Idle Surges and Voltage Problems Oh My!

[91StangGT5.0]

My 91 Mustang has been doing this for quite sometime and I'm not really sure if it's a problem or not or something typical with adding a crank underdrive pulley on, but My Voltage at idle with nothing on varies from 13.0v-13.8v.i have what appears to be a 130 Amp Alt since the whole thing is incased unlike the 75amp with the fan on the outside. Anyway, when I'm driving or when the head lights and fog lights are on...after driving for a bit and then stopping at a stop light for instance...the voltage drops to around 11.9v-12.0v minimum. I know this probably isn't good and is probably caused from having just a crank underdrive pulley. I have a brand new crank pulley. I'm just seeing what people think first before changing it or if it could be solved without reverting back to the stock crank pulley.

Second issue.....When I turn my car on, the rpms shoot up to around 1500 rpms and drops down to 1200rpms and idles like that for a minute or so, then slowly drops to around 800-900rpms. Is this normal? I do not have my EGR crap hooked up and the two spot where the hoses would go on the spacer are not hooked up and are not covered. Is this causing a vacuum leak-like effect on the engine? or do I have a leak somewhere else or other problems? I'm also experiencing an issue with my car almost dying when I get caught in neutral when changing from lets say...Reverse to Drive (vice versa). The idle in neutral is around 1200rpm....I doubt that is right at all! Because if I'm driving and I put it in neutral while driving it should go to the base idle of around 800 rpms and not 1200rpms right? I think maybe these two issues is what is possibly causing my brakes to be much weaker than they should be. (Had another thread on brakes before)

 

[HISSIN50]

Carefully see if the vac hoses and/or fittings suck air while the engine idles. If so, you have a leak.
It sounds like you do have a leak.

Have the alt bench tested and/or post up voltages seen at 2000 RPM. Anything less has too many variables due to alternator RPM (because of pulleys, etc).

If you haven't, pull the codes as well.

Good luck.

 

[91StangGT5.0]

Carefully see if the vac hoses and/or fittings suck air while the engine idles. If so, you have a leak.
It sounds like you do have a leak.

Have the alt bench tested and/or post up voltages seen at 2000 RPM. Anything less has too many variables due to alternator RPM (because of pulleys, etc).

If you haven't, pull the codes as well.

Good luck.

Is that what the vacuum fittings and hoses are supposed to do is suck air? If it sucks air how is it bad? when the engine is around 2000 rpm the voltage is around 13v-13.5v. I too think I have a vacuum leak somewhere.....What rpm is it supposed to idle when put in neutral when moving? I ask this because it shouldn't go higher than 800-900rpms but rather stay under 1000rpms right?

 

[HISSIN50]

As the name implies, vacuum hoses receive manifold vacuum. If you pop a hose off of the manifold, the fitting it was attached to will be sucking air.

The rolling idle depends upon how the idle is set. Automatic cars had a stock hot, no-load idle in drive of about 650 RPM. The idle in neutral will be higher by 1-2 hundred RPM. If rolling and in neutral, I'd expect to see over 1000 RPM (which is also true if your rolling in drive). This is because the IAC keeps the idle high so you don't stall.

You should see 13.4V at 2000 RPM with on accessories. If you drop below 13.0V at 2K RPM with all of your accessories, that's not right. Simply stated, above 12.6V is charging and below is discharging the battery.

Good luck.

 

[91StangGT5.0]

As the name implies, vacuum hoses receive manifold vacuum. If you pop a hose off of the manifold, the fitting it was attached to will be sucking air.

The rolling idle depends upon how the idle is set. Automatic cars had a stock hot, no-load idle in drive of about 650 RPM. The idle in neutral will be higher by 1-2 hundred RPM. If rolling and in neutral, I'd expect to see over 1000 RPM (which is also true if your rolling in drive). This is because the IAC keeps the idle high so you don't stall.

You should see 13.4V at 2000 RPM with on accessories. If you drop below 13.0V at 2K RPM with all of your accessories, that's not right. Simply stated, above 12.6V is charging and below is discharging the battery.

I'm still not getting what you are talking about by checking the hoses and fittings for sucking.....do you mean check the hoses externally if they are sucking from a crack or something? I don't hear anything like a vacuum leak, but then again sometimes you can't hear it.

Good luck.

If the rpms are supposed to be be higher (over 1000rpm) then why do I stall or almost stall when I put it from neutral to drive?

I don't see 13.4v with accessories on. I see close to that with just my headlights on. I definitely know I don't see that if my a/c is on or the a/c fan is on...Pretty much around 13v with just the radio and headlights on...and sometimes maybe the fog lights.

 

[HISSIN50]

If the rpms are supposed to be be higher (over 1000rpm) then why do I stall or almost stall when I put it from neutral to drive?

I don't see 13.4v with accessories on. I see close to that with just my headlights on. I definitely know I don't see that if my a/c is on or the a/c fan is on...Pretty much around 13v with just the radio and headlights on...and sometimes maybe the fog lights.

A vac leak will cause the idle to be all over the place.

Again, have the alt bench tested. It might have dropped a diode.

Good luck.

 

[91StangGT5.0]

Can the fittings on the EGR plate be left un-covered or not hooked up? Or do i have to cap them off if I'm not using them?

 

[HISSIN50]

Can the fittings on the EGR plate be left un-covered or not hooked up? Or do i have to cap them off if I'm not using them?

They can be left alone. I'd seal them just to keep crud out though.