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No. I use either HP Tuners VCM scanner (Windows, $350) or I-link 400 (handheld scanner, $150).
Originally Posted by RedValor
Ok, so I now know not to rely on spark timing advance to diagnose a worn chain on distributor ignition systems.
Not on crank sensor triggered timing. Blazer is one of few that have both distributor and crank sensor.
Originally Posted by RedValor
B: If the timing chain has wear (slop), then it could retard cam timing. True?
C: If cam timing is too far retarded, then this results in performance issues. True?
True
Probably what you see from stretch will not affect it much. 9 degrees of crankshaft timing (Eric the Car Guy) is only 4.5 degrees of camshaft timing. Retarding a cam several degrees will change the way a car runs a little bit - but not a lot to the point of misfire. Racers can change cam timing this much to tailor a cam to their use.
Originally Posted by RedValor
So because cam retard is currently at -9, and distributor gear is no longer a suspect, then the only possible remaining factor causing cam retard is the timing chain. I obviously want to replace it then.
But, you say that if I were to measure the degrees of slop in the cam gear and timing chain using the above method, it would not be so bad that it would not affect the way it runs. Doesn't this statement contradict statement B? Maybe statement B is incorrect.
With new distributor gear, other possible variables in CMP Retard are timing chain OR how precisely the $40 distributor was manufactured to position the cam sensor or hold down clamp. With a new gear, both are also possible sources of cam sensor retard being off. I don't believe I have any big contradictions. I think you are drawing conclusions without considering all the variables.
Originally Posted by RedValor
I'm still trying to understand how (if it does) the harmonic balancer fits into the cam timing puzzle.
It doesn't.
Originally Posted by RedValor
I understand that the point of manually adjusting the distributor is to get cam retard to 0, and the point of setting cam retard to 0 is to then see if the distributor lines up with the #6 mark when balancer ring timing marks line up. if not then this means a slipped balancer ring. Is that all that it means? Nothing performance related?
Imbalance from a slipped outer ring can affect the waveform and cause false misfires to be logged. It would be pointless to worry about a crank sensor relearn with a slipped balancer.
Originally Posted by RedValor
I forgot to mention that when I did pull the old distributor, I did make 100% sure #1 was at TDC. I pulled the #1 spark plug and I used my endoscope to peak inside the cylinder and watched the piston rise and fall as I turned the crank. I used this as a reference to see where the piston would stop and then go back down, and with absolute certainty I made sure #1 was at TDC before pulling the distributor. It even lined up with teh #6 almost prefectly. I used the rotor's position to tell the different between the exhaust stroke and the compression stroke.
If I did this manual distributor adjustment to zero-in cam retard, then it would no longer line up with the timing mark, which now has me a little confused.
You have to use a piston stop to get accurate TDC. But you did do enough with your endoscope to get the distributor back in on the right tooth, even though the balancer marks indicated you were not on TDC (Why I don't trust your balancer marks). The 6 mark will get you on the correct tooth, but not anything more. The rotor never seems to line up exactly with it.
Originally Posted by RedValor
TDC and distributor at the #6 mark means perfect timing, right?
It means you are on the right distributor tooth (1 of 13), nothing more
Originally Posted by RedValor
Perfect timing means the timing chain is fine and dandy, right?
So what's up with cam retard being at -9?
Sorry, but this is what I call a circular argument.
Originally Posted by RedValor
So this being the case, I'm making the assumption that any amount of chain slack results in retarded cam timing.
True
Originally Posted by RedValor
So I guess the million dollar question at the end of the day is this: Does manually adjusting the distributor compensate for a worn timing chain? I don't care so much about the timing marks on the balancer anymore since I have another way to find TDC.
Marks on balancer don't matter to you at all at this point. But if that balancer ring has slipped, it can cause you problems with false misfire detection. I described how to check it.
Please don't start getting PO'd at me. I can see you're so close to understanding it all!! I wouldn't bother otherwise!
I'm trying to find a way to relearn the crank sensor without needing a bidirecitonal scan tool.
