v8 only firing on 7 not 8

[gtomo2]

Got a 3.5 efi v8. but its only firing on 7 clys. not played with one before so before i go checking the normal things plug leads, sparkplugs, dissy. is there anything else i should check? its got lpg but mis-fires on both.
Thanks

[burgerman]

had a similar rpoblem a little while ago, checked/changed plugs,leads,dizzy cap,  checked compressoins,  all ok n no difference,    to cut a long story shorter, it was no1 injector blocked/knackered, it got changed, and bobs ya uncle,  where abouts are you located in this world ??  if ya is in travelling distance(on 7 cylinders) of me (near basingstoke, but not toooo close)  i could cast me eye over it??   but most of it should be straight forward enough to check/replace,  good luck n keep us posted,                         if  all electrics are ok, when its running, disconnect one injector at a time , and what ever the one that doesnt make any difference is the faulty cylinder

[gtomo2]

me a bit far up from you. but thanks for that wil have a look at it tomorrow. hope its something nice a simple can do without the hassle of stripping things off and trying to remember were they came from

[bilge rat]

first find which one its missing on, then swap things from another cylinder one at a time see iff the misfire moves to the cylinder you borowed the parts off. certainly a cheap way of doing it . may take longer though. good luck. alan.....

[gtomo2]

Right played with her today and its still missing check the spark plugs cant see any that are fowled up. but dont have a workshop manuel for her and never worked on a v8 with ingetion before so anymore pointers would be very well recieved. or anyone local to me who is doing nothing tomorrow fancys a trip to swadlincote to come play with it and show me around the engine would be nice
thanks

[disco-v8]

ok this might not be the answer to your problems, but after a good sort out on all my engine conponents, i forgot to plug in the air flow meter (which is atattched to the air pipe to the air box) and it seemed to be running on 7 cylenders aswell, but it was actualy running on all 8.... it must have been running a crap mixture....... it also spat out alot of black crap from the exhaust

asoon as i plug it back in it ran sweet as a nut

if there is a way you can test it then might be a good place to look

hope this may help

[Range Rover Blues]

If you get really [!Expletive Deleted!] off with it I'm up in Sheffield.  Time's a bit precious at the moment though so PM first if you are getting real stuck.

[abooth2k]

If its missing on both gas & petrol it can only be either spark or compression; you can tell which cyl it is (this may not be a recommended technique, but it works for me) when cold, start the engine up, and touch each branch of the manifold, one is likely to be noticably colder than the other NOTE ** ONLY DO THIS FOR ABOUT THE FIRST 10 SECONDS WHEN THE ENGINE IS RUNNING!! ** or a slightly more technically advanced way is one of those fancy infra red thermometer thingys, fingers work for me tho!! once you've diagnosed which cylinder it is, swap a plug with another cylinder and see if it helps, if not, as you say, standard plugs, ht leads, dissy cap, rotor arm, as its a 3.5 i am pretty sure they stall if you disconnect the air flow meter as suggested by another poster, however this can affect 3.9 (the system drops onto "limp-home" if it detects a fault such as a missing air flow meter...)

[Xtremeteam]

If its missing on both gas & petrol it can only be either spark or compression; you can tell which cyl it is (this may not be a recommended technique, but it works for me) when cold, start the engine up, and touch each branch of the manifold, one is likely to be noticably colder than the other NOTE ** ONLY DO THIS FOR ABOUT THE FIRST 10 SECONDS WHEN THE ENGINE IS RUNNING!! ** or a slightly more technically advanced way is one of those fancy infra red thermometer thingys, fingers work for me tho!! once you've diagnosed which cylinder it is, swap a plug with another cylinder and see if it helps, if not, as you say, standard plugs, ht leads, dissy cap, rotor arm, as its a 3.5 i am pretty sure they stall if you disconnect the air flow meter as suggested by another poster, however this can affect 3.9 (the system drops onto "limp-home" if it detects a fault such as a missing air flow meter...)

alright adam  

[gtomo2]

Thanks for the heads up. will be having another look at it. got it running better with adjusting the timing. will try the cold manifold touch.
Thanks agin

[Range Rover Blues]

What I'd suggest is you soak the manifolds with water before you start it up and see which if any don't heat up.

BTW whilst it seems logical that a missfire on both gas and petrol can't be down to either fuel system, it could be down to a leaky injector.  Check the colour of you plugs.

[Bulli]

Right, buy a set of spark indicators. they plug in between your ht lead and your plug. this will let you see straight away if you have a spark or not. Eliminating things is the key.
First sparks...if you are missing a spark on 1 cylinder only it can only be the plug , ht lead or the dizzy cap.
now check fuel. The easiest way is to remove the plugs and clean them then run it for a minute and take out the plugs. They should all look pretty similar . If one varies massively this is the problem cylinder.
Check the injector wiring for that cylinder, if that looks ok then check the compression on that cylinder.
Are you 100% sure it is only running on 7? V8's do have their little foibles and can sometimes change note or sound quite a bit. Im in Rotherham and could spare a hour or 2 if you fancy a run up.

[Range Rover Blues]

TBH If i'm trying to detect a misfire by disconnecting the injector plugs I have to pull 2 or 3 before I can notice a drop in revs.  V8s are so powerful at idle.


BTW, I stuck Rovacom onto the LSe the other day and it reported a missfire on bank B.  Bludy hell even I hadn't noticed it.

[gtomo2]

quick update on the misfiring. having got the exhurst silent now i can hear the engine and there is a knocking sound coming from the same cyl that was blowing badly. so me thinking maybe a broken or very out rocker arm. so will be taking the rocker cover off this week and seeing if its that. good job a have a spare head for it

[Range Rover Blues]

Weak/broken valve spring
blocked hyrdraulic lifter of faulty the same
worn camshaft
piston slap

[gtomo2]

Weak/broken valve spring
blocked hyrdraulic lifter of faulty the same
worn camshaft
piston slap

Sounds like tappets so it could be one of the first three. but dont think or hope its piston slap :roll:  but knowing my luck

[Range Rover Blues]

Piston slap is a lower pitched knocking, not as deep as the rumble of the big end but sometimes confused with it.

If ity id, don't worry.  as long as you have good compression adn don't use any oil, ignore it.

Valve train knocking has more of an, how can I put this, urgency to it? it's sharper somehow.

Oh, piston slap will get better when you stop the effected cylinder from firing, try that.

Some people recon you can detect play in the valve trian by putting pressure on the effected rocker, or stuffing a feeler guage in the gap but on these engines look for a pushrod/rocker with no preload at all, otherwise check the lifters for sideways play (sorry but that's irreparable).

[gtomo2]

Yea think it is tappet as it sounds like my old hillman sunbean but only one tappet not all of them. plus it does not get louder the higher you rev it and its only there when you press the loud pedel. but not when free wheeling. first i thought it was the exhurst blowing again but it dont get louder with revs it. think i will just put this other head on and have done. Thanks for all your help so far. and congrats on the wedding :D

[Range Rover Blues]

it does not get louder the higher you rev it and its only there when you press the loud pedel. but not when free wheeling.

That sounds more like slap then, the valve train is not loaded any higher when you push the gas pedal.