Toyota P0401 EGR Insufficient Flow

http://www.toyota-p0401.weebly.com

I own a 1996 Toyota Tercel that had this problem, and since this is a common issue, I decided to create a website and write a blog to help fellow victims of this code. This is a collection of all possible problems and remedies I have found. Most often, simply taking things apart and cleaning will solve the issue. These procedures are similar across most models of late 1980’s, 1990’s and early 2000’s Toyotas. For this job I recommend getting a can of Seafoam Deep Creep. Most work is a bolt off and bolt on procedure. Below is a list of things to check.

Vacuum Switching Valve (VSV)

Connect the two leads to your battery (12V source) and see if it clicks like a solenoid should. Apply vacuum (with a vacuum pump) to the two ends. One should hold vacuum, and one should slowly release vacuum. It should also not hold any vacuum while connected to the 12V source.

Vacuum Modulator

Apply vacuum to all 4 outputs separately to make sure the diaphragm isn’t stuck, and check all lines for carbon build up. The lines may not necessarily hold vacuum and that’s normal. Plug P, R and Q, and apply vacuum to the bottom, this should hold vacuum, if not, the diaphragm could be bad. Sometimes, just doing these steps will loosen the diaphragm and solve the problem.

EGR Valve

Apply vacuum to the top inlet, while the car is running, if the engine stalls or sputters, and you maybe hear a click, that means your EGR valve is working. You can also be relieved you don’t have to spend $150-$200 on a new one. You can also take the EGR off of the engine to check if the main inlet and outlet hold vacuum. On mine, the outlet that the hose that goes from the EGR to the vacuum modulator attaches to was completely clogged, and I spent a solid hour stabbing at the built up carbon, (bottom left). Also clean out the pipe that goes from the EGR to the intake manifold.

Throttle Body and Intake Manifold

This is a slightly more complicated job that may require some previous mechanical experience. The throttle body and intake manifold will not likely need replacing, however, will most likely need cleaning. The carbon build up from the PCV (Positive Crankcase Ventilation) Valve and from the EGR (where the EGR pipe meets the intake manifold), has most likely made quite the mess in your intake. To begin, locate the hose that connects the air filter box to the throttle body and remove it (requires loosening the hose clamps on each end). Then, take off the 2 bolts and 2 nuts that mount the throttle body to the intake manifold. You can leave all hoses attached, and just let it hang there. Be careful with the TB gasket, as it will most likely be fragile, you may have to get a new one like I did (make one out of paper). Once the TB is off you will notice all the black soot build-up. Now is the time to whip out that can of Deep Creep and spray the inside of the TB. It will be most dirty on the side closest to the engine. Make sure to let the Deep Creep sit in your idle control valve (what lets extra air flow in cold starts). Take some rags and wipe out the TB as best as possible (may require somehow keeping the throttle open). Next step is the intake manifold. Take off the EGR pipe from the intake manifold and unload Deep Creep in there. The less Deep Creep that goes into the engine the better, although it most likely will not do any actual damage. Make sure the passage way is completely free all the from the EGR to where it actually enters the manifold. Also try to keep as much carbon from entering the engine as well. When putting the TB back on, torque them to around 15 ft lbs (may vary), and check a week later. If you don’t have a torque wrench make sure you don’t over tighten. Even if this cleaning doesn’t solve the EGR issue, your car will certainly thank you and run better.