Need some help badly

[dougs450r]

Hi I got a 98 jeep cherokee sport it's lifted 6.5 inches, has fender flairs, and full exhaust system but I'm having this problem when i start it it runs great for awhile until it gets to running temp when I'm driving it and I'm at a dead stop and then take off it will start to lose power and start popping threw the air box then I have to push the gas to the floor for it to go and it gains power back. My buddy put his scanner on it and it said tps sensor and map sensor was faulty but I changed out the tps and it still does it does anyone kno if the map sensor can cause this problem before I spend $70 on a map sensor...... If anyone else has had this problem before and could tell me wat they did to fix it it would be great thanks

 

[greg92jeepxj]

The scan tool should tell you live data of the TPS and MAP sensors . A quick test for the MAP is to pull the vacuum line off and if the MAP is working the engine idle will drop like it wants to stall and may actually stall. That will tell you the vacuum diaphragm inside the MAP is good and the signal wire is feeding back to PCM. The scan tool will read voltage reference . You should be getting about 5 volts to the MAP and the TPS and the signal reference will vary with rpm change.
These references can also be checked with a computer friendly voltmeter by back probing the signal wires for either sensor. Just a question ; is the alternator putting out within specification and is the PCM ground clean ? If a pcm does not get proper voltage due to poor grounds and bad wiring harness connection AND/OR poor alternator output , any sensor will not receive approximately 5 volts to work properly. A faulty MAP sensor or TPS will produce the effects you are experiencing but please be sure you are not having issues with alternator or PCM first before needlessly changing parts . Remember , if a PCM does not put out proper reference to a sensor , it will not receive a proper signal from it and cannot adjust spark advance and regulate fuel delivery . The scan tool by either trouble code or live stream suggested poor reference from both sensors but what about O2 reference ? How likely is it that both sensors failed at the same time ?
What reading does the scan tool have for fuel trim ? If none of the readings seem right , I.e. lean fuel for an O2 not switching lean to rich , etc. , then the PCM harness connections or more likely ground wire found with the O2 grounds on the negative battery cable to engine block are not clean and not grounding properly. Before blaming a sensor or PCM itself , power ( harness and alternator ) and ground need to be checked. It's too easy to fall into the trap of swapping sensors because the scan tool or trouble codes read a fault from a sensor . Make certain of the ground and power ( alternator and PCM ) before checking sensors.

 

[dougs450r]

Well I ended up changing the tps and the map sensors and my check engine light went off so I drove the jeep and it still is doing the same thing and after over and hour my check engine light turned back on..... I haven't checked the other stuff yet like u said but I do kno that the jeep is running rich but idk how to fix that could use some help there to..... Just let me kno wat u think thanks

 

[greg92jeepxj]

Running rich could be a number of things such as lazy O2 sensors , poor reading from coolant temp sensor , fuel pressure regulator not opening fully causing fuel pressure back up in fuel rail , canister purge valve stuck open and canister possibly saturated with raw fuel to name a few. I would first look at O2 sensors to make certain they switch lean to rich and back accordingly , if they are lazy and do not then the fuel system trim is causing injector duration to stay open longer than they should. This can be read from fuel trim on the scan tool . If injectors are just leaky , than a good O2 will just read rich constantly and try to lean out system but cause loss of power. Pull the spark plugs and see if they are saturated or a rich condition such as carbonation similar to weak spark. Not that you posted any problems starting , but you may as well look for the quality of spark with a in-line spark tester . The more systems you check and rule out as good or bad , you narrow down by process of elimination and may actually find the problem but at least find and repair faults . Since we don't know yet the exact cause , diagnostic is broadened by checking all possibilities as you are doing. If you are getting good spark , you know you must look at the fuel system. That is what I meant by process of elimination. You do want to know if the coil is good and spark advance is good above idle . The PCM controls this directly , this is no longer done by the distributor , especially on engines that have no distributor but a cam sensor .

p.S. - curious to know if the same codes are logged after clearing them.

 

[TerryMason]

Did this problem happen about the same time as the new exhaust system? Any chance your exhaust could be clogged?

 

[dougs450r]

I have no idea when I bought the jeep it started doing it after I had already got way away from the guys house..... I don't think it's backed up tho it only runs like that after it gets warmed up

 

[Catoctin Jeep]

Have someone test the exhaust to make sure the cats are working properly. I have had issues with a cold engine running great until it gets to operating temp then crap out like you have stated. The issue was a clogged cat. At least then you can rule out a clogged system.

 

[54cj3b]

My 88 acted like that a while back changed everything I thought it was than it hit me it was the fuel pump going out fixed it now runs perfect now

 

[dougs450r]

So u think a like brand new cat could be clogged...... And I ran a fuel pressure test and I have great fuel pressure but I only did that when it was running good

 

[dougs450r]

Ok so I unplugged my o2 sensors and now my jeep don't stall as bad as it use to but it still does but slightly so does anybody have a clue to wat else it might be

 

[greg92jeepxj]

By disconnecting the O2 sensors , the PCM is going default on a fuel map and alternating rich/lean . Your jeep must have been running dead rich for some time to warrant a catalytic converter swap . Hope you don't have excessive blowby too , between raw fuel and oil getting into the cat , it will just burn up again. Even if the cat is just burning raw fuel , will still burn up quick. At this point we need to be sure if you don't have a burned valve or the timing chain jumped . If you can do some checks again on fuel pressure and its still strong , compression check and ignition timing timing should be next. Before it is blamed on a drivability problem , we need to know what condition the engine is in mechanically rather than just pluging in the scan tool and look for fuel system errors like injector pulse width to determine if an injector is bad and flooding the engine . You mentioned popping through the intake , its backfiring through an intake valve so we need to know is that from a cylinder overloaded with fuel , burned valve not closing , timing off mark , does it back fire in some sort of rhythm or just intermittant. But above all , we need to know what condition the engine is in. At this point , checking compression and engine timing isn't trivial , it will rule out whether problem is mechanical or a drivability problem that may even be computer related.