Hello,
I've had a P0402, excessive EGR flow, for over 4 years. This problem defied repair because the light came on intermittently, random. A simple reset by pulling the EFI 15A fuse for about 5 seconds offered a temporary solution. Over time, the CEL came on more frequently but still random.

Over the years, I replaced modulators, sensors, vacuum hoses, EGR, etc. I checked the IAC, the temperature sensor, swapped out the ECM and ran troubleshooting procedures. Still, I had no success in ceasing the CEL. At times, I could drive it several hundred miles without the CEL light coming on, and the light would return after starting the car and driving it for a mile or two.

The issue was random, with no resolution after years of trying to understand what I presumed as a vacuum loss. I began considering engine-vacuum loss somewhere, perhaps, in the engine or the intake manifold had a buildup of carbon. I could not duplicate triggering the CEL code.

I solved the issue serendipitously by replacing a slightly leaking high pressure power steering hydraulic hose, p/n 44411-33010. I found it slightly leaking at the crimp point after very close inspection of this hose. The slow dripping leaked onto the p/s pump and onto the frame. It appeared the dripping could have come from multiple sources.

This design has a pressure valve (called Power Steering Air Control Valve, not to be confused with the IAC) screwed into it, Lexus p/n 17630-74010. I immediately reasoned that the slow fluid loss from the hose affected the vacuum valve which, in turn, prevented vacuum to build up in the intake manifold. In other words, because this high-pressure hose could not maintain adequate pressure within the line, the valve reacted accordingly, causing lower pressure from the valve to the intake manifold to which it was attached. (see diagram below). The light came on once early after replacing that one valve-attached high pressure hose only because the ECM was relearning the corrected pressure. Since, the CEL code has not returned after months of driving and thousands of miles.

I didn't quit on this issue because I spent thousands on new Michelins, paint, and reupholstering the interior with 90s Toyota cloth.

Subsequently, I've replace all the hydraulic lines and the two reinforced rubber supply lines on the car as preventive maintenance. The p/s reservoir molded hoses, p/ns 44348-33090 and 44348-33100, are supplied by Rein PSH0381 (-33090) and Sunsong 3404451 (-33100), respectively.

For the rubber section of some of these high-pressure hoses, id = 10mm.

The metal hoses:
Lexus 44411-33010 equivalent Edelmann, 91861
Lexus Tube Sub-Assy, Return, 44406-33012 equivalent Sunsong 3404767
Lexus Steering Gear Outlet Return 44416-33010 equivalent Sunsong 3044478
Lexus PS hose to hydraulic fan motor 16927-20010 equivalent SUNSONG3402255, EDELMANN92171, GATES365551. I used Gates for this hose. Note, some Gates and Edelmann hoses are both made in Mexico probably from the same Mexican supplier. Go with the lower-cost hose.

Reinforced, molded rubber PS supply hoses
Lexus 44348-33090 equivalent Rein PSH381
Lexus 44348-33100 equivalent Sunsong 3404451

PS air control valve: note, these rarely fail; available but costly. Find a spare in the junkyard.
Lexus 17630-74010

Hope this will help others in considering a P0402 code with those cars that have this type of high-pressure hose with this valve attachment.

Best,
Byron

Parts image of power steering system, 96 ES300

 

You're theory is the slow hydraulic leak caused the vacuum valve to behave abnormally, causing an intermittent vacuum surge or leak or how ever you want to characterize it. Do I have that right?

 

Allow me to clarify. I presume a failing crimp (maybe the hose under the crimp) lowered the fluid pressure sufficiently to affect pressure conditions (air displacing fluid?) which consequently caused the vacuum valve to affect intake manifold pressure and thus, trigger the P0402 code. Leaking fluid is a symptom, not the cause. I can't confirm cause and affect, but surely a very high correlation exists between a new high pressure hose and no CEL P0402 code. I couldn't recreate the problem. I guess one could but one would be working with high pressure, not very safe.

I did not see fluid on the ground which did not prompt me to check leaks. It was by chance that I saw some fluid on the crimp when I was replacing the molded p/s fluid supply hoses to the pump after I found fluid on the ground. By then, I could see the fluid. I replaced the pump several years back and I thought the pump failed again or the original molded hoses finally failed. It was neither.

