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Bucking Avalon

07-06-2011, 10:08 PM
Bought a 1998 Toyota Avalon with 56K.
When 1st getting on the highway in the morning, the car bucks and jerks like the tranny is going to fall out. If I get it good and warm it runs great.
I've replaced the MAF sensor, replaced one air/fuel sensor, cleaned other two air flow sensors. Had code P0171 and P01133 and P1135. Do not see any vacuum lines off. Has new air filter and fuel filter. Any ideas? previous owner said trans was bad. I don't think so because after it completely warms up, everything shifts perfectly.
Really scratching my head on this one.

07-06-2011, 10:44 PM
Did you clean, or replace the Incoming Air Temp (IAT) sensor?
I'am not sure, it might be housed in your new the MAF.

DTC's P1133 & P1135 points towards the H02 sensor on bank 1 sensor 1.
Make sure you check the fuse for the H02 sensor.

07-07-2011, 08:07 AM
Didn't know I had an IAT. Where is it located.
I've checked all the fuses and they are good. Read a post on here about a bad EGR valve. Could that be my problem?

07-07-2011, 08:14 AM
I would consider a new upstream H02 sensor, it's the one closer to the exhaust manifold.

07-07-2011, 08:26 AM
Already did that. Darn sensor was only $169.00. Ouch! Car has California emmissions on it.
Trying to find where my IAT is located.

07-07-2011, 08:43 AM
The IAT is a two wire connection, and is usually located after the MAF sensor in the Air duct work.
The newer ones are housed in the MAF sensor.

Do a visual inspection the H20 sensor wires too!

07-07-2011, 08:49 AM
I will try to find the sensor tonight and inspect the wires.
Thank you for your reply.

Brian R.
07-07-2011, 09:41 AM
I can't be an oxygen sensor (A/F ratio sensor) or oxygen sensor wiring causing the cold bucking. It has no effect while the engine is warming up. It only controls mixture at operating temperature in closed loop. Check the ECT sensor.

07-07-2011, 10:07 AM
Wouldn't the ECT sensor throw a code? The car runs ok and doesn't run hot.
After I replaced the air/fuel sensor, still get the P0171 Lean code. I can clear it after it warms up, and it will stay off. Next morning, the same senario, it start bucking again.

Brian R.
07-07-2011, 01:58 PM
Wouldn't the ECT sensor throw a code? The car runs ok and doesn't run hot.

Your temperature gauge runs off of a different sending unit, so you can't tell what is going on with the ECT sensor by your gauge.

The ECT sensor only tells the ECM what the coolant temperature is. It will cause all sorts of running problems until the engine heats up if the ECT sensor tells the ECM that the engine is always hot and in reality, it's just started and dead cold.

The only time you get codes are when the ECM has internal limits which are outside of normal. There is no means by which the ECM can know what the engine temperature really is, so it takes the ECT sensor's word for it.

After I replaced the air/fuel sensor, still get the P0171 Lean code. I can clear it after it warms up, and it will stay off. Next morning, the same senario, it start bucking again.

The P0171 code has a two-trip detection logic. It will not come back on right away, no matter what the engine conditions. If your ECT sensor is telling the ECM that the engine is hot when it's really cold, the engine will run lean cold, and it would give you a lean DTC, P0171. The ECM probably also advances the timing if it thinks the engine is hot when it's not. Check the resistance between the two terminals of the ECT sensor. The resistance should be around 1-3 kohms cold.

If I were a betting man, I'd say you are running rough when the engine is cold because of advanced timing, the result of the ECM thinking the engine is hot - bad ECT sensor (or bad connection/wiring to the sensor).

Of course, there are always other possibilities...

07-07-2011, 03:06 PM
Thank you Brian! I appreciate the post.
Where is the ECT sensor? Is the sensor for the guage in the back and the ECT sensor in front?
I'll ohm check the sensor tonight.

Brian R.
07-07-2011, 03:53 PM
top front center of the engine. in front of gauge sensor (IIRC). Don't forget to check the connection and wiring. If the sensor is bad, it will show a low resistance, or there is a short or bad connection.

Brian R.
07-07-2011, 04:08 PM
If your interested, check out this post:
for technical articles on sensors, emission components, etc.

Everything you wanted to know, but were afraid to ask about damn near everything.

07-08-2011, 10:39 PM
Thank you for the information.
Bought an ECT sensor for the Avalon tonight. But the one I bought doesn't fit. Maybe I'm not looking in the right place. I have noticed the Avalon has 4 sensors on top of the engine. Two on the intake manifold on belt side, and two on the intake on the transmission side. Is there any place else I should be looking? I'll attach a few pics.

Brian R.
07-09-2011, 09:05 PM
Sorry, I don't have an engine like yours in front of me to figure it out. I would take it to the dealer and ask them to point it out to you. Don't trust a guy from Autozone or Pepboys unless you know them personally and you know their information is reliable. I just remember it is in the front so that I had to work on it from the passenger's side of the car.

07-09-2011, 11:37 PM
Thank you Brian. I found a local Toyota guy I can ask tomorrow.
One of the parts guys at Napa said it was on the passenger side also.

07-17-2011, 03:06 PM
Solved! Turned out to be the air fuel sensor on bank 2 sensor one. I had replaced bank 1 sensor 1, but found out it needed both. Repair guy said to not run ethanol because it's not good on these sensors.
I've been fighting it for 3 weeks and repair guy found it in 10 minutes.
Guess it pays to have the right tools. My hand held scanner didn't pick up the 2nd bad sensor. Any who, I'm happier than a dog with 2 tails!
Thank you auto forums! I appreciate all the help!

Brian R.
07-17-2011, 05:48 PM
I don't understand it, but I'm glad you solved it!

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