p0401 stalking my truck....can you help solve?

This p0401 code (insufficient egr flow) keeps coming back to haunt me and I NEED to smog my truck. I've exhausted (sorry couldn't resist) all the troubleshooting steps I know to take so I'm here seeking fresh input from smarter more experienced TW members.

Thought I had it solved last night when I took IAC apart and found that the little sliding door wasn't moving due to carbon/soot buildup - fixed that, put all back together, thrilled when idle dropped down to where it should be when engine warmed up. On way to smog station the check engine light came back on.

As of now: vacuum modulator: tests good (holds vacuum). Brand new Aisin VSV.
EGR temp sensor: err, (sht, I forgot to test the replacement! will do and update as soon as I finish this post). MAF: cleaned w/CRC mass airflow sensor cleaner. EGR: AFAIK good...neither it nor the pipe are clogged-actually whole assembly looked unexpectedly clean. Have not vacuum tested (yet)

This code popped up one night last summer as I was going down I-5. Two days later I started what turned out to be a semi-rebuild so don't know what set it off to start with.


If you have experience or input that would be helpful PLEASE PLEASE share it here. I've done extensive searches on this issue and am feeling- stymied

for reference: this is a 96 2rzfe (2-4l 2wd) auto with brand new cylinder head, timing chain kit etc.

Thank you for your time

 

I found this diagnostic for the P0401, but it's for a later model year ('03). I seem to remember there were changes to the emission system around model year 2000. It may help anyway, so might be worth a look.

 

Did you test that the vacuum modulator holds vacuum at the bottom port? That will tell if the rubber diaphragm inside is old and broken. The tests in the repair manual don't say to test this.

Apply vacuum with a hand vacuum pump to the EGR valve and make sure the valve moves and it holds vacuum.

I was going to say check that the pipe from the exhaust and the passage into the intake are clear. If the head was replaced then these were probably checked/cleaned? Did you do the work?

 

@ghs57: thank you so much! yes there were changes but I'll try all that applies.

@DrZ: ...I didn't have a vac pump- took it off and tried blowing thru bottom port. No air passed thru- and I have a pretty healthy pair of lungs, do you think that's an adequate test? Yes, I did the work.

Thank you both for taking the time to reply.

 

You don't need a vacuum pump.

Here is a good test (to see if the EGR valve can open properly).
- Detach the short vacuum line from the port at the valve
- Attach a much longer length of tube (same inside diameter) to the that same port
- start the engine
- suck on the tube

If the engine runs rough and/or stalls, that means the valve opened (as it should), creating a big vacuum leak.
If nothing happens, then the valve is stuck closed, and should be replaced.

 

No, your lungs can only generate about 2 psi max.

Borrow or buy a cheap hand vacuum pump. They come in handy for testing things like this.

 

ok guys thank you.

 

Just adding my 2cents. I had this code along with a few others on the forum who figured out it was the EGR solenoid (I believe it is another name for the vacuum modulator?).

I was able to blow through the hose on the bottom of the solenoid to tell that it was busted in my case, and verified my new one was working doing the same method with no air going through. But, as DrZ mentioned you really would want a vacuum to tell for sure on the old one.

For me changing that fixed my P0401 Code. I also changed the EGR valve and PCV valve, but on reflection I think both were working fine and the solenoid primary was the issue.

 

What about the VSV valve under the intake manifold? Its green. I had the same issue as well. I would clear the light and it would go away. Then after a couple days would light up again on a Cold morning AM startup.
I replaced that and the light was gone.

 

(solved.)

(probably).


the cause (?) :

( cue loud "D'OH !" + drumroll, this is embarrassing: )....:

I neglected to remove the 'blocker' gasket from the new exhaust manifold when I installed it. <These manifolds are all manufactured to accommodate an EGR valve even though only one state - California- requires them. A few small pieces of hardware ( a sheet- metal plate and gasket to seal off the EGR port ) make one model of manifold usable in all 50 states. >
When I received the manifold I removed the blocker plate (sans gasket) from the port, then loosely replaced the two shiny new bolts (so they'd be handy when it was time to put everything together.) Due to use of my unique (soon-to-be-patented) wrenching technique- which I've dubbed "Bolts-by-Braille" (copyright also pending)- come installation time I 'overlooked' the blocker-plate gasket....

One might think I'd have noticed this little -um, problem, if not the second but at least the third time I took everything apart -again- . But no. ....... "forest, what forest? I don't see no forest! All I see is trees, lots and lotsa trees."
My next-door neighbor never even sighted the 'forest', much less the 'trees'. Took him maybe an hour to find the mis-placed gasket while he (at my request and using top-of-the-line equipment) did a point-by-point troubleshoot on my truck.

Oy.

What a dumbs41+.

 

a belated (after the fact) "Update":

( because I didn't see Pervy and kellen619's posts until after my 'solved' post went up

@Pervy:
Excellent point, and suggestion. Actually that's exactly what I did !....
but,
( in part because I have a HUGE amount of respect for DrZ's input. !!!Guy is an unsung hero. This site is graced by the participation of many MANY great Toyota experts and enthusiasts whose knowledge and experience are unmatched- anywhere-!-and I've a long list of 'favorites' but DrZ pretty much heads the top of that list. )
I sought (and got), a concrete, equipment- verified '2nd opinion'. (I wasn't able to blow through mine-and it ended up testing 'good' w/neighbors' vacuum pump-keep reading! )

@kellen619:
I replaced mine -that's the last (maybe unnecessary) part I 'threw' at it: read post #10 for 'the exciting (nah, just embarrassing!) conclusion.



I gotta go- this bleepin computer just 'vanished' nearly 1 1/2 hrs worth typing. it's just-gone. I'll try a rewrite later-walkin away now