The tach on the dash reads the correct rpm but the scan tool (Snap On Modis)is reading random rpm from 300 to 5000 at idle.Car has random stall and no idle but drives fine

I think should be around 750.

replace the idle control sensor to get an idle speed above 800
run the fault codes as there may be more wrong that need fixing

the tach measures the RPM's of the engine. not the speed of the car. in idle park you canrev the engine and the tach will move but you are going no where.. ok.. not sure if this is what you are asking about. but if you think the tach is reading wrong the best way to find out is to attach a tach measurement tool and compare.

3cyl, or 4cyl, never ask for engine help not stating the engine options.
A/T tranny Right not STICK shift manual..
stuck in 3rd but trans is ok. (a gross contradiction if ever)
why not listen to him, if it stuck in any gear , there will be huge power loss. why not scan it."? is your mech, a non ASE? tech?
(if you mech, can say which end is bad, find a new mech)

get the errors. from the engine and the trans.
here is the easy way to see which is wrong.
1: do the FSM stated transmission stall test. this takes me 1min.
a: hand brake set hard.
b: car pointed is safe direction (mother inlaw?, LOL)
c: chock front wheels to ground. (aka; blocking wheels)
d: warm engine and cold (i try both , due to trans, fail both)
e: foot on service brake pedal. hard, say 50lbs force.
f: WOT the engine, wide open throttle (per FSM)
g: read the tachmeter, (on dash or tool) or scanner.
all USA cars 97 you can read RPM with any scan tool ever made.
h: see RPM rise and stop, this is STALL speed.
what your doing here is making poor mans DYNO.
dig?
if the tranny slips, the RPM goes out of control. tranny bad.
if the tranny dont slip RPM look like this. it loads up and goes
to near 2000 rpm. and holds, (full engine power)
if the engine is weak, the RPM will be less. (read the FSM)
google "automatic transmission stall test"
or read the FSM in the the on car test for A/T? there are many.

once you know which end is bad, we can diagnose that too.

have you done the 60k miles service yet? if not , why not do that now.
spark parts, filters, yada,yada. ,need list , its in the operators guide
and in the FSM. both.

indicates that the ratios are not shifting on deceleration
possible dirty oil in transmission , servo/ solenoid problems or TCM problem
run fault codes to check for problems

what engine, what transmission? ok ,no one ever knows....
gas, not diesel

tacho.read low, compared to what>? you have a hand held
optical tach? like me? or is this post just a post
saying why is my engine idling way below 800.
or why is my slushpump A/T stuck in 4th gear.?
ever scan the PCM? do so, or miss the boat (i promise)
lets say the tach reads, 2000 but my opto scan shows 3000.

the cluster brain gets tacho data , via a data buss (CAN) from the
PCM brain, so scan the PCM for errors
it can be 3 things.
1: bad tacho.
2: bad PCM , no way
3: bad comm buss. if you see scanned DTC errors for the comm buss fix that first.

Also the PCM , you connect any scan tool made to the 16pin DLC
and see live RPM, it its correct? and no comm errors?
then the I/P cluster brain is bad or gauge. (or scrambled)
id reset the cluster head power. the fsm covers this.
its the memory wire. to the head.
but resetting things can be dicey. if not done right.
The PCM will report all RPM errros.

the FSM is here. buggy it is...
http://www.jeep4x4center.com/knowledge-base/index.htm?utm_source=cj#service

id check all fuses.
id scan it
if head lies. and PCM scan shows good.RPM
id then reset the head, the fuse are OID named, (hot all time)
do you have your glove box , operators guide.
in mine on page 400 is the fuse box. see that.?
mine has 2, m13 fuse is mine. ill look yours up. now
ok its fuse #17 (IOD) IOD means ign. off, draw (old school hot all time) F17 is 10amps.

do not pull fuses in a random way
or ever pull the battery cables.
or you might find bad happens, lost radio codes and other issues
even security system goes nuts.

the engine can not idle right for vast reasons.
wont idle, does that mean
1: stalls.?
2: or wont regulate, hot.
3: or both.
So , no DTCs, no cell lamp glowing , running?
you replaced some very expensive parts, by guessing?
checking fuel pressure costs $20 for the tool.
do you have a $50 scan tool? if yes, you can scan all sensors.
and see if the sensors all look good. do they?
did you buy a used maf ,or new suzuki maf $$$ or the china clone.
most of the clones are no good.
best is , used, and from Cardone.(rebuilt by pros)
the China clones , read wrong, the ECU does not know its wrong
because it is not stuck at 0 or 5v. so assumes its right and is wrong.
with a scan tool you can see the MAF read wrong. do you?
what i do is plot TPS angle, RPM and MAF output and log that.
then at home read the values.

the #1 reason for stalls is leaking EGR. ever clean and service it?

The gas tank sounds like a bad ground usually located under the car at the gas tank. I do not know but maybe they both tach and fuel gague get their ground from the same place.

The serpentine belt is not the same as the timing belt! Not sure how your tach reads but at idle it will never be zero unless the car is not running. The tach tells the engine speed and if it is running you have to get some kind of reading. The idle may need adjusted or a vacuum line could be off but a zero reading at idle at a light is not correct.

To set carb adjustment, first set timing correctly, then put both a vacuum gauge and tach on the engine. adjust one idle screw to highest vacuum reading and highest tach reading, then do the other one. go back to the first and re-adjust again, then the other. I find that if you have a mechanical screw or a solenoid that holds idle speed on the linkage, turn them back so that engine is idling at the lowest possible rpm without stalling before making adjustments, then when finished, bring idle rpm up to recommended rpm using those. On an older carb, usually the idle screw covers have already been removed. If not, you need to remove them before you begin, usually best done with carb on a bench.