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Amedee
Amedee, Toyota Technician
Category: Toyota
Satisfied Customers: 21942
Experience:  ASE Master Technician advanced level specialist. Wisconsin certified emissions state inspector
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I have a 1991 toyota pu w/ 22RE 5spd. 175,000 miles. When

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I have a 1991 toyota pu w/ 22RE 5spd. 175,000 miles. When the is truck cold, the engine runs rough for about 10 sec when first started and then smooths out. First started showing symptoms in colder weather. Now seems to happen each time regardless of outside temp. but once the engine is warmed up there are now symptoms accept occassionaly at highway speeds the engine will hesitate momentarily mostly when under slight load in 5th gear. Shifting to 4th gear seems to alleviate the symptom. Another mechanic has said the rear end is going out but I don't know if that could be part of the highway speed problem or not. Any thoughts on a cause?
Submitted: 4 years ago.
Category: Toyota
Expert:  Amedee replied 4 years ago.

Hello!

 

You could have a bad mass air flow sensor. Or the snorkel from the mass air flow sensor to the throttle body could be ripped. Both are very common and can cause you vehicle to loose power along with many other drive ability issues. First look for cracks or rips or anywhere air can enter the engine that is not accounted for by the mass air flow sensor. Next comes the Mass Air Flow (MAF) sensor. This is a very important sensor input to the computer. It uses a hot wire sensing element to measure the amount of air entering the engine. The MAF sensor then outputs an analog voltage signal to the PCM proportional to the intake air mass. The PCM calculates the required fuel injector pulse width in order to provide the desired air/fuel ratio. If the sensor is bad than this air/fuel ratio will be off causing severe performance issues. In extreme cases, I have seen these two problems cause a no start condition. This input can also be used in determining transmission Electronic Pressure Control, shift and torque converter clutch scheduling. The check engine light may even pop on if the sensor is bad, the snorkel is ripped or a tube has fallen off. This will result in a lean condition.

 

 

Also, You could have a bad or lazy oxygen sensor. The heated oxygen sensor is very important to control not only the cars emissions but it also helps the vehicals performance! If it is not working properly it can result in a number of problems like rough running stalling hesitating bucking and jerking. But more common than that if it is lazy, the cars gas miledge can drop considerably, and sometimes not even set a check engine light on the dash!
The fuel control Heated Oxygen Sensor (HO2S 1) is mounted in the exhaust manifold where it can monitor the oxygen content of the exhaust gas stream. The oxygen present in the exhaust gas reacts with the sensor to produce a voltage output. This voltage should constantly fluctuate from approximately 100 mV (high oxygen content - lean mixture) to 900 mV (low oxygen content - rich mixture). The heated oxygen sensor voltage can be monitored with a scan tool. By monitoring the voltage output of the oxygen sensor, the PCM calculates what fuel mixture command to give to the injectors.

Customer: replied 4 years ago.

Is it common for either of these sensors to have problems mostly at start up with the engine cold? Gas mileage has not changed. It's not the best for a 4cyl at ~18 mpg. But I figure it's also an old truck. Can I test either of these sensors with a common test meter or something I can rent?

Expert:  Amedee replied 4 years ago.
YES... these sensors can go bad warm or cold and they both can be tested with a voltmeter.

Edited by AMEDEE on 7/25/2010 at 7:35 PM EST
Customer: replied 4 years ago.

But which is more likely to be an issue in the cold and mostly at startup - I have yet to have the rough running at engine start once it has run and is warmed up. The problem started a while ago but only occurred after the truck sat a while and it is was cold outside (maybe wet too, I can't remember) The only symptom at first was the rough running at start up w/ no runnability problems after only a few seconds of rough running. If the sensor is bad can it still operate normally except at startup? If the mass air flow meter has a leak will that only show problems at startup?

Expert:  Amedee replied 4 years ago.

If the problem is only when starting the vehicle, I would suspect the mass air flow sensor before the oxygen sensor. The computer does not even take into consideration the o2 sensor signal before the computer gets into closed loop.

 

In addition, I would also check the engine coolant tempature sensor. THis can very well cause your issue just when the engine is cold.

Customer: replied 4 years ago.
How long does it take to get into a closed loop? Just today, after completing the fuel line install, I checked the coolant level. It was ~1quart low on engine coolant and topped it off. Started the truck cold and may have run rough for just a second (it was hard to tell it was so quick). I did run quite low on coolant before and it did cause severe runnability problems (I had a bad head gasket and didn't realize I was loosing fluid). Is it likely that could have been the problem or am I being too optimistic?
Expert:  Amedee replied 4 years ago.

