P0402, EGR Excessive Flow, and my solution

miketoth1979

Well-known member
Joined
Oct 21, 2010
Location
Ohio
TDI
2001 Golf TDI GLS
Last week, my engine check light on my '01 5sp Golf illuminated while I was driving, and when I scanned the car, the error code read "P0402 - EGR Excessive Flow" The performance of the car was strangely not affected at all. No limp mode. Plenty of power, as always.

This struck me as odd, so I did a bunch of research on this site. Turns out, the EGR doesn't directly measure its own airflow, but instead uses an indirect measurement based on what the Mass Airflow Sensor (MAF) is reading, or some such nonsense. To get the P0402 code to show up, one of the following things was happening:

- The EGR was not opening/closing, maybe a leaky diaphragm in the EGR itself
- The EGR was not opening/closing because the N18 valve wasn't working
- The EGR was not opening/closing because of some other vacuum leak

OR

- The EGR really WAS opening and closing, but the stupid MAF sensor was measuring incorrectly, which was confusing the ECU when it didn't get the numbers it expected, which caused it to throw the engine code.


I checked for vacuum leaks but found none. N18 was working just fine. Since I have a bit of a weepy EGR, I wondered if this had anything to do with the problem...

Turns out, it doesn't, and it was just a dirty MAF sensor. I used some MAF sensor cleaner and gave it a thorough spraying (but NO scrubbing). Popped it back in the car, started it up, and no more engine code for the past several days and many miles. Glad I found that out before I spent any money on new valves, hoses, MAF sensors, etc!!

Moral of the story: code "P0402- EGR Excessive Flow" is bogus. If the EGR system really did have a vacuum leak, wouldn't the EGR be stuck closed? And that wouldn't be excessive flow at all...it would be no flow. So if you get this engine code, check your MAF, because it could be the culprit.
 

copbait

Veteran Member
Joined
Oct 16, 2010
Location
Johnson City TN
TDI
97 Passat TDi, 13 Passat TDi SEL
actually it is excessive flow at the maf not the egr, which is what happens when your egr is stuck closed, it is not pulling air for the cleaner because it is coming from the exhaust. But congrats on the cheap fix, I am never so lucky :)
 

UFO

Veteran Member
Joined
Jul 31, 2007
Location
A mile high
TDI
2001 Beetle
Yes, since the ECM is measuring EGR using the MAF, when the EGR is not flowing you read an excess amount of air, hence "excess flow". Nice that you can fix an issue by just cleaning the contacts.
 

cumminsfromthecold

Veteran Member
Joined
Oct 27, 2006
Location
HumCo
TDI
'84 Toyota 1Z 4WD x-cab
P0402, '02 TDI Jetta Wagon

Helpful post among all of those on the P0402. I'll resurrect this one.

I've got all of the same symptoms miketoth described - great performance, yet this code.

Car is 1 week new to me. I've ruled out N18 (12V got solenoid moving just fine), EGR is clean from previous owner, EGR diaphragm holds vacuum. There was an aftermarket boost gauge line installed that I removed and capped at the custom-made hole in the hard piping line between intercooler and intake. New vacuum lines installed where needed. The one thing I haven't done is focus on the MAF sensor.

When you say you cleaned the MAF, you mean you sprayed it's screen (electrical connection cleaner?) and/or the connection contacts? Could you elaborate? Thanks
 
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CoolAirVw

Vendor
Joined
Nov 9, 2005
Location
Kansas City Missouri
TDI
Jetta
Moral of the story: code "P0402- EGR Excessive Flow" is bogus. If the EGR system really did have a vacuum leak, wouldn't the EGR be stuck closed? And that wouldn't be excessive flow at all...it would be no flow. So if you get this engine code, check your MAF, because it could be the culprit.
This is not true at all. I've encountered this code several times and everytime its been a bad diaphragm in the egr valve vacuum pod.
 

Powder Hound

Top Post Dawg
Joined
Oct 25, 1999
Location
Under a Bridge, Crestview, FL, USA
TDI
'00 Golf 4dr White 5sp, '02 Jettachero 5sp, Wife's '03 NB Platinum Gray auto(!)
...When you say you cleaned the MAF, you mean you sprayed it's screen (electrical connection cleaner?) and/or the connection contacts? Could you elaborate? Thanks
Electrical connection cleaner would be OK for the contacts, but there is a separate MAF cleaner product that would be sprayed on the MAF sensor itself. No need to disassemble the device, just make sure the spray is directed at the sensor.
 

