I have a 2003 jeep grand cherokee, It started

Wow, no engine option stated, and no DTCs.

4.0L I-6 or 4.7Liters?

WHY DO THAT FIRST. $$$$ parts first.

why not do a tune up first.

you swap in the most expensive parts first? ouch,

The PCM tells you when ckp or cmp fail, with DTC codes

so its not that. but you never posted ALL DTC errors. why not do that.?

do a real tuneup? heat (spark plugs, filters.)

if you have P030x errors,

eg: p0301 is cylinder #1 is misfiring (3 ways to do that not JUST spark)

eg: P0300, random on all cylinders. (fuel mix wrong) or never tuned up.



Misfire 101; (example P0301 DTC)

P030x codes, are all just about spark, misfire is caused by 3 things, (at least)

1: sure spark. weak. (tuneups work as does hV wires sets if no COPS) if 1 cop is bad move it to other location, does failure follow the move>???????? buy new cop boots if engine has cops.

COPS means coil over plug, so v6 has 6, v8 has 8.

coil packs have 3 coils inside for v6 and 4 for V8. called wasted spark to any tech.

2: low compression, think of your v6 (is it) as 6 engines, not 1.

3: wrong FUELING rates. including air leaks to 1 cylinder.

if engine has EGR that too, will cause gross misfiring.

so can an air leak near one intake runner, causing 1 cylinder to misfire.





why not scan it with a real scan tool that can see ALL jeep codes

then post all DTCs found. both idling and driving for 45mph or 1/2hr.

with these facts and what engine is there we can do the remaining tests.

free book



http://www.jeep4x4center.com/knowledge-base/index.htm?utm_source=cj#service







The tune up is this and filters.

do the tune up, new wires, and plugs using correct spark plugs not some bs, magic ones, use spec, plugs first. its in the manual.

then HV wire set on 4.0L,

if cops V8 , 4.7L try new boots on them. or swap them to find bad one.

the USA car V8 has coil on plugs





misfire means ONLY that this cylinder has weak COMBUSTION

its not weak spark only, that is only 1 cause of poor combustion.



post DTC\'s driving and we can fix it

the best tool is the

Innova 3160 scan tool, that is what I use on home cars.

it does all jeep codes, ALL.

i can drive , and scan and log DTCs.

and gather lots of facts on errors.

that is always the best way,

you can not fix cars today with out owning a scan tool.

unless , luckiest man on earth. (guessing)