodiagnostics.com/gm/4.3L-5.0L-5.7L/egr-valve-tests-1

Thanks Duelin. I had already found that page and did everything it says. I still don't have a fix to my problem, but not giving up yet!!

asracing · November 2, 2015

I've worked on these trucks for many years now and I can honestly say I've never see this but with that said I think if your getting 12 volts to the EGR when the harness is unplugged you have a short to voltage somewhere. You should only get about 5 volts when read with the voltmeter, however I'm not 100% sure on this. You might want to read this article, it is the only thing I can find that somewhat explains how to go about tracking it down. I'll keep looking to see what else I can find but I hope this helps.

http://www.obd-codes.com/p0405

Thanks backyard. That's the most indepth one ive seen as far as how to test. I don't have a scan tool, so some tests are difficult. I do have 12 volts and 5 volts, but I can hook test light to positive on battery and it lights up on all the pins. Still haven't figured it out. I have a friend who was a gm mechanic at the dealership and had his own shop. He also had the same exact truck as mine. He has never seen anything like this before either. not sure what Im going to do.