I have engine hesitation between about 3k and a bit over 4k RPM, without any engine codes pending or stored. I am interested in diagnosis help from experienced persons. I am especially interested in assistance from those of you who may use the Auterra scan tool… that way I can hopefully get a baseline of measurements from a well-functioning vehicle.

I plugged in my new Auterra scan tool yesterday in hopes of catching a code. I found none stored and none pending, but I did collect data on a few time-based metrics during vehicle testing:

MAF air flow
Intake Air temp
Front and Rear oxygen sensor voltage readings
RPM
Speed
Throttle position
Engine Timing

These are all accellerations onto and while on the highway. I'm looking for symptoms that will help troubleshoot why the truck is hesitating under moderate to heavy throttle position. I will be glad to post up testing results later tonight. For now, I am looking for experienced analysis on data-based diagnosis.

MAF Air flow:
Is there an acceptable or ideal range?

Oxygen sensors:
Should these respond differently under heavy throttle (moderate/heavy accelleration) compared to throttle under maintaining speed?

Engine Timing:
What is an acceptable range for timing? Does OBD timing measure indicate relative timing adjustment? If yes, should this ideally be @ zero?

Intake Temp
This maintained steady @ about 28 degrees F. This was reasonably approximated the outside air temp. Is it possible that this would ever fluctuate with engine bay temperature?

Gasket/hose leaks:
If I had a hose leak, would this somehow show up on the oxygen sensor output and nowhere else?


Assistance from experienced persons is greatly appreciated. I would certainly appreciate any help from master mechanics or the like.

Thanks ahead of time...

 

Which engine?
Could try fuel Filter if 22re.
B

 

1996 4Runner
3.4 V6

Thank you for your reply. No guesses on solutions please. I am limiting my review to analytical diagnosis.

 

A common hesitation at the high end like that is the TPS.
Have you tried using a DVM to check the resistance value on it throughout its range?

 

I have replaced the TPS with one bought from trdparts4u.com; however, I trusted that it was correct. I have the field service manual. Does the FSM indicate appropriate resistance values?

BTW... the hesitation is intermittent. Is that still consistent with TPS?

 

Any idea where the TPS Signal gathers data for reporting to the ECU?

At first glance, the ODB charting did not indicate abnormalities in the throttle position (spikes, drops, etc, when it should have been increasing or fully applied). Charts are at home at the moment...

 

The answer is that it can be intermittent but doesn't have to be.
Is the throttle position linear on your Auterra (engine off)?
or does it jump around as you push the pedal down?

 

Throttle is linear with application of pedal. No jumps or spikes . Verified with Auterra graph.

 

Jamie,

Does your Auterra read engine coolant temp? Do you have the option to run any on-board diagnostic tests?

I feel like I'm missing a bunch of parameters and testing options... ?

 

Guess I have one thought. At 3k rpm on the 3vze, and probably on your 3.4liter, the ecu decides to push more gas to the injectors.

If true, the question becomes, is it a problem with handling the extra fuel, or, is the problem that the engine expects the extra fuel (changing timing), but isn't getting the extra fuel.

Can you tell what O2 sensor says as a result of RPM? If so, you may see it change to too rich or too lean, and that may be a clue.

 

Your hypothesis may be false and it may be true:

Under full accelleration without hesitation, the graph of oxygen sensor readings (both sensors) jumps up to 0.96V and floats around 0.95 +/- 0.01V.


More information:

This analysis is based upon a small test at highway speeds under two conditions: 1. full throttle load with overdrive off, and 2. throttle load only required to keep the vehicle at speeds around 65 to 70 mph (approx 18 - 20% of absolute full throttle, as measured). Factors reviewed are MAF Flow rate, O2 sensor 1 voltage output, short/long term fuel trim, and ignition advance. Each factor is qualitatively reviewed, compared to Throttle Load/position under conditions 1 and 2 defined above.

The ('96 4Runner, 3.4 v6, 155k miles) was accellerated to highway speed from 5mph to 75mph) without noticable hesitation. Then, upon reapplication of full throttle (65mph to 75mph), the truck was subject to random surges in accelleration. There is not yet comparison of data between surges and no surges. This analysis is to review general numerics of factors compared to throttle position.

Environmental conditions: Air intake temperature maintained at around 28 degrees Farenheight, which was about the same temperature as the outside weather. Light to no snow precipitation with slightly wet/damp highway road surfaces.

DTCs: None pending or stored at any point during testing, with or without hesitation.