Yeah I found that procedure floating around the Internet when I first got my Blazer - same reason. IIRC it is only for a VERY few vehicles (only certain trucks) and Blazers are not part of them. Your choices are get a capable scanner and do it yourself or take it to a shop with a capable scanner and pay them. Doing it on an engine with a slipped balancer would be pointless - so I say check your balancer as I described.
Lets get the cam sensor retard to zero and see how it runs. We can go on from there. You will want to wait to do the crank sensor relearn until after everything is fixed and how you want it (at least if you are paying someone).
Oh no, Les. I'm not getting PO'd at anyone. I'm just anxious about getting the job done right and making sense of the procedures. When I'm trapped at the office during my work week, all I can do is run scenarios in my head and think about what to do next if result A is negative or inconclusive like I'm backseat quarterbacking. I often over think things.
As always I greatly appreciate this conversation.
I have the CMP retard as close to 0 as I can now. Its between -1.0 and -0.7. Many attempts to get this closer to 0 were met with greatly over shooting it. Like threading a needle at how little of a movement results in +/-1 degree.
Timing marks on both the balancer and the distributor seem to line up perfectly now. So I guess this confirms the balancer ring has not slipped.
Feels like it's running better at high RPM now. No more shudder. But its still rough at idle. I ran a test were I rev'd it to 2,000 RPM for about a minute, then I let go and RPM dropped to about 300 and the engine almost stalled. Only did this once with the CMP numbers reading at -0.7. Most times rev'ing it up and letting it back down to idle causes RPM to drop to about 450 - 500 RPM then it bounces back up to 625 RPM where I normally see idle at. MAF, MAP, TPS, and IACV are all new. Throttle body is clean as a whistle. No obvious signs of vacuum hose leaks (hose going to the t-split near the brake booster is new). So could it be intake gasket or fuel injectors next?
I guess it's now time to finally test drive it.
p. s.
I tested the slack in the timing chain too. Your assumption was right, Les, there's barely any slack at all. I was just so caught off guard to see a brand new distributor read -9 CMP retard. I guess this attributes more to manufacturing quality of an aftermarket part.
Excellent! Your CMP retard is just fine now, Balancer ring is fine. So we're all done with that.
Lets move on into determining if you have a vacuum leak or other problem somewhere, using fuel trims.
From the factory, they have programmed into the PCM the theoretical amount of fuel required to feed your vehicle at the RPM and engine load and air temperature, and engine temperature, and volumetric efficiency, and other parameters it is operating with. Amount of fuel is controlled by injector pulsewidth (how long each injector it is open vs how long it is closed), and injector pulses are timed using the cam sensor readings. GM figured out how much fuel was delivered by a certain length injector pluse and calibrated it accordingly to give the best air/fuel ratio. This is how your vehicle runs when first started, before the PCM gets feedback from the PCM on valid O2 sensor readings and goes into "closed loop". Once it goes into closed loop, if the O2 sensors indicate a variation from ideal, it adds fuel or subtracts fuel from the base calibration. This is the "Fuel Trim". So if your are +10%, then PCM is adding 10% to base calibration, based on feedback from O2 sensors. If you are -15%, the PCM is taking away 15% of fuel compared to base calibration. But if you monitor Fuel Trim, you will see that it cycles rapidly for Short Term Fuel Trim. This is because O2 sensors normally cycle rapidly up and down in voltage when controlling at optimal Fuel/air ratio. It's the average Fuel Trim that you need to be concerned with. There is STFT (short term fuel trim) and LTFT (long term fuel trim) for each cylinder bank on your engine - and this follows your two pre-cat O2 sensors. If fuel trims get so excessive that they cannot compenstate for O2 sensor readings, the SES light will be triggered.
If you have normal short term + long term fuel trims at 2000 RPM but a high positive short term + long term fuel trim at idle, that can typically indicate a vacuum leak (since vacuum leak is constant, but engine requirements vary with RPM). If fuel trim is good at both RPMs, that certainly indicates no vacuum leak and no issue, but the converse is not necessarily true since mechanical problems can also cause skewed fuel trims. So this is your next task to check fuel trims on both banks, comparing fuel trims at idle to those at 2000 rpm. That should give us a direction to go.