I did not experience any issues with steering or engine cooling issues from a drop in fluid pressure before I replaced this h/p hose. The hydraulic fan worked as designed, based on cooling and ac use. I'm bothered a bit about not knowing the sensitivity of the vacuum valve and no significant drop in fan cooling speeds.

Sorry for some poor grammar in the original text. I was writing while the text spewed out of my head.

 

Hello,

Yep, you are not the first one bitten by those admittedly dubious valves, hated them ever since I learned of their existence when a guy replaced his entire engine trying to find a cause of a Blue Smoke in the exhaust, and was quite ... pleased to see it return even with the New engine fresh out of the box. It eventually turned out that the Diaphragm in that Valve ruptured badly, feeding Power Steering Fluid straight into the Intake, causing the smoke, and since they fitted the Power Steering System off the old engine, it was quite easy to work out how the Smoke made its grand return right after.

It just always seemed like a clumsy afterthought to me, one of the least reliable items out there, only beaten by the ECU problems in some older Lexus models, I was hoping they got rid of it when a 2nd-gen ES got a Facelift with a 1MZ engine, even though GS didn't get the memo right up to the Facelift in 2001. But yeah, it's pretty easy to see how a leak similar to the one described above (granted not as drastic) would ruse the ECU into blaming an EGR System, as there usually isn't much else inside an engine that could cause more Fuel to appear in the Combustion Chamber straight out of thin air.

Either way, it's nice to know that this could be one of the causes for an EGR Code, and for giving me another thread that I can use to cause one specific eye to twitch and lose some healthy sleep for a while..

 

Fortunately, my valve did not fail. I'm using the original. I think my issue was the leak dropped the pressure and the valve adjusted accordingly, but the drop was not based on engine performance. So, I think the valve allowed more pressure into the intake manifold which caused the excessive gas recirculation, P0402.

Thanks for sharing your issue, and I'm surprised. It may be a good idea for me to pull one off a junk car.

These small issues are extremely difficult to diagnose.

 

Looking up the part for a Toyota, the description states (https://autoparts.toyotaofnorthausti...074010#popup-1),

"The Air Control Valve Assembly (#17630-74010), a crucial component within the Vacuum Piping system and part of the Engine-Fuel category, is fundamental to your Toyota's performance. It monitors and controls the amount of air that enters the engine during idle. It works in concert with the throttle body, maintaining the correct engine idle speed and preventing stalling. Genuine Toyota parts like these ensure excellent compatibility with your vehicle.

If the Air Control Valve Assembly (#17630-74010) becomes clogged or broken, it may cause the engine to idle at a high RPM or stall at low RPM, leading to overall inefficiency and potential safety issues. Remember, genuine Toyota parts come backed by Toyota's parts warranty. Overall, this part contributes vastly to your engine's efficiency and reliability, making it a critical element in the overall vehicle's performance."

 

Huh, interesting..

Well, unless the leak was so bad the system emptied itself faster than you could fill it, I doubt that it was what caused the issue. In a Hydraulic System, it's the Flow that determines basically all the main parameters, a Pressure is only a result of an Obstruction to said Flow. The Power Steering System also has a Flow Control Valve that would keep the Flow more or less constant with various loads and engine speeds, dumping the excess back into the Reservoir or on the Inlet of the Pump, below is a great video of how it works. Unless the leak was big enough to dump all that excess out on the floor instead of returning it to the Reservoir, the Flow, and therefore Pressure in the System would remain Constant.


The way an Idle-Up Valve on the Power Steering Line works is it has Two chambers - the Fluid and Vacuum one, sealed completely from each other with a Membrane, per say (at least until said membrane fails), when the pressure Increases, the PS Fluid pushes on the Membrane, opening a passage between the Intake Vacuum System, and the Outside Air, essentially creating a controlled Vacuum Leak to raise the RPM, it's quite a primitive system. That is to say that in case if the pressure hypothetically gets Lower than normal, said Valve can't really get more closed than it already was, so you won't see much of a response from it either, if it is functioning correctly that is..


I am not exactly sure how replacing just the Hydraulic Line fixed your EGR code, could be that when you were unscrewing that dreaded Valve, it shook something loose in it and plugged the leak of Power Steering Fluid into the Intake, getting rid of the Code, as I can't really see any other link between those two systems..

