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00 cherokee missfire/stumble highway speed.

americanricer87

NAXJA Forum User
Location
Cincinnati
Ok guys I have been chasing this issue since i bought my jeep in February. A little background on it: 2000 Cherokee sport 4.0 guy installed a new cyl head but with original lifters. The jeep had an issue ended up being cracked head. I bought it and noticed a stumble/ missfire under a hard load and it sounded like a popping inside the exhaust. So I pulled the head replaced the lifters, it quieted the engine down but still acted up. Replaced the COIL ON PLUG, ngk plugs, crank sensor, all of the fuel injectors, and tried running seafoam in the tank. Still does it intermittently on the highway. Plugs show a sign of being lean. At idle the fuel pressure is 45psi even while power braking the jeep. Im running out of ideas as to what would be causing my issue. Someone please help maybe im missing something simple.
 
No I haven't didn't know it was possible for it to get out of time. I also haven't replaced the cam sensor. Its just a. Simple pickup type sensor, I didn't think it could go bad with out throwing a code.
 
I've had similar issues w/Mustangs in the past... turned out to be faulty O2 sensors. Not sure if that's your issue, but wanted to throw out another suggestion.
 
TPS and MAP can both cause those issues, TPS being the much more common failure.
If you replace the TPS, get one from the dealer. It's expensive but the aftermarket units i've bought are junk.
 
I am going to do the 02 sensor tomorrow. And I was thinking map or tps but didn't want to start throwing parts at it. It doesnt have a fuel filter. Just that in tank strainer correct?
 
well, dunno if you have a smart phone, but if so, $40 will get you a Bluetooth (or WiFi) to OBD2 adapter and you can pay $3 for an app to check codes & read data off your screen.
So, then it's just a matter of google searching the expected values and slowly opening your throttle while you look for dead spots / funky behavior from the TPS.
 
well, dunno if you have a smart phone, but if so, $40 will get you a Bluetooth (or WiFi) to OBD2 adapter and you can pay $3 for an app to check codes & read data off your screen.
So, then it's just a matter of google searching the expected values and slowly opening your throttle while you look for dead spots / funky behavior from the TPS.

just looked into this, hey thats pretty niffty... so u got to buy the plugin and an app and ur good to go?

seems like a "Torque" app is the way to go but is only on andriod :(
 
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I use Torque and it's got about 85-90% of what I *ever* need in a scan tool. That last 10-15% is vehicle specific stuff that is chronically out of reach for the DIYer or even a lot of shops.
Just about any OBD2 program will do the same thing, though - and I'm honestly not that impressed with torque's usability. It will do things, sure, but it makes it a PITA.
Find one for IOS & go for it. Scan tools = helpful
 
45 psi fuel pressure is too low spec is 47-52.. 50 being ideal


Also.. if the CPS is not a factory part, dump it and get one from the dealer
 
I tried a spare map sensor and intake manifold temp sensor still no luck. Pulled the cam sensor and put the engine on 1tdc and the tooth pick slid right in pointing right over the allen head screw in the middle. I really dont feel that im having a fuel issue it sounds like its loading up and popping in the exhaust. The old o2 sensor was as white as a ghost on the forward side, which would be a lean condition though right?
 
loading up i originally was thinking spark or timing or possibly too much fuel. i tried the 02 senor because i knew it had had coolant go through it and someone suggested replacing it. I am leaning toward a fuel pump i'm going to try the whole housing just gotta wait for it to come in.
 
I drove the unit and had an accurate fuel gauge attached to the fuel rail and had 47psi and the lowest it dropped was 45 only when there was a quick snap of the throttle, I don't think its a fuel problem seems more like a timing issue, crank sensor is new, cam sensor is timed correctly, could the timing chain ever have enough slop to throw off the timing under a hard pull? I have a snap on solstus and plan on trying to re sync the cam sensor.
 
could the timing chain ever have enough slop to throw off the timing under a hard pull? I have a snap on solstus and plan on trying to re sync the cam sensor.

Severely warn, yes. Normal year, no. What did the old lifters you took out look like?
 
Part of your fuel pressure test should include a volume test. I've never seen a returnless fuel system dip in pressure no matter the driving condition. It should be able to deliver the same pressure and volume in all conditions. A dip during high demand could indicate a plugged filter. (which is part of the sending unit/regulator)

Talyn could be on to something. If the bottom of any of the lifters were severely worn, you might be looking at a flattened cam lobe or two.
 
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