Been trying to figure out a P0304 misfire for a while. Checked spark at #4…good. Replaced plugs and distributor cap/rotor. Converted fuel injection to MFI…thought the old poppet injectors might be clogged. Made no difference. Compression test showed 90 psi in #4. Other cylinders were 105-110. Leakdown test…heard air loudly out of the throttle body (no bubbles in coolant, almost nil from exhaust/oil fill). I had #4 at TDC for leakdown but I’ve heard the intake valve opens just before TDC? If compression is 90 psi, could the problem still be piston rings? A little bluish-grey smoke appears on startup and gets worse…smell of unburnt fuel is terrible. Hard to believe just one cylinder isn’t burning fuel, the smell of gas is so strong. Just took intake manifold off. Doesn’t look like the gasket is bad (and the silicone sealant on the front & rear lips were so solid, I could barely get the manifold off). Valve springs and seals/guides look good. Rods aren’t bent. No “ticking noise”…just a misfire. #4 intake rod can be pushed, and “springs” back about 1/4”(lifter spring) but the exhaust rod is solid. I’m at my wit’s end with this motor. Someone please help!

 

Based on your description, #4 intake lifter is collapsed for whatever reason. Maybe flat cam lobe, maybe low oil pressure.

 

I thought maybe it was collapsed, but I’ve turned the motor over by hand probably 4 times since it’s been sitting, so I thought oil might have been “pressed” out? If it is collapsed, that means the the valve would open a bit late and close a bit early, resulting in a lean condition, not rich? I’m kinda new to these deep motor parts, so my logic might not be sound…

 

Well, if the lifter is collapsed when engine is running (or right afterwards), then lean or rich is not your biggest issue and might be an effect rather than a cause. Fix the lifter or don't worry about anything else. If you are interested in comprehensive misfire testing, Google "Running Compression Alldata" and read.

Exactly how poorly does this run? Will it idle? Video?

There is no reason for one collapsed lifter, especially when on the cylinder identified as a misfire. Take the rocker off of #4 intake and remove the lifter. Inspect the bottom. At least I think that 98's are flat tappet lifters. Someone please confirm.

 

I’ve included some pics, below.. and as luck would have it, a short video of starting the motor by mistake during my first compression test on the #4 cylinder (I had no idea pulling the fuel relay doesn’t shut the fuel supply down, just doesn’t prime it). So you can hear the motor wants to run, even on 5 cylinders (gauge is plugged into #4 spark plug hole). When I first started on this, before the MFI conversion, it idled, ran, and I could have probably driven it, but was afraid I would really cause damage because the miss was noticeable (whole vehicle vibrated). When I pulled the plug wire to #4 though, the miss was twice as bad. That was almost a month ago (had to wait for warm days off to work on it) so it’s been sitting a while. I will put new lifters on, though I’ve heard you shouldn’t either use new lifters with an old cam, or a new cam with old lifters? Again, I’m just going on what I’ve read from others…I don’t really know. I’m thinking about replacing the distributor too (not just the cap & rotor). Some others have said that a worn distributor causes odd problems like misfires. I’ll definitely start with the lifters though. Btw, that recommendation about running compression tests was very interesting. Wish I had known/done that first. Thank you for all your help. I really appreciate this!




#4 Intake rod and lifter on left..exhaust on right. Hard to see, but there are rollers under the flat bottom.

#2 (left) and #4 (right) rods, lifters

 

I wouldn't jump into replacing lifters. If you pulled a plug wire off #4 and it got worse, then #4 is not misfiring - it is somewhere else. And definitely roller lifters, not flat tappet lifters - so not a lot of chance of cam lobe being worn. Let me think a bit.

 

When you did you leakdown test you said you heard big air back through throttle body for #4. Could you have been at TDC for #4 at beginning of intake stroke instead of end of compression stroke? Desired position only happens every 2 revs of the crankshaft in a 4-stroke engine.

 

We seem to have some conflicting evidence for your misfires and having the intake off complicates things a bit. Rather than shoot the parts cannon at it, I would prefer to help you with verifying condition of your rings, heads, and valvetrain before you put that intake back on. This will involve re-doing some of the things you already have done plus a lot more. If you are game, let me know and I will try to help you. But please no short cuts on what I ask you to do. Let me know.

 

I’m definitely game to try whatever is logical. I was confused too when pulling the plug to #4 and feeling/hearing the miss get worse. I figured maybe it was missing half of time for whatever reason we haven’t discovered yet, but taking spark away killed it completely. When I did the compression test, I did make make sure it was on the compression TDC…(the distributor was pointed 180 off the “6” mark on the cap, so #1 was TDC intake and #4 TDC compression). Other thoughts: 90 psi in that cylinder means the rings are good though, right? All the rest at 105 or above. And I’m not sure what the very strong unburnt fuel smell with blue/grey smoke out the exhaust means…other than possibly more than one cylinder is missing and shoving fuel right to the Cat. At first the smoke is light and not very noticeable…then after a minute it turns thick (cat trying to burn it, I guess). What are you thinking that I need to redo?

 

I would like you to repeat the test where you pressurize #4 cylinder with compressed air. But this time completely remove both rockers from this cylinder. Report results please.