P0401 Code EGR Valve

[bcossa2001]

1997 Avalon, 246K miles. Getting P0401 code insufficient EGR flow detected. The troubleshooting from the Haynes manual says to remove the vacuum tube from the EGR valve and apply vacuum. The engine should run poorly and the vacuum remain steady. If the vacuum does not remain steady and the engine does not run poorly replace the EGR valve. If the vacuum remains steady but the engine does not run poorly troubleshoot the EGR valve.
Naturally, what happens is that the engine runs poorly but the vacuum does not remain steady. I have taken off the valve and inspected it and cleaned it and can't find anything wrong but also can't find any moving parts.....suggestions???

 

[double vanos]

EGR passage way clogged with carbon?

 

[NHGUY]

Not holding vacuum means the diaphram is no good.Bad EGR.

 

[cmorr]

Originally Posted By: NHGUY
Not holding vacuum means the diaphram is no good.Bad EGR.


+1 assuming you are certain the hose you are temporarily using to apply vacuum is not leaking

 

[eljefino]

Some (many?) EGR valves have a tiny air bleed so when the solenoid shuts off vacuum to the valve, it eventually closes after a second or two.

Possibly... Haynes is incorrect.

So you're saying that your vacuum pump pulling on the EGR does have an effect on the engine idle? It's at least sort of working, then.

IDK how the OBDII diagnoses EGR flow, if it's a change in MAP reading when the valve opens or off an o2 sensor or something. This could get complicated. I'd start with roto-rooting the carbon out of the passages.

 

[artificialist]

Originally Posted By: eljefino

IDK how the OBDII diagnoses EGR flow, if it's a change in MAP reading when the valve opens or off an o2 sensor or something. This could get complicated. I'd start with roto-rooting the carbon out of the passages.

The method used depends on the car.
Toyota usually would have a thermocouple above and below the EGR valve, and the wider the valve opened, the temperature difference between the 2 would decline. Nissan often did the same thing.

GM would often measure EGR flow the way you described. There would be a MAF sensor and a MAP sensor. As the egr opened, the MAP would detect more pressure, but the MAF would indicate no difference.

Ford used 2 methods. Earlier Fords would have a backpressure meter in the exhaust. More EGR flow equaled less backpressure at a steady RPM. Later, Ford use the DPFE system. The tube from the exhaust system to the intake would have an orifice in it. The DPFE sensor would measure changes before and after the orifice. No EGR meant equal pressure. Differences between the 2 increased as EGR flow increased.

There are probably other methods used to measure EGR flow, but I don't remember reading about other methods.

 

[punisher]

Ford used an EGR position sensor on earlier engines. It was similar in function to a TP sensor but measured the actual position of the EGR valve pintle. Kind of an OBDII no no as it had no direct feedback of EGR flow. Your EGR ports could be clogged solid, and even if the EGR valve opened up, no flow would occur and no CEL would show. FoMoCo moved to A PFE, and later a DPFE sensor. The PFE measured the pressure between an orifice in the exhaust and the intake manifold (more EGR opening = lower pressure. DPFE measured the pressure on both sides of the orifice.

It is possible there might be a metered bleed between the EGR vac solenoid/valve and the EGR valve. Most EGR vac controls are full vac when energized, and bleed when not applied. You can figure that out quick by applying a vac to the EGR side of the vac solenoid/valve. If it doesn't hold vac, then your EGR valve shoud be airtight. If your vac solenoid/valve holds vac, then there is likely a controlled bleed somewhere.

If you suspect clogged EGR passages. idle the engine, give the EGR valve at least 5-10" of vacuum, and see if the idle gets rough/engine dies. If the engine idle doesn't change, then you either have clogged passages, or the EGR valve is completely stuck (unlikely).

 

[bcossa2001]

I gave it 15 Inches of vac and the car ran rough and the vac slowly bled down.

I checked the hose that comes from the engine to the EGR valve and could find no vacuum on that hose until I revved the engine--then it jumped to 5 and then almost immediately to zero. I checked it on both sides of the vacuum surge tank with the engine warm with the same result. Could not tell any difference in the engine idle with the vac hose to the EGR disconnected. Maybe I should leave the hose disconnected and see if there is any change in the way the car runs....

None of it makes any sense to me. If the EGR valve requires vac to work and there is little to no vac from the engine then how can the valve work? I almost wonder if I have something going on with the vacuum from the engine and that with little to no vac the PCM thinks that something is wrong with the EGR.
I took the EGR valve off about 2 years ago and cleaned the passages but they were nowhere near clogged--almost zero deposits.
I can replace the valve but it sure is annoying that the only indication that the valve is bad is from the check engine light. I don't want to spend $250 plus on a valve that is not broken.
Thanks for all of the posts....I would welcome any more ideas....

 

[yonyon]

In all my years I have never encountered a Toyota with a failed EGR valve. It's always been a failed VSV or hose. Usually it's a VSV. It's been a while since I've encountered one with an EGR problem at all, so I can't be much use in helping you get a proper diagnosis down but I can tell you this:

You can test almost any simple solenoid valve off the car if you can find some test leads and a 12V battery, and can suck air through a hose. If you don't want to do the testing and you don't want to replace a perfectly good EGR valve, you can just replace every related component with VSV in it's name and all of the rubber hoses. This will almost always fix the problem and once the light is off you won't be tempted to replace the main EGR valve anymore.

 

[bcossa2001]

I took the hose off of the EGR and connected a vac gauge to it and drove it to work. It pulls a steady 5" whenever I press (here in the south, we're supposed to say "mash") the accelerator then goes to zero once I let off of the accelerator. Going up a long hill it will pull 5 and then change to about 3. Can't tell any difference in the way the car drives. I'll do some bench testing of the VSV tonight....thanks for all of the posts...

 

[bcossa2001]

Removed VSV--ohms supposed to be 33-39, reading 150. No click when I energize the relay. Ordering a new one today...thanks for all of the help.....

 

[bcossa2001]

Purchased replacement valve at Rockauto. Tested out at the proper ohms. Installed and check engine light is off. Thanks to everyone for all of the help, particularly you, yonyon

Now if my MPG would only go up

 

[The_Eric]

Thanks for posting the fix!