Manager Forum

Have a 2004 impala, 3.4 with a returning p0401. Checked original valve and was not functioning. Removed valve and commanded with pcm to open in stages and the pintle never moved. Got a new valve and tested it before installing. Pintle moves properly on the new one. A couple miles down tthe road and pop comes on the mil. Same code, p0401. Checked map sensor for proper readings and was good, replaced anyway and same readings. The egr tube is not broken or clogged and the intake is flowing well, no restrictions. Has 20hg of vac at idle, free flowing cat, power ground and reference voltage also for map. Im at a loss here and hoping im missing something silly. Mitchell repair section (only) is down so no help there although ive already read procedure and followed to a tee. Any help or insight would be very much appreciated. Car originally came in with blown head gaskets and has been properly repaired. I dont know how this posted here and cant move it to gm, maybe system gurus can put it into gm forum

Are you 100% sure the EGR passages are not plugged? I've seen it a few times.

Pretty sure. With the egr off i ran the vehicle and had good and instant vacuum. Im In the process now of removing upper intake to examine real well because its the only plausible cause. I'll keep ya posted and am still open for ideas, thanks for your input

I had one some time back that kept setting that code till I physically took gumcutter and a wire brush and cleaned out the egr passage in the intake manifold. Even though it was open it was restricted enough to cause less vacuum drop and set the code.

Frustrating so it was.

A good thorough cleaning is called for. Even a slight restriction can trip this code. Who really knows how long the EGR was closed allowing crapolla to build up. I would remove manifold and thoroughly clean. I picked up some tube style brushes at a giant yard sale that will fit into these orifices. I use them on plastic. On steel tubes i attach internal cables from e/brake cable assemblies to a drill. I take the first inch or two and hammer the crap out of it to spread it. Fill the steel tube with cleaner, force the cable into it, attach a drill, put on a leather glove and start running the cable into it. Any and i mean any restriction will cause this to trip.

The test is so critically balanced and restriction at the egr or air intake will cause an issue. If the tranny is slipping it will throw the code. I've seen filthy intakes cause this as well as crap in the air box restricting the air flow. I've pulled air filters that were fairly clean and found mice nests in the air box that would restrict the air flow. I found a leaf in the plenum tube that would get sucked against the screen causing a drive issue. The engine would exhale when shut off blowing the leaf into the tube far enough so i couldn't see it.

Another and repetitive point here is i would get the customer on board with the diagnosis and even print out the code description taking them step by step and making sure they under stand the EGR not functioning was most likely the original issue but that it caused collateral damage that needs to be dealt with. If they are open to this and i'm taking intake off i would point out the fail probability of it and suggest replacing it. Don't forget the p.c.v. happened to me once and i thought i messed up the gasket.

The original issue has been resolved (my humble opinion) and your dealing with the aftermath. Getting the customer on board is, again in my opinion, is as important as the final repair.

So, I reread the post and need to add and ask how far apart is this issue from initial repair of head gaskets? Any vehicle that we work on that is burning coolant usually (99%) gets new 02 sensors and we use our snappy bore scope the look at the honey comb of the internals of the cat. I've seen them 1/4 plugged and seem to function fine but trigger the check engine light. Is the pcm temp sensor reading correctly? It's part of the process in setting up the test for egr flow. Take oil fill cap off and see if it effects the vac reading, ive seen this where the lower part of the intake gasket was misaligned causing a leak.

Ahhhhhhhh Steve, your'e like a walking dictionary with all your ideas and possibilities.

Having said that, I agree with what you have said.

Been there, done that.

Found that the port was 3/4 restricted once the plenum was off. Soaked in de-carbon over weekend, drilled, honed, scraped, chisled, punched, kicked and a few other tricks i could think of. Came out very clean and now map and egr and 02 readings are perrrrfect. Did this one for free and got a happy customer. I guess we need to be a little more weary about any restrictions. We also should have cleaned it while it was originally apart, our bad. Bottom line is "thanks" for the input, it definately helped.

Glad to hear it's fixed. While I was doing some research, I discovered a trick in checking partially restricted ports. Apply EGR at idle slowly. Engine should just about stall at idle with 30%. With a 3/4 restriction, I would expect you could apply a whole lot more. I'll have to try it on the next 3.4 of that vintage.