Mechanics

ENGINE RANDOMLY SHUTS OFF...

1998 Jeep Grand Cherokee • 157,000 miles

Hello I'm trying to figure out what's causing my jeep's engine to randomly shut off. I can get it running for a little but inevitably the engine just dies. When I try to start it again it won't turn over, but if I wait a while it will turn over again but just do the same. I've replaced : Spark plugs, Spark Wires, Distributor cap and rotor, cam (or crank shaft sensor) my jeep only has one or the other, ignition coil, ignition switch. I've had several mechanics look at my jeep and all weren't able to really find the problem only had positive things to say. One recent mechanic suggested I replaced the Distributor pick up which is what sends power to the distributor because he's thinking when the car gets hot it's messing with that part and causing the distributor to go out and kill the engine. But I don't want to replace it until I get a second opinion and know the likely hood that this is possible and that the side effects match my issue. Any knowledge would be very appreciated! Thanks!
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JonathanP
November 8, 2013.



First, we have to know which engine you have when discussing an engine running problem. Next, you said it won't turn over. That's a starter system problem, but you're also describing a stalling problem which would be related to loss of spark, fuel, or both.

Have you checked for diagnostic fault codes? Do you know how to do that by cycling the ignition switch?


Caradiodoc
Nov 8, 2013.
Sorry about that, it's a 6 cylinder (straight 6) 4.0, I have not been able to really check for codes I can say there's no check engine light at all, I'd have to get to towed to have the codes pulled. And no I don't know how to check for diagnostic fault codes can you teach me how? Just asking because you're the master but couldn't the engine not turn over sometimes because it doesn't have spark or fuel same as it for stalling out? If it was the starter wouldn't that be obvious? You just wouldn't hear it doing anything right? Thanks so much


Tiny
JonathanP
Nov 8, 2013.
You're confusing "turning over" with starting and running. Turning over and cranking are the same thing. Once it's turning over, if it doesn't start, that's a no-start / stalling condition. I understand now that you have a no-start condition that started out as an intermittent stalling problem. Most commonly that is caused by a failing camshaft position sensor inside the distributor or a crankshaft position sensor. Either one can become heat-sensitive, fail when the engine heat migrates up to them, then work again after they cool down for an hour.

Most of the time a failure of one of those sensors will set a diagnostic fault code, especially if they stop working while the engine is running, but those codes won't turn on the Check Engine light. Only codes related to things that could adversely affect emissions must turn the light on. Chrysler makes it real easy to read engine fault codes yourself. Cycle the ignition switch from "off" to "run" three times within five seconds without cranking the engine, leave it in "run", then watch the code numbers show up in the odometer display. Hopefully there will be one to give us some direction. If not, the next step is to determine if there's spark and fuel pressure, and if the automatic shutdown relay is turning on.

I have to run out of town for a while. I'll check back later tonight to see how you're doing.


Caradiodoc
Nov 8, 2013.
The crank and or cam shaft sensors are brand new less then a year, yes the car sometimes won't start but eventually will then inevitably stall out. I tried to check those diagnostic codes but I swear to god it wouldn't work, Idk if i'm doing something wrong it's pretty hard to not do something so easy as you described. Something I did just have happen to me though when I was trying to start it it wasn't turning on you could just hear the "e eh e eh e eh eh e eh" noise (best I can describe it lol) then I put my foot on the gas and it back fired and turned on. It back fired for the first time in a long time I don't know if that's a clue to something bad. I'm thinking fuel pump right now because when it does run it runs smooth for the most part. It kind of boggers down a little revs kinda low at idle then the car picks back up when I give it gas. I'm going to get some lucas fuel injector cleaner and see if that makes any difference, let me know what you think thanks so much!


Tiny
JonathanP
Nov 8, 2013.
Let me add a couple of observations. First of all, a paper spacer is needed to set the critical air gap of the crank sensor. If the gap is just a fuzz too big, you can get the exact symptoms you're describing; an intermittent no-start, intermittent stalling, and even a backfire. Second, if the battery was disconnected or run dead recently, the Engine Computer lost its memory and has to relearn "minimum throttle" before it will know when it must be in control of idle speed. The engine will be hard to start unless you hold the gas pedal down 1/4". It also might not give you the normal "idle flare-up" to 1500 rpm when you start the engine, and the engine will want to stall when coming to stop. To meet the conditions for the relearn to take place, drive at highway speed with the engine warmed up, then coast for at least seven seconds without touching the brake or gas pedals.

