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Discussion Starter #1
Thanks for your input here guys.

I know these plenum gaskets are notorious and when I took it in to the shop a few months ago, the FIRST thing I asked was if there was any oil pooling in the intake manifold at the plenum. Mechanic said there was nothing they could see so I figured that was that.

With further thought, seeing as how I spent 300 bucks on diagnostics to get a "we-don't-know-what-the-hell-is-going-on", I figured I should look in there myself...

I went ahead and ordered a cheap endoscope to get a better view (link below - pretty freakin handy) and while I was waiting for it, I ended up doing THREE BASIC things to encourage the cylinders to fire:

-Changed the oil and put in high mileage oil.

-Put a bottle of Techron in half a tank of

-91 octane gas

It's been running WAY better since and there have been no misfire codes!!!

But, since that all felt like I was just slapping on a band-aid, AND it still chugs a bit at idle, when the scope arrived, I got a look inside the throttle body and sure enough, OIL.
(See pics attached)

Does this only get worse? Will the plenum gasket replacement just be a matter of time?
 

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fire the mechanic first and foremost. then fix your gasket problem if you dont get the hughs kit just get a thicker gasket and shorten the bolts njust a bit not very much then use blue locck tight on them suckers. i use indian head gasket sealer. its the best in my opinion if its good for steam and high presure gas its good for any gaskets .
 

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Distributor drive wear on the 3.9L,5.2L and 5.9L is VERY common at 130K+ miles and causes those symptoms, I’ve replaced well over a hundred over the last 38 years. Replace the distributor drive and bushing and it’ll probably fix your problem, you may want to check your timing chain and gears for wear
 

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Discussion Starter #5
Thanks for the help here gents. Some interesting news...

I FINALLY got around to replacing the plenum plate and gasket with the Hughes kit last weekend and all went well. Though oil was present between the gasket surfaces, there was no real break in the gasket anywhere. Not sure it had much to do with the misfires but it could have contributed. Either way, I'm glad to have the repair behind me now. Seems like it'd have to be done sooner or later.

That said, got her started up and running and she's still idling rough and fumy.

The most interesting thing I noticed this time: a knocking noise. I heard it before but it was elusive and seemed to have gone away.
It's coming from the driver side and is most apparent beneath the vehicle but a little hard to pin down.
It only happens at idle and after a few minutes of running.

After researching a bit, it makes me think it's slack in the timing chain could be the issue?
Maybe the distributor drive and bushing too like apegues mentioned?
 

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Hannibal...there's so many things that could/can cause this particular problem. I'm guessin' you want the "simple" fixes" first. Have you tried to see if there's any difference in idle with the plug wires disconnected from the spark plugs of #1 #2 cylinders, also try disconnecting the fuel injector plugs from the fuel injectore n listen/feel for any noticeable difference? Also, try running the vehicle with the sparks plugs removed from the aforementioned cylinders n see if they make a horrible popping noise (this at least means you got compression). While you got the salt to tackle such problems, try removing the upstream O2 sensor n see if the engine runs smoother. It could be your "cat" is clogged enough to cause such things. If you're not so inclined, take 'er to an exhaust shop n have them do an exhaust system pressure test. Most will do that for free.
 

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I have found that miss fires happen when plug wires run parallel.
there is a tsb about this fact. all that's needed is to reroute the wires. simple, easy, free.
factory runs 2, 4, and 6 around the back of the head, take cylinder 8 out of the guides and run it over the top of the valve cover.
other side
cylinder 1, 3, and 5 run behind the head. take cylinder 7 out of the guide and run it over the top of the head.
the objective is to separate the 6th and 5th cylinder wires from the others. these two wires should no longer run parallel and now pass perpendicular over the others. this deters bleeding and cross fires.
run the truck at night in the dark and mist the ignition system with water. look for the little rhythmic blue light flashes. if so, you need new wires all together!
how long has it been since a cap and rotor tune up?
as pertaining to you timing chain- I have had bad ones! its possible! but the distributor bushings and gears don't tend to go bad. I just pulled and replaced my dizzy with a cardone unit and I couldn't tell one from the other comparative to wear after 260,000 on the original.
 