Is it possible!

 

If you are burning coolant the first few moments on start up... then yes... chances are your head gasket is leaking coolant into the combustion chamber.

 

However, I don't think that is your problem... unless you are going through coolant on a regular basis.

Customer: replied 4 years ago.
I don't think the head gasket is bad. I think the radiator has a very slow leak. I didn't think that being only a ~1qt low on coolant was the cause of my problem but I haven't runn the truck enough to know if adding the coolant solved my problem. It did not run rough on the first start after adding the coolant but that may have been just the random nature of the problem. Is it likely being only ~1qt low on coolant could cause my problem?
Expert:  Amedee replied 4 years ago.

NO... I don't think so... I don't think that is the problem. I still think it is going to be the mass air flow sensor or coolant temp sensor.

 

I hope this helps! Please don't forget to hit the "accept" button if you have found my answer helpful. You can always accept my answer now, and repost back again later on if you have any more questions. If you want to discuss it some more just hit "reply", it's free to do so otherwise don't forget to hit "accept" and leave positive feedback if you found my answer helpful.

Customer: replied 4 years ago.
Will the repost continue to go to you or will it go to someone new at which point I will have to explain all again?
Expert:  Amedee replied 4 years ago.

If you respond back to this thread I will get it.

 

Even after you accept one of my answers, it will not cost you any more money to ask me further questions. We can still continue our conversation even after you accept my answer.

Amedee, Toyota Technician
Category: Toyota
Satisfied Customers: 21942
Experience: ASE Master Technician advanced level specialist. Wisconsin certified emissions state inspector
Amedee and other Toyota Specialists are ready to help you
Customer: replied 4 years ago.

I have a Chiltons manual which gives procedures for testing VAF (The 22RE has a volume air flow meter), O2 sensor as well as the Intake Air temp (IAT). The VAF resistance measured ~280 ohms within the range of 200-400 ohm. The IAT resistance fell right in the middle of the range allowed for a given temp (2270 ohms at 72F) but I don't understand how to test the O2 sensor. It talks about backprobing with a high impedance averaging voltmeter between the O2 sensor and the battery ground. I have a regular multimeter...will that work? Also, I how do I back probe between the sensor and battery ground.

 

I also double checked dist cap, rotor, and plug wire condition. Also checked vacuum line for obvious cracks and leaks and tightened clamps and checked condition of snorkle between VAF and Throttle body.

 

Expert:  Amedee replied 4 years ago.
Hello!

Sorry for the delay!

I just got back home from up north.

Please give me an update on where you are at with the vehicle since your last post.

Have you replaced any electrical components as of yet?

Which sensor would you like to test next. Just let me know and I will be able to help you out.

Sorry again for the delay! I look forward to your reply.
Customer: replied 4 years ago.

 

Here is repeat on where I'm at from my last post. The problem seems to happen pretty regularly now at higher speeds.

 

I have a Chiltons manual which gives procedures for testing VAF (The 22RE has a volume air flow meter), O2 sensor as well as the Intake Air temp (IAT). The VAF resistance measured ~280 ohms within the range of 200-400 ohm. The IAT resistance fell right in the middle of the range allowed for a given temp (2270 ohms at 72F) but I don't understand how to test the O2 sensor. It talks about backprobing with a high impedance averaging voltmeter between the O2 sensor and the battery ground. I have a regular multimeter...will that work? Also, I how do I back probe between the sensor and battery ground. It also looks like the O2 sensor nuts would be very hard to remove as they have not loosened in almost 20yrs.

 

I also double checked dist cap, rotor, and plug wire condition. Also checked vacuum line for obvious cracks and leaks and tightened clamps and checked condition of snorkle between VAF and Throttle body.

Expert:  Amedee replied 4 years ago.

Ok... Should we test the o2 sensor next?

 

Also have we checked the output voltage of the mass air flow sensor when this problem happens?

 

Does it feel like a cylinder misfire or just a hesitation when this problem occurs?

Customer: replied 4 years ago.

I would like to test the O2 sensor next. Can you describe how to do this and can it be done with a common multimeter?

 

It does not feel like a misfire more like a very quick hesitation but no RPM changes.

Expert:  Amedee replied 4 years ago.
Ok...

Since it does not feel like a misfire, I think we are on the right track.

