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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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Idle Issue

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Old 10-15-2013, 05:14 PM   #1
Fox Owner 88
 
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Default Idle Issue

I am new to this Forum and I have owned my 88 Mustang 5.0 for about a year now and I love it! When I start the car up cold it goes into high idle fine and then settles down to 800rpms and runs a little rough, nothing crazy. Once I start to drive it the idle smooths out fine, but then as I'm driving the car, it will at times (not all the time) start to idle at 1200rpms and stay that way until I turn the car off. When I'm at a stop it's at 1200. I put new plugs, wires, K&N air filter, and cleaned the throttle body and IAC out and it didn't fix it. I am think about changing the TPS. Would this make sense to do? Any help would be great. Thanks
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Old 10-15-2013, 07:28 PM   #2
88 orangepeel notch
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I'd check the TPS voltage first, idle and sweep. Maybe there's a dead spot. Otherwise "base idle reset".

Have you pulled codes? Seems weird that its a steady 1200rpm idle, or is it surging?
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Old 10-15-2013, 09:19 PM   #3
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Unplug the IAC and drive it around...or if you can catch it at a stop or something hop out and disconnect it and see if the idle drops back down to normal..be aware your idle might drop low enough to kill the car though so do not be afraid of that....

The IAC and TPS are notorious problems with the cars because of their age
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Old 10-16-2013, 09:11 AM   #4
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Thanks guys, appreciate it. I will try everything recommended. It does surge sometimes like it's trying to figure out where to settle but mainly just stays at 1200. I have heard of checking the voltage, pulling codes, and re-setting the idle but have never done them. I know there are instructions on here. I will also remove the IAC while it is running and see what happens. I'm hoping I have time this weekend to do it. I will let you know. Thanks again
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Old 10-16-2013, 09:56 PM   #5
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I know yours is an 88 and therefore probably doesn't have a maf unless its a Cali car; but k&n filters and the like will eventually leave an oil film on the maf coils making them read incorrectly. This is especially true if the filter is over oiled like most of them are. If your car does have a mad make sure to clean it too with sensor safe non residue cleaner.
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Old 10-16-2013, 10:01 PM   #6
88 orangepeel notch
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Great idea mongo, I've never thought of that
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Old 10-17-2013, 01:33 PM   #7
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Thanks, mine doesn't have a MAF. Could the filter oil still be a problem? Or is it just with the MAF?
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Old 10-17-2013, 07:43 PM   #8
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Nope, you'll be fine if there's no maf. I thought 87 was the last year for speed density, shows you learn something everyday.
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306 with all bolt ons, letter cam, ported E7's, T5 swap, full exhaust, 3:55's, 2" drop, and the worst paint you'll ever see or feel.
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Best ET………12.4@108, 1.66 60ft on 8" ET Drags.
24 mpg (combined)

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Old 10-17-2013, 08:16 PM   #9
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In 88 all California mustangs went maf, In 89 all others were maf.
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Old 10-17-2013, 08:25 PM   #10
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Op check all your throttle linkage as well as the cruise cable if your car is equipped with cruise. Sometimes the throttle plate doesn't close all the way especially if there is crud in there. Ive seen the cruise cable just a tad to tight and cause the throttle plate to hang just enough to create a high idle once in awhile .
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Old 10-18-2013, 06:58 PM   #11
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Yeah mine is a speed density car. I will check the throttle linkage as well. Question - someone told me that it might be a vacuum leak so I pulled one of the vacuum lines from the vacuum tree on the fire wall at 800rpms. When I did this the engine soared up and when I put the vacuum line back on, the engine went back to 800rpms. Does this mean I don't have a leak?
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Old 10-18-2013, 07:32 PM   #12
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Typically a vacuum leak will be present all the time, however I have seen instances of a plastic line being cracked and momentarily leak then seal back up. This usually causes the engine to hunt idle or surge.
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Old 10-21-2013, 04:27 PM   #13
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So I pulled the IAC off the car today while it was running at 800rpms and it died, put it back on and it was fine. The car hasn't idled at 1200 in a couple of days, been running great, strange. I also checked the linkage and it seems to be fine but I honestly don't know what I'm doing. I just played around with it and it doesn't seem to be sticking but I could be wrong. I haven't check the TPS voltage as I don't have a volt meter, have to buy one. I will check it once I get a meter. I will be shutting her down for the winter in a couple of weeks so I might wait until spring. I have learned a handful of stuff just by posting this one post. You guys have been great. I'm a newbie as you can tell but I look forward to learning much more.
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Old 10-21-2013, 08:16 PM   #14
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Possibly a bad connection at the iac , unplugging it and plugging it back in may have helped. Also Idle issues on the foxbody cars can be attributed to the salt & pepper shakers amply named for there grey and white colors. There is a bit of info on the web about unplugging them and opening up the male end terminals a bit to get a better connection.
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Old 10-22-2013, 10:28 AM   #15
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Thanks, I found some info on the salt & pepper shakers on the web about cleaning them and so forth. I will check that out
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Old 10-28-2013, 04:22 PM   #16
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I have a 93 LX 5.0. Numerous bolt ons. 70mm BBK throttle body and EGR spacer, BBK fuel rail, Accel 19lb injectors as well as Accel distributor, coil, and 8 mm wires. BBK CAI and a BAMA 4 bank eliminator chip. Replaced the fuel pump with an edelbrock unit. I am having a problem of rough idle. RPM'S drop and car may stall. Car not getting full power. Cut's back and stumbles. New TPS came with BBK throttle body. replaced the IAC valve as well the EGR. Any help will be appreciated.
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Old 10-28-2013, 04:22 PM
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