Question about 1996 Jeep Grand Cherokee

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Idles rough,run ok at road speed misses occasionally

Changed plugs,wires,cap,rotor button,cleaned throttle body,changed idle air,throttle position sensor

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Hot engine , cold or both? is hot idle speed at spec RPM?
no engine stated, so all answer go to generic, see?
4.0L I-6, one of 5 million? USA car , not the export?
skipped air filter and fuel filter, why? its part of the tune up.

that is called misfire.
I use a scan tool , first to find misfire,
even a 10 buck tool works on all 1996 USA cars, no need for the $200 tool, unless you have newer cars..
its not vacuum lines, leaking, as that makes engine only race.
but if the vacuum is lost to some devices, that sure is bad.
I already tossed coin landed on 4.0L for you.
it runs via a mAP sensor and air leaks with MAP systems, make full power and race with any air leaks at all.
(study Speed Density EFI systems)

if you suspect the front 02 as bad, or exhaust leaks near it.
unplug it, that cure the misfire?
misfire :
1: bad spark.? gap plugs wrong? bad coil?
2: bad fueling,. is it running rich? spark tips black, wet or oil soaked or antifreeze soak, yes , learn to read spark tips, works for 100 years. now. this easy simple test.
white to light tan/light gray is best. on all tips.

so it misfire parked in neutral, boy that makes this easy, huh?
fix easy first and hope for better ? hot engine, misfire. 101
1: i connect scan tool, oops DTC aplenty, need i say more. NO.
2: i then have no DTC errors. so is ECT reading 180F or more, if not fix that now.
3: i then look at all sensors. live data stream. (all PIDS)
i see all read right. but 02 , LTFT and closed loop busted.
if engine fails closed loop at idle i fix that NOW.

see why,? its simple and easy to do parked, is why.
Oxygen sensor front (02):
if the 02 reads wrong, there are some causes. say its stuck at 0v.

  1. it's bad.
  2. the exhaust is cracked near it, yes, it will suck air and make it read wrong on all cars. 1988 to today.
  3. fuel pressure wrong. preventing proper fuel mix.(use a gauge)
  4. EGR stuck open. (low idle speeds?)
  5. burn valves in head, or other issues with engine mech.
  6. map bad, tests are easy ask. (takes only a $10 voltmeter)
  7. injectors leaking, clogged, dead or stuck.
  8. FPR bad. see #3
  9. IAT reads wrong,(grossly)
  10. that is about it. there are others .... ask.

to fix cars, it takes tools : GOT TOOLS?
if not, get it serviced???????

Posted on Mar 31, 2015

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This one is tricky. Any check engine lights. Could be O2 sensor. How are your vacuum lines this could be a leak issue from a worn hose. Possibly your ignition coil. Could be getting weak. I need more information

Posted on Mar 31, 2015


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