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Matt, Engineer
Category: Jeep
Satisfied Customers: 21998
Experience:  Honors degree in Mechanical Engineering, worked 8 years as a Formula 1 engine engineer.
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Grand Cherokee: 1995 jeep grand Cherokee limited 5.2l runs

Customer Question

1995 jeep grand Cherokee limited 5.2l runs rich at 2500rpm only. o2 sensor been replaced. no result. no vac. leaks. MAP, coolant temp.air temp. TPS sensors checked OK. Fails smog at 2500
Submitted: 2 years ago.
Category: Jeep
Expert:  Matt replied 2 years ago.
I'd check for exhaust leaks before the O2 sensor as if this has been experiencing an exhaust leak then it will have been running very rich as the sensor will see the extra air pulled in as fresh / lean fuelling and be adding fuel to compensate.
this can lead to catalyst failure as the extra fuel can re-light inside and melt the catalyst
The best way to find an exhaust leak is to run the engine from cold and feel around the exhaust joints with your bare hands - you'll have about 30 seconds before it all gets too hot and you should feel any escaping gas blowing over your fingers
Customer: replied 2 years ago.
There is no exhaust leaks. OBD1. System stays in closed loop and ST /LT keeps +-2%. When engine reaches 2500 o2 sensor shows .9v but OK at idle.
Expert:  Matt replied 2 years ago.
OK thanks for the extra information
I'd next suggest checking the resistance of the throttle position sensor, this is located on the throttle body, if you disconnect the sensor plug and measure its resistance whilst your assistant slowly presses down on the throttle pedal
you should see the resistance increase slowly and smoothly in time with the pedal opening
any spike or sudden changes in the reading then replace the sensor
Customer: replied 2 years ago.
I've already checked TPS voltage KOEO. .7v up to 4v
Customer: replied 2 years ago.
I didn't noticed any glitches or spikes
Expert:  Matt replied 2 years ago.
OK if the TPS is changing voltage smoothly then its OK
I'd also check that the connections to teh MAP sensor ( electrical and physical) are clean and uncorroded and any vacum pipes between the MAP and the sensor isn't blocked / cracked or damaged
Customer: replied 2 years ago.
MAP voltage is OK. Fuel pressure is 42 OK
Expert:  Matt replied 2 years ago.
in that case I'd say to check the resistance of the spark plug leads ( 13-15Kohm / metre) and replace if any doubt or high resistance - Also replace the spark plugs if any of them fail this checklist:
You can also try spraying some water on the coil pack with the engine running. It is possible the coil is faulty and causing the misfire. If you see sparks arcing with the water, then replace the coil.
I'd check the condition of the injector plugs / connections as this could be a lean misfire if an injector isn't firing properly - the best way to check this is with a 'noid' light
Then check all the connections to the engine sensors one at a time, remove each one and examine for signs of corrosion or damage and a squirt of contact cleaner ( not WD40) will also help.
Customer: replied 2 years ago.
My vehicle doesn't have any misfire.
Expert:  Matt replied 2 years ago.
Sorry ignore the misfire comment
I still think checking the plugs is a good step though and would recommend that next