98 Cherokee Classic 4.0L cooling fan starts cycling on/off immediately after starting the engine. Cycle time is very short, 3-4 seconds.

I think that is supposed to happen - it is just likely the on cycle is a little longer at an ambient of 96 and the off cycle a little shorter. 79 is still very warm and high summer temperatures for many of us and would have to be much colder for the engine to lose sufficient heat naturally; without the fan cycling.

Cars with small radiators and compact lightweight engines often have the cooling fan running in a very short time from startup.

B1GG0RK1 - 206 TU ENGINE DW8 ENGINE
DRAINING - FILLING - BLEEDING COOLING SYSTEM
1 - SPECIAL TOOLS
[1] filling cylinder (-).0173/2 .
2 - DRAINING
TU ENGINE
DW8 ENGINE
ALL MODELS
Carefully remove the expansion chamber cap .
Unscrew the radiator drain plug (1) .
NOTE : fit a pipe to the outlet to allow clean drainage of the system .
TU ENGINE
DW8 ENGINE
ALL MODELS
Open the bleed screws (2) .
DW8 ENGINE
Drain the engine by removing the plug (3) .
ALL MODELS
3 - FILLING AND BLEEDING THE SYSTEM
Before any re-filling operation, flush the cooling circuit with clean water .
WARNING : check the cooling system for leaks .
DW8 ENGINE
TU ENGINE
ALL MODELS
Fit the filling cylinder [1] to the filler opening .
Open all the bleed screws .
Fill the circuit slowly with coolant .
Close the bleed screws in the order in which the liquid flows without bubbles .
The filling cylinder must be filled to the 1 litre mark to bleed the heater matrix correctly .
Start the engine .
Maintain the engine speed at 1500 to 2000 rpm until the end of the second cooling cycle (fan(s) starting then stopping) keeping the filling cylinder filled to the 1 litre mark .
Stop the engine .
Remove the filling cylinder [1] .
Immediately tighten the plug on the expansion chamber .
If necessary fill up to the level of the maximum mark (Engine cold) .

A term relevant only to the engine computer's operation. The computer counts drive cycles for a certain few functions, like for example, a detected malfunction may require a set number of drive cycles for the computer to set a trouble code.
One drive cycle is starting the car, and the car running long enough to warm up so it goes into closed loop operation (meaning the computer is controlling or managing engine operation through sensor information it receives). When car is stopped and key turned off-end of drive cycle. It can be just a short trip, an errand to the store, or it can be a longer drive cycle. From the time car is started and goes to closed loop operation till the key is turned off-one drive cycle.

Hi kattrinahawk, If your cooling fans do not work, take a second to understand the system: 1)Fans will cycle on and off depending on engine temperature. 2)Fans will come on when Air Conditioning system is On. 3)Fans will come on if certain malfunctions in engine electrical systems. Fans are controlled via the ECM/PCM or sub computer. Fans usually have separate fuses and or relays/resistors. Ok so lets check a few things: 1)Check your engine coolant level, fan operation is depending on correct/full level 2)Turn your A/c on max settings and wait a few seconds see if fans cycle. 3)Check for obstructions or loose wiring around/on fans. 4)Inspect fuses per owners manual locations identification.

Your engine computer is bad. It's a good thing that you mentioned about the check engine light dying also, as this is a classic sign of a bad ecm. I have repaired more than one of these Jeep computers, replacing the three power capacitors located inside the computer, and they have worked for years afterwards. If you don't have the skills or knowledge to repair the ecm, get yourself a rebuilt ecm unit and swap it out.

the fan turn on point on this model comes on at a very high temp gauge reading..not far from red or about 3/4 ways up...the engine will run quite a while before reaching this level. It should never go into the red which indicates problems but the design of the system gets it uncomfortably close. I've seen these needing to idle for 10 mins or so before the engine temp reaches the fan turn on point.One good thing about this design is when the fan does come on ,one should see immediate and noticeable drop below half in just a minute or two which indicates a good operating cooling system.

An engine temperature sensor that halfway works could be telling the ECM that the engine is already warm but the problem is more likely to be a fuel pressure problem or both. A fuel pressure leak-back within the system or lack of initial prime cycle ( the prime cycle is activated for about 3-5 seconds when the key is turned to the on position regardless of whether you start the engine or not.) To check this; have someone listen at the fuel tank (unless your pump is noisy, and most of them are) and when you cycle the ignition all the way off for about 5 seconds and then to the on position ---WITHOUT TRYING TO START IT-- you will hear the pump make a buzzing sound for 3 to 5 seconds each time you cycle the key off for about 5 sec and on for 5 seconds. If you can hear the pump cycle while doing this test then try cycling 2 times before starting and if it starts immediately each time then you will probably need to replace the pump or regulator depending on a couple other tests to find out which one. If you have very high mileage and cylinder wear then it may also take a little longer to start. it should never take more than about 4 seconds to start and 2 to 3 is even better. look into those couple things and any questions or additional information --just ask

not even close to normal, 1st replace the fan sensor on the engine near the thermostat, then retest.

Best place to start is with "check engine light". Put vehicle in park, foot on brake, and cycle key on, off, on, off, on, then wait with pen & paper ready to read flashes of check engine lamp. Will be a 2 digit code each time. For example, flash, short pause & flash, will be 11, or flash, flash, short pause, flash, flash would be 22. In between trouble codes you'll get a little longer pause, then it will start to flash again if more than one code, which is quite possible. Also note that I say key to on, not start, just on.(normal run position) Then get back to us and we'll be able to better help you.

THERE IS A THERMOSTATIC SENSOR THAT SENSES COOLANT TEMP. IF THE AC OR THE DEFROSTER IS USED THE FAN WILL COME ON BECAUSE THE SYSTEM UTILIZES THE AC COMPRESSOR WHICH CAUSE THE FAN TO COME ON AND CYCLE. THE FAN MAY EVEN RUN FOR A PERIOD OF TIME AFTER YOU SHUT THE ENGINE OFF. THIS IS A NORMAL PROCESS. JUST MAKE SURE YOU HAVE A GOOD BATTERY IN YOUR VEHICLE.