Birty Dastards Jeep Club
Tech Forum => Workshop => Topic started by: Brynjaminjones on September 19, 2018, 09:52:19 AM
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Morning all,
I'm struggling to get my head around some problems with our ZJ. I'm sure it's simple but I can't seem to figure it.
Recently it misfires fairly badly when cold on petrol. It goes away as soon as it's warmed up a bit and it never does it at all on LPG.
At about the same time it's begun to struggle to start. If you prime the fuel pump more than once it starts much easier.
All of this would point to a petrol supply issue to me, but I can't think where the issue could lie or why it would go away once warmer.
What's also weird is that the misfire goes away at the EXACT same point in the warm-up cycle every time. It's like somebody flicked a switch.
I'd wondered about the O2 sensor, but my understanding is that it operates in open loop mode when cold and only starts using the sensor once warmed. This switch over though would probably be at about the same point that the misfire goes away.
Is anyone able to help me get my head around this? Please feel free to point out if I'm blatantly being stupid...
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Connect a pressure gauge to the fuel rail and check that the fuel system retains some pressure when the engine is off. If it drops to zero after a few minutes the pump is leaking back.
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Cheers Mike, I'd wondered if that was the hard-starting issue. Any chance this could be related to the misfire?
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The switch from open to closed loop can be as little as 20 seconds after starting. It all depends on initial temps.
I don't know the ins and outs of LPG systems so not sure how LPG fuelling is controlled and how rich it can go.
With petrol, the PCM switches from open to closed loop when it thinks, based on an internal timer and initial temps that the O2 sensor has reached it's operating temperature. If the O2 sensor heating element is bad, the O2 sensor will not be within it's operating temperature range, but the PCM assumes it is and fuels the engine accordingly. The misfuelling that results causes running problems until the exhaust gases heat the sensor to it's operating range. At that point normal operation resumes and all is good till the next cold(ish) start.
Normal diagnosis of this is to just unplug the upstream O2 sensor and see what happens. If there are no running problems with the sensor disconnected the sensor should be replaced.
Normal heater resistance is somewhere around 5 Ohms.
Hard start could be fuel pump or filter/regulator. It could also be leaking fuel injector(s). If you can get a fuel pressure test kit that goes between the fuel line ane rail, and has flexi hoses each side, you can clamp off each side to test fuel injectors of pump in turn. Other ways to check for leaking injectors are removing plugs and looking for wet plugs or pulling the fuel rail and looking for drips from any of the injectors.