4m4 engine has low rpm (+/- 800 )

there is an idle up solenoid on the the carburetor. adjust the idle speed at 800-900rpm. RPM up and down is due to a/c compressor engaging. if the rpm drops too low, a/c compressor disengages. observe your idle speed with engine warmed up, turn on airconditioner and adjust the idle compensating vacuum diaphragm adjustng screw underneath the carburetor to the same rpm when a/c was off. that should correct your rpm problem

55 is max (pressure relief)
20 is typical hot engine at 800 idle. and rises as RPM rises.
your post tells neither , temp, or RPM

my guess, car unseen.
car has an A/T trans, and all A/T jump from park to drive
selection is made on a cold engine, due to the very fast idle speed.
what was the park and cold idle speed. and out door temps.
the lower out door temps, make idle speed , cold engine, even higher
but when the engine reaches 180F or more and holds, the RPM
is about 800 RPM (the FSM tells the exact amount and does vary for the unstated engine option) so at 800 the tranny don;t lurch, did it on a hot engine. so if idle speeds are wrong say so and say it with engine temps. or its all meaningless.
next :
runs rough, the engine (even parked?) is called misfire.
misfire on a good engine (full compression)
is:
1: bad spark (or never had a tune up ever) every 60k miles.
2: bad fueling. too rich, too lean (low idle) bad injectors (stuck or clogged, or wrong fuel pump pressure or bad Fuel pressure regulator)
3: bad fuel.

there is a test for each possible failure. all very simple test.
it takes $80 in gauge tools to find engine problems.

cold engine idle surge. (problem)
RPM range of surge not stated
300-700 or 800 to 1500? (range is critical to diagnosis)

bad fuel
running summer fuel in winter (running old fuel?)
needs a tune up bad, see the list in the operator guide.
idle speed controller,sticking or EGR valve same. (clean them)
location not stated, so cant guess about local air temps.?
no other symptoms, lack of engine power? etc?
was it ok last week, or better, when was the last time it was OK?
1week, 1year?
surge , causes, on a tuned engine might be:
1: too lean. so lean the ISC can not control idle. rpm low, stalling
2: too rich, flooding, stalling, spark plugs foul (look at them)
3: weak compression in 1 or more cylinders.
4: bad spark.
5: air leaks, in the plenum causing , ISC to lose control.
why high RPM?

especially?
well that means it fails hot too.
and there is a simple tests. for hot surge.
is engine fully hot? 180F and holds solid there?
say rpm is about 800
and you turn on head lights, heater fan and (not AC) and defrost.
the RPM must hold or the ISC is failing.
ISC -= idle speed controls. (all EFI has this function)

run the fault codes . Check for idle speed sensor and MAF sensor. It should be idling around 900rpm and the oil light coming on ( assuming you have enough oil in the sump) will be because of the very low idle rpm not pumping enough oil to maintain pressure.

I guess that you have a manual transmission. If that is the case you have a clutch on the way out. Slow slippage that gets worse as the plate gets hotter. If an automatic it is possible that the overdrive is not holding due to low pressure in the box. Have a pressure test done on the box--- have the fault codes read. From experience 120 kph is around 3200rpm for most cars and the 800 rpm difference indicates that the o/dive is not working as it should.

when was the last tune up or fuel filter chage and oil change done..if you cant remember, its time..repost please...

The normal idle speed is below 1,000 rpm...between 800 to 900 rpm. I suspect that the idle-air control motor has seized and can no longer automatically adjust the engine's idle speed. This part is also responsible for slightly increasing idle speed when the airconditioning is switched on to compensate for the added engine load. Some cars even have a "step-up" solenoid to increase idle.

When replacing the idle-air control motor, remember to clean the port that IAC motor goes into; there will be some carbon build-up that may prevent the part's cone-shaped valve from seating properly. A can of intake valve spray helped along with some Q-tips works well.



Hope this helped.

Have a great day !!!

Try and clean the throttle body. Carbon will gum up on the blade and the vaccume passages, which will cause a surge and stall.

Sounds like it may be low on freon.