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amateur mechanic needs advice on jeep not starting
Hi, first post here, just joined so be nice. I have a 94 grand cherokee laredo, 164000 miles. I try to do whatever I can repair wise at home, since I am a teacher and cant really afford the crazy labor prices that my area shops charge. I have done basic things like brake calipers and rotors, replacing fuel filter and starter, but I am far from a certified mech. So keep that in mind if you respond, I need things in simple terms! thanks in advance.
Anyway, my jeep was running relatively well til the other day when I tried to start it in my driveway, and it was dead. had a friend who is a bit more mechanically minded then I am come by, and he told me my fuel pump was bad. So I went ahead and ordered an inexpensive one off ebay, and put it in, along with replacing the fuel filter, as I dont know when they last time it was replaced. I also replaced the fuel pump relay and checked the fuses, which all seem fine. After putting the new pump in however, it doesnt seem to be getting any power, as I dont hear anything when I have someone tun the key ( while listening at the gas door). There also isnt air or gas coming out when I check the shrader valve on the fuel rail. The relay does click when I turn the key, but otherwise the car wont start.Does anyone have any ideas what I could try next? Like I said, I am an amateur mech, so if you have any advice, keep that in mind. thanks in advance. |
amateur mechanic needs advice on jeep not starting
What engine do you have?
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Welcome to CF!
One thing I'll mention right off. The fuel pump only runs for less than 2 seconds when you turn on the key. That's just to charge the fuel rail with pressure. It won't run continuously until the computer senses that the crankshaft is turning. The computer gets this information from the CPS or Crankshaft Position Sensor. Three thing happen then. The fuel pump runs, the injectors start to pulse, and the spark plugs fire. All this happens when the PCM (computer) (Powertrain Control Module) turns on the ASD (Auto Shutdown) relay. Just for jollies, try putting one of your headlight relays in place of the ASD relay. The relays are all the same. This will prove or disprove that the ASD relay is good (or the PCM isn't turning it on) Come back and let us know what happens. It is possible there's a problem with the wiring at the fuel pump also. All the above information was just to let you know a little about how the system works. |
Originally Posted by JPXJMOAB
(Post 3133278)
What engine do you have?
I have the 4 liter inline 6 model |
Originally Posted by dave1123
(Post 3133350)
Welcome to CF!
One thing I'll mention right off. The fuel pump only runs for less than 2 seconds when you turn on the key. That's just to charge the fuel rail with pressure. It won't run continuously until the computer senses that the crankshaft is turning. The computer gets this information from the CPS or Crankshaft Position Sensor. Three thing happen then. The fuel pump runs, the injectors start to pulse, and the spark plugs fire. All this happens when the PCM (computer) (Powertrain Control Module) turns on the ASD (Auto Shutdown) relay. Just for jollies, try putting one of your headlight relays in place of the ASD relay. The relays are all the same. This will prove or disprove that the ASD relay is good (or the PCM isn't turning it on) Come back and let us know what happens. It is possible there's a problem with the wiring at the fuel pump also. All the above information was just to let you know a little about how the system works. Thanks! I will definitely try this out. My wife is having a baby tomorrow, but I should be able to get to it a bit this weekend. thanks again for the info, it is much appreciated. |
HEY, NO! Stay with the wife and baby! Otherwise, she'll be ragging on you for the rest of your life about deserting her to work on your jeep! I didn't have a choice. My mother-in-law told me to go work in the garage because she had it covered. Later that night was MY time! My wife woke up and panicked! She found my son and I asleep in the Lazy-Boy.
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Originally Posted by dave1123
(Post 3133578)
HEY, NO! Stay with the wife and baby! Otherwise, she'll be ragging on you for the rest of your life about deserting her to work on your jeep! I didn't have a choice. My mother-in-law told me to go work in the garage because she had it covered. Later that night was MY time! My wife woke up and panicked! She found my son and I asleep in the Lazy-Boy.
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Yeah, that was my one and only. Sorry about that. I guess I'm used to talking to young kids who just got married. LOL! I was 49 when I became a dad!
Okay, here's the way the jeep system works in a nutshell. When you turn the key on far enough to light up the warning lights, all the modules and sensors become live and the fuel pump runs momentarily to charge the fuel rail. When you start cranking, the CPS sends info to the PCM that the crankshaft is actually turning. The PCM then turns on the ASD relay, energizing the injectors, ignition coil, and fuel pump. Once the engine starts running, the PCM reads the data from the MAP sensor, throttle position sensor, intake air temp sensor, and once it get hot enough, the 02 sensor. Until the 02 sensor comes on line, the PCM relies on preprogramed mixture settings. It uses the camshaft position sensor to tell which spark plug to fire and when. Confused enough yet? I'm sure if I am mistaken, someone will jump on me about it! LOL! The CPS is the most critical part in this whole mess because if it's not right, the whole system falls apart. |
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