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Marty
Marty, ASE Certified Technician
Category: Mercedes
Satisfied Customers: 53941
Experience:  Bosch Certified Master Technician with over 35 years experience in the automotive industry.
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I have a ML320 Petrol 2000. It developed a miss fire when hot.

Customer Question

I have a ML320 Petrol 2000. It developed a miss fire when hot. Changed the crank position sensor, still the same problem, OK when it is cold as soon as it get hot starts to miss fire and becomes un drivable and cannot restart. Once cooled down after 3/4 hours starts first time
Submitted: 1 year ago.
Category: Mercedes
Expert:  Marty replied 1 year ago.
Fault codes? Fuel pressure?
Customer: replied 1 year ago.
No Engine management lamps and the only faults were random misfire on cylinders 4 & 6. There was a code for the throttle position snsor but we don't know how old these are
Customer: replied 1 year ago.
The car starts and runs when cold so it's not likely to be a fuel problem? Also I disconnected ach coil pack in turn and the engine revs dropped so it doesn't appear to be the plugs or coil packs either. When hot it doesn't even start.
Expert:  Marty replied 1 year ago.
Fuel pressure can be good cold and go away when hot due to a bad fuel pump. I'd make sure you have at least 50 psi fuel pressure when it runs bad. I'd take a fuel sample from the fuel rail and make sure there is no water in the fuel. A misfire on 4 and 6 could be a sign of a plugged left side catalytic converter. A TPS code would not cause poor running, just lack of throttle response. I'd pull the front O2 sensor and drive it with the sensor removed and see if it runs ok. If it does it indicates a plugged converter. If it won't start hot you need to test for spark and fuel pressure to see what is missing.
Expert:  Marty replied 1 year ago.
From what you are telling me I would be leaning towards a bad fuel pump. Maybe a plugged cat but more likely a bad fuel pump.
Customer: replied 1 year ago.
Where is the front O2 sensor?
Expert:  Marty replied 1 year ago.
It's in front of the left side cat but I'd check fuel pressure when it won't start first. It's sounding more like a bad fuel pump.
Customer: replied 1 year ago.
It starts now, slightly lumpy but otherwise revs ok. Originally I though it was rubbish petrol from the garage. Would the fuel pump give the same problems only when hot though?
Expert:  Marty replied 1 year ago.
If the car had bad fuel it might still have some bad fuel. If the car had bad fuel it could also have gummed up the pump. The fuel filter should also be checked.
Expert:  Marty replied 1 year ago.
And yes, the pump can work well when cool but start to slow down and not put out enough pressure when it gets hot.
Customer: replied 1 year ago.
I am no mechanic I am an electrician. Where would you recommend I start, I know it's a difficult one but I seem to think that you think its a fuel problem rather than a sensor etc
Expert:  Marty replied 1 year ago.
Because the car runs ok when cold and progressively gets worse to the point that it won't run I'd bet on the fuel pump. The cranks sensor would not cause this problem. It either works or it doesn't but the fuel pump can work but provide too little pressure. As the pressure drops you lose power and develop misfires until the car eventually dies. I'd recommend connecting a fuel pressure gauge to the fuel rail to monitor the fuel pressure when the car dies or when it won't start. If it drops below 50 psi or it starts fluctuating back and forth I'd replace the pump and the filter.
Expert:  Marty replied 1 year ago.
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