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map sensor??

B

byrinxx74

NAXJA Forum User
Location
oakland
Ok so i have been having trouble trying to figure out my idle shake. I have posted on so many threads what to do. Though everything that i have tried has made no diffrence. cap,rotor,cables,plugs,iac,tps,seafoam,fuel pressure test,fuel filter,clean throttle body, motor mounts. Still nothing. No codes, drives fine when you hit the gas. I have had other guys with jeeps take a look to see if i was crzy and they say the same thing that there is a slight miss. So today i tried to unplug the map sensor by the throttle body, while the jeep was running, it really did not seem to make a diffrence though when i went back into the cab the check engine light came on!! I plugged it back in though still check engine light? So my question is this. Could it be the map sensor that is causing the rough idle. Also should the check engine light come on for the map sensor. :doh:
 
MAP Sensor will set a CEL. P0107 - Map Sensor Voltage too low, P0108 - MAP Sensor voltage too high.

If you just unplugged the vacuum source to the MAP Sensor with its electrical connector still plugged in, while the engine is idling, the PCM would see its input voltage from the MAP Sensor of between of 4.8 and 5.1 volts; in this case you would have minimum vacuum with a high voltage output from the MAP sensor with a resulting P0108. Normally, at idle the vacuum should be high with minimum voltage output from the MAP Sensor.

If you have the MAP Sensor vacuum source plugged in and ran the engine through various RPM's (loads) and no CEL was set the MAP Sensor may be okay. The PCM will see the highest voltage input from the MAP Sensor (5 volts +/-) at WOT and the lowest voltage at idle (.03-.05 volts).

Make sense?

Edit: You will have to clear the set fault to get rid of the CEL.
 
Shake at idle:

How long have your fuel injectors been installed? If they've never been changed since 1993 they may be defective (plugged or just lazy from age). Defective fuel injectors could cause a lean condition resulting in a rough idle.

If all cylinders compression check okay and your regulated fuel pressure is within specs, you may want to consider your fuel injectors.

See this thread concerning regulated fuel pressure at the rail: http://www.naxja.org/forum/showthread.php?t=1079299
 
Sidewinder CC said:
MAP Sensor will set a CEL. P0107 - Map Sensor Voltage too low, P0108 - MAP Sensor voltage too high.

If you just unplugged the vacuum source to the MAP Sensor with its electrical connector still plugged in, while the engine is idling, the PCM would see its input voltage from the MAP Sensor of between of 4.8 and 5.1 volts; in this case you would have minimum vacuum with a high voltage output from the MAP sensor with a resulting P0108. Normally, at idle the vacuum should be high with minimum voltage output from the MAP Sensor.

If you have the MAP Sensor vacuum source plugged in and ran the engine through various RPM's (loads) and no CEL was set the MAP Sensor may be okay. The PCM will see the highest voltage input from the MAP Sensor (5 volts +/-) at WOT and the lowest voltage at idle (.03-.05 volts).

Make sense?

Edit: You will have to clear the set fault to get rid of the CEL.
Click to expand...

OOPS...P0107 and P0108 are OBD II Codes. I see you have a '93, pre OBD, so I have no idea what the codes would be for MAP Sensor, if any, or if you need to clear the fault to get rid of the CEL. Never the less, the MAP Sensor data I stated is correct to the best of my knowledge.
 
No its a 1996 obd2. I unplegued the map sensor from the wires. I have cleared the codes. I will check now for bad fuel injectors
 
byrinxx74 said:
No its a 1996 obd2. I unplegued the map sensor from the wires. I have cleared the codes. I will check now for bad fuel injectors
Click to expand...

You didn't state in your original post what year your XJ was so I went to your public profile, where it says 1993 Jeep.

Unplugging the MAP Sensor electrical connector is a whole different ball game. Sorry for misleading you.
 
Ooh sorry. Yes it's a 96 I hope that this miss is not a big deal. Usually with a miss does that mean that u need motor wirk?
 
I've had the same problem and changed just about every sensor (sometimes with junk yard parts), but always with the same miss, both at idle and when the RPM is held steady at any RPM. Just cant figure it out. Tomorrow I am going to take out the injectors to see if any of the orings are cracked.
Hope you figure it out though.
 
bucklanddj said:
I've had the same problem and changed just about every sensor (sometimes with junk yard parts), but always with the same miss, both at idle and when the RPM is held steady at any RPM. Just cant figure it out. Tomorrow I am going to take out the injectors to see if any of the orings are cracked.
Hope you figure it out though.
Click to expand...

You could spray around the injectors with throttle body cleaner to determine if there's a vacuum leak past the o-rings. Much simpler.
 
Sidewinder CC said:
How long have your fuel injectors been installed? If they've never been changed since 1993 they may be defective (plugged or just lazy from age).
Click to expand...

I don't think I'd call something that lasted 19+ years "defective" if it's plugged up or worn out.

I definitely agree that they are due for service or replacement if they are the originals, though. Moving parts wear out over time and need replacement eventually-- injectors are definitely one of those things that can cause mixture problems at idle that go away with RPM.
 
How slight or big is the miss? Have you check the compression on all the cylinders yet?

I had a slight miss (among many other more major issues) for years with my 87 that I finally solved recently, it was a poor electrical connection on one injector. Found it by accident, once I finally had 500 other things all fixed, LOL.

The 89 had a rough idle miss that turned out to low compression between two cylinders, 4&5.
 
I had a slight miss at idle that was gone when I gave it any throttle.

I swapped in a known good map I had lying around and it fixed the issue. I also got better throttle response.

Initially I thought the MAP was lazy but Im now Im thinking the little tube running from the map into the TB was leaking a tiny bit.

I threw no codes at all just had a little miss and shake intermittently at idle.
 
wow ive never replaced injectors on my '94 xj limited, its done 390,000 km

and only had the o2 sensor and temp sensor replaced, idles nice and runs ok

maybe i should start replacing this stuff just incase
 
Don't mean to drag this thread on but I have a stock 2001 cherokee with the same low rpm(idle) miss, also fowls out two or three plugs every two months and gets 11 mpg in the city. Could the map sensor be my problem too? I started it today and unpluged the map sensor and there was no change in the idle. Thanks
 
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