Tystoy

2001 Grand Cherokee 4x4 4.0. I’m new to the jeeps and needing help. I have a decent understanding of engines and how they work. However, I’m needing troubleshooting help.

I bought the Grand Cherokee a couple weeks ago and drove it home. Other than needing an alignment, it ran perfect. We made a couple short trips to the store and just around the house everything was good. Then it sat for a couple days and when we went to move it, it moved about 100 feet and started missing and acting like the timing was all over the place. I put a scanner on it and got 17 codes. I cleared the codes and started it. It ran fine for about 15 seconds and then did the same thing. This happened three or four times. With the scanner hooked up I went to live data and saw that the timing advance was at zero. After some research I realized the cam position sensor had failed (or so I thought). I replaced the cam sensor and all six plugs. Now while watching the timing advance I started it and it ran fine for about 15 seconds with the timing staying at 1.5 to 3.0. After about 15 seconds the timing jumped to 20 and it shook, sputtered and died. Tried again and it wouldn’t crank. The timing is WAY off. After letting it sit for a minute or two it starts and runs about 15 seconds and repeats its horrific process of dying. It blows air out the intake, shakes and sounds like it’s gonna throw a piston out of the cylinder wall. Sometimes when it does there’s enough pressure in the cylinder to actually spin the motor backwards about a full rotation. Scary! As I said in the beginning of this post, I’m new to these motors and needing help properly troubleshooting this thing before I just throw money and parts at it. Also, I have a $165 scanner that does do live scanning but I’m sure it’s not the best in the world. To start, I guess my question is this, I replaced the plugs and cam sensor so where do I go from here? What’s the next troubleshooting step? What’s causing the timing to be fine for a short time and then go haywire? All of this is happening at idle. No idea what happens if I bring the RPMs up because I’m afraid it will actually sling a piston or bend a rod.

dave1123

The cam sensor has nothing to do with ignition timing. It give the PCM (computer) signals as to which cylinder is going to fire and when to time the injector pulse. The ignition timing is controlled by the PCM reading openings in the flex plate called windows so it knows exact positioning of the pistons. If I recall correctly, there are 4 windows at each of 3 stations in the flexplate and the leading edge of the 4th window is 4* BTDC.

SO, it's the CkPS (crankshaft position sensor) that senses timing and the advance is set by the computer by interpreting data from manifold pressure, throttle opening, and crank speed. The CkPS is a hall-effect magnetic switch that switches state when the magnetic field changes caused by metallic objects passing close by it. It is powered by 5 volts and creates a square wave if seen on an ocilloscope. On your engine it is mounted on the top left side of the bellhousing below the brake booster and is a b**** to change.

Tystoy

Wow, exactly the person I was looking for. You were able to take my best explanation (which probably sucked) and troubleshoot from there. Outstanding, sir! If you are saying the CkPS is my issue, I’ll dive off into changing it. At this point any tips other than “it’s a bi***” would be appreciated. Lol

dave1123

Okay....The harness is attached to the sensor and feeds behind the block and connects on the passenger side below the ignition rail connector. The reason for that stupid design is the CkPS on the 4.7 V8 is above the starter on that side and they didn't want to change the harness any more than they had to. When I changed mine, I tied a string to the connector and pulled it out on the driver's side, changed it to the new wire, then pulled it back thru. I guess I didn't have to do that, but it was easier for me at the time. We had the jeep up on a lift and loosened and removed the trans crossmember bolts, dropped the trans down on a jack and got a lot more room to get to the sensor bolts.

I'm not saying the crank sensor is the entire fault, but it's looking pretty much like it is. When the engine first starts, it's running on preprogrammed information. Once the PCM starts getting data flow from the rest of the sensors, it doesn't like what it's getting and shuts the engine down. At least that's my read on the problem. The 4.0 also uses the "waste spark system" in which 2 spark plugs fire together every revolution, one on compression and one on exhaust. Did you notice there are only 3 coils in that rail? The crank sensor usually doesn't die suddenly, but gets funky sending signals when it gets hot. If the PCM doesn't get strong signals from any sensor, it will shut the engine down. That even includes low voltage input from the battery. All relays operate on 12v, but sensors are usually only 5v. Clean electrical connections are vital.

I bought my 2000 WJ 4.0 in 2012 and the first thing I bought for it was the Factory Service Manual, downloaded PDF files from the net. It cost me $15 then, but you can get it free from several sources now. This is how I got to understand how the system works. That and my basic digital electronics knowledge.

Here's a short rundown on how the engine control system works. Turning the key to "run" activates the PCM. It turns on the fuel pump for 2 seconds to charge the fuel rail with pressure, then shuts off and waits for a signal from the crank sensor telling it the crank is actually turning on the starter, at which time it energizes the ASD and fuel pump relay. The ASD relay energizes both the ignition system and the injectors. The injectors are supplied with a constant 12v and are pulsed by digital drivers in the PCM being grounded. The ignition coils likewise are turned on with 12v and are caused to fire by the PCM breaking the ground connection to fire a spark. Once the engine begins to run, the PCM reads the other sensors for timing and fuel mixture. That's only a bare bones explanation, but something you can use for troubleshooting.

Tystoy

Great information bro!! All good stuff!! On the way home today I grabbed a crankshaft position sensor and you were right, it’s a bi***!! However, I did get it installed and it seems to have fixed the problem! Can’t thank you enough for you’re help and great explanations!! Didn’t want to say this to soon but I’m actually a Toyota guy. This is my first real dabble into a Jeep. We’ll continue to see how it goes. Lol. Thanks again!!

dave1123

Word of caution. Don't buy any Jeep newer than the one you've got because they get more complicated as time goes on. This one is on the edge of being able to be worked on without an expensive scan tool or diagnostic computer. In my opinion, once Fiat got control, quality and customer support took a nosedive. Check into the Wranglers that had factory installed death wobble! Also, when it comes to sensors, Mopar parts are the way to go. They cost more, but work properly and last a long time, especially the CkPS. It's made with a neodymium magnet whereas aftermarket one aren't.

You can always private message me and I'll help all I can.