My Jeep(4

MICA1057

MEMBER
1994 JEEP CHEROKEE
150,000 MILES

My Jeep(4.0 L)runs very rough from ±1200 to ±2500 rpm. Idles smooth most of the time. As the throttle is applied, the engine shakes & is reluctant to rev up. I have to feather the throttle to slowly increase rpm. There is almost no power to move the vehicle. Almost all sensors are new, fuel pump & injectors replaced. All ignition(cap, rotor, wires, plugs, coil)parts are new. Converter removed. ECU replaced. Throttle body cleaned & rebuilt. Can't detect any vacuum leaks. Head recently rebuilt. Intake manifold gasket replaced. Compression 120-125 psi on all six. Thank You.

3 Replies

HMAC300

MECHANIC
48,601 POSTS

Check intake manifold vacuum if 15" or belwo at idle suspect clogged exaust. Also check to make sure cannister purge valve isn't hung open pull off hose not going to engine andsee if vacuum there if it is valve is bad. Or you can plug engien vacuum line an see if that helps. Rescan for codes as well. Also check cranks sensor when ccranking it should have 500 mv also check firing order. Pic attached.

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Tuesday, March 5th, 2013 AT 4:28 PM

MICA1057

MEMBER
2 POSTS

I removed the cat converter & am confident the exhaust is clear, but I haven't checked the vacuum. I am unfamiliar with the purge valve & have looked but don't see anything resembling a valve. There is no EGR valve on this jeep. The crank position sensor has 3 pins. I checked for voltage on the lead going to the sensor & checked from the sensor itself, moving meter to different combinations of pins. No voltage reading(the sensor is new). The engine smooths out from 2500 rpm on. Idle is smooth, but sometimes speeds up or slows down, then settles into smooth idle once again. I have a Nova OBD1 &2 scanner & have checked & re-checked.I also did the "self-diagnosis codes" the 91-97 models have & it shows nothing.

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Tuesday, March 5th, 2013 AT 5:54 PM

HMAC300

MECHANIC
48,601 POSTS

The cannister purge valve sits atop oyour cannister on theobd1 models.-If the sensor no sense checking it. What you are describing really soundslike a vacuum leak more than anything. Also check teh vacuum line leading to the map sensor if it's soft replace it. I'm sending a vacuum diagram alongfor your truck. Also to check for a vacuum leak use choke spray and spray around anyplace there is a gasket or hose. You might try wiggle tessting sensor connections to see if that does nanything different while idling. As it can be a poor connection this wold involve more like crank, cam sensors maybe map sensor more than anything also check fuelpressure if not in the 39-41 psi range at idle with vacuum disconnected it may be a fuelpressure regulator problem other than that I can't really think of anything else.

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