Question about battery/alternator function at idle.

Wow, great question. I see similar activity on a 1996 5.2 car. Curious to see what others say.


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Regarding the battery and a stumble - I would think it to be unlikely an old battery would cause this, as once the car is started, it should be running purely on the alternator's output. I've always been taught that the battery of a car is like a reservoir, and the alternator is the dam upstream controlling the river flow to the reservoir (thus keeping the 'water' [voltage] level at a predetermined 'height' [a certain voltage]). If the alternator cannot provide enough power, voltage will be drawn from the battery when running.

I'm curious as to the voltage readings with the engine off, at initial startup, and fully warmed (should be >12.5v off, and between ~13-14.5v running). With the battery in the RH nose of the car, I imagine it's kind of difficult to get a voltage reading directly from the posts. FWIW, it's common for a high-load component (i.e. a 27A fan draw) to affect the idle speed of the engine.

My dad was also recently noting a low-idle stumble (he described it as a 'miss').

He's got a set of Kingsborne wires (in rossa, of course - lol) on their way. Thinking that it might be as simple as a bad wire - they are original to the car...

 

If everything is on, how many amps is that?

sjd

 

There is positive post from the battery in the engine bay under the right side trim piece.

 

A) with an idle speed of 1050, your motor should never be under 1000 RPMs at idle.

B) with the fans running the alternator is pumping out 50A to keep the fans running and another 5-10A to keep the motor running. Lights will be another 20-40A if on.

50A at 12V is 1HP and at typical alternator efficiencies maybe 2 HP, with the belt takeoff from the crank, maybe 3HP from the motor.

So, while the alternator pulls a load on the engine, it should not pull it down "that much".

C) as to the battery, it has often been noted that a fresh new battery eliminates many undiagnosed gremlins. The real question, is; after sitting for a week, does the car start on the first or second turn of the key. If it does, the battery is probably not at fault.

 

Yes, this I knew - but what is the potential voltage drop from the nose of the car to the battery post? I only suggested testing directly at the battery to eliminate the potential of a false low reading due to this.

 

Get a set of compresson and leak tests done on the engine. With the engine on tickover, you are asking it to produce more torque when the fan is switched on, to maintain the tick over. Also how well is the fuel delivery balance on the engine. It sounds fine on tick over but when asked to do a bit more it objects.
Does the engine have some form of fuel pressure regulation that might need a bit of attention ?

 

This is another 10-15A

Also note, the alternator may be current limited at low engine RPMs whereas it will not be at higher RPMs.

Try turning off the fogs and park lamps and report back if there is any change.

 

Sounds like a fuel supply issue to me.

 

It is possible the off idle stumble is more related to unbalanced or misadjusted throttle bodies.

 

Interesting because uconnhusky has had them close to 10 years and I have never heard him complain or have problems.

 

My car afterI did the major idled better than it does now. My guess like others have suggested is the injectors are no longer perf ct at idle.

It was perfectly smooth now a slight stumble hardly noticable but there.

As Dave said likely a fuel issue or carb balance is out, idle control valve, etc.

 

Dennis - this is one of those times when real time, real life diagnostic will be needed.

 

A slightly dirty MAF can also cause a hiccup from Idle, or even uneven idle.
Easy to clean inside with electrical component cleaner in a can. (first remove from the inlet system of course). If that does not help, my cards are on the ignition wires. But you can check those a bit by starting the car in the dark, and see if sparks are crossing between the wires. (you do need to remove the covers first as that is where the wires are stacked closely to each other).

 

I agree, clean the MAF and the throttle bodies. Easy to do and I recently had an experience with one of my other cars that had idling issues where doing that took care of the problem.

Go to the auto parts store and get a can each of TB cleaner and MAF cleaner. Follow the directions on the cans. DONT touch the MAF wires, just spray as directions say.

Give it a try.

 

Right on - Dennis

Glad it's solved

 

Interesting. I had an issue recently in which I considered replacing the fuel pressure regulator and in looking at it in the engine I had no idea how you could get at that without some significant disassembly. How did you change it?

I called on a replacement also and it was $700. Good for you having one laying around!!

 

Probably not the most fun job but it's accessible...

Only picture I had handy..
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