Why does my engine run very rich on cold idle

  • 1999 FORD F-550
  • 80,000 MILES
Engine is actually an F53 in a motorhome (but not on your list). Engine runs very rich and lumpy(exhaust fuel smell) when started cold, until it reaches about 75 degrees, then runs perfectly.
It is dual fuel, petrol or LPG, and exactly the same happens on both fuels (separate injection systems).
I have replaced plugs, COPs, boots, wires, injector o-rings, all EGR parts, O2 sensors (it only has upstream and no cat.), PCV and hose, fuel filter, air filter.
There are no codes, I have cleaned the MAF, a vacuum test is exactly as it should be, spraying around head gaskets has no effect, performance seems very good, live scanner readings are all sensible, although one O2 sensor is much slower than the other.
I still suspect MAF or O2.
Do you think I should remove plugs again to check them, and to do compression tests?
Should I replace MAF even though readings seem sensible on scanner?
Many thanks.
Do you
have the same problem?
Sunday, May 20th, 2012 AT 9:18 PM

1 Reply

Probable causes of an engine running rich:

1. Excessive fuel pressure can be cause by restricted fuel return line

2. Defective Oxygen Sensors.

3. Leaking injectors and fuel pressure regulator.

4. Coolant temperature sensor out of calibration.

5. Restricted air intake system.
Was this
Sunday, May 20th, 2012 AT 9:23 PM

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