For checking fault codes, try cycling the ignition switch four times, just to be sure, but three is all that should be needed. If the starter ever tries to engage, even for an instant, that aborts the test. You'll have to wait a few seconds, then start over.

Chrysler fuel pumps almost never quit while they're running. That's common for GM pumps that let you sitting on the side of the road. When the brushes are worn in the motor, Chrysler pumps fail to start up. You won't hear the hum for one second after turning on the ignition switch, and you won't have any fuel pressure. The clue to looking at the pump is you'll still have spark.


Caradiodoc
Nov 8, 2013.
I have heard the fuel pumps on Jeeps are very reliable as well... I can hear it prime although I haven't had the opportunity to check the fuel pressure yet which will be next on my list. I can't speak for the guy who put in my cam/crank sensor but that info is good to know and i'll have my buddy take a look at it that knows a little more then I do about cars. Something really interesting happened tonight though that i'm hoping with help shed some light for you and narrow this down. This noise and response has happened in the past as well but I find it very strange so I took a video and posted it on youtube to show you. Thanks again let me know if this means anything to you. http://www.youtube.com/watch?v=ZzVke_k1OAI


Tiny
JonathanP
Nov 8, 2013.
My guess is that noise has something to do with a shorted starter motor or corroded battery cable, but to be sure, look for the starter relay in the under-hood fuse box. Bypass that to see if the same noise occurs. You can remove the relay, then use a piece of wire or a stretched-out paper clip to jump terminals 30 and 87 together, or you can remove the cover from the relay, reinstall it that way, then squeeze the movable contact.


Caradiodoc
Nov 8, 2013.
So I'm guessing that will trigger the starter and if it's a bad starter I'll hear the same noise? Also any comments about the interior light flickering on and off rapidly when I was trying to start it?


Tiny
JonathanP
Nov 9, 2013.
Also could a bad starter be the cause of my issues or do you think it's multiple things possibly?


Tiny
JonathanP
Nov 9, 2013.
Think of the fat battery cable going to the starter as a garden hose. If you step on the hose and partially kink it, the water will just dribble out the end. That water won't have enough power to do anything. Also, if you unscrew the nozzle, the water will run wide open but won't be able to build any pressure either, so it won't have the power to do anything. A corroded cable is like standing on the hose. You can't get enough current through it to run the starter motor. When you try to run water current through the restriction in the hose, you lose all the pressure. When you try to energize the starter, it draws so much current that the voltage drops. Voltage is electrical pressure. When the voltage drops enough, other things that are turned on are also affected. In this case you see the results of low voltage when the interior lights get dim, but the starter solenoid is also affected. There's actually three switches involved with the starter. The last one is that solenoid which also does mechanical work. Because of the low voltage, the spring-loaded solenoid doesn't have enough power to stay engaged, so it releases. When it does, it turns off the very high-current switch to the starter. If you turn off the faucet feeding the hose, standing on the hose has no effect on anything. When the solenoid turns off, that high current to the starter stops trying to flow. As a result, voltage goes back up. That higher voltage is what the solenoid needs to engage, so that's what it does, then the process repeats again. That solenoid typically turns on and off multiple times per second, and the lights will flicker in time.

You can expect the same results when you bypass the starter relay. We're just doing that test to verify it's a starter circuit problem and that we don't overlook something unusual. The ignition switch can not handle much current without overheating. It passes a fraction of an amp to run the starter relay. The contacts in that relay can only handle about 15 - 20 amps, but that's enough to run the solenoid built into the starter motor. When the solenoid engages, it first runs a small gear into mesh with a ring gear, then it turns on a high-current contact to pass about 150 - 200 amps to the starter motor. You have to have really beefy cables and solid and clean connections to pass that much current.

Excessive resistance anywhere in that circuit can cause a buzzing noise from the train of events I described, but besides bad connections, corroded cables, and the rare shorted starter, a bad battery will do the sane thing. You can find that by measuring its voltage while under load. It's hard to do that when running the starter because that voltage will be pulsing, but you may be able to determine the battery's condition by turning on the head lights and heater fan. A good battery that's fully-charged will read 12.6 volts. Without going into a lot of unrelated details, if the battery has a shorted cell, you'll find it's around 11 volts.

The sound in your video does not sound like the normal buzzing caused by any of the things I listed. That's why you should bypass the starter relay and verify the same noise occurs. By leaving the ignition switch off, nothing else will be trying to run.


Caradiodoc
Nov 9, 2013.
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