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I have a miss that has me stumped....

1K views 22 replies 7 participants last post by  gmcxt 
#1 ·
My beater "farm" truck is an 84 K20 with a 454/465/205/10/14FF. i have a miss that somewhat dissappears above 2500 rpms. Its got something going on in number 8 cyl. I have verified even compression on all cylinders, no excessive resistance on plug wire, good plug, no flat cam lobes, valve springs on that cylinder dont appear to be broken. I have replaced the cap, rotor, plugs, and wires seeing as it needed them anyways, no change. Swapped parts around to different cylinders, miss does not move. I have orangish blue flame at the plug, but it doesnt look like this plug has ever fired. Both valves are sealing well for an engine that I dont know history on ( I put it in there a couple months ago, used to be a 350/400/208 truck). It ran great for a while, and just picked this miss up all of the sudden. Its real noticeable at idle, and goes away slowly as you increase the revs.

Any thing you guys can think of?? I'm no stranger to diagnosing problems, but I'm not seeing anything that stands out. Her it is! I dont really know why, but this is one of my favorite trucks I have ever owned.
 

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#3 · (Edited)
How did you check the plug? How much compression was in each cylinder, actual numbers? How do you know the cams not flat, how did you check? Have you done a cylinder leak down test? Plug gap correct, plug wire not arcing somewhere? Firing order correct? How's the condition of the distributer? Have you adjusted the valves? Wondering if a weak lifter? Double check your bases, it's easy to overlook or assume things.
 
#4 ·
Checked plug by swapping it to another cylinder, and also by cranking with the plug out and grounded.
Compression varied between 155-170psi
Pulled the valve cover on that bank and compared lobe lift with a dial indicator, was even with other cylinders.
I ran it with the cover off and tried to compress the lifter both while running and imediatley after shut off, couldnt collapse it
All the valvetrain on that bank is right.

BTW, i determined it was number eight by killing cylinders at an idle with a test light.
I haven't checked for compression in the cooling yet, as it has no heating issues, but I checked compression hot so any issues SHOULD have shown up in the compression test. See why I'm stumped??
 
#8 ·
haha!! I suppose it COULD be. the engine does sit for periods at a time. Little mouse may have cralwed up the tailpipe, thru the 'mofasters, up a header tube through the exhaust port, shit in the cylinder and is mad as fuck ever time i start it since hes sleeping in the intake.
For shits and giggles, try reinstalling the dist. 180 off and wire accordingly. That "should" rule out a pick up coil or something in the distributer.
I dont think its possible for a single cylinder to die with a failed pickup or module??? I guess one of the points could have broken off of the pickup, but then why would i have fire at the cylinder??
 
#10 ·
try adjusting lash looser and tighter on this cylinder. also, watch the plugs after dark to see if you can see if the electrode shorts out before it gets to the cylinder. between the boot and the plug is about 1/4 inch of plug, if it glows, the plug is no good. sure, it might spark, but it grounds before reaching the electrode.

check the plug wire to ensure good contact with the cable end. touching isnt enough, sometimes the audible click is the end pushing around the electrode and missing contact.
 
#13 ·
try adjusting lash looser and tighter on this cylinder. Also, watch the plugs after dark to see if you can see if the electrode shorts out before it gets to the cylinder. Between the boot and the plug is about 1/4 inch of plug, if it glows, the plug is no good. Sure, it might spark, but it grounds before reaching the electrode. if it was the plug itself causing the problem, the miss should have followed to the other cylinder when i swapped the m around????? Will play with the lash if nothing else gets me anywhere, but its perfect as is.

check the plug wire to ensure good contact with the cable end. Touching isnt enough, sometimes the audible click is the end pushing around the electrode and missing contact.its definetly locked down on it tight and making good contact, i skinned the shit outta my hand trying to pull it off several times
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#11 ·
Seeing as I work 2nd, I'll fire it up when I get home and look for arching. I have fought many a miss, but this one has me beat at the moment. I have never had a head cracked in the combustion chamber that didn't cause an overheating issue, anyone ever expierence that?

If I see no arching, i think i'm gonna swap wires on 8 and 7 and see if the cylinder will fire, even if its in the wrong order. The way the plug looked, its almost if no fuel is getting to that cylinder. it had no discolor and was not wet at all. Literally looked brand new. Maybe there is a fucking mouse in there:(

Might be time to bust out my fancy dancy bore scope and drop it down the intake port??
 
#15 ·
Edelbrock performer 2.0. I know very little about this engine. It was pulled to do a cummins swap. I did hear it run and drive it before it was pulled , but the PO bought the truck specifically to do the 12V swap and had no knowledge of its history. Looks to have been overhauled at some point in its life based on the fact that it is pretty clean inside and has positve style valve stem seals. This miss just showed up in the last week or so. I have had this engine in this truck for 5-6 months now, but few miles on it.
 
#20 ·
So I found it. The nipple on the intake that the brake booster hooks to had broken. Except that it was still standing up and looked solid. Whan I hit the intake gasket and carb base gaskets with carb cleaner, nothing happened. I never thought to look at that nipple, since it appeared intact. Seems odd to me that just one cylinder was lean because of it though.

anyways, replace that pot metal piece of shit with a brass one and have no miss now. Thanks for making me look twice at that area Grumpster!!!
 
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