Hello all, I'm having a rough idle problem with my '96 Z-28. Here's the situation. 1996 Z-28, pretty much all stock, under 50k miles, driven 1-2 times a week on average. I was hearing an rpm-related noise at idle which turned out to be a dying bearing in the OEM alternator, so I bought and installed the usual cheapie rebuilt alternator at the local Auto Zone back in the April/May timeframe. I also changed oil & filter in that timeframe. A few weeks later I started noticing a rough idle, with bobbing voltage according to the instrument panel, with the roughness and voltage fluctuation going away somewhere between 1000 and 1500 rpm. At 2000 rpm and above, perfectly normal engine performance. Happens with cold or hot engine, with little electrical load or with stereo & lights on. Tried different gas grades, gas stations, added a bottle of STP gas treatment, ran the tank almost dry and refilled, drove some highway miles, etc. Nothing tried has made a difference.

The other symptom which may or may not be related is that the check engine light would come on during idle sometimes and remain on for days, then go off, then eventually come on. I got the local shade tree mechanic to put their code scanner on it, and they came up with the code for the damned CAGS system being inop (which still seems to work most of the time, I don't pay attention to it much as I've trained myself to blip the throttle when making the 1-2 shift). Otherwise, normal oil pressure behavior, likewise coolant, clean air filter, battery seems to charge OK. I thought low voltage might be causing the CAGS sensor to squawk. AFAIK it has no impact on reliability, and certainly none on driveability, but I'm a worrier so having the check engine light on is bothersome.

Aside from the shade tree guys, who are the best in town for basic mechanical work, nobody has touched the engine except me for several years (last repairs I did were new water pump and new coolant level sensor last summer).

Before I go charging down to Auto Zone in a rage to demand another alternator, I figured I better see if the F-body community has any ideas for me to check out first. Any other simple tests or checks that I should be doing?

Thanks much and have a great holiday weekend!

You may want to return that alternator and pick up a good unit such as a Denso. Now I'm not saying that this is your problem but clean adequate voltage is essential for proper operation and I have solved tons of driveability problems by fixing the charging system on these very cars. You also want to check and make sure that all the Bat cable to the alternator is clean and tight and also that the connections at the battery are clean and tight, alot of time the battery connections will build up corrosion inside of the rubber boots that you can't see.

Thanks, when the weather clears I'll take another look at the cables. I didn't notice anything when I replaced the alternator, but I wasn't looking for anything either.

Checked and cleaned the battery terminals and the output cable from the alternator. Cleaned the ring terminal on the latter with Flitz, tightened the hell out of all three. Thought it helped, but a few days of driving showed that it was false hope.

Could it be a tired battery? I'm thinking that at idle, the battery has to supply some of the current for the field coils (I'm getting way past my grasp of charging systems here) and a tired battery might not produce a steady 13.7 VDC or whatever it is.

How old is the battery? If it isn't used often it can sulphate plus having a continuous drainage, e.g., stereo, etc., doesn't help either.

Electrical system runs from the alternator once the engine starts up and the alt gets turned on. Battery becomes a load and the over voltage you measure, 13.7 volts vice 12.6 volts (engine off), is the charging voltage to recharge the battery. Today, the alternator is of sufficient electrical size, the voltage should not drop enough to bring the battery into service to "make-up" for power loss, or not enough current being generated.

With this said, either the alternator is of poor design(?), as stated earlier, or battery may be faulty, dead cell or old, always taking a heavy charge and will continue to do so until either the battery gives up or the alternator quits.

Belt may be slipping also and you would never know it.

I run a shop in Texas and I used to have a lot of problems with Autozones alternators and starter. I have used a couple lately(duralast) and have had no problems with that. I was told that Delco-Remy manufactures duralast.

you might just try using a hand held voltmeter on the alternator output to ground, maybe test across the battery too.

It sounds like your alternator just isn't "kicking in" until you get to 1500 or 2000 rpm's. If you read a low voltage at idle, take that sucker back and get a new one.

How old is the battery? If it isn't used often it can sulphate plus having a continuous drainage, e.g., stereo, etc., doesn't help either.
...

Belt may be slipping also and you would never know it.

Battery is probably 5 years old now (its an Interstate basic model). I've probably only put 15k miles on the car since I replaced the OEM battery. It gets driven 2-3 times a week now, mostly around town. Lots of cold cranking power still, almost instant start every time. Even when I'm running errands and doing very short drives, battery always has excess muscle to crank the engine so I believe it still has good capacity and takes a charge quickly. I topped off the cells with distilled, and peered down into each cell and didn't see anything out of the ordinary.

Belt tension feels OK, and I use belt dressing spray since the serpentine system seems to squeek whenever possible.

I'll dig up my old DVM and check voltages. Aside from battery voltage with cables disconnected, and alternator output when idling, what other measurements should I check? Voltage across battery terminals when idling?

what other measurements should I check? Voltage across battery terminals when idling?

Yes.

Battery over 12v motor off = good fully charged battery (usually)

Alternator output and battery same at idle = wiring good.

Alternator and battery low (below 14v) at idle =bad alternator

As long as she always cranks over ok hot or cold I wouldn't worry about the battery. It's just a storage device and shouldn't effect the voltage when the car is running. Jinxing myself here, but I still run the same battery that was in the car when I bought it 7 years ago, don't know how old it was then. Always cranks strong and holds a good charge all winter long too.

[quote=pdq123;635306] I topped off the cells with distilled, and peered down into each cell and didn't see anything out of the ordinary. [\quote]
Any time water is added to replace electrolyte, battery capacity is reduced, I believe.

[quote=pdq123;635306]
Any time water is added to replace electrolyte, battery capacity is reduced, I believe.

It shouldn't. Its not replacing electrolyte, its just replacing water. Batteries lose water out of the electrolyte due to evaporation, but the H2SO4 itself doesn't evaporate. Adding water simply returns the electrolyte to its design strength.

Where batteries lose capacity, as I understand it, is when the electrolyte level gets lower than the plates and the lead oxidizes due to exposure to air. Once lead oxide is formed on the surface of the plates, that surface area is permanently lost to the lead/lead suphate reaction process that stores and releases electrochemical energy.

Okay.

Finally got my DMM straightened out, had recently put a new battery in it but the battery was bad.

Voltage across battery terminals, cables connected, engine off; 12.4 VDC, steady.

Voltage across battery terminals, cables connected, engine on; 14.6 VDC most of the time, but fluctuating downward to maybe 14.0 periodically.

Bad alternator/regulator?

Wow 14-14.6 Volts with the Engine Off! Thats amazing :)

14-14.6 is just fine at idle, alternator wasn't designed to output full charging power at idle.

Wow 14-14.6 Volts with the Engine Off! Thats amazing :)


Arrgh!

Took the rebuilt alternator back to the store and got a new one. Have only driven it a few miles so far, but the idle is smooth and the charging system voltage is no longer fluctuating. So it would appear the alternator was bad from the get-go. Of course, this one is a bit noisier than the 1st one, but I'd rather have a little bearing noise than the stupid check engine light coming and going.