at 150k miles the egr valve cooler assembly should be cleaned

Good question - If you understand that you understand the EGR system. At part throttle settings (mostly city driving) less HP is needed and more EGR is used to dilute the mixture for better emissions and fuel economy. Whenever the gas pedal is mashed the EGR is supposed to close.

Less flow through the EGR = less deposits inside the EGR cooler.

It's also interesting to know that combustion with low oxygen produces soot. Whenever the fuel mixture is richer than the available oxygen (if you understand stoichiometric mixture) you'd have more soot production. This means that when a fuel molecule does not meet up with enough oxygen molecules instead of water and CO2 it turns into something else like CO (carbon monoxide) or a soot particle.
Possible scenarios for soot production:

1. During a cold start the fuel mixture is richer partly to prevent a misfire and partly because the O2 sensor is unreliable until it's hot. This is called "open loop" control.
2. Whenever the engine's in the power band. I'd have to look at the fuel maps to know how bad this is.
3. I suspect another scenario is when the EGR valve is open it dilutes the oxygen with C02.
4. Bad O2 sensor?

 

LOL, that's a good tease.

It also doesn't help that the previous owner lived 3 miles away from work.

 

Not so in Prius. EGR is active at WOT, and that cools combustion and no extra rich mixture is needed unlike regular cars. That limits the peak HP of course.

 

That's not what I want to hear: a lot of our driving is lower speed, in town.

Now that's what I want to hear.

I guess time will tell. Only 68K km's, and I've check the connector pipe (between EGR valve and intake manifold) it's had only very minor carbon. I'm thinking with the coolant change (at 10 year mark) I'll also remove the cowl and check out the egr cooler, intake manifold, etcetera.

 

Do you have a link for EGR flow rates or EGR valve % open versus throttle for a Prius?

 

It might be 950~1600 rpm?? That's mentioned on page 1 of the attachment, lots of reading.

 

Do you (goes both ways)? My take is based on the info posted here by people way more knowledgeable than myself. Feel free to search.

 

I think you read too much into my question. My intent was not to debase you only to learn more why the Prius has a crappy EGR system that doesn't set a DTC when it plugs up. Yes I have searched and found nothing related to the EGR step position / RPM.

Good find, Mendel. This proves that there is a sensor for the EGR cooler and that the computer tries to monitor it from 950~1600 rpm. It could be that the EGR is still open >1600 RPM but that the manifold pressure increase from the EGR flow is too low to measure for the DTC.
I'm also curious why the DTC is set so late in the game when the cooler is plugged up for some people. Is it because the computer program has such lax trigger points for this DTC? Or is it the increase in blow-by (due to thin oil control rings) increasing manifold pressure around the same time that the cooler plugs up?

 

fyi
In Torque I found this - "EGR Step Position"

Earlier this evening I added this to my Torque settings.
I just got back from some surface street driving, stop-n-go, 35-50mph max.
Prius was already warmed up from driving all day.

When the ICE was providing almost any power (anything above ~1200rpm) the EGR step position would register ~20-45.
The lowest I can recall seeing was an 18 one time.
The value was constantly changing depending on the slight variations in throttle & road elevation.

 

My Prius spends nearly its entire life at highway speeds, 65-75 mph. Flow rates and operating temps should maybe keep the cooler sort of clean? Now I want to spring for a couple EGR gaskets and take a look.

 

Cool, I may need to do that. have you done a WOT trial?

And a question for all who have done it: did you replace the gaskets per the FSM?

 

Nice. With some careful study and a few datapoints we may be able to come up with a magic math equation that can tell someone approximately the restriction in their EGR cooler.

As soon as I get my Prius back on the road I'll record "EGR Step Positions", and MAP sensor for a 100% cleaned EGR cooler and intake manifold.