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my wires are custom and run the exact polar opposite of the tsb. not to confuse you-
all my wires run over the top except 6 and 5 run behind the head. like I was saying the objective is to separate one from each side and run it out of the way.
clear as mud?
 

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Discussion Starter #9 (Edited)
Updates from the misfire caper...

I went ahead and ran my own compression test and all look good. Attached is a pic of the results with plugs.

I tried Spun360's advice with plug wires and uncrossed per the tsb.

Removing the 2nd fuel injector connector with the engine running doesn't make much difference (on the already chugging idle) but removing the first cylinder's connector seems to make things chug a bit more.

I tested the fuel injector connectors with a test light for power and they are all flashing. I listened with a stethoscope and the injectors themselves are all clicking away.

I disconnected the upstream 02 sensor and it just made things worse - more fumes and foggy exhaust.

I removed the 02 sensor to see if there could be any change. No dice. Still fumes. Plumes of exhaust out the exhaust pipe and the rough idle.

Cat seems fine. I measured temperatures upstream and downstream with a laser and they seem up to spec.

I'm still getting the knocking noise which I'm pretty sure is the timing chain slack. People have told me that if the timing chain was really the issue, then the misfires would be more random. I'm not sure where to go from here. Before replacing the timing chain and distributor gear, it'd be good to be sure that's the culprit.

Thanks all for chiming in.
 

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#1 &2 plugs are black compared to the others, What's that telling you? People forget old school ways of doing things and expect to just go by what the codes tell them. Get a timing light and put it on both plug wires then run the engine and see what the light looks like. Any cutting out of spark/no spark...Also put it on the coil wire to see if there is any issue there. The so called "mechanic" that looked at it should have done this! You need to be sure there is spark to both plugs when it's supposed to be before going any further. Could be a crank or cam sensor going bad(it won't always throw a code either).
 

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Discussion Starter #12
New twists in the misfire caper!

I purchased a cheap wifi OBD2 unit ($15) a while back but couldn't get it to read much on the apps I'd found. Almost bought myself a much more expensive scanner until I just started using another app called OBD Fusion ($10) and whoa, access granted...
Take a look-see:

TACH: 500-690rpm ...After warming up, a narrower fluctuation: 550-630rpm.

FUEL TRIM:
SHORT TERM immediately goes negative: -11 to - 5% then corrects when the LT settles a bit and hangs out within +/-3%
LONG TERM goes negative when it catches up (-9.4 to start) and continues to go negative the longer it is at idle. I let it idle for about 20 mins and descended to -24%. !!!
When I rev it up and hold, long term evens out and increases to closer to 0 (-0.8% after 5mins of 2500 RPM).

TIMING ADVANCE: The 1st cyl timing advance is jumping all over the place at idle between 7 and 24 degrees! (This could be seen with a timing light too.) Very erratic.
Jumps to 40 at 2500 rpm.

O2 SENSORS
- UPSTREAM: The upstream sensor startss oscillating between .2 and .9V at idle. Not the most constant, but seems like it's doing its job. Normal too at 2500 RPM
- DOWNSTREAM: The downstream sensor starts oscillating between 0.34 and 0.8. Then it stays around 0.7 - 0.9V after warming up. Not a good sign. Normalizes to less than 0.2V at 2500 RPM.

MAF: 1.3-1.5 lb/min... Settles down to 1.0 to 1.1 after warming up. Keeps steady at 2.8 at 2500 RPM

MAP: 12.7 - 12.9 inHg... Settles down to 11.5 inHg after warming up. Keeps around 7inHG at 2500 RPM

As the fuel trims adjust, the exhaust becomes increasingly clearer and less fumy but the chugging, rough idle continues.

So the question is now, why so rich at idle???

