RPM/voltage drop at idle

[shifuharris]

Hi everyone,

I'm new to this forum and I hope someone can point me in the right direction. I've been scouring the net for an answer and haven't had any luck.

I have a 96 GT that while idling (usually around 650 rpm's) will suddenly drop to around 500 rpm's or a hair below and then go back up to a little above normal and then settle back at normal. When this happens, immediately afterwards the voltage will drop and the lights in the car will dim. I suspect this also happens while driving sometimes but I can't confirm that (seems like there is an occasional stumble when accelerating). This is a intermittent problem and doesn't happen all the time and the amount of time it sits at idle before it happens is not consistent either.

I recently had some work done but nothing that should have affected the idle (new clutch, shocks, struts and springs installed). I have already replaced the idle air control valve (the common answer to idle problems on the net) which did nothing. Other than that, all that has been done is a new stereo deck added and the center console replaced with one for a '98 stang (gotta have some cup holders).

Other than this issue the engine seems to fine except that I found out the car is burning a little oil.

Any advice or help anyone can offer is greatly appreciated. I'm anal about my car's performance and this is driving me crazy.

 

[squeak93]

My car does this exact same thing (with a low idle like that). Set the idle to 750 or so, reset the pcm (unplug batter for 30 min or so) and it should be ok. Seems your idle is a bit too low.

 

[streethorse]

weak battery.....UD pullies.......dirty fuel filter or any combination of the 3.
Is it auto or 5 speed?

 

[shifuharris]

thanks for the input guys. i will check all of these and see if anything makes a difference. I will post back if it does.

 

[shifuharris]

It is a 5 speed and I don't have UD pullies on it. The car is stock under the hood.

 

[MaxedGT]

When you installed the deck, did you disconnect the battery?
I'd take the battery terminals off and make sure that the contact surfaces are clean and shiny.
After you reconnect the battery, grab the terminals and make sure they are secure and don't wiggle. Poor battery connections can cause all sorts of stupid problems.

 

[shifuharris]

I replaced the battery this weekend which means new connections and I made sure they connections were tight. Turns out is was the original battery from '96. However, the problem still persists. I have done some investigative work and figured out that the RPM drop coincides with the engine fan kicking on. Now I know that will draw more current but it seems like quite a big noticeable drop for a fan. Does this seem normal to everyone?

Thanks again for all the input.

 

[kendoo130]

If you just had the clutch done-the exhaust had to be removed. Make sure all of the O2 sensors are plugged back in.

 

[shifuharris]

O2 sensors...now that is probably a good call. I know this problem didn't happen until after the clutch work. Also, the clutch was not done correctly (still chattering a little and sounds like there is a cricket under the car). I have already replaced the battery and the fuel filter\ so the O2 sensors sound like a good call.

I will report back on that one.

Thanks again.

 

[MaxedGT]

You mentioned your cooling fan is kicking on when it does it. I was wondering if your a/c compressor is kicking in and causing it? See if it does it if you leave the a/c control in the off position. The a/c does kick in in the defrost positions by the way.

 

[Greenscobie86]

I have the same exact thing in my 96GT...

This was a problem becasue the fan used on the 96's had too much of a current draw while not being able to cool the vehicle effectively...

Do you ever notice your engine temps vary as the fan kicks on BTW?

Ford changed the cooling fan design/radiator/shroud for 97...

Hope this answers your question...

 

[shifuharris]

MaxedGT, the problem still happens with the a/c controls in the off position. The idle gets rough and drops a little and then the voltage will drop, the lights dim and then it recovers.

Greenscobie86, the temp in the car is amazingly consistent. Once the car gets up to temp, it is pretty much stays in the same place. I will keep watching it though.

Thanks again for all the input.

 

[stangnutlx]

My 01 does the same thing. I raised my idle up another 100rpms using an Xcal3 and it fixed it.

 

[shifuharris]

Seems like raising the RPM's is the way to go. I will try that this weekend. Squeak93, I did try the PCM reset yesterday (without raising the idle RPM) and it didn't have any effect.

I appreciate all the tips.