 

I didn't replace the gaskets. But then the gasket between the EGR valve and EGR cooler leaked, making a hissing sound that I pinpointed with rolled up thin cardboard as a stethoscope. I took it apart and coated the three metal gaskets with copper coat gasket compound, and that fixed the problem. I chose not the replace the metal gaskets because my local Toyota dealer didn't have them in stock, and never called me back on the special order. I didn't want to pay as much for shipping as the parts, so I didn't try mail order.

The rubbery/silicone gaskets on the intake manifold should be totally fine to reuse if they look good upon inspection. Unlike the metal gaskets, you're not going to stress the material beyond the yield strength in this application and get permanent deformation.

That said, the EGR pipe to EGR valve metal gasket survived a couple disassemblies without leaking, and the EGR cooler to exhaust manifold gasket also didn't leak. The EGR valve to EGR cooler interface has more curves to it and more play. I might have avoided the leak by using the service manual's tightening sequence, which I didn't look more closely at until I had finished.

 

catch can after 800 miles

Pixel XL ?

 

Finally got a chance to try WOT - getting onto interstate accelerating from ~35-80mph.
The RPMs peaked around 4700.
"EGR Step Position" started around 17 before dropping to 8 just before I stopped accelerating.
"EGR Step Position" was never 0 during WOT for sure.
So EGR is being used during WOT.

Two more data points:
1) Another WOT test to 70mph, "EGR Step Position" stayed at 17.
2) Steady state 70mph, ICE ~1900 rpm, "EGR Step Position" 45-50.

For the last two data points I was carrying a bicycle on a hitch mounted carrier.

 

Thanks for doing it. I'll try to get my EGR numbers too. There is no doubt if Toyota was shooting for Max HP, RPM would be higher (6k), EGR would be off, and mixture would be rich. No need in a hybrid with electric power reserve.

 

Hey guys you'll want to make a table of EGR position vs. MAP. I'll explain later - busy today.

VS985 4G ?

 

I'll give you all an "A" for effort.

VS985 4G ?

 

Fluid dynamics is a complicated subject. After looking at a website that explains HVAC equations I have a new-found respect for those engineers.

Basically, the pressure drop from additional laminar airflow is related to the square of the airflow. So if the airflow doubles the pressure drop inside the manifold compared with the atmosphere is multiplied by 4.

Unless the air flow changes from laminar to turbulent. Then you're talking about another multiplication...

If you're eyes have not crossed yet, the problems of more engine blow-by and intake deposits will each cause an increase to the pressure read by the MAP sensor. This is because the deposits shrink the cross sectional area inside of the manifold leading to an increase in air velocity downstream and pressure increase upstream at the MAP sensor. I believe this is one reason (if not the reason) why Prius owners can get a completely plugged EGR cooler without setting P0401.

We could calculate an approximation for EGR cooler restriction with only one contingency. Ideally, everyone should have the same amount of blow-by and intake restriction or the approximation may be way off. I'm assuming since the pipe for the PCV is tiny compared to the cross sectional area of the intake manifold that PCV may contribute little. IMO any sludge depositing in the manifold would have a much larger effect. So anyone contributing datapoints would need to have a clean intake manifold.

Anyone interested?

 

Just some side notes and a question: First, I did this service earlier this week. The removal procedure document posted by Mendel Leisk is slightly off. You do NOT have to remove the stud that's hidden under the EGR towards the back. You only have to remove the nut on it. I was having difficulty with this part because it's impossible to see and hard to reach and get tools in, so I did it last. Once the nut was removed, the EGR slid right out. I don't even want to think about what it would have been like to try and get that stud back in during the reinstall.

Second, even trying to clamp the hoses, the EGR leaked a lot of coolant. Make sure you put down rags underneath all the hoses. The repair document posted here recommends to drain the coolant prior to removal. Since it had been a while since my last coolant change, I wound up doing that. Not a bad idea to line the two services up, if you can.

I was wondering how often those of you who know would recommend this service? Do you suggest checking the pipe for buildup periodically and cleaning the EGR when it's very dirty, or should I be doing this regularly? I'm at 150,000 miles and it was very dirty. Probably 2-3mm of carbon buildup on everything.