Vince Waldon

Top Post Dawg
Joined
Apr 25, 2009
Location
Edmonton AB Canada
TDI
2001 ALH Jetta, 2003 ALH Wagon, 2005 BEW Wagon
...and it's a hot film sensor rather than the more traditional hot wire sensor (needs a different can of cleaner... ie NOT the one with the picture of a a hot wire sensor on it :) )... and many will say spray-cleaning a hot film sensor is a crap shoot right from the get-go. :) :)
 

CoolAirVw

Vendor
Joined
Nov 9, 2005
Location
Kansas City Missouri
TDI
Jetta
Then why did cleaning the MAF fix the OP's problem?
Then why did it fix the ones I've had here? Mine we diagnosed with information gathering from the car. The valve doesn't open, the diaphragm was ruptured. The original poster never did any real info gathering. Did he pull the elbow pipe and see if the valve opened? Did he cycle the solenoid with vagcom to see if the solenoid applied vacuum to the EGR pod? Did he pump up the actuator and see if the diaphragm was ruptured. NO he just assumed it was a dirty maf and cleaned it. If he's right then fine, but I'll bet you have to clean the maf sensor at least a dozen times to fix a ruptured diaphragm.

Why did it fix his??? How should I know? (who cares)

Theory one. He posted that it was fixed then a few days later the code reoccured.
Theory two... His code was a fluke and clearing it would have been enough.,
Theory three... who cares.

But for every one reading this... certainly, clean your Maf sensor first, just dont be confused when the code keeps reoccurring. And if you have access to a mityvac then see if the diaphram is ruptured or maybe even make some of the checks I mentioned above.
 
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Votblindub

Veteran Member
Joined
Dec 22, 2010
Location
NY
TDI
MK4 Jetta Wagon
I'm signing up for more info and going to try some maf cleaner. You never know *shrugs*, it's cheap and worth a shot, at least.
 

CoolAirVw

Vendor
Joined
Nov 9, 2005
Location
Kansas City Missouri
TDI
Jetta
So what happened?
Send him a PM and ask him.

It would be nice if there was follow up.

Maybe,..,., been driving year and its worked the whole time. Car is fixed. No other work was done other than cleaning the maf.

or .... sorry I made this post as the code reoccurred 2 weeks after making the post. I eventually fixed it with a vacuum hose (or egr replacement).
 

ukle

New member
Joined
May 31, 2008
Location
Houston, TX
TDI
2001 Jetta
My solution

I got an excessive flow error code and thought I'd share how I ended up fixing it for the archive.

I connected my mityvac to the out port of the N18 valve and saw there was maybe 4-5 inHg which told me either the vacuum pump wasn't generating vacuum or the valve wasn't passing it...

Then attached the mityvac to the EGR valve and manually vacuumed. It took about 10 inHg to actuate--I knew it was working because I saw the requested and actual flows equalize on the vagcom. EGR ruled out.

Then measured the vacuum on the line going into the N18 and saw 20-25 inHg, so I pulled the solenoid out and played around with it. Applying a voltage (12-13V to the "flat side" on the polarized connector) I'd hear a faint clunk, but nothing when the voltage was released. Vacuum wouldn't hold after releasing the voltage, and even applying vacuum to the in and closing off the out port.

Using the head of a nail, I cracked the valve open by prying apart the top half, and pulled the head off. The inside was really dirty and caked in a white powder which I thought might have been some kind of dried up lithium grease, maybe? The valve is about 14 years old after all. After gently cleaning the diaphragm and lightly coating in lithium grease, it still wouldn't work after reassembly. I figured the plunger wasn't returning to the fully upright position, perhaps the pressure exerted by the diaphragm had been reduced over the years since there wasn't a return spring.

So I cut a tiny piece of spring and dropped it down into the solenoid's hole. It just fit around the peg at the bottom and when compressed was a bit shorter than the peg, but when extended, above.

I think the spring gave it just enough of a boost to push the diaphragm back up against the tiny metal tube connected to the in port, thus sealing it.

No more code and no $200 replacement. Hoping the hack holds up...
 
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