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Ted G.
Ted G., ASE Certified Technician
Category: Chevy
Satisfied Customers: 6159
Experience:  20 years auto repair experience, ASE Master Tech, Mechanical Failure consultant, Expert Witness
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I have a 1998 Chevy Silverado with the 5.7 l v8 and I keep

Customer Question

I have a 1998 Chevy Silverado with the 5.7 l v8 and I keep getting the same two codes P0405&P0404 and I have replaced the egr and the plug and the map sensor and I do have my 5v reference and I'm on my second egr
Submitted: 2 years ago.
Category: Chevy
Expert:  Ted G. replied 2 years ago.

Okay, most likely you have carbon blocking the EGR passage. Probe the C terminal on the EGR and turn the key on. You should read between .6 and .9 volts. (Engine off.) If over .9 volts you need to clean out the carbon from the EGR passages.

Customer: replied 2 years ago. this is the test I did and I get .82 v
Expert:  Ted G. replied 2 years ago.
Okay then, you need to perform Test 5 and see what happens, if the EGR valve voltage moves smoothly and comes back to .82 then move to Test 7. If it does not return to .82 or does not move smoothly then the sensor/valve is bad or carboned up.
Customer: replied 2 years ago.
I did those steps and I get to step 8 and pass that one two the thing is when I plugged it my works teck2 scanner and looked at my egr the only time it sent those two codes is when it gets to 100% duty cycle
Expert:  Ted G. replied 2 years ago.
Well, that could mean that you have a clog in the EGR passages. The ECM will watch for either a change in manifold vacuum and/or a change in the front O2 sensor to see if there is sufficient flow regardless of what the EGR sensor is telling it. Try putting a vacuum gauge on the manifold, run at 3000 warm and open the EGR valve. You should see at least a 3 in/hg drop when the valve opens, if not then you could be clogged.
Customer: replied 2 years ago.
1 where do I attach the vac Gage
2 I put a kn air filter on it when I got it and it is a max air flow could that be messing things up
Expert:  Ted G. replied 2 years ago.
Any vacuum port behind the throttle body, should measure full vacuum with a closed throttle at idle. I don't think the K&N would do it, but it might be an idea to change it back. Do the 3000 rpm vacuum test with the K&N and without and see if the reading changes, if the reference reading is disturbed the ECM might see it as a fault. (Like when an O2 goes bad and the ECM thinks the converter is not working correctly).