Thread: Misfire at certain revs

i had a similar problem on a 2l rover i had in the UK... it turned out to be the HT leads leaking at those particular revs.. they were a pretty new set too (about the same age as yours)... and they were leaking where a rough edge on the head casting had had only just scored the rubber surface.. not chewed through it..

dont see how that could be related to aircon tho.. unless it was dripping water on them?

probably not the case but a simple thing to check..

thanks

Steve

Originally Posted by Grimace

Well my rangie (1991 3.9V8 auto) has always had a noticable misfire if I turn the aircon on.
Simple solution, don't use the AC!

Now I spent the weekend at Bribie and the trip home was a PITA as the vehicle has developed a misfire between 2000-3000rpm.

The car has always felt abit sluggish to me, and I have always thought that it was a electrical fault related to the AC. But the misfire is very bad now even with the AC off. With the AC on it exagurates the miss even further.

So highway driving was done by selecting first gear and getting past the 2-3thou rpm barrier as fast as possible, then reving it right out to select 2nd after 3thou rpm, then revving 2nd right out to get straight into drive with the lockup on and the revs under 2000rpm.

The car has new leads, AFM, cam and timing gear all done 6 months ago.

Anyone had a similar problem or could help shed some light so I have some idea as to what could be causing the problem. I am going to book the car in with Rick but am afraid he wont be able to look at it for a week or two.

Cheers
Grimace

Try a new set of plugs, make sure the gaps are correct before you install them, when it gets dark run the engine with the bonnet up and see if you can see the leads arcing anywhere, check around the dissy cap and the coil for any arcing as well, replace if you can, go to an Auto Electrician and get him to check that the coil is not breaking down, when you get it fixed can you reply post here so we all know what the problem is, Regards Frank.

ah yes had the coil break down on a mini too with similar results and to top it off it was intermittent also.... and only at certain revs

thanks

Steve

Another thing that may be wrong if it occurs at the same rev range regardless of gear is the distributor it self, I had a crack in the hall efect that is on the side of the dizzy, it used to coff fart miss and generally drive me crazy until I worked it out replaced and it went fine.

Blythe

Question.
has the car had a Bosch alternator fitted?
Regards Philip A

Originally Posted by PhilipA

Question.
has the car had a Bosch alternator fitted?
Regards Philip A

A bosch 120 amp alternator caused similar problems in my Rangie. The higher the electrical load, the lower the revs that the missing would come in. The problem was rectified by fitting a magneti marelli alternator.

Originally Posted by PhilipA

Question.
has the car had a Bosch alternator fitted?
Regards Philip A

yes.

Originally Posted by BigJon

A bosch 120 amp alternator caused similar problems in my Rangie. The higher the electrical load, the lower the revs that the missing would come in. The problem was rectified by fitting a magneti marelli alternator.

GREAT.... just fuggen great , It's not old (12 months) and now your saying its the problem

I am goin to kill some one, if it is the alternator how can we explain why it was fine for so long until now? (well it wasnt fine cause i never used the aircon, so i never found out about the ac drama until about 3 months ago)

I really hope I dont have to turf this alt as it cost me $300 , I will be disconnecting the alt and going for a drive this arvo! Hope its not the problem... but don't have high hopes at this stage.

When you disconnect the alternator, remove the belt altogether. Just unplugging it might not work for diagnosis (I have no proof of that, just my thoughts).