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Old 11-07-2021, 02:53 PM   #1
Drives: 2008 Yaris Hatchback Automatic
Join Date: Oct 2020
Location: Naples, FL
Posts: 7
Rough running, idling, and shifting issue. Please help!

Hi all, and thanks in advance to anyone who can help me with my problem(s).

  • '08 Yaris,
  • 2 door hatch,
  • Automatic transmission,
  • 200,000 miles on odometer.

Description of problems:

For several months, the car had been idling rough if the air conditioning was on. It then would become worse when the (radiator)fan kicked on.

Then, a couple months ago the check engine light started coming on sometimes. The code was P0505 (IAC System problem). It would usually go off overnight or after I drove around a while.

Yesterday it started idling really rough and would stall at idle. It became really hard to restart. The check engine light came on but I didn't have the code read.
  • I replace the air filter.
  • I sprayed off the MAF sensor with carb cleaner.
  • I put a bottle of Gumout Multi-Symptom gas treatment in the tank. Full 16ounce bottle for 11 gallon tank. (for possible injector issues)

It was still hard to start and the engine light was still on. I decided to take it on the freeway to start working the Gumout through the injectors.

Now, it seemed like it was much worse. It was sluggish on the on-ramp and when I put the pedal down to put it into passing gear the transmission would shift erratically. I pulled into a lot and disconnected the battery, I thought this may have some positive affect on the ECU. The car would not restart.

I looked on my phone and read that a bad Mass Airflow sensor could cause the shifting problem. I walked to AutoZone, and bought a new one. I thought I may have ruined the one I had by cleaning it.

I was then able restart the car, but it was still hard to start. It was still running poorly and the shifting problem was still there. I managed to get it home...once it was cruising it ran OK in the freeway.

What I will try next:
  • Clean the throttle body. I did not do this right away because the hose to the air filter box is brittle and it will break if I try to take it completely off. It does look like there is a bit of carbon on the butterfly at the top and bottom, but I can't imagine this is the sole problem causing all of this.
  • Look for a PCV valve to clean.
  • Check for vacuum leaks.

I believe I am looking at multiple points of failure. Maybe if I had cleaned the MAF sensor and throttle body back when it was just idling a bit rough that would have fixed that...then this thing about the bad shifting in conjunction with the rough idle would have shown itself to be a completely separate problem.

So a couple questions for anyone who can give me reasonable feed back:
  • Does anyone think the Gumout treatment could cause problems like this? The worst of it coincidentally started right after I put it in.
  • Right now the engine light is not lit, but what other sensors, valves, vacuum lines can I check or clean?
  • What are the chances of this being the ECU itself? If this is the problem what is the cheapest way to replace it?

Again, thank you for any advise you can give! This car is my livelihood. I am a delivery person. So I am unemployed until I figure this out.

JCPepper is offline   Reply With Quote
Old 11-25-2021, 08:40 AM   #2
Drives: 2007 Yaris 240000
Join Date: Dec 2018
Location: Southern Maryland
Posts: 2
One thing that I see is a possible problem. The can says never spray the maf sensor with carb cleaner, only use cleaner for sensitive electronics or maf sensor cleaner. It’s very possible the maf got damaged from the harsh chemicals. Not sure if there is any issues with aftermarket maf sensors but that could be an issue also. May be a good time to check spark plugs as well as pcv valve. My 07 Yaris has 286,xxx and has similar issue where the a/c running causes to car to idle a little rougher than normal. I just cleaned my throttle body last week and upon inspection my pcv valve was slightly gummed up and looking where the hose attached to intake there is a lot of looks like carbon buildup I’m guessing from the oil being introduced to the intake. I sprayed the pcv valve with throttle and intake cleaner and used a que tip to clean valve and seems good as new.

Also does your scanner support live data? Not sure if it will pick up fuel pressure but probably good to check the basics. Also I highly doubt any fuel additive would harm anything, I could be wrong but I don’t think that would cause a problem.

When is the last time you changed the spark plugs? I’m sure the rough idle is causing the bad shifting but have you checked your trans fluid? I know at one point of the many drain and refills I underfilled the transmission and it made it shift really weird and I was thinking I damaged the transmission but after adding a quart of fluid everything has been good for probably 5000 miles.
Austinowa360 is offline   Reply With Quote
Old 11-27-2021, 12:44 PM   #3
Drives: 2007 Toyota Yaris Sedan
Join Date: Feb 2019
Location: Harrisburg, PA
Posts: 74
To identify a performance issue, add fuel or subtract fuel and see what happens. Do a cylinder drop test. You're gaining clues as to whether it's fuel, spark, compression, or timing related. Since you have a code for intake air valve I'd go there first with a diagnosis. I'm sure you can test the idle air control valve with just a battery and some wire. You can cause a vacuum leak and see if it improves, if so I'd just gamble with a new iac
IndestructibleYaris is offline   Reply With Quote

engine help, rough idle, sensors, shift, stalling

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