This is kind of a bounce-it-off-wall post. My adventure using a DIY First Look Pressure Sensor to help figure out a rough idle.
My friends tell me I over think things...
I wonder what gave them that idea? :banana: :lol:
Feel free to comment...
It's been a few months now since installing the reman'ed engine and I'm still experiencing a "shimmy" rough idle.
I've heard of the expression "Thrown everything but the kitchen sink at it". Safe to say, I've actually thrown the kitchen sink at this one. Before I pull the engine, I'm down to my last ditch efforts to understand what's going on. So....
I made my own version of the First Look Sensor (too cheap to buy one). It's simply a pressure sensor that I can stuff up the tail pipe or use it on a vacuum line.
The intake waveform was unremarkable and didn't show anything unusual.
BUT... the exhaust waveform tells the story. The waveform shows 12 exhaust pulses (twice around the "block"... if you will). The first pulse is not sync'ed to any one cylinder (it could be any cylinder). The firing order is 1-6-5-4-3-2. There are no "dead" misses. All cylinders are firing, but some are weak in torque (more than one), thus more of a hum or shimmy symptom in the idle, not a dead miss type vibration..
The Torque App shows very random misfires on cylinders 3 (mostly) and somewhat on all others. Oddly noted that cylinder 6 has never shown a misfire (see below). They are only shown at idle and less when in gear. The general "hum" or "shinny" can be detected throughout the entire RPM range under load, but not under a no-load (neutral) RPM rev.
Understanding how the misfires are detected helped me a lot. It only looks for the rotational speed change in the crankshaft... it really doesn't "know" if a cylinder really fires or not, just the resulting torque for each cylinder's power-stroke.
So, here's the exhaust waveform. The way I read this, I've got 2 (maybe 3) weak-fire cylinders. I'm also looking to see if they are on the same bank. I don't care right now which bank is which right now, just if the misses are on the same bank or not. This is what I got:
Exhaust O-scope Pulses:
1 -> High -> Bank(x)
2 -> Low -> Bank (misfire?)
3 -> High -> Bank(x)
4 -> High -> Bank
5 -> Low -> Bank(x) (misfire?)
6 -> High -> Bank
(The next cycle on o-scope repeats same)
Fire -> Fire -> Mis -> Fire -> Fire -> Mis (repeat)
So waveform pulse 2 on bank is low and 5 on bank(x) is low. Knowing that cylinder 3 has been the most (by far) reported misfire, I can plug in cylinder 3 to pules 2 or 5 (the two low pulses) and count the firing order from either starting point and I come up to the same results. Cylinders 3 and 6 are weak. Now I know (best guess really :think the misfires (weak power strokes) are not on the same bank and are on cylinders 3 and 6.
This explains the fairly high frequency of the "rough idle" and humming vibrations when engine RPM and torque increase... and why "shimmy" might be a better way of describing the rough idle. Blurry-image-in-the-rear-view-mirror syndrome.
Now, what the heck to do next?
Done (as well as much more) spider, plugs, wires, cap, rotor, distributor, lower manifold, upper and throttle body gaskets (all twice), inspect upper manifold for crack (off-vehicle twice), vacuum 19" at idle, compression at 145-148 (not too low for new rings), PCV, idle air motor and other assorted things, like O2, clean MAF, check MAP and TPS, etc...
Maybe I should look closer at the vacuum waveform again (I didn't take picture)? I'm questioning the sweep time scale I was using on the o-scope.
Somewhere in this house I have a 250 PSI sensor (I just can't put my hands on it today). If I can find it, I'll look at the cylinder(s) with it.
My friends tell me I over think things...
I wonder what gave them that idea? :banana: :lol:
Feel free to comment...
It's been a few months now since installing the reman'ed engine and I'm still experiencing a "shimmy" rough idle.
I've heard of the expression "Thrown everything but the kitchen sink at it". Safe to say, I've actually thrown the kitchen sink at this one. Before I pull the engine, I'm down to my last ditch efforts to understand what's going on. So....
I made my own version of the First Look Sensor (too cheap to buy one). It's simply a pressure sensor that I can stuff up the tail pipe or use it on a vacuum line.
The intake waveform was unremarkable and didn't show anything unusual.
BUT... the exhaust waveform tells the story. The waveform shows 12 exhaust pulses (twice around the "block"... if you will). The first pulse is not sync'ed to any one cylinder (it could be any cylinder). The firing order is 1-6-5-4-3-2. There are no "dead" misses. All cylinders are firing, but some are weak in torque (more than one), thus more of a hum or shimmy symptom in the idle, not a dead miss type vibration..
The Torque App shows very random misfires on cylinders 3 (mostly) and somewhat on all others. Oddly noted that cylinder 6 has never shown a misfire (see below). They are only shown at idle and less when in gear. The general "hum" or "shinny" can be detected throughout the entire RPM range under load, but not under a no-load (neutral) RPM rev.
Understanding how the misfires are detected helped me a lot. It only looks for the rotational speed change in the crankshaft... it really doesn't "know" if a cylinder really fires or not, just the resulting torque for each cylinder's power-stroke.
So, here's the exhaust waveform. The way I read this, I've got 2 (maybe 3) weak-fire cylinders. I'm also looking to see if they are on the same bank. I don't care right now which bank is which right now, just if the misses are on the same bank or not. This is what I got:
Exhaust O-scope Pulses:
1 -> High -> Bank(x)
2 -> Low -> Bank (misfire?)
3 -> High -> Bank(x)
4 -> High -> Bank
5 -> Low -> Bank(x) (misfire?)
6 -> High -> Bank
(The next cycle on o-scope repeats same)
Fire -> Fire -> Mis -> Fire -> Fire -> Mis (repeat)
So waveform pulse 2 on bank is low and 5 on bank(x) is low. Knowing that cylinder 3 has been the most (by far) reported misfire, I can plug in cylinder 3 to pules 2 or 5 (the two low pulses) and count the firing order from either starting point and I come up to the same results. Cylinders 3 and 6 are weak. Now I know (best guess really :think the misfires (weak power strokes) are not on the same bank and are on cylinders 3 and 6.
This explains the fairly high frequency of the "rough idle" and humming vibrations when engine RPM and torque increase... and why "shimmy" might be a better way of describing the rough idle. Blurry-image-in-the-rear-view-mirror syndrome.
Now, what the heck to do next?
Done (as well as much more) spider, plugs, wires, cap, rotor, distributor, lower manifold, upper and throttle body gaskets (all twice), inspect upper manifold for crack (off-vehicle twice), vacuum 19" at idle, compression at 145-148 (not too low for new rings), PCV, idle air motor and other assorted things, like O2, clean MAF, check MAP and TPS, etc...
Maybe I should look closer at the vacuum waveform again (I didn't take picture)? I'm questioning the sweep time scale I was using on the o-scope.
Somewhere in this house I have a 250 PSI sensor (I just can't put my hands on it today). If I can find it, I'll look at the cylinder(s) with it.