MJ Comanche Tech. Stock & Modified. MJ (86-92)
All discussion regarding the Jeep Comanche, the pickup truck related to the Cherokee platform.

back firing need help!

Old 05-04-2014, 11:45 PM
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Default back firing need help!

hey guys im new iv got an 88 comanche... i got it stuck the other day and after we pulled it out it back fires really bad and wont even drive. it will idle but anything more than that it wants to die. i dont know much about the 4.0s but i am know my way around a motor. i check the fuel rale pressure and its around 30 psi . i checked all my vacume lines and there are no leaks. what would you guys recomend?
Attached Thumbnails back firing need help!-img_20140401_201323.jpg   back firing need help!-img_20140401_201313.jpg  

Last edited by grovercamanche; 05-04-2014 at 11:48 PM.
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Old 05-05-2014, 08:21 AM
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I would check the fuel pressure at the rail. 31 at idle with vacuum hose connected. If you disconnect the hose, it should shoot to 49 PSI.

Also check to see if the fuel pump ballast resistor is okay.
Got a volt/ohm meter?

Behind the driver's side taillight is the ground for the fuel pump. Only on there with a self tapping screw...
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Old 05-05-2014, 10:08 AM
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ok il check that and let you know thank you.
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Old 05-06-2014, 02:45 PM
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i replaced that and it helped a little bit but it is still doing the same. my manual says it could be the EGR? but i dont want to pay 110$ to replace it cuz im on a budget. is there a way to get rid of it?
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Old 05-06-2014, 04:30 PM
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Originally Posted by grovercamanche View Post
i replaced that and it helped a little bit but it is still doing the same. my manual says it could be the EGR? but i dont want to pay 110$ to replace it cuz im on a budget. is there a way to get rid of it?



TESTING THE EGR VALVE FOR 87-90 4.0 ENGINE

Valve Opening Test
1) With engine at normal operating temperature and at idle,
rapidly open and close throttle. Open throttle sufficiently to obtain
at least 1500 RPM. Movement should be noticed in EGR diaphragm.
2) If diaphragm does not move, probable causes are: faulty
vacuum signal to EGR, defective EGR diaphragm or defective
backpressure sensor diaphragm (if equipped), or leaks in vacuum lines
or connections.
Valve Closing Test
1) With engine at normal operating temperature and at idle,
manually depress EGR valve diaphragm. RPM should immediately drop,
indicating that EGR valve is not leaking and had been properly cutting
off exhaust gas flow at idle.
2) If there is no change in RPM and engine is idling
properly, exhaust gases are not reaching combustion chamber. Check for
plugged passage between EGR valve and intake manifold.
3) If engine idles poorly and RPM is not greatly affected by
manually moving diaphragm up, EGR valve is not closing off exhaust gas
flow. Check for carbon between pintle, leaking EGR valve gasket or bad
EGR valve.
***************
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Old 05-07-2014, 08:12 AM
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Nice truck...besides checking what cruiser set for you for the egr you might as well check the cps...as per his instructions in his write up....
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Old 05-07-2014, 12:54 PM
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thanks i really like it. and ok i will check that too thank you. its raining here so i finally get a day off to work on it.
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Old 05-10-2014, 01:52 PM
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i got rid of he egr. and replaced the cps and now it wont even try to start.
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Old 05-10-2014, 02:11 PM
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Originally Posted by grovercamanche View Post
i got rid of he egr. and replaced the cps and now it wont even try to start.
Test the CPS.

How did you "get rid of" the EGR?
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Old 05-10-2014, 05:15 PM
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i pulled a plug and checked for spark. i have spark. i checked the fuel rail pressure again and there wasnt anything. i have put in a new fuel pump and filter in it. took a break to eat and then im going to check all the grounds on it and il update it when i get that done. do you have any other sugestions?
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Old 05-10-2014, 05:25 PM
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i pulled the egr off cut a plate to match got a new gasket and put it on.
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Old 05-10-2014, 10:34 PM
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Originally Posted by grovercamanche View Post
i pulled the egr off cut a plate to match got a new gasket and put it on.
IIRC, now you have eGR passage wide open always.
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Old 05-22-2014, 02:19 PM
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no i have puled off that hose and put in some plugs so it isnt free flowing. sorry i havent been able to get on for a while iv had a full schedule
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