I don't think you ever did these, did you? Could be revealing at this point.

https://www.alldata.com/us/en/suppor...pression-tests

 

No I never did a running compression test Les, I have seen the running compression talked about on other threads, I saw on another thread you said you have done this while leaving the injector connected so it just dumps fuel down the exhaust on the cylinder being tested? I guess my cat has already seen its days anyway LOL. I'm also wondering the best way to ground the plug wire on the cylinder being tested? Looking at the alldata website it explains the test very good and seems like it would be beneficial for me to do at this point.

 

Extra fuel during this test would be inconsequential and not harm the cat converter because it is for a short time. I would just use an old spark plug wire and clip an appropriately sized screw driver into the spark plug end. Then clamp the screwdriver to something metal. Then you can take off a spark plug wire at the distributor and replace it with your alternate one that goes to ground. Much easier than trying to work with the spark plug end of your real wire. Know that when you do the snap test you must do absolutely full throttle for that brief instant - so don't be shy.

I doubt you will find a problem with the running compression test (actually hope you don't) - but I think there is value in knowing what is NOT the problem so you can more readily identify and focus on what IS the problem.. Doing this testing of comparative volumetric efficiency will rule out all the mechanical things that can cause the cylinder to misfire like many different issues with the valvetrain as well as plugged exhaust. If you supply a proper fuel mix to a cylinder with decent volumetric efficiency and light it off at the right time, then it will not misfire. Static compression does not take volumetric efficiency into account. Too bad this testing is such a pain to do on the Blazers, but your lifted one may be a bit better. If it passes, you can relax and forget about major engine issues.

Good luck

 

It normal for O2 sensor cycle time to be slow at idle and faster at speed. There is no synchronization between the banks. As long as they are cycling across 0.45V, the frequency doesn't really matter. And there can be brief times where they don't cycle across 0.45V (like when changing speeds). Some of the waveform happens to be due to at what point during the cycle it is sampled. The more data points, the better defined the waveform is. It helps to sample fewer PIDs if this is important, as sampling gets spread between all PIDs. So those readings all seem quite normal to me.

Maybe you haven't found the SAE fuel trims in Dash Command yet?

Think about 2 cases.

1) PCM is correcting 8%-10% for an actual rich condition

Fuel trims go negative.
O2 sensor voltage still crosses 0.45V and vehicle runs at proper mix.

2) PCM is correcting for a rich condition by mistake

Fuel trims go negative
O2 sensor voltage goes lean and vehicle runs lean. I might suspect this, but your O2 sensor voltages do not support it (at least it doesn't seem so).

Seems like you have case #1 for both banks, but not all the time as seen in various fuel trim displays that you posted. Are the fuel trims going negative at idle only? At speed only? Both? Can you establish when this happens? You could always set up Torque Pro to log MPH, RPM, 4 fuel trims, and 2 primary O2 sensor voltages into a file. We need to establish the pattern (there should be one) and also give the PCM time to adjust the LT fuel trims between conditions. Or you could just take logs after going down the highway at speed for some miles, or after idling for an extended time.

 

Misfire counters are known for false positives, but are designed to not give false negatives. However if you would like to evaluate the operation of the misfire counters, just monitor the misfire PID while you pull each plug wire (in turn) and start the engine. If it detects the misfire by the counter for that cylinder rapidly rising, and does that for all cylinders I would not waste money on a crank sensor relearn. In fact, I wouldn't relearn an engine (without the light on) until I thought it was running good - since that waveform is the reference that is stored and used to compare against.

What I wish we could do at this point is something none of your scanners can do. It's called a cylinder balance test or power balance test. The scanner locks the IAC and evaluates how far the RPM drops when each injector is removed from operation and displays the results. If there is a particular cylinder or group of cylinders that are weak, that can help direct things. I have a handheld scanner that will do this as well as crank relearn, but it's a bit pricier at $150US but price is unknown in Canada. Still I think we can work around your problem using what you have now.

