Timing chain and distr gear:



George

 

I'm getting the feeling that you don't really understand how the ignition timing and misfire detection works on these Blazers. Admittedly they're not easy things to wrap your mind around at first.

In 70's distributors the ignition was generally triggered off of the reluctor wheel on the distributor shaft inside the distributor, advance was done by springs and weights that moved the rotor position, and a vacuum cannister moved the reluctor position for additional advance. In any case, since all was inside - if you turned the distributor you adjusted the entire timing curve either up or down.

On your Blazer, the reluctor wheel is on the crankshaft, and the coil is triggered by the PCM. Distributor does nothing regarding how or when the coil outputs a spark. However, the 4.3 retains a single coil and a distributor to "distribute" the spark to the different cylinders. This requires the rotor to be pointing at the right terminal at exactly the right time for the spark to jump the gap. Enter cam sensor retard. Camshaft position sensors generally tell PCMs the exact position of the engine for the purpose of timing sequential fuel injection pulses to the different cylinders. But since this one is located (and cam sensor retard measured) in the distributor on this engine (which is run off the camshaft), it can also be used to determine ignition rotor position as a secondary function. By putting the cam sensor in the distributor, you involve both the timing chain and the distributor gear as possible reasons for it not remaining synced exactly with the crank sensor since the vehicle was built. So by having a cam sensor retard of -18.5 degrees, your PCM was timing injector pulses a bit late and the rotor was getting past the terminal for each spark plug before the coil was fired. This is why it is important to set cam sensor retard to zero +/- 2 degrees for optimum performance. Provided the distributor lock down is manufactured exactly in correct position, with your new distributor your last 10 degrees of cam sensor retard has to be in the timing chain and gears. You can either replace the timing chain and gears (they seem to always be worn/stretched) or you can turn the distributor to correct. To turn the distributor you will need to either remove the locking hold down and replace it with a standard v-8 hold down, or dremel the hold down bolt hole oblong so you can turn the distributor enough to correct the cam sensor retard. If you replace the hold down with a V-8 one you will need a longer metric bolt and a flat washer.

Now lets talk about misfire detection. When the crankshaft position reluctor spins past the crankshaft sensor magnet, it generates a electrical waveform that is read by the PCM. By the crankshaft speeding up a little and changing the waveform each time a cylinder fires, the PCM can see each cylinder firing. When the vehicle was new, the PCM "learned" and stored a normal waveform for your vehicle and sensor setup. Since then it has compared the current waveform to this stored waveform according to some engineering specifications and counts anything abnormal as a misfire. It can tell you which cylinder's waveform has a problem by analyzing the camshaft sensor position. But there are other things that can screw up the waveform and get falsely counted as a misfire. Replacing or even disturbing the crankshaft sensor can cause false misfire detection, as can some other things. In my own Blazer, bad engine bearings were getting counted as misfires enough that the SES light flashed and set codes. According to factory info (see attached), things that affect crankshaft balance like pulleys, cogs off of belts, bad balancers, flywheels, torque converters can also be a source of false misfires.

A certain amount of false detection is normal during operation. I don't believe you said your light was on and a code set? So first I ask you if you can actually feel a misfire at idle? If not, you are probably overly-concerned about falsely detected misfires. But then again, maybe you are seeing a result of a bad front balancer screwing with the waveform. This is why I wanted you to set cam sensor retard to zero and then see where the rotor points with the balancer marks lined up. If it doesn't point to the "6" with cam sensor retard set to zero, then the balancer ring has slipped and the balancer should be replaced.

Hope this helps.

Les

 

Nice write up.


George

 

Thanks

 

Wow, very much appreciate the info. Really I appreciate everything you guys are helping me out with. I understand the mechanism a lot better now than when I started down this rabbit hole 3-4 weeks ago.

So just to establish a few things, what started me down this rabbit hole to begin with was low power on acceleration and poor MPG. I suspected an obstructed cataylst at first because the one in my Dad's '93 S10 went bad so he punched it out, and he claims the one in his '01 Blazer is going bad. So I did tests to confirm or deny this. Which reminds me I need to go back and update that thread. I bought a single new Walker O2 sensor and I was doing trial and error with the new sensor in various places and test drives with one sensor pulled out before the catalyst to see if there's a power difference. Of course having an O2 sensor disconnected long enough throws a SES light. I cleared it, put the old sensors back in, and did another test drive. I did not notice any power difference with or without a single O2 sensor, so I deemed nothing is wrong with exhaust back pressure. What did happen on this final test was that my whole dashboard litup light a Christmas tree, shutdown, and then resumed normacy. Engine kept running the whole time. It's like the the system rebooted itself. This is when I decided to pull out my BlueDriver and scan for troublecodes. To answer your question, Les, no there was no SES light when I got home from that test drive where everything rebooted itself, but the strange behavior prompted me to check anyway and that's when I found a pending P0300 that as led me down this current rabbit hole. After all the reading, learning, and hand-on testing I've done on this subject so far, I'm now convienced that the source of my low power / poor MPG issue is because of how retarded CMP is. This engine has 185k miles on it and has never been tuned since the factory. I've so far done everything else possible short of actaully pulling the engine apart. I'll have pull the engine apart soon because it does need new gaskets and seals. Oil leaks and I'm finding tiny bits of gasket material in the coolant.

