Does this symptom statement seem good enough? Your own ideas are welcome.

I'm attaching this letter when I drop my car off to a reputable mechanic in my area. Before I do, I'm hoping you can pre-guess the issue before I go in case it's something silly I can do myself. I do not own gauges of any kind. Thank you for your time.

2013 Toyota Prius

227K

I am sometimes experiencing a very specific issue with my vehicle under very specific driving conditions. At normal freeway speeds and 25-50 mph access road speeds, the Prius runs very well. It also starts up and idles nicely, whether it be from a cold start or from when the battery gets low, and the motor comes on to charge it up. EXCEPT when the AC is on…then, it really seems more likely to have a problem. Explained below.

I have replaced or cleaned a great many things during its life for prevention and better performance.

(THIS IS THANKS TO YOU ALL HERE ON PRIUS CHAT!) You guys are more famous than you know.

ISSUE:

1 - While using the gas pedal to maintain 12-20 miles per hour (like in a stop and go school pickup line or making my way through a neighborhood with a low-speed limit) the Prius engine seems to grumble/stutter in some way. It’s most noticeable, sometimes only noticeable, when the AC is on. I can lessen the effect by turning the AC off or simply accelerating faster.

2 - Gradual Coolant Loss - No leaks on garage floor or that I can see around the hoses. However, I sometimes can see evidence (teeny, small wet spots) of coolant puddling right next to the main reservoir. If I let it, the coolant will eventually empty past the low mark after several drives spanning days. Could it be steaming out slowly? That would explain the wetness sometimes in that area, but I don’t know.

3 - Too lean engine code P0171 - This code has been noted for a very long time, pre-dates the current issue.

4 - Decreased fuel efficiency (displayed at least but if I do the math myself, it’s not as bad)

The other codes were cleared when I put on the correct upstream air-fuel ratio sensor.

Please note that this does not happen all of the time. Most mornings, if not all, I can get in the car, start it up, and immediately drive away with no issue at all. Zips away quietly and quickly. I can even drive for several hours in many changing conditions and will never notice anything. But when the ac is on and the engine wants to go from battery-idle to motor coming on? It may or may not have a hard time at first. It’s like the engine can’t take the extra load when the AC kicks on. Or WHEN I PRESS THE GAS to maintain some really low speed, it may or may not like that - AC is on makes it worse.


Symptoms NOT Observed: (by me)

• No other codes. - I check daily. I’m sure there are stored codes from the past but my little reader shows nothing new but the too lean condition which has been there forever.
  
  
• Does not overheat under any condition.
  
  
• Both the AC and heater run very well without fluctuations or delay.
  
  
• No milky oil on the dipstick, the cap, or seemingly too much oil.
  
  
• Oil smells like oil.
  
  
• No air bubbles in the coolant reservoir that I can see after the water pump kicks on. When the cap is off, some obvious steam after running for a while.
  
  
• No white smokey exhaust.
  
  
• No strange smells coming through my vents.
  
  
• No leaking/dripping exhaust.
  
  
• Not a start-up issue except as described.
  

I’m for sure suspicious of a vacuum leak because of the code itself but now with the missing coolant and occasional low speed shutter, I wonder if my manifold gasket is the culprit. Perhaps the gasket at the manifold to the EGR passage? I’m unclear how all of that works. Is it all just a dirty mess in there that needs to be cleaned, making the engine a little too hot, steaming out my fluid? Seeping through the gasket?

RECENT WORK - Past 500-10,000 miles in order from most recent.

Upstream Air/Fuel Ratio Sensor - Denso - This sensor was why I had a bank one code. I learned that the oxygen sensor that was put on a year ago was just that, an oxygen sensor. NOT the more parameter specific air/fuel ratio sensor the Prius wants. Code is gone.

Coolant Reservoir Cap replaced.

Denso Coils - Self Install - Wow, what a difference for overall stability!

12V Battery Check - Full and healthy.

Denso Water Pump - Sell Install - The other was failing - Problem solved immediately after install. What a pain. Feel free to check my torque on that. Been operating well.

New CAT - Your business did this. Amazing performance difference. New downstream sensor attached too. Thank you!

Spark plugs, MAF sensor, general things as needed.

