Alexhemming097

Thanks for the replies all. I checked with the garage when the car was in for its service/MOT Paul, the alternator installed is manufactured by Hella. They plugged it in to diagnostics and couldn't see anything but I guess that's not surprising with there being no warning lights on. Also as is typical the fault did not replicate itself either when it was in the garage. Before Christmas I bought a voltage meter that plugs into the 12v socket so have been keeping an eye on what its reading. Under normal conditions its sits at 14.2 (+/- .1 depending on what electronics are being used etc.). When it was doing the rough idle/pulsing, it would drop to 13 and the fluctuate similar to in Paul's case. But then if the lights were switched on (side or dipped), the voltage would stabilise back at 14.2. Having said that, it seems to have settled now and hasn't been pulsing since I got it back on the 2nd. For now I'm going to keep an eye and see how it behaves but if the problem comes back I think it'll be a case of changing the alternator, going by what's been discussed. Cheers for all the info/advice. Most appreciated.

Thanks for the reply Steve! It was a brand new alternator, although I must admit I'm not sure whether it was an OEM or aftermarket part. Didn't think to ask at the time (in hindsight I quite probably should have!). As the problem only started after the alternator replacement, it does feel like the problem may well be in that area (whether alternator, regulator or connection/earth as you suggest). It's quite intermittent and the curveball I noticed today was switching the lights on (or blower on full) seems to stop the rough idle/fluctuating voltage. Turning the engine off and back on also stops it for a bit. I bought a cheap battery and alternator tester from Argos a little over a week ago (Buy Hilka Battery & Alternator Tester | Car battery maintenance | Argos) and this shows the alternator as working when at idle. But when I engaged drive and the rough idle started, the output did appear to be fluctuating with the change in RPM. It's a bit crude though as it only uses coloured led's. She is booked in for the 2nd Jan for a service and to have this looked at again. If the part was aftermarket and is playing up, I think perhaps going for a Denso part may be worthwhile (pretty sure the OEM part is Denso?), unless it does turn out to be a connection/earth.

Hello Lexus Owners Club! First time posting! my 08 IS250 has started playing up after some recent repairs. I had my auxiliary belt tensioner replaced earlier in November and then a few days after I got the car back, the alternator failed and so that was replaced also (both at a local independent garage). All seemed fine once the car was back. Didn't replace the battery as whilst the battery was completely drained when the alternator failed, its about 2 years old and seems to be holding charge ok. On my first commute to work after getting the car back (20ish miles), I noticed at a set a set of lights the idle was rough. The rev counter was bobbing up and down between 500 and 900 rpm (ish). On the drive home, all was fine. I know when the battery is completely dead for a period of time the engine management can 'forget' certain parameters and needs to re-learn them, so assumed it was that. I've also cleaned the throttle body and MAF sensor looks clean. (both as advised by other posts on here and elsewhere when others had similar issues) A couple of weeks on and the car is still idling rough when held in drive or reverse (so happens when you stop at lights, or are parking, etc). Its intermittent and does not always happen. When it does happen, I notice that the clock appears to pulsate and if the interior light is switched on, this also pulsates (as do the brake lights). when the rough idle has started and the engine is under acceleration, the interior light also pulsates. Interestingly, if I put the car in drive and hold it on the brake and I'm getting the rough idle, it seems to stop if I turn the lights on or put the blower fan on full. As soon as I turn either off (whilst in drive/reverse and foot on brake), the rough idle starts again. I've managed to capture it happening if that helps: I've got the car booked in at the start of Jan but any ideas would be greatly appreciated. I did think about a new battery but if that's holding its charge then not so sure that's the issue? I would think alternator but that's just been replaced (when the idle is rough the alternator voltage drops but that seems to be in line with the RPM dropping). Otherwise, there don't seem to be any notable changes when actually driving. Any help/ideas appreciated. Cheers.