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P0401 1996 Corolla 1.6l....again

by Rich, February 27, 2008 in Pre-1997 Toyota Corolla and Geo Prizm



Hello All,

First post here, so please be patient. I have a 1.6L '96 Corolla with 189--- miles on it. 3 or 4 months ago (~5000 miles) my CEL came on - code P0401 Insufficient EGR flow.

Originally, I checked the EGR Valve by applying a vacuum and it caused the motor to stall. The EGR modulator passed a small amount of air when I held ports P and R. Additionally, the bottom port held a vacuum so I believed the internal plastic piece was not torn. I then purchased a VSV from Toyota and cleared the code.

The CEL remained off until last night when it came back on. A quick run down to AutoZone revealed code P0401 again. This weekend I will repeat the above procedures and recheck the EGR and modulator. I am quite confused now because of the duration of time between the two errors. Does anybody have any suggestions about what is going on or what else might be suspect? Thanks for the input!

EGR system is notorious in getting completely clogged with carbon deposits - could be in the time you were diagnosing the EGR vacuum system, you might have did enough to let it drag on a few more months. Similar thing happened to our Camry - P0401 codes every couple of months - ended up replacing the EGR valve and the problem has not returned in over 2 years. You might get lucky with a cleaning - can't hurt to try. Good luck.

Rich,

Since the car did stall with vacuum applied it would suggest that the EGR valve is functioning and the exhaust and intake ports are clear. If it idles smoothly it can also be assumed that the EGR valve seating area is clean.

My guess is that the EGR modulator failed.

If you haven't read it yet, we discussed it pretty well here .

https://www.corolland.com/forums/index.php?...mp;#entry132780

You might go to a junkyard and pick one up to give it a try. The same modulator is used across a broad number of years and it is also used on Subarus.

Good luck,

Jay in MA

Hello All,

 

First post here, so please be patient. I have a 1.6L '96 Corolla with 189--- miles on it. 3 or 4 months ago (~5000 miles) my CEL came on - code P0401 Insufficient EGR flow.

Originally, I checked the EGR Valve by applying a vacuum and it caused the motor to stall. The EGR modulator passed a small amount of air when I held ports P and R. Additionally, the bottom port held a vacuum so I believed the internal plastic piece was not torn. I then purchased a VSV from Toyota and cleared the code.

The CEL remained off until last night when it came back on. A quick run down to AutoZone revealed code P0401 again. This weekend I will repeat the above procedures and recheck the EGR and modulator. I am quite confused now because of the duration of time between the two errors. Does anybody have any suggestions about what is going on or what else might be suspect? Thanks for the input!

Thanks for the help. I will try to clean everything out this weekend too. Guess it can't hurt.

TRcar54: Interestingly enough, the car is not idling smoothly. This is a recent change, as for the last 4 or 5 years it has been really smooth. Coincidentally, the rough idle occurred about a month ago, after taking the car into Beaverton Toyota to have the t-belt changed and valves adjusted/checked. Figured something happened there that made it idle roughly. Perhaps its the EGR valve seating area as you mentioned.

Also, I will be going to a junk yard this weekend and picking up at least a modulator. Thanks for the help!

You're welcome Rich.

You could remove the EGR valve and see what condition the seat is in. Generally an EGR is going to either fail to open or not fully close. The latter can also cause a hard start, or no start scenario depending on the severity of the carbon buildup on the seat. The former can cause a pinging symptom.

On the rough idle, I suppose it is possible that the repair shop screwed up the valve timing by misaligning the cam and crank a cog or two on the pulleys. They also could have adjusted a valve too tight.....not good.

Good luck,

Jay in MA

Thanks for the help. I will try to clean everything out this weekend too. Guess it can't hurt.

TRcar54: Interestingly enough, the car is not idling smoothly. This is a recent change, as for the last 4 or 5 years it has been really smooth. Coincidentally, the rough idle occurred about a month ago, after taking the car into Beaverton Toyota to have the t-belt changed and valves adjusted/checked. Figured something happened there that made it idle roughly. Perhaps its the EGR valve seating area as you mentioned.

Also, I will be going to a junk yard this weekend and picking up at least a modulator. Thanks for the help!