Because fuel trims cycle rapidly up and down, it is not useful to take spot readings - but you can look at the range/average with the two STFTs and two LTFTs in the Dash Command data grid (so it follows closely). Unfortunately, I do not think Dash Command is the best program to do this evaluation of fuel trim - since I have never found a plot function. But you can watch the fuel trims vary up and down and take the max and minimum that you observe. The way I like to do it is to use a program like Torque Pro that can plot multiple things on the same graph. Yeah I know it is another $10, but Torque Pro is a great addition to Dash Command - its just that Torque Pro doesn't do Cam Sensor Retard. If you use them both, make sure you completely exit one before starting the other or they will not connect.
Below is collected using Torque Pro. You can see why static Fuel Trim values are not valid - they vary so much (same with O2 sensor voltages) and need to be plotted to evaluate them.
On mine at idle:
STFT + LTFT Bank One = about +0.8
STFT + LTFT Bank Two = about zero
On mine at 2500 rpm:
STFT + LTFT Bank One = about -3.0
STFT + LTFT Bank Two = about -5.0
Why the difference from zero at 2500 rpm? Most likely variation in sensors that affect the PCM calculation being compared to. I suppose I would suspect maybe Mass Air Flow is off just a touch. But this much would be considered to be completely normal and does not cause rough running or any kind of misfire. See how that works?
Blue Driver has graphs I can use, so that's how I logged this. Unfortuately There was a mishap in saving the data, so I lost the CSV file it generates where I can load it into a spreedsheet and plot a line chart.
Thoughtful of me I made a screen recording of the test session so I have fuel trims at least. Lost O2 voltage though. I didn't scroll down to look at those because I though I have the CSV data to observe after the fact.
I'll share screenshots of the video too highlighting the important zones.
At the beginning, I started her up and was just waiting for fuel status to close. Once that was warmed up, I raise the RPM to aboout 2000. I struggle a bit with my foot to keep the throttle stable. Then I let it fall back to idle. Then I raised it to 2000 again for about a minute. No weird RPM drops at or when return to idle this time. These "near-stalling" RPM drops usually only happen when first starting out for the day and if the engine core temperature had time to reach ambient temperature the night before.
The data I got from this seems to be the complete opposite of what you said a vacuum leak would look like, Les.
I get normal fuel trims at idle. Still rough idle. But when raising the RPM to 2000 fuel trims go way into the negative.
Also got SES light when doing this.
P0301 #1 misfire. I guess ignore this until a CKP relearn can be done.
P0341 Camshaft Position Sensor ”A” Circuit Range/Performance (Bank 1 or Single Sensor). Based on what I read about this I think its also related to the CKP relearn.
The scale of the data on the BlueDriver charts is not very good, but you can see the general trend is to go negative when RPM is raised. Exactly how much negative (average) can't be determined by a spot check of data like it shows, but I can see in the plots there is definitely a trend to go negative at higher RPMs. That does seem abnormal and doesn't appear to be a vacuum leak. Is this new data or old data? Let's check some sensors out that the PCM uses in the fuel trim calculation. I'll come back later with some different checks to try. But got some stuff to do right now. In the meantime if the fuel trim data is old, probably should get some new. You could try with Dash Command as well.
Check engine air inlet duct where it comes through radiator support. This can get folded over and blocked when removing air filter box.
Check air filter for cleanliness
Check MAF sensor - should be 6-12 g/sec at idle, going up to >130g/sec at full throttle 1-2 shift.
Check MAP sensor - should be ~103 kPa at sea level (key on, engine off). (- 3 kPa for each increase of 1000 feet elevation)
Check coolant temp sensor - should make sense.
Check idle air temp sensor - should make sense.
Check TPS - 0 to 100% and smooth.
If these are OK, then check leak down time on Fuel system (looking for leaky fuel pressure regulator inside of intake plenum).
Also recheck fuel trims with purge hose to EVAP system disconnected and plugged to make sure this is not a source of additional fuel into engine.
02-24-2022, 01:18 AM
