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Discussion Starter · #1 ·
It used to be that occasionally my YJ wouldn't start when it had been running and engine warm. But its gotten bad now. Now it strats right up but dies after a minute. I have new fuel tank, fuel pump, fuel lines, CPS, cam shaft sensor, cap/rotor/plugs/wires and cleaned the Air idle sensor and throttle body. I have also tried swapping in a buddys YJ computer but Jp still dies after a minute. I have checked fuel pressure and press regulator and that's all fine. What else?
 

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Discussion Starter · #3 ·
Tried a new fuel pump relay to no avail. Seems to stay running if I keep the rpm up. I'll clean the IAC again....this time get into the hole where the plunger fits in
 

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Sounds like an air leak after the throttle body.
Check around your intake manifold gasket/cylinder head area.
You can lightly spray fogging oil into small areas at a time and see if it sucks it in.
If it does, it will start to smoke out the tailpipe, that will verify the air leak.
 

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Take the IAC off completely but don't remove the electrical connector. When you key on, (not start, but key on), the EEC will command the IAC to retract its plunger, thus providing the most "idle air flow" possible, make sure it retracts. It could be stuck in the extended position and thus choking off the air flow too early.

Also, if you have a new IAC or one you could temporarily install and cover up the hole in the TB, you could start the Jeep up with the other IAC off and connected electrically to see if it extends when you rev up, (closing the idle air flow post idle speeds).

Although you can check for leaks at the TB, a leak will cause the engine to idle higher (bc there is more air getting to it past the IAC). It's seems that your problem is too low of an idle? Also verify that you fuel line hose clamps aren't choking the fuel flow since you installed a new filter. Keep troubleshooting the sensors though. I had this problem last week where a brand new $60 IAC wasn't working properly.
 

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Discussion Starter · #6 ·
Thanks Defcon1. I'll give it a try. I have no check engine lite on but when I read codes I get 54 Sync signal gen. So I replaced the Cam shaft sensor but no fix. Checking the wires for shorts/ground too.
 

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Discussion Starter · #7 ·
I get proper voltage output to the distributor (9.0V at supply and 5.1V at signal output) and ground is good. Moving to the Idle sensor.....I removed it from TB but left it plugged to computer as u said. When key "on" the AIS does not retract the plunger and the AIS motor feels hot. Gonna go replace it.
 

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Discussion Starter · #8 ·
Air idle sensor replaced and after an initial startup rpm of 1900 it settled down to 800rpm then hesitated then motor died. Back to Cam shaft and Crank pos sensors I guess since that's what codes I'm getting. Any other suggestions? How to check throttle position sensor?
 

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Hmm that's pretty interesting. Did it stay at 800 rpm for a bit before it died? If you're getting codes for the CrankPS and CamPS go for them and their wiring. The throttle PS is more of a "work / don't work" sensor. You can't really check it unless you ohm it out electrically. They're only like $30 but again if your getting engine light faults for the other sensors go for them first. Keep us posted on how it's going and good luck man.
 

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Discussion Starter · #10 ·
Just getting code for Cam sensor but the crank and cam sensors talk to each other and coordinate spark. The Cam shaft wires consist of Supply, signal output, and ground wires. Continuity back to ECM is good.
 

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Discussion Starter · #11 ·
And yes it idled at 800rpm for a while (maybe 30s -1min) before hesitating, sputtering, finally dying
 

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Discussion Starter · #12 ·
Last week the YJ stayed running for 30mins before dying. Now it won't start (just cranks but doent catch). On occasion it'll start for 30secs then die. So this is what I did. I waited til I had a no start situation and checked fuel and spark. Fuel pressure is a normal 31psi but there was no spark at the coil (coil is relatively new). The intermittent nature of the problem makes diagvnosing difficult but could it be caused by impending death of the alternator?
 

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Last week the YJ stayed running for 30mins before dying. Now it won't start (just cranks but doent catch). On occasion it'll start for 30secs then die. So this is what I did. I waited til I had a no start situation and checked fuel and spark. Fuel pressure is a normal 31psi but there was no spark at the coil (coil is relatively new). The intermittent nature of the problem makes diagvnosing difficult but could it be caused by impending death of the alternator?


Ok good troubleshooting man keep at it. It definitely sounds like an electrical issue for sure, 31 psi is great. Is the defective alternator currently installed? Sometimes the commutator inside the alternator will fail intermittently, it'll work some days and not work sometimes lol. Since the alternator is most upstream, start with that and have it tested. You said the coil is fairly new but new doesn't always mean working correctly. I bought a new $60 IAC and it was trash lol. Did you check the rotor in the distributor yet? Sometimes if they're dirty or corroded they'll prevent the current from getting across. But, like you said since the spark is faulting at the coil, it can only be something upstream of that that's causing this. Keep us posted man.
 

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Discussion Starter · #14 ·
Took the alternator in for testing and even though it passed the guy said there was a portion of the test that checks the rectifier (communtor?) and that this portion he heard the alt bog down and so it may intermittently not put out correct voltage. Hmmm, I guess that could explain why my coil wasn't firing. So even tho it tested fine, I decided to get a new alt (went with the 90amp vs 75amp factory original). With new alt and battery fully charged, jeep starts right up and runs! But, when its running I only measure 11.8V at the battery, as if alt isn't charging battery
 

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Took the alternator in for testing and even though it passed the guy said there was a portion of the test that checks the rectifier (communtor?) and that this portion he heard the alt bog down and so it may intermittently not put out correct voltage. Hmmm, I guess that could explain why my coil wasn't firing. So even tho it tested fine, I decided to get a new alt (went with the 90amp vs 75amp factory original). With new alt and battery fully charged, jeep starts right up and runs! But, when its running I only measure 11.8V at the battery, as if alt isn't charging battery

Awesome man that's great to hear! Glad to see that Jeep is up and running whoot whoot. The 11.8v isn't too far off, are you taking this reading at the battery or before the battery? Just wondering. And yes the rectifyer and commutator are basically the same unit, a rectifyer works threw the process of commutation so to speak. Keep an eye on the battery itself too maybe since the alternator wasn't putting out the correct amps/volts, it could have messed up the battery. It's the idea called "memory" like if you only charge a battery so far all the time that is the max it will limit itself to. Give it some time to try to see if the volts go back up, if not have the battery tested but it shouldnt be too big of a deal. But anyway good troubleshooting man and good luck :)
 

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Discussion Starter · #16 ·
Well, I figured out why battery wasn't charging. Apparently when I was removing old alt my tool shorted across alt contact points and sparks flew. I blew 2 50A fuses in the power distribution center (which is in between alt B+ and Battery+. Replaced those fuses and battery reads 14.4V when car is running. I ran it for 40mins w/o a glitch so I think its fixed! Funny how the alternator behaves when its starting to die and how it affects everthing. I had Cam shaft sensor codes and AIS idle codes. Now I wonder if they were truely bad sensors or just a fault stored by computer caused by bad voltage output in these circuits from bad alternator.
 
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