Misfire

[yzBailey]

My 720 is misfiring both coils are working all the plugs are routed correctly and have been triple checked also have new plugs and wires.

[bottomwatcher]

New quality cap and rotor too?

[datzenmike]

This happened recently or been getting worse and worse over time?

 

Under load or just cruising along steady, or both?

[yzBailey]

I just did a new head and all new timing components and it has been doing it since then but there seems to be nothing wrong in the motor and I did it all by the book, thought it was timing related but I’m not sure now

[datzenmike]

 Check the valve lash. A too tight valve won't close properly.

[yzBailey]

I recently did a valve adjustment when I put the head back on so that should be good 

[DwayneOxford]

7 hours ago, yzBailey said:

I recently did a valve adjustment when I put the head back on so that should be good 

Springs checked?

[yzBailey]

Had a machine shop resurface the head they said all the parts looked good

[datzenmike]

I would quickly confirm that the valve clearances are ok. As it was recently adjusted/disturbed it's possible that a lock nut came loose. Easy cheap fix if it is.

 

Intake BP6ES NGK

Exhaust BP5ES NGK absolutely MUST be NGK plugs, and not those counterfeit ones on Amazon

 

 

 

Describe miss fire.

 

Random all the time? Only at speed? Only under load? Only at idle.

 

If only at idle, turn the idle speed screw down and turn the idle mix screw in and out stopping when the idle quality drops off in either direction. There's probably half a turn or more where it runs fairly well, just set roughly in the middle. If idle went up from the improvement then turn it back down and repeat as many times as needed till you can't improve the idle quality and the idle is about 700. This only adjusts the idle mixture say 550-1,100 and not the entire range.

 

If random all the time, pull and inspect the plugs for a cylinder that may be struggling. It may be carboned up from miss firing, or oil fouled. Look for plugs that are different from all the others.

 

Try idling while it misses and one at a time pull each spark plug wire off. You might have to turn the idle speed up slightly to keep it running. It should run rough but evenly on a single plug but the miss should be even more apparent. When you unplug the 'bad' wire/plug it should idle better than the other 7 . 

[yzBailey]

I’ll check the valves and try pulling the plugs and see what happens, it’s missing all the time consistently when idling when under load especially but it’s pretty consistent 

[yzBailey]

I’m also running all BR6ES instead of 4 BR5ES and 4 BR6ES, but I didn’t think that would be a big enough difference to cause a misfire 

[datzenmike]

The intake side plug actually faces the exhaust valve so they use a colder #6 plug. The exhaust side plug points right at the intake valve and runs cooler and because of this they run a warmer #5 plug. A colder plug is not as self cleaning and a hotter plug.

[yzBailey]

My head gasket might be leaking I did 2 because the first one didn’t seal at all but there is drops of what look like water on top of the piston and the plugs in that cylinder are a little wet I think I might be screwed.

[datzenmike]

Pfffft. It's a replaceable gasket.

 

Did you have the intake off ???? There are coolant ports under each runner. Intake vacuum will suck any leak into the cylinder.

 

 

 

Was the head checked for being level? I use a drafting straight edge I have. Place diagonally in an X and also parallel along the length. You're allowed 0.004" feeler gauge to pass under. More and it should be resurfaced.

 

Also when bolting the head on it's

 

R..A..D

7....8

3...4

1....2

5....6

9..10 sequence. Tighten all bolts in this sequence to 20, then again to 40, and last 60 ft. lbs. They don't require a re torque but you can in a few weeks if you want. Z24s tend to blow gaskets every 100K so definitely re torque once a year. Do on a cold engine. Completely loosen only one bolt and tighten to 60 ft. lbs. Only then go to the next bolt. Do in any order you like.

[yzBailey]

I know it’s a replaceable gasket I’ve replaced it twice in the last month this being the 2nd and it appears to be leaking in the same cylinder it did last time, my book says 22 58 loosen 22 58 again which is what I did but the liquid isn’t green like coolant it’s black and there’s just little drops of it and the valve reliefs on the pistons have a small puddle in them it doesn’t really smell like coolant either so it could be unburdened gas but I’m not sure why it wouldn’t be burning when my ignition system seems to be working perfectly 

[datzenmike]

 

 

Merely saying you aren't screwed, it's just a gasket.

 

You have the head off??? Water oil or gas. Not coolant you say. Is it oil? Should be able to tell buy handling it.

[yzBailey]

It’s All together right now nothings apart

its hard to tell with the head on all I can do is use my bore scope

[datzenmike]

 Was it smoking blue? If oil droplets there may be damage to the piston ring lands and the compression would be down. Plugs would be black and oily.

 

Black water could just be coolant mixed with carbon. That would most likely be a blown head gasket or water from the intake runner(s) being sucked into the combustion chamber. In which case the exhaust might be steamy white and the rad would need unexplained topping up. A blown head gasket might also pressurize the cooling system, compression alone is at least 150 and when the cylinder fires, briefly 1000 PSI.

 

Unlikely gasoline. It would be in vapor form from the engine heat and blow right out the exhaust valve if not burned. Unlikely to be unburned with two spark plugs even if one was not working.

[yzBailey]

I’m pretty stumped I cranked it and the droplets were gonna could’ve just been me bumping the key and getting fuel in it but it’s still missing on that and I’ve verified both plugs are working in that cylinder but when I pull the wires off that cylinder it affects the engine less than all the others also put a new cap on my distributor and it didn’t change a thing

[yzBailey]

I’ll do a compression test tomorrow 

[datzenmike]

It would seem that all external things are accounted for. Fuel and air come from the same source as the other three cylinders and (you say) has spark so something in that cylinder is off.

 

Problem is intermittent. You could try...

Swap both plugs into another cylinder and see if problem moves there.

Swap both plug wires to another cylinder and see if the problem moves there.

 

 

Check valve lash so you know the valves are closed.

Compression test may show that cylinder is low and not carrying it's weight. Cause has to be bad valve sealing, bad sealing rings/piston damage or failing head gasket.

[DwayneOxford]

On 3/27/2023 at 11:33 AM, yzBailey said:

Had a machine shop resurface the head they said all the parts looked good

They check tension?

[yzBailey]

Did a compression test lowest cylinder was 168 there were all 168-180 and I gapped and put the correct exhaust side plugs in along with a new distributor and nothing changed 

[datzenmike]

Got a timing light? Clip to each plug wire one at a time. See if you can find one that does not flash when it misses.

[yzBailey]

I do not have a timing light but I did unplug both wires on each cylinder on cylinder at a time to figure out which one wasn’t firing and number 4 seemed to affect the idle much less than the others, and I’ve verified my plugs wires and coils are working so I’m not sure if it’s a spark issue