Misfire ? Ideas ?

[Gearhead]

Here's the deal

1994 YJ  2.5 , 50,000 km on full rebuild.
New spark plugs , distributor cap & rotor - this week
New sensors -  TPS , O2 , coolant temp - this week

Starts good , idles good , accelerates good up til 2000-3000 rpm .
It will pull through the rough spot and run properly , continuing to accelerate. The 2000-3000 range is where the stumble / missfire occurs.

More things I've checked , 35 psi fuel pressure , checked at the fuel rail, while running. Good healthy spark while cranking.
I've got an OTC monitor , pulled three codes on the weekend , TPS , O2 , CTS( volts low ) .  It's got 271,000 km on it so I figured a new TPS would be nice , the O2 sensor , normally , is a maintenance item that should be replaced periodically , was new at time of engine rebuild , went ahead and changed it , I pulled a coolant temp sensor code once before , went ahead and changed that too , just because.

Additionally , this Jeep has had intermittant hard starting , rough running issues for quite some time now  BUT , get this ........ when this happens ( almost always while starting) , you shut it off , re-start it , it then runs fine !

Some additional info ....... this Jeep has a 4.0 throttle body , no catalytic converter , a flowmaster muffler , 4.88 gears and an electric fan. All of this , in an effort to make it run sweet , which it does , most of the time.

The 2000-3000 rpm stumble is just in the last week.

I reset the computer back to factory default settings , put some fuel injector cleaner in the tank and then ran it out to strathmore and back today (hammer down ), hoping it would straighten  itself out. It still breaks up and stumbles , the little sucker will still pull in 5th gear , will do 120 in 5th , once you get through the rough spot.

Any ideas ?

Please no dumb responses , and no I'm NOT putting a 4.0 in it.

[blitherskite]

take it to a shop with a 5 gas analizer and an ignition scope ...

Its the only way to figure out whats going one.....

[yjcanibul]

I would loosen/retorque the intake/exhaust bolts incase air is being sucked in.... on exhaust this will mess up the shiney new o2 sensor ... after I replaced my head the bolts backed off a bit and it ran a little rough at start and in certain rpm ranges ... extra info - in my case the eng wont run on anything but premium ...

[Gearhead]

I would loosen/retorque the intake/exhaust bolts incase air is being sucked in.... on exhaust this will mess up the shiney new o2 sensor ... after I replaced my head the bolts backed off a bit and it ran a little rough at start and in certain rpm ranges ... extra info - in my case the eng wont run on anything but premium ...

Checked all the manifold bolts the other day , while messing with it , I check them periodically , my 4.2 used to need attention here often.

Installed a new fuel filter this evening , no improvement  :cry:

[frenchy]

Please no dumb responses , and no I'm NOT putting a 4.0 in it.

Not sure if this response meets your requirements... but.. try a different coil ?

[Gearhead]

Please no dumb responses , and no I'm NOT putting a 4.0 in it.

Not sure if this response meets your requirements... but.. try a different coil ?

I might do that , there's a used one here. The one that's on it is not very old but it's a Blue streak (USA). A couple of years ago , I had no start issues , which ended up being a weak coil and a cheapo distributor cap , the resistance , when measured between the coil terminal and the spring loaded button on the cap was wayyyy too much , changed both items ,...... vroom .


Additional bad stuff , Comanche is down ! , looks like the water pump , which is less than a year old  , is siezed , , last winter I put a new water pump , fan clutch , thermostat , radiator and some hoses on it . That's what I'll be fixing tonight .

In recent years , I've experienced a decline in quality of remanufactured replacement parts , new ( cheapo) replacement parts and such. I only buy name brand or premium components anymore , I could go on and on about the short life / low quality of replacement parts issues I've seen.

[Gearhead]

Update !  


Comanche - fixed , it was the alternator , the front bearing piled up , my good friends over at Amperage technologies took care of me . No stock on a new alternator wasn't a problem , Rich replaced both bearings quickly , tested it and had me headed for home in minutes !  

A big thank's for the awesome service !


94 YJ - Went over to Mopac , bought an MSD coil and MSD custom fit plug wire kit .  Built a nice set of 8.5 mm wires with the kit. It now runs better than it ever has.   :twisted:

Serge wins a prize (?) for suggesting to try a coil.