Going back to the problem at hand... Ok, I now see the point of physically adjusting the distrubutor to calibrate CMP retard. I initially thought this to be a permanent fix and I was going to ignore the timing marks on the balancer ring.
I understand the next steps now. When I free up some time later this week, I plan to take the hold-down on the old distributor and elongate the bolt-hole on that and install it on the new distibutor so I can tune it's position. I'll get CMP retard to 0 and then check the timing marks. I'm already mentally preparing myself to replace the timing chain and balancer anyway though.

Eric THe Car Guy has a video on how to check timing chain slop, and this is something I want to test when I can.

Scotty Kilmer mentions in several of his videos that if the ignition advance has variance at idle speed, then that's a sign of a worn timing chain. I notice some variance in timing advance on my Blazer at idle in it's current state, but how much variance is considered normal? Right now I see anywere between 16 and 19 degrees at idle. It did see timing advance have variance at normal driving RPM too. I took measurments while out on a drive one day: cruise set at 45 MPH on a smooth strech of road and I saw about 5 degrees of timing advance variance. That has since been resolved with a new crank sensor I installed, but timing advance still has variance at idle with the new sensor.

 

That variation in timing thing meaning bad timing chain doesnt fly with your vehicle. Remember that ignition is triggered directly off the crankshaft by the PCM, not something run with the timing chain that can vary back and forth because of slop.

Honestly, best way to check timing chain slop in your vehicle (with cover on) is to determine how many degrees of crank rotation does not move the rotor (with a good distributor gear). Turn crank backwards until mark lines up with something. Then turn forwards until rotor just starts to move. Measure the distance the balancer mark has moved and use math/balancer diameter to calculate degrees of slop in the chain and cam gear. You can do it several times and take the average.

I suspect it will not be so horrible that it affects the way it runs.

 

When you install a new crank sensor, logged misfires are most likely invalid and cannot be counted on. Should have a crank sensor relearn done. However know this will not change the way that it runs - only how it detects misfires.

Guy in the second video knows just enough to be dangerous. I'm sure he thinks he's an expert. Bet his meter doesn't even display cam sensor retard. I think its hilarious that he is in front of a vehicle that doesnt have a distributor of any kind, talking about a 1970s distributor timing variation issue. This vehicle has a LS engine with individual coils!

The first video is valid way to estimate timing chain + distributor gear slop so that one is good.

 

Is the balancer ring not keywayed?

 

No the outer ring is mounted in rubber which can deteriorate and slip in relationship to the center.

 

Never thought I'd hear someone call out Scotty for being "dangerous". I've been 100% trusting his advice so far on many things. Guess I need to take it with a bit more salt.

Doh... I goofed on that. Explains the misfire data I've seeing. Does Dash Command do a relearn? I'll look into it. There still is a misfire I can feel. Probably not as much as the data is saying.

Ok, so I now know not to rely on spark timing advance to diagnose a worn chain on distributor ignition systems.

I want to run through the logic based on everything I know and learned so far...
A: The crank and cam are in sync with each other because of the timing chain. TRUE.
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?

So because cam retard is currently at -9, and distributor gear is no longer a suspect, then the only possible remining 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.

I'm still trying to understand how (if it does) the harmonic balancer fits into the cam timing puzzle.
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?

Lets say for scenario sake I did this distributor adjustment and lined everything up. I put a new balancer on and the marks line up perfectly, both on the cover and on the distributor. I then return the distributor to it's original factory poisition with the original unmodified hold-down. If the distributor lined up prefect with the modificaiton, then obviously removing the modification would upset the timing mark on the distributor... I'm confusing myself in the mental gymastics. Nevermind this part.

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.

TDC and distributor at the #6 mark means perfect timing, right?
Perfect timing means the timing chain is fine and dandy, right?
So what's up with cam retard being at -9?

Fast forwarding into the future because doing math...

Ok, so the diameter of these rotors is about 8.5 cenitmeters, and I know the computer is complaining about it being 9 degrees off. So plug these into a formula to find the arc of a circle and that's 0.66 centimeters.
Wow, ok, very, very low tolerance. I didn't realize this at first until I did the math, but +/- 2 degrees is milimeters of precision getting these things line up with the timing mark. I see why even a small adjustment to the distrubtor can make a big difference.
So this being the case, I'm making the assumption that any amount of chain slack results in retarded cam timing.


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.