Idle problem (KOER code 91)
 

Hi, thanks for reading. I have a 90 LX coupe 5.0 T5 with 110,000. My car is having an idleing problrm, on the rpm the needle constantly moves up and down (1000-500rmp) when i come to a complete stop or when its idleing. I read the computer codes and it gave me code KOER 91. I replace the 02 sensors and no help. The power as i'm driving doesn't seem affected, it's just when its idleing where the problem is noticed. If anyone has a suggestion please let me know. Thanks in advance, Rick

Check your EGR valve and its operation. There is a solenoid that controls vaccum to the valve and it may be malfunctioning or your EGR valve may be seized (If you look at the diaphram of the EGR valve while the engine is idleing and goose the throttle you should see a slight movement in it).

The EGR valve vaccum solenoid also has a small air filter in its top part, check it to see if it is clogged.

If your EGR checks out okay then check the TP voltage (should be somewhere around .85 to .99 volts KOEO I think)

That's all I can think of right now hope this helps.


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1968 Fuel Injected Mustang Coupe

If none of that helps, clean your IAB on the TB, this helped my surging idle last time.

Try resetting your idle....it works for me when I get a surging idle...

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1992 5.0 LX, Edelbrock 6037 Heads, Cobra Intake,E-303, 1.6 Rollers,65MM Holley TB, 75 MM Pro M Mass Air, 190 LPH FP w/ 24 lb injectors,BBK equal length shorties,offroad h-pipe,Mac Flowpath Cat-Back, Pro 5.0, 3.73's,Motorsport "C" springs,welded subframe connectors

Daily driver: 89 notch with 200K plus, BBK shorties,offroad,2-chamber flowmasters with dumps, cut and welded stock shifter...and it runs like a champ.

Clean everything from and including the upper intake to the air filter. Cleaning the upper isn't mandatory, but should help as well. That should fix your problem.

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90 Mustang LX 5.0
My Ride*My Site

All the mention items already list are good advice you might also check the cannister purge selnoid, it is located passenger side of the engine, front down low near the frame. The vacumn hose coming out from under the intake behind the distributor is it vacumn source. If you use the idle screw on the throttle body to increase your idle besure to check your TPS voltage as it may (probably will) throw your TPS out of limits. Good luck.

Army

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89 Notchback 347 Trick flow heads and intake bbk full length headers 3.73 Pro Charger with 3 row intercooler and much much more never enough got to go faster!!!