Remember: Compression is good. Cam/push rod lift is good. Distributor and rotor are new. Wires are good and new. Plugs are new and functioning. Coil just got replaced and tests better than old. Injectors are getting power and functioning. Fuel pump was replaced not long ago. Coolant temp sensor and thermostat are new. Plenum gasket is new and holding tight.

I'm thinking it's something to do with the timing on account of the erratic advance correction at idle that smooths out at high rpm. Since I'm hearing the knocking/clacking noise at idle (only after warming up) coming from the front of the engine, I'm thinking it's at least partially due to timing chain slack. At almost 180k mi, thinking I should change the chain.

It could also be partially due to high fuel pressure and/or a leaky injector. Since the fuel pump has been an issue before, I want to check the fuel pressure and see if it could be too high. Maybe it's the regulator too? Also I want to see if it holds pressure just in case I do have a couple of leaky injectors.

Other than that, anyone else think they can solve this mystery? Thoughts?
 

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First, your cat is fine, or so it seems, if the downstream steady's out in voltage, then the catalyst is actually functioning. To tell for sure, do you have one of those laser aiming thermal probes. The downstream side of the cat should be somewhere round 100 degrees hotter than the upstream...that's if your worried.
Second, the fuel trims r meaningless until the engine is at normal operating temp AND at steady load condition. Any change in throttle position, RPM or engine load will cause changes in the trims.
Third, don't sweat a timing change too much...that only means the HAL 9000 is doing it's job.
Fourth, the upstream O2 looks like it's doing it's job, when it stops moving up n down, then it's time to worry.
Fifth, the fuel trims are indicating that HAL is leaning out for some reason. The long term is a base line set 'cause it has to start at a leaner condition for long periods of time (thus the name) and the short term continues to compensate from there. If the long and short terms return to round "0" at higher RPM, but at idle they go lean, I'd say start by checking something as simple as your air cleaner and remove your PCV and crankcase breather from the intake and see what happens then. Idle is the focus now.
 

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Plug the hole left in the intake where the PCV line went into the intake and then pull it out n see if HAL corrects for the leak and record both readings.
 

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Discussion Starter #16
Haha, thanks Jet. Spending some more time with HAL here...

Disconnecting the pcv line to the intake I immediately get a smoother situation, timing advance stays at 0, trims are looking way healthier.
But the idle is way higher now and rpms go up and down in a 2 second rhythm between 900 and 1000, oscillating with the MAP and MAF readings.

Intake temp is about 155 F. Engine at 203. Outside it's high 50s.
Also, pcv valve and hose and air filter are new.

Plugging the pcv hole at the intake kills the engine. Not immediately sure what this means.
 

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Discussion Starter #17
Instead of suddenly plugging the pcv intake, I tried slowly plugging it instead. The engine slowly adapts and mellows out and returns to the really rough idle like before.
 

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Intake temp...waaaay too high for that outside air temp...pull the intake air temp sensor n see if it's soaked in oilly residue. If it is, clean it off n try it again, if not...replace it. The intake temp shouldn't be anywhere near that hot on a 50 day, ya know?
 

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If HAL thinks it's that hot, he'll do something he shouldn't..."I'm sorry Dave, but I cannot open the podbay door". Now,...I'm guessin' here but, about 60-80 degrees cooler n much denser air is present in the intake if your numbers are correct, yet once again HAL is being lied to by his sensors and HAL doesn't like being lied to Dave!!!
 

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Discussion Starter #20 (Edited)
Intake temp...waaaay too high for that outside air temp...pull the intake air temp sensor n see if it's soaked in oilly residue. If it is, clean it off n try it again, if not...replace it. The intake temp shouldn't be anywhere near that hot on a 50 day, ya know?
Definitely. In fact, I was the liar there - all apologies... WITH the pcv hose connected, the intake temp was 155. Disconnected, 110 degrees. Before starting up it was same as the ambient so I imagine at least we know IT'S telling the truth.
 
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