To test the o2 sensor, we first must know how it works and then check it to see if it is doing what it is supposed to be doing.

The o2 sensor will go from rich to lean depending on how the engine is running. From there the computer will change the injector pulse width to compensate for the positive or negative fuel trims. It does so to keep the air fuel mixture in the cylinder as close to stoichiometric as possible 14.7 to 1. This is the best air fuel ratio to get the best gas mileage. THis is what the computer is pre programed to do.

So, we are going to back probe the signal wire from the o2 sensor connector with your voltmeter (or rather a paper clip) to get a reading from the sensor to see what kind of signal it is producing when this hesitation happens.
Customer: replied 4 years ago.

How exactly do I "back probe" the signal from the O2 sensor?

Expert:  Amedee replied 4 years ago.
Just go to the sensor connector and back probe the signal wire. You dont even have to disconnect the connector. Just use a small paper clip and go into the back of the connector enough to reach the bear wire to get a reading.

Here is a picture I found on google.

graphic
Customer: replied 4 years ago.

What exactly am I trying to do, just see what the signal is coming from the probe? My book says back probe between the sensor and battery ground. Which wire am I trying to back probe? What color will it be? Also how will I do that at highway speeds when I'm driving the truck. Is there a way to test the O2 sensor with the engine off?

Expert:  Amedee replied 4 years ago.
Nope... the engine needs to be fully warmed up and it will need to be running when you test the o2 sensor.

You can just carefully run a jumper wire from the o2 sensor connector to your voltmeter inside the vehicle and watch the signal while you are driving.

The only other alternative would be to purchase a scan tool that can read live data from your vehicle and watch the o2 sensor signal while this porobelm happens. The only problem with this would be that you would be spending hundreds of dollars on this scan tool. I am trying to help you by getting around a expensive repair.

I will get you a wiring diagram.
Customer: replied 4 years ago.
I'm resourceful enough that I can rig it up. I just need to know what exactly I'm trying to tap into. The wires coming from the probe are blue, white, black and black. Also, if we're reasonably certain it is the O2 sensor, I'll spend the money and replace it. I'm just not sure it's going to come apart all that easy and would not want to try and replace it if not necessary. The nuts are pretty rusty and need to get a deep well socket that fits to take it apart. I think it's an 11mm but it seems odd because all the work I've done on this truck I have encountered a lot of 10mm, 12mm and 14mm bolts.
Expert:  Amedee replied 4 years ago.

I believe the signal wire is going to be the black wire. Here is a wiring diagram.

 

graphic

Expert:  Amedee replied 4 years ago.

YES... that would be correct... that is why I like Toyota's... the manufactures make it easy to work on this engine which makes my job a lot easier!

 

Anyway, without knowing what the reading is coming from the o2 sensor it is hard to say that this is the problem. The MASS AIR FLOW SENSOR would be a more likely culprit to cause this problem then the o2 sensor. But the o2 sensor still could be bad.

Customer: replied 4 years ago.
I have two black wires a blue and a white coming from the sensor. Is there a mass airflow sensor and a volume airflow sensor? My book only talks about a VAF only and says to test the resistance of the sensor. The resistance of the VAF was 280ohm which was right in the range of 200-400 ohms my Chilton manual gave.
Expert:  Amedee replied 4 years ago.

I think one of the black wires is going to be your signal wire. You will just have to check both wires to see what kind of signal you get from each. The signal will go from .1 to .9 volt. .1 is lean and .9 is rich.

 

YES... this is a VAF.... I just have a habit of calling it a mass air flow sensor... They serve the same purpose.

 

The resistance of the sensor is not reliable... you need to check the output voltage of the sensor while it is in use.

Customer: replied 4 years ago.
How do I test the output voltage while it is in use?
Expert:  Amedee replied 4 years ago.
Same way you are testing the o2 sensor.
You can try to test it with the engine running in park and see if you get a good reading. If not, it is bad and will need ot be replaced. If you do get a good reading, you will need to test it while the hesitation is occurring.
Customer: replied 4 years ago.

Is this scan tool the same one that reads the check engine error codes?

 

Expert:  Amedee replied 4 years ago.
Sorry for the delay!


There are many scan tools out there. The type of scan tool that you would need would be one that reads live data. There is a difference between code readers and scan tools.

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Amedee
Toyota Mechanic
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ASE Master Technician advanced level specialist. Wisconsin certified emissions state inspector