Manifold Gasket Replacement - a “mechanic comes to you” type person did a driveway install over a year ago. It wasn’t failing, I just wanted it cleaned up around there, and replaced before issues happened due to mileage and constant use. Not sure of the skill level of the gentleman or gasket quality, but I think I picked out the gasket. I know he did clean up some areas of the manifold somewhat, but I don’t know if he changed the gasket to the EGR too. Those little ports in the manifold and the intake itself? Those ports can get a good amount of build up, so I wonder…

EGR Valve itself and the EGR Cooler below it were installed maybe 2 years ago now. Never was sure if that was sealed up properly. (I have a story as to why I think this) I feel like my “too lean” code and hint of exhaust smell from the engine bay itself started shortly after that was done. You should have heard and smelled it when they tried to give it back to me NOT AT ALL SEALED UP! (No, I don't go there anymore)

PCV Valve changed at some point but can’t recall when.

I did not change the thermostat when I put in the pump. No issues pointing that direction.

Whatever the issue, I really appreciate your professional diagnosis. Maybe it’s a combination of a few small issues happening at once making it feel like one big thing.

Sorry that was long. I like to feel helpful by providing good information.

 

hahahaha I'm scared! And I don't even know if that's it. Is it? You think that's it? This is embarrassing, omg.

OH, I see what you mean now. Well, I had him do the gasket and clean up the EGR pipe, etc some time ago before I knew how to do anything at all. I figured with the 200K plus miles, it was needed. The water pump, I just did a couple of months ago after watching and reading a ton of stuff. Sorry. I thought you were implying "Well, if you can do the pump, what makes you think you can't do the gasket?" I don't even know if that's my issue. Thank you.

 

The Air/Fuel Sensor took 15 minutes on the lift at the local shop. Soon as it was done and I drove, I cleared the codes and it never reappeared. The problem with the pump was I would get an occasional flash of the over-heat light. Did it about 6-10 times over the course of many drives. I use my car for deliveries and so I drive far more than most people do. That light would blink, then stay on for about 20 seconds or so, then go away for the day or even days. No over heating light issue after that or even now, with this. Thank you.

 

Spark plug coils - All four. I had never did them ever or when I changed the plugs a while back. Couldn't afford. I feel like when I did the coils, the engine seemed to sit in idle much better and the Prius battery level doesn't move around so much. Hard to explain. Lets just say it feels nice and stable.

 

Wow, you ask great questions. Man, that would suck if the stupid water pump did the damage. You know, many miles ago, I called a mechanic because I had read a few times EXACTLY what you just said about the pump. I probably read the discussion threads on here, actually. I've read so many. I wanted to replace it then, before it became an issue, but he gave me so much grief with, "Well, why would you want to do that if it's not broken!? I would wait because unless you're getting a code, it's fine!" I felt like as soon as I said, "I've been reading online..." that he thought, "Oh great, another YouTube Expert Mechanic" and wanted to set me straight. I reluctantly let it go. I should have just had it done anyway. So, okay - I'll have my newer awesome mechanic track down this leak first and maybe check to see if I have the pump on fully tightened. I can't see what I could have done so wrong. I took my time installing it. Got some of those bolts from the top and the rest by taking off the tire. Good times. Ugh.

Yes, the engine does shut off like it's supposed to. If it's not shutting off, then it sure the heck is quiet at the light. I can see the little display showing the hybrid battery arrows going to the engine while I sit there. When I get going again, all the arrows go the other way towards the battery. I feel like it never used to be so stable like that at lights. I do not know...I do know that I can sit in park for a very, very long time before the engine will even need to kick on as well. Hey, so while all of you were replying, I had to do a delivery route. Minimal "rumblies" this time, very light maybe once, but the car for sure has lost coolant since yesterday.

 

Thank you, sir. This is good to know. I'm so happy to have this information. So, do you think my letter will be good enough to get the mechanic going in the right direction? What do you think I should expect for him to want to do first or what would YOU do first?

 

I'm so thankful you came back to close this out. Okay, I'm taking it to my mechanic tomorrow 9am. I think these are very prudent steps to take and in the right order. I won't leave ya'll hanging! I'll post a reply to this thread to memorialize the outcome for others. Big thanks from South Texas!