 

Thanks for this Les, a lot of good information here. I think I should be good to go with the running compression test this weekend, maybe I will get it done sooner I just always find time flies after I get home from work LOL. The new distributor should also be going in this weekend along with a new cap, if it comes in time, I called my local parts store and they told me $90 for a Delco cap! Crazy. So I just ordered one from Amazon for a bit cheaper. About the fuel trims, I'm not sure how to log stuff in torque pro, I will figure that out. However, I have plotted all 4 fuel trims while driving and found this, while idling or not on the throttle (cruising) the trims will stay near 0, once I start putting the throttle on ONLY while actually moving (putting a load on the engine) the long term trims will go way positive until I let off the throttle. You can see this happen in the video below, at the end of the video I was in park and holding the rpms at various amounts and the trims would not change as they do when I'm driving. I'm sure you will have better input after watching the video Les, I know it's long as it's the whole drive home from work but I hope it gives some insight into how it's running under different conditions. Also, is it normal for the long-term trims to change so drastically so fast like that? I mean I have monitored my fuel trims before but don't remember the long-term jumping around so much? This really has me stumped here, as much as I hope the running compression test doesn't turn up something I almost hope it does because then I can be done with this game of "finding the rough idle" and at least have an answer for what's causing it. I have seen you talk about the cylinder balance test in other threads but was a bit confused about what it was so thanks for explaining! I kind of shot myself in the foot with the wired scanner I have as it's around $200 Canadian and can only read fault codes, basic live data with an option to use a botchy graph for one pid at a time only, and this is decent I guess but it can read ABS codes as well. My point is I should have just gotten a scanner like yours from the start that can do everything for around the same price, if only I had known. I am happy with my tablet setup now though excluding the fact I still can't do a crank relearn or anything bidirectional like the cylinder balance test.

 

I watched the entire thing. Didn't see anything I thought was an aha moment, but the fuel trims did get a little positive sometimes as high as 15 and managed to stay there for a while in what seemed to be cruising. Before we talked about negative fuel trims (not positive), but that was in Dash Command. I'll capture the same thing on mine this afternoon for comparison and that hopefully will answer the rest of your questions. Engine seemed to sound good in the recording. Did it seem rough to you?

As far as the scanners, I really bought the Vident handheld for doing the ABS brake bleeding procedure and found out it really does a lot of bidirectional stuff. But 95% of the time, I just use the bluetooth OBD2 adapter and phone software because it is so convenient and will handle most everything. I started with Torque and then added Scan XL Pro for Windows to do Cam Sensor Retard. Then I found Car Gauge Pro which did cam sensor retard with the bluetooth and also would display/clear codes for other systems in the vehicle like the ABS, SIR, HVAC, BCM, etc. Added Dash Command because for some reason neither bluetooth software would connect with my 2006 Pontiac G6 GTP (first year of a new communication protocol). Then added HP Tuners VCM scanner for Windows to do crank sensor relearns and some bidirectional control like the cylinder balance test. Finally bought the Vident I-link 400 for the ABS and found it did all functions nicely except for the plotting and logging but was not so convenient to use. I even unlocked a junkyard factory CD changer/radio for my G6 with the Vident. I keep finding out more and more that it will do. But I just pick the one I think will do the best for me at the moment. I suppose I have collected various OBD2 softwares with a focus on the Blazers. To me they are all interesting

 

Looks like all bets are off unless throttle is held steady at whatever engine speed you are monitoring. Mine seems to cycle more rapidly at speed than yours. I have read that O2 sensors cycle more slowly when old, but don't know that for fact. Could be that old systems like (1996) cycle more slowly than late model systems. I don't know the context or the reliability of that info. I'm still not seeing a smoking gun on yours.

George are you seeing anything with the fuel trims or comparison of fuel trims?

 

Thanks for this Les, your video is also much easier to follow with the callouts of how fast you are going at different times. With that said it looks like your long-term fuel trims always stay around the same when accelerating, cruising, and idling. I see they change a bit like when you let off the throttle and slow down but when your accelerating they stay about the same. When you were cruising at steady speed and the long terms were around 0 then stepped on it and downshifted the fuel trims didn't change at all hardly, also to note at the end when you did wide-open throttle, the fuel trims jumped up a bit when you first stepped on it but then stayed the same until you let off the throttle. On my blazer, I have found when I step on the throttle while cruising making it downshift, the long-term trims will go more positive and this seems to continue until I let off the throttle. It seems at idle my long-term trims will stay just under 0 then once I step on it they will go positive, how much throttle I give seems to determine how positive the trims go. I will test this theory more later today and also do a wide open throttle run to see what happens to my fuel trims. On the note of how the engine feels, you can definitely feel there is something wrong when you're sitting in it and idling, the simplest way to put it is it feels rough. I only notice something is wrong while idling though, once I'm on the throttle it smoothes out and has good power.

 

We may have to figure out how to do a manual cylinder balance test to prove it's either a misfire, or just a balance shake being felt through the polyurethane motor mounts. Probably a good idea for us to let George have a look at the fuel trim comparison and get another opinion.

Is there also a starting problem?