I rechecked everything today. The EGR valve seemed to be functioning right and there was no carbon buildup in the hoses. The modulator though didn't appear to be working right. It would not pass any air through one of the ports to the outside (Q maybe - if that's the one that's supposed to expel air through the filter to the outside). After a few minutes of playing around with it, it become unclogged. I put everything back together and reset the CEL. I have driven the car it several times since then, allowing it to warm up fully with no CEL. For good measure, I will be replacing the modulator to *hopefully* avoid re-occurrence.

As a side note, after unclogging the modulator, the idle became very smooth again. Not sure why it became rough right after it was taken into the dealership for t-belt and valves. Hopefully it was coincidence. Again, thanks for the input!

You're welcome Rich.

You could remove the EGR valve and see what condition the seat is in. Generally an EGR is going to either fail to open or not fully close. The latter can also cause a hard start, or no start scenario depending on the severity of the carbon buildup on the seat. The former can cause a pinging symptom.

On the rough idle, I suppose it is possible that the repair shop screwed up the valve timing by misaligning the cam and crank a cog or two on the pulleys. They also could have adjusted a valve too tight.....not good.

Good luck,

Jay in MA

that modulator part is used on ALOT of import cars...pretty much every toyota with an EGR from 93 onward, i've seen them on honda, subaru, nissan, mitsubishi, etc etc.

you might also replace the vacuum lines that go to the EGR system, sometimes the lines can go bad and flake little bits of rubber internally. also not a bad idea to use a small pick or drill bit to make sure the vacuum nipples are clear and clean on the manifold.

and while you're in the area, pull your throttle body from the intake manifold and give it a good cleaning on both sides and the edges of the throttle plate as well as getting into the manifold. dont worry about getting some carb cleaner down into the cylinders, seafoam also works well to clean the manifolds. i wrapped a rag on a stick like a giant Q-Tip and soaked it in some chemicals and used that to clean things out in there. mine wasnt very dirty since i do not have an EGR. you'd be surprised at all the carbon buildup left by the EGR system!

Again, thanks for all the help guys. I will write a brief follow-up post for people who experience similar symptoms.

As mentioned above, the EGR modulator failed (aside from now passing air, the plastic piece inside was torn) about four months after the VSV failed. After replacing it though, I got the CEL again. Last weekend, I pulled off the throttle body and intake manifold. They were both quite easy to remove. I was surprised how much carbon buildup was in the intake manifold. I clean the throttle body plate about every six months, so that area was fairly clean. The ductwork between the intake manifold and EGR valve though was quite full of crap. It's quite possible my problems were partially caused by this buildup flaking into (or filling up) the other EGR parts or restricting the vacuum/air flow. From that time, my CEL has remained off.

So, for people with high mileage cars with CEL code P0401, I would suggest you first pull the intake manifold/throttle body and clean everything quite thoroughly ($3). While you are at it, pull the EGR valve and clean that out with carb cleaner (seafoam, etc.). Replace all vacuum hoses ($2). Then proceed with replacing the more expensive parts. Just my 2 cents after working through my EGR problems.

I am curious if anybody knows, what sensor(s) detect EGR flow?

Again, thanks for all the help guys. I will write a brief follow-up post for people who experience similar symptoms.

As mentioned above, the EGR modulator failed (aside from now passing air, the plastic piece inside was torn) about four months after the VSV failed. After replacing it though, I got the CEL again. Last weekend, I pulled off the throttle body and intake manifold. They were both quite easy to remove. I was surprised how much carbon buildup was in the intake manifold. I clean the throttle body plate about every six months, so that area was fairly clean. The ductwork between the intake manifold and EGR valve though was quite full of crap. It's quite possible my problems were partially caused by this buildup flaking into (or filling up) the other EGR parts or restricting the vacuum/air flow. From that time, my CEL has remained off.

So, for people with high mileage cars with CEL code P0401, I would suggest you first pull the intake manifold/throttle body and clean everything quite thoroughly ($3). While you are at it, pull the EGR valve and clean that out with carb cleaner (seafoam, etc.). Replace all vacuum hoses ($2). Then proceed with replacing the more expensive parts. Just my 2 cents after working through my EGR problems.

I am curious if anybody knows, what sensor(s) detect EGR flow?

i beleive it to be the MAP sensor that detects the pressure difference when the EGR opens. i know cars with AFM's/MAF's use a pressure sensor to sense EGR function. its also possible that the O2 sensors come into play since you're adding combusted gases which would displace fresh air.

 

 



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