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Another look at my rough idle - documenting

9K views 57 replies 11 participants last post by  norohs 
#1 ·
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? :confused: :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(y) (misfire?)
3 -> High -> Bank(x)
4 -> High -> Bank(y)
5 -> Low -> Bank(x) (misfire?)
6 -> High -> Bank(y)
(The next cycle on o-scope repeats same)

Fire -> Fire -> Mis -> Fire -> Fire -> Mis (repeat)

So waveform pulse 2 on bank(y) 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.
 

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#52 ·
I'm still learning much about pulse readings and there is more to it. I should not only cosider the pulse heights, but the valleys as well... and together they tell a clearer story.

The pulse in number 3 is telling. The lower swing especially.

I've got a 200 psi sensor coming in soon. I'll be doing an in cylinder pressure cranking test. I'm worried about the best psi of cylinder 3 and yet it seems its the worse performer. A worn exhaust cam lobe can cause the psi reading and poor performance. I wish I'd done a cam lift test the last time it was open for the lifters. I would be ahead if I had.

I'll keep at this until its figured out.

Any bets before its found?

Mine is a spun camshaft bearing. Makes sense... metal shavings, #3 and #4 next to each other on the cam shaft, slowly getting worse, new lifters solved worsening lifter ticks but not completely ... etc.
 
#53 ·
I went and talked to the engine shop today. I re-capped all the events, observations and diagnosis and they agree, that it's likely something has happened to the camshaft. They are stopping short of my level of confidence about the problem as I see it (a spun camshaft bearing), but they are sure something inside the engine has let go.

So, I'm pulling the engine this weekend.

The shop has agreed to re-man the engine fully if the 4k miles of metal shavings has caused damage (duh). They are also going to let me watch the tear-down so I can see first hand what's been going on. Pictures to come.

My enthusiasm for seeing light at the end of this tunnel is tempered by the fact that "The fat lady hasn't sung yet", but at least I feel as though I'm moving in the right direction.

Thanks for letting me "sound this out" on this forum. I fear I've been a pain with this thread popping up all the time. Sometimes I get self conscience about talking to myself in front of others. :ty:
 
#54 ·
6 hours later... :banana:

6 hours.jpg


The mid-front cam bearing looks good...

front-mid bearing.jpg


The mid-rear bearing looks like trouble. Overheating and a gap. :think:

rear-mid cam bearing.jpg


rear-mid cam bearing2.jpg


It will be interesting when it's removed. The shop said they didn't want me to pull it since it's under warranty, but I can watch as they pull it out.

When it's slung I might get a better picture.
 
#55 ·
Final post on this... engine was toasted. Compression rings flat and washed out. Pistons collapsed 5 thousands. All cylinder walls scored. All valve guides run out. Valves has star pattern of blowby. All valve seals leaking oil. Main rear seal leaking. Cam burnished on lobes. Front seal on harmonic balancer had cut into balancer shaft. The list goes on and on.

Found #3 cam bearing spun at the race to block seat. Cam bearing stuck to camshaft. Much metal shavings in the oil.

Warranty engine at hand and should go back in this weekend. It too has corrosion issues at the cooling ports. I might start another thread on that repair.

Anyway, I'm back to square one and hope this nightmare is soon over.
 
#58 ·
A final post...

The warranty rebuilt engine is running today. No more idle misfires. Not perfectly smooth idle, but way better and acceptable.

BTW, this is the oil plug magnet at 3200 miles, 4x oil changes, 1200 miles on the last change. I'm looking forward to seeing a much cleaner magnet on this engine.

Oil plug at 3200 total miles (1200 since cleaned):

3200 miles metal shavings.jpg


Oil plug magnet cleaned and ready for this 'new' engine:

cleaned magnet.jpg
 
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