Well, considering that it is a dubious nature of the Valve that causes the issue, chances are that the one you get off the Junkyard will be the same or worse than what you have right now, the choice is to either get a New OEM one, or bypass it completely.

That sounds more like a description for an Idle Control Valve, which it's not. Its sole function is to Increase the RPM of the Engine when the Load on the system increases, preventing the Engine from bogging down, which would cause some Vibration throughoutthe car, if anything, that's all there is to it. It is actually part of the reason for my beef with it, for such a simple function, it creates a lot more trouble when it fails than it can solve. There are people who bypassed it and the cars went on for years without it, you just need to remember not to force the wheel too much.

Hope this helps and best of luck!

 

An update, I am wrong about my original post, still an unsolved P0402 code. It had returned twice since I worked on the car in April or May.

I still don't know why I throw this code. It occurs mainly when I start driving the car for the first 5-10 miles after starting. I can drive hundreds of miles and it won't trigger a code. It doesn't throw this code often. I can drive the car for days.

Since it's random, I haven't been able to troubleshoot the problem. The next step, a guess, is to remove and clean the intake manifold which I have Toyota parts to do this work.

I've replaced just about all the parts with OE or OEM parts related to this code. Texas will end its inspection in 2025 and the car has passed the 25-year mark for inspections, too, so I can just erase the code and drive it.

 

I would still suggest bypassing that Idle-Up Valve from before for the time being, just take the Vacuum Lines off of it and plug both ends with something to avoid a Vacuum Leak, get a spare Vacuum Line and connect both Valve Ports together as well so that nothing gets trapped in it, the only concern after that would be to not push the Steering Wheel too hard into the lock, though it's just generally a good advice. While at it, get a Scan Tool capable of reading Freeze Frame Data, I am pretty sure your 1996 Model Year should support it, and check if there are any Parameters that are out of spec, pay close attention to the Fuel Trims and the EGR Position Sensor Data, it may prove useful in case one of the sensors fails intermittently for a second at a time, however unlikely.

If none of that will yield any results, you can try Unplugging the Vacuum Line for the EGR Actuator and see if it will trigger a different code this time, like P0401, which would indicate Insufficient, instead of Excess exhaust flow, then you will know to look in that general area.

Hope this helps and best of luck!

 

I know it's a very long-shot but I'd replace the gas cap if it hasn't already been. With the various systems tied together perhaps something in the vapor control system is connected with the EGR system. My '05 ES began throwing a bunch of unrelated problem lights on the dash every so often including traction control and similar, and it actually did turn out to be a bad gas cap. I found that apparently MANY people had the same thing happen and it seems maybe Toyota wanted to be sure you'd get the car checked out so they tripped a bunch of warning light. I could clear the trouble code (I forget which one it was) and it might stay gone for a day or a week, but it always came back til replaced the gas cap. Given the age of the OP's car, if the cap is original it wouldn't be a bad idea to replace it anyhow. Since replacing mine a year ago and clearing the error code, not once has the problem recurred. Just a thought. And if the OP did mention having done that already, I must have missed seeing it.

 

Hi, thanks for the tip. More ideas always help and can't solve the problem alone. Yes, it's the original cap. I'll add to my order to the other parts list.

 

Thanks for your reply! I'll try that, but I've been reluctant in doing just that.

I have a scan tool with freeze frame, and it does collect these data, but I don't know what I'm looking for--how to check what is out of spec. Where can I find these specs?

My EGR is vacuum-actuated. I have unplugged the vac line to the EGR and yes, it pulls a P0401 code.

Thanks to all, most grateful.

 

Again, the sole purpose of said Valve is to raise the Idle RPM when there is an excessive load on the Power Steering System, and unless you are one of those people that only turn the wheel when the car is stationary, or try to force the wheel to do an extra turn with all their might, you shouldn't feel any difference, if nothing else, it may actually help you better understand the Steering System and help prolong its life.

If possible, post everything that it stores here, so we are not missing anything.

Interesting, makes me wonder if there is indeed an issue with the Valve or the Solenoid that controls it, Freeze Frame Data may really come in handy here if it stores the Position of an EGR Valve among other things.

Hope this helps and best of luck!