Rough Idle - KOER Codes 21, 41, 91 - Ford Mustang Forum
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post #1 of 8 (permalink) Old 06-22-2004 Thread Starter
Hokie D's Avatar
Joined: Jun 2004
Location: Charlotte, NC
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Unhappy Rough Idle - KOER Codes 21, 41, 91

One fine weekend I decided to clean my IAB & TB and change my plugs (.54 gap) as I've done in the past. Well, decided to also clean my MAF and gunk in the EGR while I was at it. After doing this stuff, my car would no longer start cold and stay running without me tapping the gas. After the car warms up for a couple mins, the car idles at around 1000-1200 RPMs. If the car sits for a couple more mins the idle drops to around 700 and the car shudders. I never had this problem before. I could always count on the car to start and idle smoothly on the 2nd turn of the key (1st one would always die).

Also related...when at a traffic light the car will stop and idle at 900-1000 RPMs, then after a minute or so of sitting idle at the light, the car will drop 200 RPMs and start shuddering again! Then when I try to leave the light I get a slight hesitation when I hit the gas. The car pulls harder since cleaning the MAF at WOT...why does it develop problems sitting at idle? It used to be very smooth before I cleaned stuff and changed the plugs.

I had no KOEO codes, so I ran KOER codes and got 12, 21, 41, and 91 after heating up the O2's for 2 mins. I have checked my TPS, I have replaced the ECT sensor, and tried resetting the computer after each change. I checked all vacuum lines to see if they were all connected snugly and as far as I can tell there isn't any leaks (haven't tested). Next, I plan to have the fuel filter changed and then I will swap out the O2 sensors. Is this the right approach?

Please help!

Reef Blue 1993 5.0L Coupe - AOD w/ Baumann Shift Kit, 3.73 Gears, MoSport Pulleys, 2.5" Flowmasters, Subframes, Eibach Springs, Steeda Camber Plates, ADS Computer Chip, K&N
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post #2 of 8 (permalink) Old 06-22-2004
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You didnt use carb cleaner on the MAF sensor right ? You need to use rubbing alcohol on that tiny sensor. I suspect your maf is probably dirty. Did you remove the throttle body from vehicle to clean those sensors ? I would double check the throttle position sensor to be at 0.98v. Try hooking up a vacuum gauge to make sure you dont have any kind of leak. Should be atleast 15 inches of vacuum at idle. Did you hook the vacuum back up to the EGR valve ? Also the MAF meter is easy to install the wrong way.

89 GT Hatch: Custom M90 Supercharged w/ Ported Trickflow Trackheat Heads, Stage 1 cam, Streetheat lower / Sheet metal upper, Tweecer RT & PLX Wideband, 75mm TB, 1.6 Roller rockers, 3.73s, MSD 6a, Canton 7qt Pan+ Windage tray, 36LB Injectors, 200 amp Alternator, 75mm Pro-Mas MAF, 190lph fuel pump,Spec Stage 1 Clutch,SouthSide LCA's, Pro 5.0 Shifter, Steeda CC Plates / Springs, FMS Aluminum Driveshaft, New T5 w/ 2.95 first gear, Underdrive pullies, Zirgo 3300Cfm Elec. Fan, AFCO alum. radiator
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post #3 of 8 (permalink) Old 06-22-2004 Thread Starter
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Location: Charlotte, NC
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LOL, first time I cleaned it yes I did use carb cleaner. But, I realized that it left behind a residue and the q-tip left behind some strings so I went back and used brake fluid like everyone suggests. It had enough pressure to blast away the q-tip crap and it shined the MAF filaments right up. I could take the electronics out and inspect more closely for damage.

I did remove the TB to clean it. I had to buy a new gasket because the old one tore away when I removed the TB. I'm positive the TPS is within range. That was the first thing I tested. Bought a voltimeter and made sure it was adjusted in the .90's. I even had to even enlarge a bolt hole to let it adjust higher.

EGR vac line is hooked back up. When I took off the EGR I tried my best to clean it, but I can tell you that the inside was so gunked up that the best I could do was spray some brake cleaner in the valve by manually pushing the little cone open and spraying inside. The cone was black so I cleaned it with brake cleaner and a rag. The bowl that bolts up to the TB was cleaned as well.

How much is a vac test kit and where do you hook it up? Got any links on how to test?

Thanks for your help.

Reef Blue 1993 5.0L Coupe - AOD w/ Baumann Shift Kit, 3.73 Gears, MoSport Pulleys, 2.5" Flowmasters, Subframes, Eibach Springs, Steeda Camber Plates, ADS Computer Chip, K&N
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post #4 of 8 (permalink) Old 02-07-2005
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Location: Staten island
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just wondering if you ever fixed your idle having the same one please let me knowwhat you ended up doing to fix it
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post #5 of 8 (permalink) Old 02-07-2005
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Joined: Jun 2003
Location: Eau Claire
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I had a rough idle problem I fixed recently, I started the car grabbed my insulated spark plug boot puller and started pulling spark plug wires. Sure enough the second one I pulled the spark plug wire was bad, now I just installed these ford racing wires about a year and a half ago. Put a new set of wires on and it ran great.
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post #6 of 8 (permalink) Old 02-08-2005
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.54 might be a little too wide of a gap. That could be causing the rough-
If your coolant wasn't warmed up, that would cause the Engine Running
Check the fuel pressure for 39 psi with the fuel pressure reg unplugged.
Check for an exhaust leak between the heads and the O2 sensors. I am
also getting the 41 and 91 KOERs, and the exhaust leak is the only thing
I can think of, that would cause that.

'88 GT convertible, 5 speed, 3:08 gears, A9L Mass Air conversion. Mac shorty headers and H-pipe, Magnaflow catback.
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post #7 of 8 (permalink) Old 07-18-2008
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Location: pensacola
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I also have a idle problem,mine stays idled up between shifts and when i get to a red light it jumps up to 2 grand sometimes and will stay there for a min or sometimes it will drop back down and cam like it is supossed to.I had a code for the o2 sensors and fixing to replace them but somone told it that it wouldnt cause it to do that much?????any help
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post #8 of 8 (permalink) Old 05-17-2010
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Joined: May 2010
Location: Kentville
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idle at 2000rpm

Hi 95 sleeper, did you ever get your car running ok? My idle jumped up to 2000rpm as well and i have a rough idle with smelly gas smell exhaust. Any help would be great!!
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