For the last three years I have been helping a neighbor track down the cause of random stalling in his 97 Jeep Grand Cherokee 4.0L, which runs like a dream otherwise. There are no other driveability problems. The problem has been temporarily alleviated by changing the PCM and other fixes. He has gotten to the point where he regularly swaps the PCM for another at Napa just to keep the warranty alive. The problem predates my involvement by several years. He gave up taking it to shops. They have replaced all the usual suspects with no lasting results. Researching this problem reveals that it is not uncommon in this vintage of Jeep, but no online discussion I have read, and their have been scores, has ever resulted in a solution, which is disheartening, to say the least.
I thought perhaps a real-time scanner might give a clue. Are their any recommendations for one under $250 that might be helpful? Thanks, Jack
Terminology note fyi, the more common term for that gadget is “scan tool”.
Given the problem history, OP’s proposed method seems like a good one; i.e. figure out what the stalling problem actually is caused by, rather than replacing stuff that might be the cause, on a hope. One problem however is a 97 Jeep is just barely into the OBD II phase, so current scan tools may not work, or may not work with full functionality with the vehicle. Hopefully somebody here has some scan tool experience on early OBD II vehicles… hmmmmm… so what else? …
Are you able to read the OBD II diagnostic codes stored in memory without a scan tool? Like a dashboard light blinking the number off in a sort of Morse-code? Some early OBD II vehicles came with that functionality I think. If so, any diagnostic codes?
Unexplained stalling is understandably very frustrating. The most usual causes are problems with the vehicle’s electrical power, fuel pump intermittent failure, or ignition system problem. The first objective is to narrow down which of these three. When it stalls, does it restart immediately? If not, it cranks but won’t start , might be possible at that time to check for a visible spark at a spark plug. If no spark, that points to ignition system problem. If spark is ok, then could spray starter fluid into air intake. If it then starts briefly, then stalls, points to a fuel system problem. For the electrical system, monitory the battery voltage while driving is helpful. Should never fall below 12 volts. A diy’er battery charging system test: Before first start of the day, battery should measure close to 12.6 volts; then immediately after starting engine, 13.5 - 15.5 volts. If you post your measured voltages here, you’ll get some more ideas.
The relationship with replacing the PCM is concerning. Could indicate a corroded electrical connector. Has vehicle ever been flooded or driven in very high water?
Concur, you seem a very good neighbor for trying to help.
I presume owner has already taken Jeep to a dealership shop for a diagnosis, right?
Wouldn’t a dealership be sort of the last-chance? They might keep out of date/use specialty diagnostic equipment on hand for this sort of problem on older Jeep products.
Most OEM parts are discontinued after 10 years, by then the aftermarket has a pretty much taken over anyway… Dealers don’t like to install aftermarket parts (always an exception to every rule), and most dealer mechanics are spoiled (lol) by the newer more rust free or less rusty anyway vehicle that they work on normally… Dealers are or are supposed to be continuing education/training on the new stuff coming out and as they find common issues and how to repair them, NOT training them to work on the out of date technology no matter how much you and I may or may not like it… So unless they still have an older tech that is familiar with the older vehicles and willing to work on them, the service department may not even take it in for running issue…
I am not doing it for my neighbor. I am doing for myself. He is just benefitting from my tenacity. I am very curious about this problem. I have not read of anyone fixing this problem. I have read no discussion on the internet that has resulted in a fix for the stalling Jeeps of this vintage.
I have read many discussions in which CPS was changed on these Jeeps, but it failed to solve the problem. Should I suggest he throw $15 at a generic, no-name CPS, or $150 at a Mopar (supposedly a Mopar)?
The Jeep usually starts right back up after stalling. I have personal experience where this occurred on one of my vehicles. A few years ago it was a faulty fuel pump. New one solved that. The other was a faulty MAF sensor, just two days ago on the same vehicle. A known good one solved that problem. The bad fuel pump did not throw a code. The bad MAF did. This Jeep is throwing P0320.
I asked the question because the Jeep will often stall without throwing a code. I know from experience that a bad fuel pump will stall an engine and not set a code on some vehicles. There could be multiple causes for the stalling. Hence the interest in what a real-time scan tool might be able to do to help pinpoint the cause of a particular stalling event.
If it always started perfectly after stalling, I’d be looking for a possible throttle position problem. But the fact that sometimes it cranks but won’t start immediately after stalling, a more likely guess is one of the suspects if my first post above: electrical system, fuel pump, or ignition system.
Given this new information, there’s a stored diagnostic code in ECM memory, a faulty crank position sensor would have to be near the top of the suspect’s list. It might not be the sensor itself, could be a problem in the wiring, connectors, or grounds, even shielding.
Any of the three problems mentioned above can cause engine stall without setting a diagnostic code.
hmmm … if I had this problem myself, as the first step I’d probably use a lab o’scope to probe the output of the crank position sensor during cranking.
