2k-3k misfire!

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discomark10

discomark10

Well-Known Member
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Location
Milton Keynes
my 91 disco 3.5 efi a while ago developed a misfire between 2000-3000 RPM
I cannot for the life of me figure out what is wrong with it.
I installed a Volt meter to see what was going on.
It seems that if the voltage is below 13v it starts to misfire and kangaroo between 2k n 3k revs.
I've done a pressure test and all the cylinders are pretty good.

so far i have replaced...

Spark plugs
HT leads
Dizzy cap
Rotor arm
Ignition amplifier (lil box on the side of the dizzy)
Coil
Fuel filter

And yet the misfire is still there...

I'm starting to lose hope and cant think of anything else i can test or replace.
Any help at all would be appreciated.
 
Its kinda odd with mine mate, in that if i hold the revs up in neutral it misfires, yet if you rev it up its absolutely spot on.

it generally causes no problems though, and drives fine, but i do think it misfires if at steady revs.

im suprised the voltage is causing a problem, at 13 volts it should be fine.

check the ignition timing, make sure it is advancing properly as the revs rise.

if the igntion timing is stuck retarded until after 3k where it suddenly advances, that could cause your problem.

cheers.
 
well...

tbh mate if you're having an issue where the vehicle is missing when there are heavy electrical loads, id be checking the alternator.

one day by alternator packed up, the car drove for about an hour before the voltage got too low and it started cutting out.

it would have been igntion or fuel pump.

either check/replace/borrow another alternator, or fully charge the battery and then run the car on a battery charger, and see if it makes any difference.

if you get an old style non electronic battery charger it should keep the voltage up whilst the car is running.

monitor you voltmeter and try the fans again and see what happens...

cheers.
 
the igntion timing is tested with a strobe light.

borrow one off someone who knows how to use it.

remove the vacuum advance from the dizzzy, and as you rev up the car the mechanical advance should advance the timing fairly smoothly.

any major jumps and you timing will be out until it jumps again you see, and at this point before the jump you could misfire.

cheers, sam.
 
well, ive cleaned the alternator out and replaced the brushes i get around 13.4 - 13.8 volts on idle and up to 14.2 volts while driving.
 
Last edited:
Mr Noisy said:
well...

tbh mate if you're having an issue where the vehicle is missing when there are heavy electrical loads, id be checking the alternator.

monitor you voltmeter and try the fans again and see what happens...

cheers.
Click to expand...

When i put the duel 16" fans on it drops by about 0.5 -1 volt.
 
I'd say gotta be a lame alternator then.

My fans are 80w each, they draw less than 10 amps each - alternator should he able to handle that and keep the voltage up...
 
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