MAF air flow: Proportional to throttle position... full throttle @ 3500 RPMs and above results in 100 to 105 gm/s. speed-maintaining (70mph) throttle results in 25 gm/s

O2 Sensor: Heavy to Full throttle results in O2 (sensor 1) voltage in the range of 0.91V to 0.96V with minimal to no sine wave present. Partial/speed-maintaining (70mph) throttle results in typical sine-wave 02 (sensor 1) readings swinging from 0.39V to 0.74V.

Fuel Trims: Under full throttle, short term fuel trim is 0% and long term fuel trim is first at 3.9% and then 3.1%. Under speed-maintaining throttle, Short-term trim followed a wave-form from -4% to 4% with no significant observable trending.

Other observations:
Observation 1. Moderate to heavy accelleration resulted in instantaneous fuel consumption ratings in the single digits. Light throttle resulted in around 17mpg, instantaneously.

Observation 2. During testing, I noticed a slight popping sound once--similar to a backfire, which I don't recall hearing in the last 5k miles of ownership.

Further testing should be conducted to gradually increase/decrease throttle load and observe trending of factors. Additional testing should be conducted to review comparison of data between states of hesitation and no hesitation.

Questions of items not understood:
1. Do higher levels of 02 voltage indicate richness or leanness states? Does rich/lean refer to oxygen content, or does it refer to fuel content?

I don't know how to host an image, or I would. Anyone interested in hosting a screenshot to reference for this review can PM me and I'll send you the file.

 

Bonk, good info, looks like you've been doing your homework on this!

I'll try and answer some of your questions......

1) Yes, voltage levels do directly correspond to rich or lean states (the closer the voltage is to 1V, the more rich it is running and the closer to 0V, the more lean you are) when the engine is running. Your front O2 sensor seems to be a little on the rich side but that isn't that unusual seeing as 14.7:1 is about 0.45V at the O2 sensor (and the vehicles power range under acceleration will be more like 13:1). The O2 sensor actually measures oxygen present in the exhaust gasses being expelled (this is why if you have a hole in your exhaust just in front of the cat, it can often give you a false lean reading) since the more raw fuel there is in the exhaust, the less oxygen there will be along with it.


I think this document might be helpful for you to read:
http://www.autoshop101.com/forms/h37.pdf


2) Hosting an image is easy and free on the web. Just use one of the many photo hosting sites like www.photobucket.com or www.webshots.com or www.imageshack.com and then just use the insert image icon when you go to post the link. That way the photo will automatically show up in the thread.

 

I'm going to go against the grain on this one... I'm reviving this thread to share my story of success...

Here's what I did:

1. Replaced the MAF
2. Cleaned the throttle body and intake tube between the throttle body and MAF
3. Replaced the coil packs and spark plug wires
4. Replaced the spark plugs
5. Tightened a loose ground connecting somewhere behind the intake area coming up to the rear of the engine bay at the top and center of the firewall.

I suspect that Replacing the MAF and cleaning the intake tube got rid of a chugging that presented itself upon heavy increase in gas pedal application. This chugging / lugging / bogging was typically independent of RPM. However, since the largest change usually occurred upon initial accelleration from a stop, the car would sputter / bog / lug / chug at that time. It would also happen at 2500 and 3500... usually at the same point of shifting from one gear down to a faster one in an attempt to accellerate faster.

Wires, coil packs, and spark plugs were changed as a precautionary / preventative measure. The spark plugs were most likely fine, since they were replaced in the last 10k miles. The wires seemed clean, but they were easy enough to change with the spark plugs. I suspect that worn out coil packs may have been a culprit of higher RPM hesitations / power fade (NOT to be confused with Bogging / chugging / lugging / etc) typically found in the range of 3000 to 5000.

Lastly, in the process of working on the truck with my brother, my brother found a ground wire that was loose. I have no idea why or how it got loose... or even what it is designed to ground. But we snugged it up. It is possible that this may have caused all the problems; or it may have caused none of the problems: like I said, I have no idea what it is designed to ground.

This was all completed Wednesday morning. Thursday and Friday the truck ran great... I was towing an enclosed U-haul trailer without any issues at all... Cruise control worked on all sorts of uphill highway @ 70mph without any noticeable problems... not even a need to upshift or force the transmission to exit out of torque lock at highway speeds.

I'm very happy with the results. Needless to say, I plan to keep my 4runner indefinitely now.

As a side note, remember that I verified no obvious problems existed with my OBD II scanner. And yes, buying an OBD II scanner was WELL worth the peace of mind and the money... considering the potential for aimless troubleshooting costs I might have had otherwise.

Cheers... good luck with your troubleshooting everyone...