Bucking

[east302]

No, he’s referring to the GM factory manual. It usually is written around the tech2 scanner. It would have the factory repair and diagnostic procedures, along with the wiring diagrams, etc.

Someone scanned in the 98 model here...

http://67-72chevytrucks.com/vboard/showthread.php?t=700011

But alldatadiy.com is nearly the same thing. It’s about $30/yr per VIN and has the factory diagnostics in it. It’s well worth it.

I meant to mention that your situation reminds me a little of when my catalytic converters were damaged. Any significant load on the engine (such as a fast merge onto an uphill interstate) would make it start chugging, preventing any acceleration until I restarted it. It would also lug in fourth gear at around 40-mph. It felt like I had shifted up too soon with a manual transmission.

You can check the upstream pressure by removing the oxygen sensor in front of the converter. This, by the way, is from alldata. I haven’t checked, but presume that the GM manual has a similar table.





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[swathdiver]

No, he’s referring to the GM factory manual. It usually is written around the tech2 scanner. It would have the factory repair and diagnostic procedures, along with the wiring diagrams, etc.


I meant to mention that your situation reminds me a little of when my catalytic converters were damaged. Any significant load on the engine (such as a fast merge onto an uphill interstate) would make it start chugging, preventing any acceleration until I restarted it.

^^^^ This is a good possibility! ^^^^

 

[Danneman]

No, he’s referring to the GM factory manual. It usually is written around the tech2 scanner. It would have the factory repair and diagnostic procedures, along with the wiring diagrams, etc.

Someone scanned in the 98 model here...

http://67-72chevytrucks.com/vboard/showthread.php?t=700011

But alldatadiy.com is nearly the same thing. It’s about $30/yr per VIN and has the factory diagnostics in it. It’s well worth it.

I meant to mention that your situation reminds me a little of when my catalytic converters were damaged. Any significant load on the engine (such as a fast merge onto an uphill interstate) would make it start chugging, preventing any acceleration until I restarted it. It would also lug in fourth gear at around 40-mph. It felt like I had shifted up too soon with a manual transmission.

You can check the upstream pressure by removing the oxygen sensor in front of the converter. This, by the way, is from alldata. I haven’t checked, but presume that the GM manual has a similar table.





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Great info, thank you! I've downloaded the manuals for the 98.

So, on the exhaust, I had the full stock exhaust replaced in 2017, and it was a few months later that I first noticed my current issue. When I replaced the exhaust, I got new O2 sensors. Then, in 2018 I had my catalytic converters replaced under warranty as I thought I had heard some rattling - it turned out to have been something else, but in the process I was hoping it might help this situation, but it did not. But, for that warranty replacement we reused those two O2 sensors on the cat portion of the exhaust. Possible that they were bad all along? That, or for whatever reason I've got a backpressure issue. Thanks for the info and I'll investigate all of the above soon enough here.

 

[Danneman]

Ok, got the scanner today, data time.

Successfully reproduced my issue and managed to capture the HO2S data. Here are three, 5-second snapshots of me in a valley just at the bottom of a hill, heading into it on cruise control at 39MPH, and then heading up the hill (roughly 3-4 seconds in).

For these plots I chose to show engine load and the two pre-cat sensor readings. It looks like the recording captured all data on this HO2S page and I can plot whatever, that's pretty cool.

Not sure what I should be making of this, if anything. I can say that at idle it seems like these are both fluctuating steadily between 0-1V, and opposite each other (one is high while the other is low) - whereas it looks far less consistent when I'm experiencing the issue.

I'll try unplugging the sensors next, just wanted to see any clear conclusions could be drawn from this in the meantime.

 

[east302]

So what do you think of that scanner? I’ve been wanting to get one for a while, just haven’t pulled the trigger yet.




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[Danneman]

Ok, let's try this again. I've had a lot of problems this afternoon trying to get these pictures added to the post correctly.

 

[Danneman]

So what do you think of that scanner? I’ve been wanting to get one for a while, just haven’t pulled the trigger yet.




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I love that I've got all this data now, I think it will help. No basis of comparison to other scanners of course...and on most data points I'm not sure what I should be looking for!

 

[Danneman]

OK, general update:

Tried unplugging O2 sensors, on first test drive up my hill same results as usual. My snapshot showed that the computer plugged about .5V in (presume this is the “default”).

With O2 sensors still unplugged, tried unplugging MAF. Lurched forward a bit when putting into gear, and gear changes were harsh, and overall felt pretty underpowered...but believe that's normal? Anyway, this time, going up my hill, it started to lug but then dropped to 3 gear before it could start to buck. That doesn’t usually happen, it usually holds out in 4 at low RPM and starts to lug and that’s when I get my bucking. Not sure what I should take away from that, if anything.

I did my test drive probably 8 times or so, each time capturing different data, but of course I don’t know exactly what to look for.

I did look for misfires and there were none, so at least we know that now.

Can the tech 2 measure fuel pump pressure? I can’t seem to find that. I was told by a mechanic that it reached and maintained at spec, but that was just regular driving around and maybe I’d see something different going up a hill.

Thoughts on any of that? What else should I try next?

 

[alaska_guy]

Bucking sounds like a transmission issue. Possibly the pump. Does your scanner allow you to test transmission line pressure? Sounds like possibly a bad boost valve in the transmission pump.

 

[Danneman]

Bucking sounds like a transmission issue. Possibly the pump. Does your scanner allow you to test transmission line pressure? Sounds like possibly a bad boost valve in the transmission pump.

Does anyone know for sure? I’m thinking that this (and the fuel pressure test) both involve connecting a pressure gauge to a test port, and that they can’t be tested electronically through the OBDII.