 

Hey Ya’ll,

I am here again to say thank you for the years of amazing information and for replying to this thread. I also want to post the results so it will help others someday. If you have issues like mine, well…. just like the folks on here say over and over again, do this:

I can’t find the first sheet they gave me with all the explanations and findings, but basically it went like this:
Leak Test. They even did the one where they measure the exhaust for the presence of ??? None found.
Block Test.
Full diagnosis with the gizmos.
For this issue on this thread, if your mechanic is not recommending this from the start, you should leave.

Results:

First, I had a lower oil pan gasket leak. I’ve always wondered why I was losing oil somewhere with no visible leak and I never associated the sagging under panel in that area with anything because I don’t get under there that far, and those under panels can really suck to put back on correctly in your driveway. The sagging was the collection of oil. All this time I thought, “I guess my car just drinks oil now because it’s old.” I’m a dummy. — Fixed

My downstream 02 sensor was a hunk of junk. Well, I wouldn’t say “junk” but it certainly wasn’t a Denso AND when the “other” shop put it on, it took 10 mins and I think they are supposed to do some techy stuff that I know they didn’t do. — Fixed

My do-it-yourself water pump install - Perfection! Just wanted to brag. Hey-OH!!!!! Haha

The Head Gasket Suspect

Drumroll…

It’s not leaking! But it’s mildly “sweating” as they put it. They don’t see a need to do anything now. I wish I could find the way they described in the Tekmetric link, but just take my word for it, it’s good…. for now. I may have headed off disaster because of this next thing.

Those DANG EGR PORTS and the intake manifold mess! I’ll see if I can attach photos to this.

I don’t know if they took a hose to the entire thing but man…

The shop I’m using is a place called EurAsian in San Antonio. They’re not cheap. I had sticker shock for sure but maybe that’s because I’ve been so used to dirt cheap, cheaper if you pay cash, type places. I’m so glad the other shop down the way totally blew me off the other day when I got there before they did, waited as they ignored me, then put my “get home” bike back in the hatch and left without them saying so much of a “Hey, where ya going!?” I guess it was a sign.

I’m going to enjoy my car for a few weeks delivering and see if I still lose coolant. I kinda think I won’t, but who knows. The availability of a new 2023 Prius or Corolla hatchback is terrible right now. Perhaps I’ll order it soon, then when I get it in a year, I’ll be done with this car or I’ll keep it and ride it till the wheels fall off.

My two cents for owners: Don’t have people look at your Prius the way I’ve been doing it and stop using "good enough" parts.


I’ve been lucky. I used this forum and YouTube to do as much as I could myself and often chose non-ASE Certified Mechanic shops with no real specialty training, a pull up shop who mostly fix flats, or a driveway mechanic.

It has mostly been me telling them what I think needs to be done and I don’t know Wth I’m talking about either. My advice to any Prius owner is to put away some bucks each month to afford certified mechanics. Not the dealership really, but just a solid shop with the good coffee in the lobby and who charge money to look it over. There are very fine, fully trained mechanics out there with the latest software to do it correctly and then some. Use them.

Your BIG GOTO HIT LIST is what they say on here all the time. EGR, EGR COOLER, INTAKE MANIFOLD, and the EGR PORTS on the manifold. Keep them CLEAN or pay to install new.

Lastly, I did print out your replies to my issue and showed it to the manager when I got there! He was in complete agreement. His thoughts were given to me, (based on my notes) before I showed what you all said. Almost a perfect match. Thanks for saving my car and costing me an arm and a leg to do it. - Kel

 

Thank you for the information. Congratulations on the 300K! It takes a lot of care to get it there, huh? I have about 70K more to get there. EGR cooler is clean, actually it's fairly new. PCV too.

All vibrations are gone. He mentioned later "...once the cycle drive was completed, she coded for P0607. All other codes are gone. Turns out it only codes when below 450 degrees and fault is situated in ECU. It, in turn, was compromising the air fuel mixture by misinterpreting signals from emission sensors. She's running clean, a LOT better. But at some point, she will need to go to a dealership for ECU "lobotomy" or even replacement. Not a huge cause for concern in the greater scheme of things so far as drivability."

I would like to point out that I have never had a conversation like this or any recommendation like this since I've owned the car. I have been VERY fortunate with my junky shops and trying to be a YouTube Toyota Master Mechanic. I'm telling ya...it's the forum. Like, holy smokes. The information on here is crazy, crazy gold.

 

Found the quote. This also came with a phone call.