Misfire at certain revs

[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

[5teve]

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

[Tank]

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.

[5teve]

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

[blitz]

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

[PhilipA]

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

[BigJon]

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.

[Grimace]

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

yes.


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 :mad:, 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 :mad:, 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.

[BigJon]

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).

[Grimace]

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).

mate all i did was remove the belt ;)

and seems the alt is my problem :(:mad: as the misfiring is no longer there...

[Pierre]

PhilipA gave me a clue about a month ago - that was to shield the chopped signal in a shielded cable. He suggested the BW cable be shielded from the distributor plug to the coil plug.

It's being done this coming weekend and I'll report the outcome.

BTW, it's got a Bosch alternator. The miss is at about 2200 rpm.

It will be much cheaper than an alternator - at about $3.

FWIW

Pete

[PhilipA]

And the shielding must be earthed at both ends.
Regards Philip A

[Lucus]

Also doesnt hurt to twist the wires at aprox 1 twiwst per 25mm of cable. This helps to shield the signal from noise.

[GeorgeZ]

Exactly my problem. I retrofitted a bosch alternator after I couldn't get a spare regulator for my MM. I have a miss at 2200 rpm esp when I increase the electrical load, air con, lights, etc. Took the alt belt off, prob went away. Interestingly not noticeable when running on LPG, only on petrol. I haven't fixed it yet, as it is fine on gas. But will in near future. Tried the extra shielding/earthed at both ends as PhillipA suggested to no avail.


93RR3.9

George

[Grimace]

But how bad was the miss you guys were experiencing? Mine is extremely bad, as in if i had it in drive and tried to drive normally it would never get past the miss, I have to drive it hard, and it still struggles to get past the miss :(

I hope this sheilding works as I would much prefer to keep my alt, I might just shield everything :D

[PhilipA]

I also suggest that you route the wiring away from the alternator, eg run the shielded wire straight back along the injector wiring on the LH bank to the firewall then along the LH guard.
I have 2 Bosch alternators and no problems.
My wiring is shielded and goes back to a Unichip piggybacked on the ECU, then forward along the LH guard to an amplifier mounted on the LH guard then to the coil.
Regards Philip A

[Grimace]

So what size shielded wire do you reccomend and where abouts did you earth the ends to??? I am assuming I am just to replace existing wiring with higher quality shielded core. Sorry I am a noob when it comes to motor vehicle electrics.

I can wire up a stereo and maybe an amp or two but never done anything else. But would really like to learn :cool:

[PhilipA]

I didn't install mine and it looks a size that is not readily available. Maybe it came with the Unichip.
AFAIK , you can get 2 sizes at Jaycar, and both are intended as speaker cable (aside from the TV antenna cable). I would use the larger size . If you can only get two core, just pull one wire away from the other to make a single core.
To earth the outer, I usually bare about 80MM and twist it into a wire. I then seal the end where it enters the outer insulation with shrink tube. It should then be earthed as near as possible to the source, eg a mounting of the amplifier on the distributor or another convenient bolt and on to the coil mount.
I didn't do mine but that is how I did my O2 sensors.
Regards Philip A

[Grimace]

two easy mate thanks :cool:

[GuyG]

Hey Grimace

Could also be a computer or as simple as an earth issue