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Mike65

ZETEC MISFIRE UNDER LOAD

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Late last year on a track day, the 2 litre black-top in my Mk1 Corty developed a misfire under load, knocking about 30 mph off the top speed and particularly an issue up inclines. I found the throttle pot was not reading, so changed it and that works ok. Changed the plugs. Leads seem fine. The engine develops a misfire under any load in any gear but none when not under load.  Can't put it down to any single cylinder dropping. Does this sound like a coil pack issue? Any other ideas?

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Not plugs. Not coil pack. Not leads. Beginning to think it might be fuelling (twin 45s) but playing around with fuel pressure hasn't made any difference. Going round in circles............

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This is driving me nuts. Changed plugs, coilpack, leads. Checked carbs. Got fuel, got spark but No.2 cylinder is not running all the time. OK off-load but seems to stop firing under load. Only happens on this cylinder. Changed and checked the throttle position sensor.  It isn't fuel supply. There is a spark at the plug, but much of the time the cylinder doesn't fire, the header stays cool.

HELP >>>>>>>>>>>>>>>>>>>>>>>>>>>

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Have you checked compressions? Is it on hydraulic lifters if so any rattles any undue noises ? Cam lobes on no2 cylinder

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How do you KNOW its not fuel? Could be a blocked jet? If you've changed most of the ignition then that leaves fuel! Seems very cylinder specific so that's the one I'd be looking into. Also no load running / revving don't mean jack - has it been RR dyno'd for carb and Ign?

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As has been said before. Check the compressions as a start. Most Zetec misfires are from Ht side of things so leads and coil pack. What are you using to control the ignition?

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Update:  Got it on a rolling road. Don't know if its because the rollers don't apply a 'normal' road load but the engine ran pretty much ok for the whole time. Webers were re-jetted and it seemed to be running fine until it was back on the road when the problem was immediately back again.

So I've changed the coil pack, swapped the leads, new plugs. Carbs are working perfect, all stripped and checked by CCK Racing in Uckfield. No2 is the only problem cylinder. On the road under light throttle, seems to run fine. Push throttle down beyond 1/4 or quickly under load and it misfires. Tested the TPS and voltage seems to be within a 1.5-4.0 range which appears to be ok.

Krypton machine shows engine is not burning fuel in No2 (so its going in alright) but there is a spark there. Plug stays dry so its firing sometimes just not under throttle.

What else is there? Surely if it was a dodgy valve I wouldn't get it to rev up to 6k off load would I?  My instinct says electrical but why only No2 going down, not a general misfire on all 4 pots? I am getting a lot of backfire in the exhaust on the over-run.

Its doing my head in.  The only things I haven't looked at are the Edis4 module. Could it be this or something with the crank sensor?

I don't want to pull the head off only to find all the valves are fine.

Grrrrrr.............

 

ps. compression check seemed fine but this is only at low revs. Should I try a leak-down test?

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Would have thought if it's to the degree of the header staying cool ie not firing correctly it would have shown up on emissions ? So if the plug is staying dry you have two things no fuel under load when it needs it or lacking in compression to make it burn effectively. Had this once on a mere Astra would run fine till put under heavy load and it had a slight blow by between cylinders 2/3 on head gasket. In my experience when crank sensors play up they show up more on starting.

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OK finally got to the bottom of this problem and it makes for interesting reading. It was the 45 Weber. Having checked all the jets and stuff, I went back to the venturi opposite No.2 cylinder. This had come loose before and started this whole problem off. Once the venturi was re-tightened, the problem went away but only for a short time, then re-appeared. Swapping the No.2 venturi into another barrel moved the problem to another cylinder. There is very slight distortion of the venturi where it was misaligned when I tightened the grub screw and missed the locating notch. For some reason, putting the venturi into cylinder 4 had no adverse effect, so I left it there. Now the engine is running fine throughout the range.

Reading an old 80's book on Weber sidedraughts I found that the 45's in particular are susceptible to minor distortion in the venturi and can cause poor running on the relevant cylinder. Seems this was my case.

You live and learn.  

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Hi Old Skool Newbie,

My first post! THANK YOU!!

I've spent £2,000 trying to sort out the misfire at c. 3500 revs, had my Tiger Super 6 (sorry, not a Ford, yet) at CCK and another garage and browsed endless other forums for help in identifying this really annoying problem. Read in some posts that you just have to "live with it"! Really? No way. Switching cylinder 1 and 2 venturi in Weber 45s has done the trick. Now no flat spot / spluttering / hesitation. 

Only concern now is that as no 'limiter' at 3500, that it'll goad the right foot downwards :-0

Seriously though, thanks for sharing your fix!

A local from Henfield in red Tiger but on look out for a Capri / Cortina ;-)

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