1998 Jeep Laredo

Hi vern8650,

Thank you for the donation.

Hope the following helps.

BACKFIRING 

Check for vacuum leaks. 
Ensure ignition timing is correct. 
Ensure timing belt or chain has not jumped. 
Check distributor. 
Ensure fuel injectors are not leaking. 
Check spark plugs and plug wires. 
Check ignition and coil circuit. 
Check crank and cam sensors. 
Check TP sensor and connector. 
Check exhaust system.


HESITATION - ACCELERATION 

Check for vacuum leaks. 
Check ignition and sensor wires for proper connection. 
Check distributor cap for moisture. 
Check idle air control motor. 
Check minimum idle airflow. 
Ensure spark plug wires are okay. 
Ensure spark plugs are not fouled. 
Check MAP sensor. 
Check TP sensor and connector. 
Check ignition and coil circuit. 
Check crank and cam sensors.

© 2007 Mitchell Repair Information Co., LLC.
TESTS W/O CODES - 4.0L -1998 Jeep Grand Cherokee Laredo 9/7/2008 http://www.ondemand5.com/mric/common/asp/printart.aspx

ive been through all that stuff and no luck. i think it may be the valves or a head gasket because i can hear a puffing out of the exhaust every other second. i may have to do exploratory surgery

Hi vern8650,

I would suggest getting a compression test done first.

i did a compression test a while ago. two of the adjacent cylinders were a little low. but im not burning oil or getting no coolant in the oil. il probably have to pull of the head for inspection. im thinking it may a bad head gasket or leaking valves, its only a slight misfire at idle and a cold backfire. thats my only option now.

Hi vern8650,

Did you check and compare all the individual spark plug conditions? Any burning too rich or lean?

Low compression can be due to worn pistons/rings or leaking valves. A wet compression test would give a better indication of which is causing the problem.

i did replace the plugs and cap and rotor because the cap was a little burnt. the wires are from mopar which i replaced a few months ago. but the problem has been there for way longer than that. the plugs were pretty clean actually. just a little brown. it may be running lean. it has to be internal. could a lifter be failing? maybe not pumping up to let the valves open and close? just some suggestions? thanks

Since it is running lean, I would suggest paying more attention to vacumn leakages, low fuel pressure, clogged fuel filter or faulty injectors.

i did a compression test heres what i got. #1 155 #2 165 #3 160 #4 160 #5 150 #6 160. the first stroke went up to 90psi so i dont see any signs of super low compression. how do i check the coil circuit as far as voltages with key on and cranking?

make sure plug gap is .035 and also it could be intake manifold leaking also a leaking throdle body gasket throdle poistion senor intake air sensor

ive tried all them,plugs are new and gapped. is there supposed to be power at the coil at all times?. i see power for a second but the asd relay turns it off. is there a possibility there could be bad lifters? i did have the head redone last week and there was no abnormal wear on the bottom of them but i couldnt tell if they were bad on the inside.

hope you don't mind if i jump in here. if the coil secondary is working i wouldn't worry so much about the coil. your getting spark at each plug, correct? it's possible the coil has intermittent problems. unlikely but possible. in that case test the coil. have you connected a timing light to each wire and looked for a large variation between cylinders? with all you've done so far, wouldn't it be worth doing a cylinder leak down test? you mentioned the exhaust was making an abnormal sound.

thanks for chiming in... yeah the exhaust makes a puffing sound. like a misfire when warm. it does have a cat that i replaced a year ago and a magnaflow muffler. ive been chasing this problem for over a year now. its wierd, the engine backfire out of the throttlebody a few times then runs fine.

well there's a little more info there. popping throught the t.b. sure sounds like a timing problem doesn't it? whether it be ignition timing or valve timing. the ignition timing on your vehicle is controlled by the pcm. can you state the symptoms again? first you said it has a cold backfire but if you goose it it goes away. then you said it sounds like a misfire when warm through the exhauxt. just so we're clear on all the stmptoms. be as specific as you can. thanks

yes thats exactly what its doing. and i am getting spark as well as checking each cylinder with a timing light. the fuel pressure is also fine it doesnt even move from 49 psi. im going to put my old coil back on and see if the coil i bought may be going out

i found a new problem. my engine has low vacuum at idle. its only getting 14.5 to 15 hg. could this be a sign of a partially clogged cat?

here is a diagram of your vaccum system check all hoses for cracks or getting cloged

that diagram is somewhat close to mine. but i did check my vavuum lines and all were good. the only reason im thinking the cat is because the engine sometimes lacks power and it sounds the engine chokes down then picks up.

When the engine has low vaumn and backfires, you need to check the valve timing and test for valve leakages.

i had the head redone and all the springs were checked so i dont think they are bad,but anything is possible. next i may try to unhook the exhaust pipe to see if there isnt a exhaust restriction.

hey guys just wanna give you an update. i changed the plug wires out and it runs smoother. but there is still a occasional backfire when first started. and when getting on the tollway i feel the truck trying to hesitate to accelerate. i feel a slight jerlking too. could it be a transmission problem?.

ok now check you timming marks and also check the cam and crank sensors

i did replace the crank and cam sensors. could the fuel pump not deliver enough volume? i have 49 psi at all times. i would like to check the pump for volume because the engine does hesitate to accelerate and the engine use to run strong but in the past 2 years it started to run with not as much power.

44-54 PSI(Pounds per Square Inch) this is what the fuel pump should be putting out also check the fuel pressure regulator

its putting out 49 psi. with no drop. the only thing i didnt try is putting the gauge up to the windshield and drive because my gauge is short. ill try that next. if not i may bring it to a shop. sometimes the hardest things for us do it yourselfers may be easy for a shop.

there is a service bulliten for the pcm software to be updated the pcm controls the ignition and other functions