P0401 Code - me too... - Toyota Nation Forum : Toyota Car and Truck Forums

kocho · 04-09-2008, 11:48 AM

Just posting to share - have been getting the EGR Flow Insufficient code P0401 lately for the past about a thousand miles or so. Car is at 122K approximately. Reset the DTC - comes back in a few hours/days. No other issues that I have noticed - car drives fine and the engine is smooth.

I found tons of info on diagnosing and fixing the problem and printed the repair manuals, so I'll go from there and share what I find.

From just reading-up, it seems I will approach in the following manner (since I am lazy, I will take my time and do the minimum at first). Please, comment if you have other suggestiongs.

(1) Check and clean the filter in the EGR Vacuum Modulator (page 1409 in the manual)
(2) Reset the DTC and see if it reappears
(3) If DTC reappears, inspect/clean/replace the VSV for the EGR valve. Clear DTC & wait
(4) If the DTC comes back: remove, inspect, clean/replace the EGR valve and gaskets (2 of them - the manual says the old gaskets should not be reused); look/clean where the valve connects to the manifold for carbon deposits; look/clean the pipe.
(5) If DTC reappears, do all sorts of vacuum checks & other diagnostics as needed

If anyone has other ideas - share.

Thanks!

Kep · 04-09-2008, 02:53 PM

Sounds good to me.

Kep

Creeps228 · 04-09-2008, 10:03 PM

i hate that code

Maim Menu · 04-10-2008, 06:59 AM

Well, it looks good, but #5 should be #1 IMO, never start throwing a bunch of parts at your car before checking all the pesky hoses and tubes first. Learned that the hard way.

EGR and EVAP codes are a pain in the ass to diagnose, you gotta be real methodical about it, otherwise you might miss something.

There should be a tubing diagram somewhere under the hood if I recall correctly, be sure to inspect every little hose before going any further...

kocho · 04-10-2008, 08:00 AM

I've looked at most hoses and they look good. I'll follow the diagram once over to make sure I did not miss any. While looking at these, I found a small oil leak (see my other post) so I will be replacing the valve cover gasket and in this process I will have even better access to the area.

I did not mean replace parts without diagnostics - removing testing & reinstalling/replacing based on the results was implied in the checks for the VSV/EGR components. By looking thru posts here, I seem to see that the most comon cause reported was the VSV (they did not say which of the two VSVs though!), then the Modulator, then the EGR valve itself or clogging at the opening to the engine.

My thinking is to check/diagnose the easiest pieces for which I do not need equipment or too much time. If the EGR valve gaskets are cheap, I am willing and curious to look inside the valve to see if it is clogged. Buying a new EGR valve will only come if I find it to be bad as I think it is close to $200 at many places and I'm not looking to spend that much if not needed...

Last night I opened the modulator valve cover to clean the filter and it was pretty clogged-up - there was a thick black spot where the air comes out of it and into the modulator, but the rest of the filter was not too bad, just not pure white.

Are these filters for sale as a separate item? Looks like a thick piece of felt...

I blew thru the filter with compressed air and rotated it or so to move the black spot away from the tiny whole that feeds the modulator. DTC reset and will wait to see if this cures anything. I doubt it, but if it does it would be the easiest fix I've done in a while...

Quote:

Originally Posted by Maim Menu

Well, it looks good, but #5 should be #1 IMO, never start throwing a bunch of parts at your car before checking all the pesky hoses and tubes first. Learned that the hard way.

EGR and EVAP codes are a pain in the ass to diagnose, you gotta be real methodical about it, otherwise you might miss something.

There should be a tubing diagram somewhere under the hood if I recall correctly, be sure to inspect every little hose before going any further...

nukem678 · 04-12-2008, 09:03 PM

Its the VSV almost 100% sure trust me. 70 dollar and 20 min fix.

kocho · 04-13-2008, 11:20 AM

Quote:

Originally Posted by nukem678

Its the VSV almost 100% sure trust me. 70 dollar and 20 min fix.

It's been 2 days without the code since I cleaned the modulator filter. But I have not taken the car above 40 mph (city traffic only) and I think it has not had a chance to go thru all self-check cycles.

I'll scan for DTCs tonight to check and may be take it for a spin on the highway.

If the DTC comes back, I'll check the VSV - are you talking about the EGR VSV or the other VSV?

Thanks!

kocho · 04-20-2008, 07:09 AM

The DTC returned a couple of days ago. So I started checking the components more carefully. The rubber hoses all seem to be in good shape. The Vacuum Modulator also seems to be operating OK. There is vacuum going to the modulator (I can feel it suck harder with my fingers when the car is running at 2-3K RPM).

Bypassing the VSV and the modulator and connecting one of the two vacuum hoses directly to the EGR valve (while blocking the other one) so that vacuum goes directly to the EGR does nothing - the engine runs as usual (at 2-3K RPM).

With the engine was running at 2-3K RPM I also applied a little "manual suction" to the EGR vacuum port as well - just connected a hose and sucked by mouth, which should be even stronger than the intake vacuum. Still no change in engine operation.

Based on the diagnostics notes I have, the engine should run rough or die when vacuum is applied to the EGR vacuum port. Right? This seems to indicate that the EGR may not be working, so I will remove it and inspect it. Was postponing the removal since the gaskets need to be replaced for reinstallation, but I think now is the time. Or am I doing something wrong with my tests (done on warmed-up engine)?

Thanks!

================
EDIT 15 minutes later...

