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5.0L (1979-1995) Mustang Technical discussions on 5.0 Liter Mustangs within. This does not include the 5.0 from the 2011 Mustang GT. That information is in the 2005-1011 section.

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Old 05-19-2008, 10:19 PM   #1
93 LX FiveO
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Default really bad idle surge

i know some people get the common 600-800 idle surge

well.. i got something a bit worse. my idle will drop to about 400 and shoot back up to about 1500 over-and-over again....

most of the time it stalls the car out if i dont catch it.

usually reving it to about 3k for a few seconds settles it out.. but doin this at stop lights gets me unwanted attention.

i took the IAC off today only to find the lower bolt is broken off inside the plate. tried to use the ez-out with no luck (its too small).

in my desperate attempt, i slobbered RTV around where the bolt should be and fastened the other one pretty tight. everything was good, and i set idle to about 800 and it was very steady.. untill i left work.

im out of idea's. maybe the broken bolt has to come out?

any help appreciated, thanks.
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Old 05-19-2008, 10:41 PM   #2
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Default RE: really bad idle surge

i am still trying to figure this out ...i have the same problem, mine wouldnt stall out but very close.... in order to get mine under control i replaced the IAC, ignition module, Retimed it and advanced it to 12*, reset the computer not to menchion a COMPLETE tune up. ive spent all this money and at this point it is idleing at 1000 but will still surge up to 1250 sometimes and only drop idle every once in a great while.

so before you start replacing stuff, just clean the IAC, check your timing out real quick and reset the computer and i hope that helps. cuz ive spent a good 250-300 trying to figure it out. good luck.
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Old 05-19-2008, 11:30 PM   #3
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Default RE: really bad idle surge

sounds like we've all had this problem. iac would be the common solution, however i checked mine and its working and clean. i do get no reading on my tps sensor. so i bought a new one and will replace that soon. ill keep you all posted. maybe the computer cant tell where the trottle postion is at bc of a faulty or broken sensor. causing the idle to jump around. thats my hope. lets keep our fingers crossed.
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Old 05-20-2008, 01:06 AM   #4
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Default RE: really bad idle surge

I had a similar problem that spiked the rpm's about 200-300. It was because I put my cold air intake on and didn't have a heat shield/deflector in place. I also have 2 huge cooling fans hardwired to a switch. (which I always turn on when I start the car) My problem was the fan wash. The fans were messing with my K&N intake which was just inches away, so I made a deflector around it. However, it never stalled the car out. I don't think a fan would cause that much trouble for you. I was just throwing that out there, in case it might help. If the IAC and TPS are in good working order, I don't know off the top of my head what else it could be. Maybe Mass air sensor. Have you messed with anything on the car recently? Timing? intake? etc.?
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Old 05-20-2008, 02:01 AM   #5
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Default RE: really bad idle surge

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ORIGINAL: MustangMan628

I had a similar problem that spiked the rpm's about 200-300. It was because I put my cold air intake on and didn't have a heat shield/deflector in place. I also have 2 huge cooling fans hardwired to a switch. (which I always turn on when I start the car) My problem was the fan wash. The fans were messing with my K&N intake which was just inches away, so I made a deflector around it. However, it never stalled the car out. I don't think a fan would cause that much trouble for you. I was just throwing that out there, in case it might help. If the IAC and TPS are in good working order, I don't know off the top of my head what else it could be. Maybe Mass air sensor. Have you messed with anything on the car recently? Timing? intake? etc.?
i thought the shield was just to keep hot air out of the fender
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Old 05-20-2008, 03:04 AM   #6
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Default RE: really bad idle surge

check your vaccuum hoses man. thats all mine was. the tiny red plastic one from the egr to the ... whatever the hell it runs to was split inside of its rubber shield thing.
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Old 05-20-2008, 08:56 PM   #7
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Default RE: really bad idle surge

its was hard to get a reading on my TPS as well... you really have to stick the needles into the sheilding....i also adjusted mine to the debatable .98-.99 volts and i do think that helped....
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Old 05-20-2008, 09:10 PM   #8
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Default RE: really bad idle surge

a rolling idle like that is a vacuum leak and a half. the engine doesnt know how to react to all this extra air coming in so it will add a bunch of fuel and see that the idle is too high and it will drop it back down and it will keep doing this till or stalls out of till you turn the car off
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Old 05-20-2008, 10:48 PM   #9
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Default RE: really bad idle surge

I was actually having an idle problem to. I thought it may be my tps but I had it hooked to the computer and I have bad oxygen sensors. I was running rich on oneside and lean on the other. I would be in a fast-food drive-thru and all of a sudden my engine would start to idle between 200 and 300 rpm. It got really annoying because people started to think I was being an *******.
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Old 05-23-2008, 10:42 PM   #10
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Default RE: really bad idle surge

put on the o/r pipe with new 02 sensors, and car still idles like crap.

only does it after driving for about 30 minutes.

while coasting at 40 mph if i push clutch in car basicaly dies...

would this still be the iac?
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Old 05-23-2008, 10:42 PM
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