Erratic RPM

Well I did a few nice things for the old girl this week. First I changed oil in gearbox, transfer box and both diffs. Engine oil was done a month ago. Went for Mobil 1 - yes i know its synth and yes, pearls to pigs and all that but I like how some of it stays up on the cams and rockers and how cold starts are quieter. I used Dexron IID in gearbox and transfer box (as per manual but not as per later versions of RAVE, I know). Caltex EP80W 90GL5 in diffs. Slightly different from manual but closest match I could find. Next I took down the petrol tank and flushed it, servicing the fuel pump and lines too. There was a LOT of crud in there, mostly fine brown silt like stuff and all so some foul smelling stuff. First time its been cleaned in 14 years. Changed the fuel filter again, second one in 4 weeks. Lastly, fitted new magnecor leads.

Its a new car. I honestly have never had it drive so quietly, smoothly or eagerly. The only problem is at idle, sometimes, the rev meter wobbles, sometimes dropping to almost 0. Normally it idles around 700 - 750. The needle drops and then picks up again but I don't feel it from the engine. It doesn't go to stall or anything like that. But there can be a very slight intermittent vibration.

So my question, what causes RPM to fluctuate? Air leak in intake somewhere? A bad lead in the magnecor set? A bad plug perhaps? And what would be the symptoms if I accidentally got the leads in the wrong order? Would the ecu allow the engine to run at all? (I'll check this last out again in the morning). Second, what feeds the rev-meter? Crank speed sensor? Engine ECU counting sparks? Could it just be my meter that's dodgy and not the engine?

Thanks for your thoughts folks!

alternator???

i don't follow? how did you come to that?

rasheed said:

i don't follow? how did you come to that?

Click to expand...

rpm gets its feed from the alternator,if the needle is acting and the actual engine speed is not affected,this fault will be followod shortly with "alternator fault".if the engine speed is varying with the needle,put the testbook and read the faults,it could be the tps or the stepper idle.
i believe if the leads are put wrong the engine will fire but it will be very rough like missing on a cylinder.
good luck.

i think rpm gets its feed from the alternator in a diesel but not sure about petrol. certainly it doesn't in mine. i've been using an aftermarket alternator that only has leads to the battery and starter. i've had the "alternator fault" message for years because there's no feed to the ECU but the alternator actually charges just fine and works just fine and more to the point, the rev counter also worked just fine till yesterday. for what its worth, i'm starting more and more to suspect my crankshaft speed sensor. will try to replace it tomorrow and report on any progress.

jorjio - thank you for stepping forward to try and help!