Wasn’t sure whether to post this in the electrical section or engine… but starting to think it’s more electrical related since testing the voltage across battery (see below)
Problem:
Turning on heater blower/lights etc causes revs to drop 100/200 or so when usually sitting nicely at 850ish idle. It even fluctuates slightly to the beat of the indicators!
When coming to a stop the revs drop slightly more, but this is not always the case.
I’ve got a K&N Induction kit and think perhaps it was happening before fitting this but can’t be 100% sure…
Testing
I’ve tried resetting the base idle (no change needed). Sprayed a load of intake cleaner into the throttle body (though didn’t take off the large pipe, just one of the little ones and sprayed with a straw). Quickly tested for air leaks by spraying intake cleaner at joints and listening for stuttering.
After some research I began thinking it was electrical related so today disconnected the battery for 30 mins and then took the car out of a drive. Returned home, engine off. Started again shortly after and the engine fired up OK, revs dropped to around 400 or so then up to 1000+ then dropped again… and continued this pattern a few times until it stood at a steady 850ish! New battery few weeks back as car had failed to start a couple of times at Christmas so figured it wasn’t holding charge - but now thinking maybe the alternator is the issue? Measured the voltage between battery terminals today and it was around 13.8v - dropping to about 13.25 with lights on. When revving up to ~3000rpm, the max voltage seen was about 13.91v. This seems low and suggests an alternator issue? Especially as the battery is new.
Also, not sure if this helps, but with Ground+10 connected on the diagnosis box, switching the lights on dropped the revs slightly and they stayed at that point whilst the lights were on. Voltage across the battery at this time was about 13.5v (when normally ~13.8v).
Thanks for reply! Are there any other tests you can recommend or is the alternator pretty much confirmed dead?
The belt tension seems OK, I need to check the electrical contacts on the alternator though. I’ve also read about bypassing the voltage regulator to see if this is causing an issue?
I think it could alternatively be a duff ISC valve, which normally compensates for additional electrical load.
You can test it. Find the ISC under the front of the throttle body. When the engine is warm and idling, unplug the connector. The revs should climb immediately to around 1200 rpm, or it is shot. While the connector is off you can confirm by measuring the resistance across the ISC terminals, which should be 12ohm +/-1ohm…or it is shot.
I think it could alternatively be a duff ISC valve, which normally compensates for additional electrical load.
You can test it. Find the ISC under the front of the throttle body. When the engine is warm and idling, unplug the connector. The revs should climb immediately to around 1200 rpm, or it is shot. While the connector is off you can confirm by measuring the resistance across the ISC terminals, which should be 12ohm +/-1ohm...or it is shot.
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Thanks for the reply. I removed the connector with little electrical load and yes, RPM raised to around 1200. With the alternator fully loaded and connector disconnected, RPM dropped to around 1k. Though please note it's quite intermittent, and revs weren't dropping when coming to a stop or loading the engine before testing this (perhaps very slightly but not noticeable)
Resistance between connectors seemed to be 13.8-14 ohm or so (though not sure how accurate this is... very fiddly when still attached!).
Would an alternator fault be intermittent? Been looking through old receipts and it had a new alternator in January 2008 (and interestingly new batteries Feb 2008 @ 91000mi, July 2009, Feb 2010 @ 92580)
Eventually got round to a new alternator (recon) and having some issues. The charge light on the dash keeps flickering when revving high… though seems to be getting worse.
Got home today and the light remained on. Idle drop issue still present. Volts at battery whilst idling 14.8 (was 14.28 when first fitted).
Could this be a dodgy alternator or belt slipping? It feels pretty tight (can’t twist the belt more than 45 degrees).