P0401 problem

[gankin247]

Hello and thank you for helping me.

I have a 1996 Grand Prix 3100 V6 with 150K mi.

Recently my SES light came on, code P0401, insufficient EGR flow. My car has a linear EGR valve. I took the old valve off and ran the engine to check for plugged exhaust intake or vacuum. The engine ran rough, but I noticed exhaust puffing from the exhaust hole and there was suction from the intake hole, and when I plugged the hole with my thumb the engine smoothed out. There wasn't much visible carbon buildup on the valve or either of the ports, but I cleaned them using carb cleaner (the engine sped up when carb cleaner was sprayed in the port, so I think obstruction is unlikely). I put the old valve back on but the code returned.

Next I replaced the EGR valve itself, ($150 from Autozone) including a new gasket, cleared the codes, but the SES light came back on after about 20 miles, same P0401 code, so, assuming the new valve is functioning properly, the valve is not at fault.

Even though there was vacuum through the EGR intake port, could there be an obstruction somewhere else inside the intake manifold that would cause the insufficent flow?

Next I removed the vacuum line from the UIM to the MAP sensor, blew air through it, and it was not plugged.

In reading posts regarding EGR valve and a 3800 V6, I read that a P0401 can be caused by a PCV valve if it doesn't have a black gasket. Is this also true for the 3100 V6? My current PCV valve doesn't have a black gasket on it, the metal lip just lies flush with the black "mount" on top of the engine.

Should I try replacing the PCV valve? I would be surprised if this was the problem because I have not replaced my PCV valve recently, and my current problem is a recent problem.

Assuming the PCV valve is not the culprit, does anyone else have any ideas as to what the problem could be?

Thanks again.

[richtazz]

The 3100 doesn't have the black o-ring on the PCV valve, so that can be ruled out. The PCM uses MAP sensor vacuum during the test to determine EGR flow, so I would start by checking all vacuum line branches related to the MAP sensor. The EGR itself is fed exhaust gas from a small metal pipe that runs from the rear exhaust manifold up to a plate on the upper plenum. These rarely if ever clog, but they can crack or rot out causing a leak that might trip a p0401.

[gpchris95]

its probably clogged in the UIM happends all the time.

[gankin247]

Thanks for your replies.

richtazz, I didn't notice any cracking or leaking on the plate that is used to attach the small metal exhaust pipe, which the two screws go into that are used to fasten the EGR valve to the UIM.

gpchris95, is the UIM difficult to clean out? The screws that hold the UIM are right on top of the engine, so taking it off should not be a problem, but could you tell me what I should be looking for, or if there is a website that gives good instructions/pictures on cleaning the UIM. Alternatively, would a treatment such as Seafoam do the trick?

Thanks.