I removed the EGR valve and it was indeed stuck. After wiggling it about a few times and blowing out carbon dust with compressed air, it seems to operate smoothly again. I stalled the engine by sucking at it with my mouth after reinstallation, so it is at least not stuck now. But I see not much change in engine operation by applying the vacuum from the intake directly to the EGR valve (bypassing the modulator and VSV), so I'm not sure what's going on. There is vacuum but it is weaker than my "suction by mouth" and its effect is at all apparent

Not sure if this is the end of the story, since the EGR valve, while not stuck, may actually be out of speck. Will drive about till the checks are done and see if the code returns. I also felt that the vacuum modulator increases the supply of vacuum with RPM increase, so that is probably working as well, but how well ... who knows...

WIll update you as things progress.

Maim Menu · 04-20-2008, 12:46 PM

Perfect, keep us updated.

kocho · 04-20-2008, 06:23 PM

Quote:

Originally Posted by Maim Menu

Perfect, keep us updated.

All checks cleared OK but then about 10 miles after my second start the DTC came back.

Am I right in thinking that the EGR Valve should cause the engine to stall/run rough if the vacuum house from the intake is connected directly to it? And if yes, does it mean I somehow have a vacuum leak somewhere and thus not enough vacuum is created to drive the EGR Valve? Or does it mean that the EGR valve is just too tough to operate and needs replacement?

The valve is not clogged at all - it is blackened but there are no deposits enough to clog any passages. And it now activates if it suck on it with my mouth. But the vaccum from the intake is not strong enough to activate it...

Here is again how I tried to bypass the Vacuum Modulator and the EGR VSV. There are two vacuum houses coming out of the intake and into the P & R ports each on the vacuum modulator. I disconnect these two at the vacuum modulator and leave them connected to the intake as they were. Then take the P hose and plug it in the EGR top, while I keep my finger on the R hose to block it. All else is connected as it should be. I can feel vacuum with my fingertips, but that is apparently not enough to activate the EGR with the engine running at any RPM.

Should I be blocking the EGR to VSV hose while I do this test? The hose that goes to the bottom of the Vacuum Modulator?

I guess my next step would be to rent the vacuum meter and see the precise values of the vacuum created at various points...

Thanks for any suggestions!

68 Malibu · 04-20-2008, 07:27 PM

Quote:

Originally Posted by kocho

All checks cleared OK but then about 10 miles after my second start the DTC came back.

Am I right in thinking that the EGR Valve should cause the engine to stall/run rough if the vacuum house from the intake is connected directly to it? !

Yes. Forget about hooking a vac hose to it. Use your finger to compress the diaphram. If the engine is warm, the egr will be hot so be careful. Compressing the diaphram will make it run rough.

sheepish_fixer · 04-20-2008, 08:34 PM

may not be the egr valve:

http://www.mpt.org/motorweek/goss/2709.shtml

The egr system should be cleaned every 30k...my mothers car has ever been cleaned...here's a blurb from the link above:

"Now in the past it was next to impossible to clean these without taking the car apart. Well now there is Snake Bit, a flexible drill that can go into a variable speed hand held drill and it'll go down through that clog, clean away the carbon, the carbon then passes through the engine and out the exhaust and you have saved a lot of money and aggravation"

I hope this helps, and thanks for keeping us posted

kocho · 04-21-2008, 06:28 AM

Quote:

Originally Posted by 68 Malibu

Yes. Forget about hooking a vac hose to it. Use your finger to compress the diaphram. If the engine is warm, the egr will be hot so be careful. Compressing the diaphram will make it run rough.

Yes, this happens - when I manually open the valve the engine stalls. But when I hook-up the hoses from the intake to the EGR directly nothing happens...

Kep · 04-21-2008, 09:36 AM

You might try finding a vacuum gauge to see what vacuum you actually have. Going on memory, doesn't the VSV for the EGR determine whether or not there is a vacuum to the Modulator / EGR? I believe that you have already checked this but if you don't have a supply vacuum at the EGR you may want to check again, as well as verrifying that the hoses in between are unobstructed and leak free.

By the way, there are hoses between the EGR and the modulator. When I cleaned my 90 the lower hose was completely obstructed by carbon (I twisted a small screwdriver like a drill bit to ream it out). Don't know if your model has the same plumbing but it is likely similar.

Kep

kocho · 04-21-2008, 10:45 AM

Quote:

Originally Posted by Kep

You might try finding a vacuum gauge to see what vacuum you actually have. Going on memory, doesn't the VSV for the EGR determine whether or not there is a vacuum to the Modulator / EGR? I believe that you have already checked this but if you don't have a supply vacuum at the EGR you may want to check again, as well as verrifying that the hoses in between are unobstructed and leak free.

By the way, there are hoses between the EGR and the modulator. When I cleaned my 90 the lower hose was completely obstructed by carbon (I twisted a small screwdriver like a drill bit to ream it out). Don't know if your model has the same plumbing but it is likely similar.

Kep

On your second note, yes, there is a rubber line b/w the modulator and the EGR. Mine seems in good shape and completely unobstructed. I had to disconnect that to remove the EGR valve.

My next thing to check are the two little metal tubes coming out of the intake, to which the two rubber lines to the modulator connect. These are very thin and might be blocked on the inside of the intake by sludge. If these are plugged with sludge, no wonder vacuum would be weak. Sludge does accumulate in this area a lot (around the throttle body plate) to such an extend that if not cleaned every 40K miles or so, my gas pedal begins to stick while I stop to idle.

If cleaning them does not help, I'm off to AutoZone for the vacuum gauge/vacuum pump to do further checking.