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1997 Jeep Grand Cherokee: backfires when accelerating..cranking…

Customer Question
1997 Jeep Grand Cherokee Idles...
1997 Jeep Grand Cherokee Idles fine but backfires when accelerating. Also will not start while cranking engine over it start once you let off the key
Submitted: 8 years ago.Category: Jeep
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Answered in 4 minutes by:
7/3/2010
Jeep Mechanic: Ricardo Hohl, Master Certified Tech replied 8 years ago
Ricardo Hohl
Ricardo Hohl, Master Certified Tech
Category: Jeep
Satisfied Customers: 4,343
Experience: Automotive instructor at Porter and Chester institute
Verified
hi, this problem seems to be a bad ignition switch, have you replace the ignition switch?
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Customer reply replied 8 years ago
No I have not. Why would a bad ignition switch cause a back fire when I accelerated but Idile fine ?
Jeep Mechanic: Ricardo Hohl, Master Certified Tech replied 8 years ago

the ignition switch has to powers one to crank the engine and the other to start the engine in your case you have power to crank the engine but the power to start is not there until you release the key and go back to accessory power, this is a sign that the ignition switch is shorting out the coil primary circuit. and causing the back fire .

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Customer reply replied 8 years ago
Yes, I agree with you on the switch causing the starting problem. But it does not explain the smooth idle to backfire when I give throttle. It will run fine for hrs till I give it gas
Jeep Mechanic: Ricardo Hohl, Master Certified Tech replied 8 years ago

if you don't have enough electrical power going to the coil primary winding wish comes from the ignition switch, then the secondary winding on the coil will also be affected ,not at idle but at hi rpm because is when the coil needs to be able to deliver the voltage constantly and if not enough then this can happing . lest take a test at the primary winding connect a voltmeter positive lead to the positive terminal of the coil and the negative end to engine ground lest check the voltage at the coil on two steps of the throttle at idle and then mid throttle or when the backfire is duplicated ,then see what voltage you get.

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Customer reply replied 8 years ago
12 v at idle and 12v when it starts backfireing. I will replace the ingnition switch tomarrow but I am certian it will not fix the backfire issue

Jeep Mechanic: Ricardo Hohl, Master Certified Tech replied 8 years ago
did you do the test is it 12 volts what is showing ? it should be 13 to 14.5 volts can you double check , and please don't replace the ignition switch if is showing the right voltage, we simply have to look some were else like the secondary coil winding may be braking up but first confirm voltage at the primary.
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