Hesitation is Back!!

[Marcos Beltran]

Ive been having hesitation problems since ive bought this truck. Its a 96 gmc v6, 4.3. Got it running really well for the last month and a half recently started to hesitate slightly. Put on my scanner and the short and long term fuel trims are all over the place. Sometimes the short term is high, sometimes the long term is high but never really a pattern thats consistent. During the initial acceleration is when i get the most erratic readings. While cruising it stabilizes for the most part and down to normal values. I did find that one of the down stream o2 sensors, the one right in front of the catalytic convertor is not working properly. It idles at around .045 to .070 and never goes higher. But on the initial acceleration it will sometimes jump to .800 but will go back down and will never reach above ..150-.200 during cruising. My question is could this be causing my problem im having now? Reason i ask is cause ive heard from people and read of people deleting their catalytic convertor and having no issues with the way their truck runs. Although they could be exaggerating, but i thought only the 2 upstream o2 sensors affected the driveability.

 

[Schurkey]

If it's in front of the converter, the ECM is probably using it to adjust fueling. "Downstream" 02 sensors are behind the converter.

You need that sensor to be working properly. How old are the sensors? Anything over 50K miles is suspicious. Does the scan tool show appropriate O2 crosscounts?

 

[Marcos Beltran]

If it's in front of the converter, the ECM is probably using it to adjust fueling. "Downstream" 02 sensors are behind the converter.

You need that sensor to be working properly. How old are the sensors? Anything over 50K miles is suspicious. Does the scan tool show appropriate O2 crosscounts?

I have 3 o2 sensors before the catalytic converter and 1 behind it. Its gonna be O2B1S3, which i assume means O2 Bank 1, Sensor 3, thats giving the low voltage readings. This is the one right before the CAT. Yes the vehicle has 170, 000 plus miles and im amost certain that the 02 sensors have never been replaced.

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I dont know if i mentioned this but it is not throwing any type of code and im getting green checks across the board in the I/M rediness function.

 

[Schurkey]

I have had bad experiences with O2 sensors. I fought one on my '88 for two years, on and off. Had a lean-code that only set at highway speed--but it set every time I got to 55--60 mph. I tested everything; and it all seemed "good".

Over time, the "highway speed" got lower and lower. By the time the code was setting at 40 mph, I got desperate. I just shoved in a new O2 sensor, despite the readings from idle to ~40 being "good".

Problem fixed. Two fookin' years, twenty hours of occasional diagnosis...and a thirteen dollar sensor took care of it. The sensor I took out was only about 80K miles old.

From that point forward, I've made it a point to not be shy about cramming new O2 sensors in place. They're not that expensive, but they can be a real pain in the *** to remove. And then you need to clean-up the threads in the hole, because often as not it'll be at least slightly buggered.

O2 sensors are the only sensor on many vehicles that has a "service life"; there's a point where the thing is just worn-out and doesn't work properly even if it hasn't failed entirely. And 80K miles is about that limit. The sensor becomes slow to react, (lazy) and may not go to the voltage extremes that it should.

Among the first things I did to my '97 was to replace all four O2 sensors. I bought the truck with 160K miles; and given the trouble I had getting them out they may have been original. All four were throwing codes for their heaters.

 

[Marcos Beltran]

I have had bad experiences with O2 sensors. I fought one on my '88 for two years, on and off. Had a lean-code that only set at highway speed--but it set every time I got to 55--60 mph. I tested everything; and it all seemed "good".

Over time, the "highway speed" got lower and lower. By the time the code was setting at 40 mph, I got desperate. I just shoved in a new O2 sensor, despite the readings from idle to ~40 being "good".

Problem fixed. Two fookin' years, twenty hours of occasional diagnosis...and a thirteen dollar sensor took care of it. The sensor I took out was only about 80K miles old.

From that point forward, I've made it a point to not be shy about cramming new O2 sensors in place. They're not that expensive, but they can be a real pain in the *** to remove. And then you need to clean-up the threads in the hole, because often as not it'll be at least slightly buggered.

O2 sensors are the only sensor on many vehicles that has a "service life"; there's a point where the thing is just worn-out and doesn't work properly even if it hasn't failed entirely. And 80K miles is about that limit. The sensor becomes slow to react, (lazy) and may not go to the voltage extremes that it should.

Among the first things I did to my '97 was to replace all four O2 sensors. I bought the truck with 160K miles; and given the trouble I had getting them out they may have been original. All four were throwing codes for their heaters.

Curious, where are you getting your 02 sensors from cause ive been looking around and they are pretty expensive. All the ones i found are over $45. Thanks.

 

[Schurkey]

Amazon, Denso brand. I refuse to support Bosch if I have a choice.

Bear in mind that the O2 sensor that fixed my '88 was purchased ten years ago; and it's a single-wire sensor. The same sensor today is much more expensive--almost eighteen dollars.

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I have the sensor, and the wire harness kit, to convert this truck to a 3- or 4-wire sensor (I forget which.) The only thing I'm missing is the motivation.

The sensors for my '97 also came from Amazon, and also Denso brand.

Two of these @ $25/ea. for the downstream positions:

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...and two of these @ $35/ea. for the upstream positions.

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[Marcos Beltran]

I made my own diagram cause i couldnt find anything on the internet. Now the O2 sensor that im thinking im having a problem with i think i was mistaken in an earlier post. It is not the O2 sensor before the CAT, its the one right after. O2B1S3 is the one that was reading low. Looking at the diagram i drew up, did i label it in the right location? If so correct me if im wrong any O2 sensor after the CAT would not give me the problems i am

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having right? Anyone with knowledge on this i would greatly appreciate any help.