M20 Weak spark, misfire on cyl 5 only

[schlafr]

Haven’t had the car on the road as I bought it with a head gasket leak… replaced head gasket, decked head, valves adjusted, valve seals replaced, replaced all sparkplugs wires, distributor and rotor, all of that… had the car running somewhat fine until I magically now am getting a misfire on cyl. 5, already swapped injectors, checked compression, checked and changed spark plug, swapped coil wires, thing still doesn’t hesitate when I remove coil wire from cylinder. Makes no sense at all as I’ve replaced a lot

Eyes raised on the CPS or coil itself but it should be misfiring on all if it was the coil… I don’t know what else it is as the head is fine and compression is good, any ideas lmk, maybe CPS?

[tomstin]

Have you checked the wiring to the injector on #5? Agree, strange symptoms.

[packratbimmer]

Coolant getting into cylinder? Pressure test?

[bangn]

what colour is the plug on number 5? is it fouling compared to the others?

[schlafr]

Have you checked the wiring to the injector on #5? Agree, strange symptoms.

Yup, checked ground and and positive continuity, no wires internally messed, like I’ve checked everything and this is making no sense lol…

[schlafr]

what colour is the plug on number 5? is it fouling compared to the others?

The old plug I pulled was completely black, electrode and everything.

[schlafr]

Coolant getting into cylinder? Pressure test?

Yup, compression test was 120, still got decent compression.

[packratbimmer]

Yup, compression test was 120, still got decent compression.

I was meaning a cooling system pressure test.

[bangn]

Wont be an injector wiring issue unless youre unlucky enough to have a busted ev1 connector. These cars have a common power for all the injectors and the injectors fire off in pairs. So it would be two not getting a signal not just one.

If you've got a black plug then you're getting fuel. If you've swapped injectors you can eliminate a badly atomising injector.

I feel this will be either a bad lead, a badly seated/burnt valve or you have water being dragged in under the headgasket. If you have tried NEW leads on it and you've swapped round the lead with number 6 but the issue stays in pot 5 then you can eliminate an ignition issue providing youre 100 percent the contact inside your dizzy cap isn't worn way worse than the others on no5.

When you do your compression test are you testing each cylinder exactly the same and not having to crank one more than the others to get the same pressure? Tbh 120psi isnt fantastic anyway but so long as they all take the same time to crank roundabouts the same pressure thats all that matters.

Does it ping or did it ping? Did you flatten the block and check with a straight edge across the bores before putting the head on?

[packratbimmer]

I have seen M20 engines carbon up bad enough to cause a serious lack of power. I agree that 120 PSI is starting to get on the low side. BTW: are you using copper NGK or Bosch spark plugs? I would stick to copper unless you are willing to experiment.

[schlafr]

Wont be an injector wiring issue unless youre unlucky enough to have a busted ev1 connector. These cars have a common power for all the injectors and the injectors fire off in pairs. So it would be two not getting a signal not just one.

If you've got a black plug then you're getting fuel. If you've swapped injectors you can eliminate a badly atomising injector.

I feel this will be either a bad lead, a badly seated/burnt valve or you have water being dragged in under the headgasket. If you have tried NEW leads on it and you've swapped round the lead with number 6 but the issue stays in pot 5 then you can eliminate an ignition issue providing youre 100 percent the contact inside your dizzy cap isn't worn way worse than the others on no5.

When you do your compression test are you testing each cylinder exactly the same and not having to crank one more than the others to get the same pressure? Tbh 120psi isnt fantastic anyway but so long as they all take the same time to crank roundabouts the same pressure thats all that matters.

Does it ping or did it ping? Did you flatten the block and check with a straight edge across the bores before putting the head on?

I got black plugs for all cylinders, suspicions on FPR or block warped, lost cause lol, tried everything, it still pings but absolutely no change disconnecting good wire

[schlafr]

i have seen m20 engines carbon up bad enough to cause a serious lack of power. I agree that 120 psi is starting to get on the low side. Btw: Are you using copper ngk or bosch spark plugs? I would stick to copper unless you are willing to experiment.

ngk

[digger]

I am confused as to the symptoms. how did you verify weak spark on #5? is that what you meant? now all plugs are black?

[bangn]

So all plugs are fouled?? First you'll need to ensure your TPS is adjusted correctly, test your ecu temp sensor isnt faulty, ensure your ignition timing, gaps and spark plug heat range is correct.

Low fuel pressure could cause the fouling, does it buck and surge when you put your boot into it? Drop power around RH Corners?

[schlafr]

I am confused as to the symptoms. how did you verify weak spark on #5? is that what you meant? now all plugs are black?

I thought it was weak spark but it seems to be u known lmfao, it sparks but literally doesn’t fire, pull the coil wire and no difference, swapped injectors everything.