1996 Jeep Cherokee Classic XJ with 4.0 Liter, 6 cylinder engine. 200,000+ miles

Symptoms:

1. · My Cherokee’s normal idle is 750 – 800 rpm. I can tell when this problem is about to happen again because the engine remains at higher than normal idle (1000+ rpm) once it is at operating temperature. Idle increases to an even higher idle (about 1,200 to 1,500 rpm or so) when turning a corner and the clutch is depressed.
2. · If the engine is at normal operating temperature and then stopped for a short time (like at a store parking lot), the engine does not start up easily when it is time to leave. It has to crank for a while with the gas pedal depressed. If the car is parked long enough for the engine to cool down, it usually starts right up.
3. · Once the above symptoms have shown up, another major symptom also shows up: if the gas pedal is depressed to accelerate after cruising at a constant speed, there is a brief but noticeable hesitation, followed by a strong, jerking engine surge.
4. · On two occasions – the engine just quit when in stop and go traffic at the top of a mountain pass. I almost ran the battery down trying to get the car restarted. I got towed to a stopping area on the downside of the pass. After the engine cooled down, I was able to get it started again when pointed downhill by popping the clutch. Then it ran fine downhill 50 miles to get back home.

No ‘’check engine” light came on. And although the engine was obviously not operating efficiently – the engine threw no trouble codes! Thus, the onboard engine computer did not help at all in identifying the problem.

Trouble-shooting Steps taken – but didn’t solve the problem. I tried multiple trouble shooting steps aimed at what seemed to be a spark or fuel flow problem, or other sensors, after doing some internet research and looking at posts on the Jeep Cherokee Forum.

1. · Tuned engine – new spark plugs and distributor rotor
2. · Installed new spark plug wires
3. · Replaced the fuel filter
4. · Put BG 44K in fuel tank to clean fuel injection jets
5. · Replaced both upstream and downstream oxygen sensors
6. · Pulled out the ECM and checked that all contacts fit and performed correctly (by my mechanic).
7. · Cleaned idle air control valve
8. · Cleaned out the air intake at the manifold.

All to no avail. The engine ran a little better after the tune up, but the fundamental problem – rough running, overly high idle, surging when accelerating, and hard starting after the initial start – did continue to happen. Not all the time. But a lot of the time. Maddening. This went on for over 6 months. I was tempted to sell my Cherokee at this point I was so frustrated!

The Solution (s). The solution actually turned out to be a series of solutions:

1. The first solution was to replace the MAP (Mainfold Absolute Pressure Sensor) sensor. After doing all the above trouble-shooting steps, I asked my mechanic – what else is there to try? My mechanic said he took an educated guess and replaced the MAP sensor. The problem was immediately solved (for about a year)! With a new MAP sensor in, the engine ran great. Almost as good as new. Excellent, reliable power and starting. No more idle problems or engine surging problems or hard starting problems. (FYI – the 1996 Jeep calls for a MAP sensor that has a female electrical connector. However, mine had male connectors, and this meant my Jeep had a 1997 Jeep MAP sensor as its original equipment.

However, after about a year, the same symptoms came back (high idle upon putting the clutch in, surging, hard starting). I thought maybe I had a faulty MAP sensor. I put in a new MAP sensor and again, the problem was immediately solved. However, after about a week, the same symptoms slowly came back. I tried several more MAP sensors with the same results. The same problems returned. Again – no “check engine” light or trouble code ever showed up. After consulting with another expert, my mechanic said his expert only had one other suggestion (follows).

2. Replace the connector for the MAP sensor – that’s the second solution. I found a connector with female prongs for about $30 from NAPA, and my mechanic installed it. It immediately solved the problem. The engine worked perfectly again. But as a result of this, I got a spare MAP sensor and keep it in my Jeep should the same problem MAP sensor problem arise again.

My Jeep again worked great for the next year and a half. But recently, the same symptoms came back – hard starting after the engine warmed up, surging when accelerating, and high idle when the clutch is engaged. So naturally – I thought the MAP sensor went bad again. So, I replaced it (about a 10- minute job) with the spare I keep in my Jeep and expected the problem to be solved. But it wasn’t. It didn’t solve the problem at all. So if a new MAP sensor wasn’t the solution – what was? A new MAP sensor connector (seemed unlikely)? I just let things be for a while - hoping an idea would come to me (my Cherokee is a spare car, not my main car). For an unrelated reason, I was checking through my Jeep’s file looking for something else. While in the file, I came across a repair order for a problem I had with my Jeep back in 2002 that I had taken to a Jeep dealer to fix. I’d completely forgotten about that issue and repair 15 years earlier. I noticed on the repair order the symptoms I had reported – hard starting after initial start, surging when accelerating, and high idle when the clutch was engaged. The same problems I was having now. That Jeep dealer at the time came up with a solution (replacing the coolant sensor for the ECM - see below), which fixed the problem.

3. So the third solution was to replace the coolant temperature sensor – the one which goes to the ECM (electronic control module, or on-board computer). This is the sensor at the front, driver’s side, of the 4.0 liter engine and which goes into the thermostat housing. It gives the ECM feedback about the coolant temperature and adjusts the engine performance as needed. (There is a second coolant temperature sender sensor at the back of the engine on the driver’s side which is what sends information to the temperature gauge on the dashboard. That’s NOT what I replaced.) So I decided to give it a try. I found the sensor at Advance Auto Parts for about $25, and It took only about 5 minutes to remove and replace it. Simply unhook the connector and use a 19 mm wrench to unscrew it and replace it. And to my surprise – it solved the problem immediately. Again – the engine is working great.

Conclusion - So my conclusion is there are two sensors – the MAP sensor and the coolant temperature sensor to the ECM, or possibly the MAP sensor connector - that could be causing the symptoms I described above. All are relatively cheap to fix: the MAP sensor was about $50 - $100, the connector was about $30, and the coolant temperature sensor was about $25 - $50. Both sensors are very easy to remove and replace (although my MAP sensor requires a Torx 25 socket to remove it from the manifold). I had my mechanic remove and reinstall the new connector, but that was only a half hour job.

I hope this helps others struggling with the same symptoms.

 

Good work!

 

Excellent info!! Thanks!!