Description/symptoms:
I have a dedicated cng Ford (2003 F150 5.4L V8 108k miles) that the idle rpm seem to vary or surge, more so when the engine is warm. Doesn't matter if its in gear or not. RPM seems to drop low/rough for 2 seconds and then up to "normal" for 5 seconds. When coming off a 65mph interstate drive, sometimes at the first stop the engine will almost die, but automatically rev up to catch itself, then continuing to do its variable surge idle as described above. I have no check engine light, so I assume it hasn't thrown a code (unless the light is burned out!!)
Things I have tried:
1. Checked for vacuum leaks, found a small cracked boot on pcv valve line. got a new boot, did not help. Possibly more leaks, but unlikely.
2. New coalescent filter.
3. Cleaned fuel injectors - using motorcraft PM5 injector cleaner. Placed inside the coalescent bowl as per ford tech service bulletin (TSB_03206.pdf) Seems to have a little more power now, but did not rough fix idle.
Things I have not tried but could be the problem?
1. Replace o2 sensor(s) I can only find 2 and they are pre catalytic converter. I thought there were 4. Anyone know how many their should be on a dedicated cng? Should I replace all of them? or is their a way to test for a faulty unit? Should they be replaced with stock or CNG specific sensor?
2. Bad fuel regulator?
3. Bad injector(s)?
4. Bad TPS (throttle position sensor)?
4. Any ideas of your own?
Thanks in advance for anyones help
I have a dedicated cng Ford (2003 F150 5.4L V8 108k miles) that the idle rpm seem to vary or surge, more so when the engine is warm. Doesn't matter if its in gear or not. RPM seems to drop low/rough for 2 seconds and then up to "normal" for 5 seconds. When coming off a 65mph interstate drive, sometimes at the first stop the engine will almost die, but automatically rev up to catch itself, then continuing to do its variable surge idle as described above. I have no check engine light, so I assume it hasn't thrown a code (unless the light is burned out!!)
Things I have tried:
1. Checked for vacuum leaks, found a small cracked boot on pcv valve line. got a new boot, did not help. Possibly more leaks, but unlikely.
2. New coalescent filter.
3. Cleaned fuel injectors - using motorcraft PM5 injector cleaner. Placed inside the coalescent bowl as per ford tech service bulletin (TSB_03206.pdf) Seems to have a little more power now, but did not rough fix idle.
Things I have not tried but could be the problem?
1. Replace o2 sensor(s) I can only find 2 and they are pre catalytic converter. I thought there were 4. Anyone know how many their should be on a dedicated cng? Should I replace all of them? or is their a way to test for a faulty unit? Should they be replaced with stock or CNG specific sensor?
2. Bad fuel regulator?
3. Bad injector(s)?
4. Bad TPS (throttle position sensor)?
4. Any ideas of your own?
Thanks in advance for anyones help
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