darren509

Im working on finding a problem for my friends 98 Cherokee 4.0 the idle fluctuates between 500 and 1000 RPM Starts up just fine and seems to be cutting out or misfiring throughout the rpm range when you rev it. I have cleaned the iac valve and throttle body tested the tps ( .084 volts at idle at 4.0v at wot signal voltage pin has 5v at idle and 5.10 at wot and with a digital vm i did not see any dead spots but dont have an analog meter. I sprayed the intake mate surface with throttle body cleaner to check for leak because it was all I had at the time no signs of leaks. Checked map sensor vaccum line and its good. Cap and rotor look almost new as do wires. Have not pulled plugs yet(edit plugs are good).The ECM has been taken apart and soldered the first 3 pins inside because prior to that the car would die randomly and if you slightly touched the connector it would die. Car does not die anymore at all. NO ENGINE CODES either. What else am I missing besides a better way to check for intake leak ? Rest of the history of the car is unknown as it is not mine. Thank you..

darren509

update. So I checked the fuel pressure and key on is 40psi and running the needle bounces rapidly between 45 and 50 PSI. steadily and rapidly bounces. May be my gauge. The car now runs like crap. Cuts out and hesitates all the time. I unplugged the MAP sensor and the check engine light came on. reset code and unplugged IAT sensor, check engine light came on, reset and unplugged TPS and it ran worse but it did not throw a code. Is there a problem with my TPS or possibly an open in the signal wire

darren509

Anyone have any ideas ?

97grand4.0

If the old gal has over 100k and the needle bounces like you say it likely fails the 'leak down test' for fuel pump. The bouncing needle means the regulator/filter is shot too , all part of the same assembly. You should do the leak down test but I would tell you you're looking at replacing the fuel pump module which contains all the above and the tank level sensor too. Don't do what I did and try to replace one at a time you will just end up replacing them all in the end. You can get a new module on ebay for about $60 and they are fine quality. Fairly easy install with the plastic fuel tanks. watch some videos and get an idea what's involved first. leak down test ..key on then off and should hold pressure for 5 mins at least 25 lbs. If it goes straight down then you are looking at hte regulator for sure. Change the whole module you will be starting right up and revving hi again.

darren509

Tested the pressure. Key on 40 psi and shut key off, waited 10 minutes and psi was at 35

97grand4.0

Ok, so the leak down test passes,.. but you really want to see the needle hovering around 50 when running. The injectors fire around 50....40 with key on engine off sounds right....how many miles on this? Cutting out and hesitating, even when revving at idle, that 'could' be fuel pressure related, but sounds more like ignition..has the crank sensor been changed? Usually that will be hard to start though...Major mechanical, like bad valve? worn timing gear with missing teeth is somewhat common on high milers, but even that wouldnt make it just cut right out...can you maybe isolate a misfiring cylinder?
he really should be throwing the whole tune up on it just plugs, wires cap rotor...then go from there...cant tell a plug by looking or wires either..they cheap $2 at walmart..get the champions that are recommended...gap at .35..also the pressure should be more like 50, does the pump sound healthy when you first turn the key to on? tps sounds ok, iac probably needs to be changed but that wont make it cut out and buck..hope that helps

dave1123

So now you know your fuel pressure regulator is good and your injectors aren't leaking. It could be the coil is shorting internally. Do you have a known good coil to try?

97grand4.0

His pressure seems a bit low though, I wouldnt rule out the regulator/filter as the filter could be blocked. The factory lists it as filter/ regulator. Mine had the steel filler neck, which had rusted to the point the filler hose fell off it, rust in the tank big time. Got the same filler nek at a bone yard, but in plastic. no probs. The elephant in the room is that he mentions the PCM had been worked on and soldered.

darren509

Prior to me working on this it we taken to 4 different mechanics to fix a random problem with the car dying but would start back up easily. All of them had a problem reading the computer and none bothered to find out why. So I researched it and found out if I touched the ecm connector it would die. Re soldered the first 3 pins and fixed random die problem. So communication issue was always there.not sure what brand plugs will check today. Do not have an extra coil and one .mechanic replaced fuel pump because he thought it was bad . So it's only a year old

Khawk2

D509, I chased a random miss in my 97 GCL 4.0and after many of the same things you have checked I found if I "wiggled" the TPS connector at idle, the rpms would go up or drop off. I got a replacement connector from Auto Zone (about $35) installed it and that solved the problem. Just another thought.

darren509

I wiggled the tps sensor the day I checked it and it seems fine as far as I know the crank sensor is stock.

darren509

After reading all the posts again I realized I left out a key piece of info. The scanner will not read the ecm while the car is running only with KOEO. The 4 mechanics working on it prior all could not read it while engine was running and my solder job was done after them working on it. I have a separate post in the electrical section regarding this. This has made troubleshooting very difficult. I apologize. And thank you all so far for your help by the way.

rangermonroe

Check and clean both battery terminals. It sounds silly, but I have had two GCs absolutlely go haywire with corrosion between the stud and the cable terminal. They looked clean from the outside, but had a white film that was causing just enough resistance to make the thing run like it had a bad timing belt.

Its a free, quick, and easy check that just might save you a ton of headache.

darren509

Did the battery terminals about 4 days ago

97grand4.0

Yep, you right that does sound silly. The alternator would keep it running.