hey yall. Well I bought my 99 GC about a month or so ago. its got 72k so I thought I was getting a steal. well a week or two after buying it it showed a code for cyl 5 missfire, i replaced all plugs. ran fine for a week, then read code for coil A,B,C, and cyl 1-6 malfunction....replaced coil rail and it ran good for about a week. Then it read a code again for random missfire....rocked on for a couple days and then it wouldnt start..wouldnt start for a couple days so I repaced the crank sensor...started up and ran good for two days..now its not starting again..The battery is fine, the starter is working and the alternator seems to be fine...its turning over just not starting. Im lost now...
wont start
- Thread starter WJLEO
- Start date
greg92jeepxj
Official Supporter
Real sorry you guys are having a tough time with no start issues. A code reader is good to have or at least have autozone or someone check for trouble codes any time there is a driveability or no start problem. A scan tool is useful if it runs but won't help and better than checking for trouble codes if it's not. When we are dealing with computer controlled issues , we need computer compliant equiptment to check it. Noid lights will tell if the injection harness is firing the injectors for fuel and we still will look for spark at the number one plug wire with a good spark checker. Most modern fuel and ignition systems are computer controlled so that just makes it easier or harder , depending. If no pulse is read at the injector harness plugs , the PCM is not firing . If no spark , may be a coll(s) but voltage is controlled by PCM directly to control advance and retard. This may prompt you to want to cut to the chase and check the PCM but we should always start from the beginning so we don't miss something. Jeeps don't get started when we overlook something. As far as the PCM goes , we check,for good ground(s) which are found on the harness in the usual ground locations. Shop manuals are usually helpful to find grounds. The negative battery typically grounds on the engine block and oxygen sensor grounds and PCM grounds can be found there on some models. A PCM MUST have a good ground. Next , you'll want to make certain any fuses that feed ignition , fuel pump and PCM power are good . To fing these check points , a wiring harness is a must. You can try autozone's online site as there are wiring diagrams on it. Maybe the store can print one too. You'll want to probe what terminals are power and ground but you need a computer safe test light . They're not expensive and really good to have to avoid damaging sensitive electrical modules , PCM's and PCM driven items.
One good way to see if the PCM is feeding back with engine sensors is to check ground , reference (5 volts ) and signal ( voltages varies ) . That will tell if the PCM is communicating to sensors that are responsible for starting an running the engine. An O2 , TPS , CTS ( coolant ) cam and crank sensor , etc. are places to look for PCM reference , voltage and ground for driveability and no start issues . There is always more check,list items that vary per vehicle , engine and year but these ought to be a good place to start. With a good computer safe digital multimeter and computer safe test light , you can go far in diagnostics. Scan tools are great time savers but pricey . We have to get to know our systems and we can diagnose manually just as well but it takes longer , sorry about that. Try giving it a shot before forking over cash to the tech guy , it always angers me to find they will perform tests WE could do , AND THEY CHARGE US !
One good way to see if the PCM is feeding back with engine sensors is to check ground , reference (5 volts ) and signal ( voltages varies ) . That will tell if the PCM is communicating to sensors that are responsible for starting an running the engine. An O2 , TPS , CTS ( coolant ) cam and crank sensor , etc. are places to look for PCM reference , voltage and ground for driveability and no start issues . There is always more check,list items that vary per vehicle , engine and year but these ought to be a good place to start. With a good computer safe digital multimeter and computer safe test light , you can go far in diagnostics. Scan tools are great time savers but pricey . We have to get to know our systems and we can diagnose manually just as well but it takes longer , sorry about that. Try giving it a shot before forking over cash to the tech guy , it always angers me to find they will perform tests WE could do , AND THEY CHARGE US !
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