I just purchased a D150 that has been sitting for around 2 years..The motor sounds good but has a misfire on cylinder #5 at any rpm..The spark is good and i pulled the valve cover all rocker arms are on..I do not see any thing such as a broken valve spring..Someone has replaced the manual pump with a electric fuel pump..I guess my question it could low fuel pressure perhaps cause this or possibly blown intake gasket?I sure hope it's not a burned valve but i guess if it is i'll just have to tear it down..Just looking for some ideas ..Thanks in advance i am new to the forum..Pic below got it for 300 bucks
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Check for a flat cam lobe.

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1976 Feather Duster /6 4sp
1984 W100 318 727 np241
1972 'Cuda 340 4sp
1985 D250 360 46RH

Could be a lot of things, bad valve, bad seat, gasket leak. Do a compression check first and go from there.

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Official Cookie and Mater Tormentor.

Will do compression check and get back with you

On cylinder #5 only has 8lbs of pressure...The rest have 150 lbs ..Apparent burned or bent valve..Piston looks ok with my snake camera..

Wow, you have a snake camera?! Do share...

Anyway, it could be a stuck or warped valve, broken rings, or (ugh) a cracked valve seat...hopefully it's just a head gasket. A leak down test will tell you which path(s) through which the gases are escaping. Did you do the typical diagnostics, check the coolant for bubbles, excess water in the exhaust and/or coolant in the oil (emulsion) - characteristics of a blown head gasket?

Brian

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Have not done leak down test but coolant and oil look very good..Engine never has smoked and had no blowby in the crank case..

When it's cold, open the radiator cap and fire her up. Do you see bubbles in the coolant? this would indicated a blown head gasket leaking into the coolant jacket...

Don't open the radiator cap when hot!!!

I suppose if you have to pull the head, you'll know if the head gasket is the issue or a flat cam lobe. The fact that there is no compression in #5 is telling.

Brian

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You could pull the rocker arm assembly, the #5 push rod and lifter to see if that lobe is flat. If you pull more than one lifter, make sure to put them back in the same spot from which they came.

brian

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Are the valves adjustable on all slant 6's?Mine is in a 86 d150

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No. All slant sixes were built with hydraulic lifters beginning in 1981. If your truck has the original engine then it will have hydraulic lifters.

Try to use your camera to check for cracked valves. I find cracked valves on hyraulic lifter era slant sixes more than the earlier years. I think it has something to do with lean burn ignition and carburetions systems failing combined with the air injection right on the exhaust valve.

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Casually looking for a Clifford hyperpak intake for cheap.

Ok i removed the valve cover and started the engine..Still with #5 Missfire everything looks ok but one thing i noticed..#5 exaust valve is Not Oiling at all ..Others are fine..

Pulling the lifter won't be that easy on a peanut plug head... he will have to pull the head.

At a minimum the oiling problem needs to be dealt with. The rocker arm should be removed from the shaft and the oil passages cleaned (keep all parts in order!).

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Casually looking for a Clifford hyperpak intake for cheap.