Missfires When the AC is Turned On
09-05-2017, 09:15 PM
#1
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Year: 1998
Model: Cherokee (XJ)
Engine: 4.0
Missfires When the AC is Turned On
I've been chasing this missfire issue for awhile now. First it was in the ignition so she got a full tune up, (plugs, wires, cap, rotor, & coil pack). Ran better but not 100% so both TPS & CPS were replaced & she ran great. Now she's misfiring when I turn the AC on! I haven't solved it yet & its getting worse. The check engine has come on only twice & went away an hour later. I scanned it & got a 300 & a 306 which is the 6th cylinder. The plug is good & i unplugged the injector & she ran way worse so it's working. Gave her a fuel system cleaner & it ran a bit better but only for a week.
It is still mostly when I turn the AC on so I figured I'd ask you fine people! Thanks!
It is still mostly when I turn the AC on so I figured I'd ask you fine people! Thanks!
09-05-2017, 09:49 PM
#2
CF Veteran
Poke around with a multimeter with the AC on/off and see what is happening to the system voltage (start by measuring voltage at the battery terminals). Between the AC coil, the electric fan, and the blower fan, it puts quite a load on things.
When you have the AC on, do you have the blower on full blast?
Check the voltage on all fuses in the PDC, as well as the internal junction box. See if any of them are lower than the rest. The entire system at the battery terminals should be between 13.5-14.5v with engine running. Some circuits will be lower, but anything greater than about 1.5v less than what you measured at the battery terminals (while) running with AC on you should take note of.
I don't think a 98 has the same "loaded" ignition switch that a 96 has (where major amperage is running right through the ignition switch instead of relays. But if in doubt, you can pull the steering column plastic and check the voltages there as well (at the back of the connector).
When you have the AC on, do you have the blower on full blast?
Check the voltage on all fuses in the PDC, as well as the internal junction box. See if any of them are lower than the rest. The entire system at the battery terminals should be between 13.5-14.5v with engine running. Some circuits will be lower, but anything greater than about 1.5v less than what you measured at the battery terminals (while) running with AC on you should take note of.
I don't think a 98 has the same "loaded" ignition switch that a 96 has (where major amperage is running right through the ignition switch instead of relays. But if in doubt, you can pull the steering column plastic and check the voltages there as well (at the back of the connector).
Last edited by jordan96xj; 09-05-2017 at 09:54 PM.
09-06-2017, 04:04 PM
#3
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Year: 1998
Model: Cherokee (XJ)
Engine: 4.0
Will do.
I should also add that there's no power loss at highway speeds or side roads. However as soon as I stop for a light it starts misfiring & there's a bit of hesitation only at the point of acceleration from a dead stop.
I should also add that there's no power loss at highway speeds or side roads. However as soon as I stop for a light it starts misfiring & there's a bit of hesitation only at the point of acceleration from a dead stop.
09-06-2017, 09:42 PM
#4
Worth an hour to remove and clean the IAC (idle air control) valve. I think it's only mission is to increase the idle speed when the AC is on. I find it easier to remove the whole throttle body than to fit a small torx in there. Take off the air intake hose. Unplug the TPS and IAC connectors. Remove the 4 bolts holding the throttle body. Read up on it first. You don't want to move the pintle. Just spray it and wipe it down. This will also give you a chance to clean the throttle body completely too.
I hd a similar problem. The distributor shaft was wobbly so replaced the distributor. Made it a little better. Tried to get the TPS (throttle position sensor) out but wasn't happening. I kept getting codes for cylinder 3 so went to the junkyard for replacement injectors and a throttle body with a TPS. Replaced the injector and throttle body with TPS and my problem went away. I was unable to determine if it was the TPS or the injector. The TPS screws must be put in with loctite red.
I hd a similar problem. The distributor shaft was wobbly so replaced the distributor. Made it a little better. Tried to get the TPS (throttle position sensor) out but wasn't happening. I kept getting codes for cylinder 3 so went to the junkyard for replacement injectors and a throttle body with a TPS. Replaced the injector and throttle body with TPS and my problem went away. I was unable to determine if it was the TPS or the injector. The TPS screws must be put in with loctite red.