Who would have known this would cause a surgeing idle!

[93RiceKillinGT]

Hey guys I got something good to add to the list of possible cuases of a suregeing idle in a 5.0 mustang! Your battery! Yes your battery and im not joeking around. I spent the last 3 days farting around looking for vacume leaks, setting timeing, pulling engine codes, checking sensors, replaceing sensors, etc, etc on my brand new 10:1 compression 306 with high lift cam and I found nothing! What the car was doing was it would start up fine and hold a good idle until everything warmed up when naturally the computer would drop the idle to around 700-800 RPM at warm idle. Well when this would happen the car's idle would surge from 500 to 1500 RPM and you could tell the computer was trying really hard to keep it running normally but then it would die, and when i would step on the gas the car would rev up but it would do it hesitantly and it would sputter almost.

I did everything to try to fix this problem untill last night when I went to turn the car over after messing with the timeing the car wouldnt turn over and i'd here the trypicall click click click come from the starter. This indicated my battery was dead so my friend tested it and it wasn't holding a charge or putting out a good voltage, and i noticed my voltage gauge was reading alot lower than where it should be when the car is running.

So this morning I went to the parts store and bought an optima yellow top battery and installed it. Presto my voltgage reads normal as it did when I had the old stock engine, and the car held idle at about 900 RPM with no surgeing, but a little lumpy becuase I have a lumpy idle cam.

What I believe was going on was the car was idleing below the rpm that the altornator would kick on thus the car was sucking what little juice was left in the battery and that wasn't enough for the engine to run right, supply voltage for the fuel pump, computer, ignition, ect ect. I've experienced this once before where my battery was near death and the car would suppter like crazy when i stepped on the gas and then it stalled out.
This may sound crazy but it did fix my problem.

 

[331 stang91]

sometimes it seem sthat after you change something to fix a problem you need to unhook the battery for 20 min or so . Did you do this after you changed ex. the censors? If the only time the battery was unhooked was when you changed it then unhooking the battery/clearing the comp of the memory of a bad censor then maybe that was all you need to have done? Just thought I thrught that out there may have or may not have been the problem?

 

[Curse]

Never heard of taht one, but if it works for you then cool. Glad you got your problem worked out.

 

[TheUser]

what kind of alternator do you have? A stock alternator should kick on w/ the ignition, I believe. If you have a one-wire alt that kicks on at 1500 rpm or whatever, that might be the case. Never-the-less, the Optima is an awesome battery. I have a 960cca red top in my GT

 

[HISSIN50]

So at idle you think you were outrunning the fuel supplied by the pump?

Otherwise, this is the reason the puter uses a 5 volt reference instead of 12, etc. I am wondering if clearing the stored adaptive strategies had more to do with it. Let us know if the surging comes back once the puter adapts.

Anyhow, I am glad it fixed your issues.

 

[93RiceKillinGT]

I'll keep you guys posted on what happens in the next few days while I break the engine in. I didn't have the battery disconected long engouh for the computer to loss it's memory. I know it takes at least 30 min or so. I changed battery very quickly within 2-3 min so.

I also have a big exhuast leak at the header/x-pipe union to fix becuase that car has absolutly no torque. After that I'll really be able to break the engine in and fully test it out to see what it has for nuts and to see if the surging idle comes back.

 

[HISSIN50]

I didn't have the battery disconected long engouh for the computer to loss it's memory. I know it takes at least 30 min or so. I changed battery very quickly within 2-3 min so.

More internet myth IMHO. The KAM discharges very quickly - yours likely did.

Good luck with it.