3.9 misfire on 1 pot - can't find fault

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MattMatt

MattMatt

Active Member
Posts
122
Location
Swindon - ish
How can I check the injector wiring on the 3.9Efi. I have a problem with one cylinder that won't fire and it is driving me mad
So far I've checked and done:
  • New plugs and leads
  • Good spark at all the plugs
  • Nothing is wrong when you run the tests on the ECU Plug as per the book
  • The injectors are all working - I've had them out and cleaned them on the bench
  • Compression on all pots is good
Still I have a problem

The engine starts and the one dodgy cylinder (it's no 1) seems to fire very occasionally just once in a while so I am suspecting the wiring to that injector is broken

Can I check it with a meter? If so how and what voltage do I need

Any help appreciated
 
basics first , what brand of plugs, leads , cap and rotor did you get? did you change them all at the same time?

lots of stories of dodgy bits so let us know and we can go from there!
 
OEM Cap and rotor, Magnecor leads, NGK sparkys (can't remember the number but the recommended ones) all replaced at the same time. The spark to all the plugs is good and strong
 
hmm, should be any probs there mind you I have heard of ngk's (and others) even when new being a nightmare. my mate changed everything else and I mean everything and was at the point of giving up when he put his correctly gapped new ngk's on a spark checker and found they were sparking totally wrong. new set of champions (i know these can be dodgy too nowadays) and all was well.

I run ngks as both can be ****e lol.

so did this problem start after you fitted all these bits? are the oem bits the lucas ones? as they are not the same- only gen L/R cap and rotor in LR boxes will do. I learnd this the hard way :doh:
 
There is no problem with the sparkys. If you swop the plugs around or the plug leads it still won't fire on No1. I'm almost convinced it is an injector problem - just need to know how to check the wiring as the continuity test via the ECU plug seems ok
 
If you suspect the wire running to the injector, try plugging in a noid light. Would be the best way to confirm or deny the wire and/or ECU signal;
here's one on ebay (I am not connected with the seller, though have used one and can thoroughly recommend);
Noid light Bosch EFI | eBay
 
Thanks Joe - I've ordered the Noid light and will see if that is the problem. Never heard of these before so thanks for the info
 
they look like a good bit of kit.

and cheap enough to eliminate easily.

you said you had tested the injector itself? I agree it only leaves you the wire to it and they can look pretty scabby after 20 odd years on the road.

if its not that though then dont think it cant be ignition side as its been proven many many times on here.
 
fett said:
they look like a good bit of kit.

and cheap enough to eliminate easily.

you said you had tested the injector itself? I agree it only leaves you the wire to it and they can look pretty scabby after 20 odd years on the road.

if its not that though then dont think it cant be ignition side as its been proven many many times on here.
Click to expand...

Yeah, those two lil bits of kit have proved there worth for me so far. I had similar issues with injector number 8. I ended up replacing the actual injector and all was well after that.
Obviuosly the kind of gear a well sorted specialist injection centre/ garage has will give em info about each injector, how well its opening, time open then how it closes etc....but to be honest if it gets that far I'd be inclined to seek professional help.

Incidentally, if you end up disconnecting the fuel rail to remove an injector, get a new o ring set to replace the seals on ALL the injectors. I ignored this to start off with, and starting up produced a tidal wave of petrol running down over the manifolds!:eek::eek:. Fitted new seals as I shouldve done in the first place, and touch wood, all has been well ever since.

Good luck with it
 
Just one to throw in if you have practically eliminated the electrical side what about the mechanical? Worn cam is a fav number one is very prone to wear how many miles have you done
On the electrical side the ecu switches the earth on the injectors to pulse them.
Also you can use a multimeter to do a continuity test on the injectors not the ecu as it can harm it, check a lll the injectors you can to see if it is a winding failure fault rare but do happen
 
Don't think it's mechanical - the rockers and pushrods move up and down the same amount as the others. I have had the cam problem on a 4.0L and it ticked badly - this one doesn't

I've had the Noid light on and all the wiring seems fine. I've also had the Injectors off again. the solenoids pulse and you can get carb cleaner through them.

What I didn't do is take Joe's advice and replace the seals so when I hooked it all up again I had the fuel fountain. (I'll never learn). Seals now on order.

The continuity test on using a multimeter through the ECU plug was fine 4.6 ohms and I unplugged the offending injector and the resistance changed accordingly.

I wait for the seals and try again. Hopefully recleaning the injector has done the trick
 
Ok I'm going to hold my hands up now........

I got all the injectors off and made up a brill test rig, cleaned them all, worked them through on the rail etc etc etc.

Got it all back on and still got a misfire - bugger!

Started at the beginning again with the ignition.
Plugs - check
HT leads - new
Dizzy - checked
Timing - WTF!!!!
No2 and No1 leads were back to front on the Dizzy. Swapped it over the BINGO it runs just perfect.

So before you all say it for me
I'M AN IDIOT!!!!!!!:mad:
 
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