Could a bad alternator cause a surging idle?
03-08-2009, 05:49 PM
#1
Could a bad alternator cause a surging idle? I have a '98 A4 with a mild cam (234/111) and a 4200 stall. I've had probably 5 different cams in my car over the years but this latest one seems tough to get a decent idle out of. It's been tuned by 4 different knowledgeable people, all trying to get rid of the idle surge with no luck. The last person to tune it (was a mod in this section) spent hours on it, trying everything, and got it to idle decent but could not get rid of all of the surging.
Before I put in this most recent cam (3 years ago), I had to change out the alternator for a generic aftermarket one. Well the idle has always sucked, but it gets alot worse when there is a greater load on the engine, like the AC or the high speed fan. Whenever the high speed fan comes on, the idle really goes to hell. If I give it enough gas to bring the RPM up a few hundred, it's fine. But anything around idle speed is nothing but surging. I literally have to put the car in neutral when I'm at a light just to keep it from surging and dying.
So my question is, would a bad alternator possibly affect idle surge? Or do I probably have another issue that the high speed fan just worsens?
Before I put in this most recent cam (3 years ago), I had to change out the alternator for a generic aftermarket one. Well the idle has always sucked, but it gets alot worse when there is a greater load on the engine, like the AC or the high speed fan. Whenever the high speed fan comes on, the idle really goes to hell. If I give it enough gas to bring the RPM up a few hundred, it's fine. But anything around idle speed is nothing but surging. I literally have to put the car in neutral when I'm at a light just to keep it from surging and dying.
So my question is, would a bad alternator possibly affect idle surge? Or do I probably have another issue that the high speed fan just worsens?
03-08-2009, 06:41 PM
#2
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If you can tune the ECM, you can set the idle speed to come up a bit higher automatically when the AC comes on (not sure about the high speed fans).
Without actually seeing how far your voltage drops when the fans come on, its going to be very hard to see whats going on with it.
Battery might cause it too... if you think its the alternator, pull it out and have it tested for free @ Autozone.
-Eagle
Without actually seeing how far your voltage drops when the fans come on, its going to be very hard to see whats going on with it.
Battery might cause it too... if you think its the alternator, pull it out and have it tested for free @ Autozone.
-Eagle
03-10-2009, 12:31 PM
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The idle problems with load and big cams, probably
relate to how bogus the motor torque model has
become. The best you can do is get the lower-
RPM part of the VE table cleaned up at the higher
MAP ranges where loads put you (but may not be
hit during plain driveway idle tweaking unless you
think about it). But the relation of torque to air
mass and timing is pushed around by the cam and
heads, so the add-X-g/sec figured from the drag
torque models for accessories is also whack and
compensates improperly. You can mess the fan
air-bump values and there is an A/C torque vs
IAT vs pressure table you could tweak on (but
info on what to do, is none). I would hit the
basics (VE @ high MAP) first. And if open loop,
be sure your EQ (OLFA) table is not pushing a
non-stoich mixture as a result of elevated idle
MAP.
If the alternator output falls off at low RPM and
loaded, this could bother the injector delivery
(offset table vs volts, the offset table is a wild
guess for non-stock injectors to begin with) or
the head pressure from the pump (hot wire kit
might help that?). Might have a look at rail
pressure and see if it changes with the A/C or
fan load.
relate to how bogus the motor torque model has
become. The best you can do is get the lower-
RPM part of the VE table cleaned up at the higher
MAP ranges where loads put you (but may not be
hit during plain driveway idle tweaking unless you
think about it). But the relation of torque to air
mass and timing is pushed around by the cam and
heads, so the add-X-g/sec figured from the drag
torque models for accessories is also whack and
compensates improperly. You can mess the fan
air-bump values and there is an A/C torque vs
IAT vs pressure table you could tweak on (but
info on what to do, is none). I would hit the
basics (VE @ high MAP) first. And if open loop,
be sure your EQ (OLFA) table is not pushing a
non-stoich mixture as a result of elevated idle
MAP.
If the alternator output falls off at low RPM and
loaded, this could bother the injector delivery
(offset table vs volts, the offset table is a wild
guess for non-stock injectors to begin with) or
the head pressure from the pump (hot wire kit
might help that?). Might have a look at rail
pressure and see if it changes with the A/C or
fan load.
