Vehicle is a '90 Jeep Cherokee 4.0 I6. The vehicle does have 395K on it so I don't expect it to run like new but the only reason it is still running is because my dad ordered it and was the original owner until he handed it down to me. Other than the bolt-on accessories (starter, alternator, battery, etc.) the engine is the only part that has been rebuilt and that was done at 225K. All in all it has been an amazing vehicle and it actually still runs and drives great except for this one issue.

When driving at speeds of 65mph or greater (around 2200rpm) it will have a random misfire. It doesn't do it continuously or every time I drive on the highway. It acts like the coil stops producing spark for a split second and is very noticeable. My Dad says the Jeep has done that for a while now and it's never concerned him. It wouldn't concern me either but my daughter loves riding in the p.o.s. and I'd hate for the issue to escalate suddenly and leave me stranded over something so preventable.

The Jeep idles and runs fine with no misfires at all every other time. It also won't have a misfire at the highway speeds if the vehicle has a small load on it e.g. a slight incline or me trying to accelerate. It only does it if there is minimal load e.g. flat ground @ constant speed or on a decline. I did replace the plugs, wires, cap/rotor, fuel & air filter, and thermostat along with all coolant hoses when I got it just to make sure it would be more reliable and since it hadn't been done in 100K or so. The fuel pressure regulator was replaced about 20K before I got the Jeep.

Where would y'all point your fingers?



[Edited on January 27, 2010 at 9:59 AM. Reason : -]

1/27/2010 9:58:30 AM

TPS?

[Edited on January 27, 2010 at 10:07 AM. Reason : check codes also?]

1/27/2010 10:03:34 AM

my Dad says he checked the voltage on that trying to solve a no-start issue (1 out of every 500 or so attempts to start and it wouldn't give any spark. he said if it happened to wait about 10 minutes or so and it would be fine. it hasn't happened to me yet and i've put ~5K miles on it) and it checked fine. i could go back and recheck it but i assumed if it were the TPS it would have a constant problem. spark plugs look good indicating correct mixture.
i'm not getting a CEL so i haven't even thought about checking for soft codes.

[Edited on January 27, 2010 at 10:09 AM. Reason : -]

1/27/2010 10:08:12 AM

i just said TPS because it started out very sporatic on my 4runner and got progressively worse

plugs, wires, distributor, rotor and even EGR valve got replaced..

on mine, the TPS has to be set..

try resetting it?

1/27/2010 10:24:54 AM

Are you positive its a misfire. Maybe the fuel pump is weak

[Edited on January 27, 2010 at 11:19 AM. Reason : new filter]

1/27/2010 11:17:40 AM

lots of CPS problems on those.... just throwing that out there as another sensor to check.

1/27/2010 11:41:01 AM

^actually i don't think that has been checked. i would've guessed if a sensor were faulty it would run rough at idle not just randomly at 65-70 mph under no load.

1/27/2010 1:25:56 PM

^^true and definitely worth checking just to be safe, but i'd say the chances of that being it are slim. most of the time tps failure is usually presented by different symptoms, though. in my experience, they either go completely bad, or they'll work fine from a cold start until the sensor becomes heat soaked. also, generally speaking, misfires caused by faulty ignition parts will only occur under load. or, if they do happen to exist with no load, they get worse as a load is introduced.

if it was me, i think i would look to the maf system first. the sensor/meter itself, the wiring, etc. be sure to check the air intake tubing for cracks/leaks and any other vacuum leaks in general. as i'm sure you know, any amount of unmetered air entering the engine has the potential to cause problems, especially when load is low/vacuum is high.

1/30/2010 7:29:39 PM

I have had to replace my cps because of a similar/same problem. my jeep was doing the same thing. highway speeds, jeep acted like it cut off. then boom! started working again.

no check engine light came up, but when i did the key on, off, on, off, on trick it showed a cps code. replaced it and it went away.

something to check out for sure, as mentioned previously.

cps basically tells the coil to fire so if it is having intermittent problems it could cause a no start or the "hesitation" feeling at speed or whenever. basically you're losing spark/power from the coil.

[Edited on January 30, 2010 at 8:01 PM. Reason : .]

1/30/2010 7:59:44 PM