M20 Weak spark, misfire on cyl 5 only

[bangn]

TPS needs to be adjusted correctly so its on the right fuel map. ie.. not at WOT when at idle. Pretty common on these cars to be full of oil and muck.

If your fueling is off and you have fouled plugs then its likely pulling leads will show no difference as tested. Needs the right mix of air and fuel. Also to note fouled plugs will not clear themselves up and will need cleaning up to continue testing.
I trust you can hear the spark jumping when you pull and push the lead back on? I assume this is the distributor type on the LH side of the block?

How hard is the fuel hose FEED to the rail (not the one exiting the fpr) at prime and during engine operation, it should be pretty rock hard so long as your hose isnt old as hell. If its squishy then I think somethings up with your lift pump. If it takes a while to crank over that's another nail in the coffin

[schlafr]

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?

I think your getting onto something, haven’t adjusted TPS but I don’t know why that would ultimately cause 1 cyl misfire, haven’t tested temp sensor nor heat range, I think I’m getting low fuel pressure due to a clogged fpr, haven’t had it running on the street yet but yes it bogs/stalls when I give it a blip of gas then returns to idle, only revs if I slowly push the throttle, no blips or stall.

[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.

[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?

[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?

[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

[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

[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.

[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]

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

I was meaning a cooling system pressure test.

[schlafr]

Coolant getting into cylinder? Pressure test?

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

[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]

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…

[bangn]

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

[packratbimmer]

Coolant getting into cylinder? Pressure test?