Tip & How-To about Mercedes-Benz E-Class

Bad fuel economy and lumpy idling - possibly causes

Excessive fuel consumption and 'lumpy' idling engine

Possible faults:-



Coolant sensor - engine is signaled as being 'cold' all the time, not just at start up, causing ECU to set longer injection cycle

ø Low coolant level can prevent sensor being able to detect coolant temperature

ø Faulty 'open' thermostat allows coolant to circulate without regulation and perhaps prevents the engine from achieving normal running temperatures.


FPR - broken diaphragm allows fuel to enter the vacuum line and then into the inlet manifold. Additional source of fuel makes the fuel air mix richer causing a faster and lumpy idle.


Vacuum leak - 'high oxygen' signal from O2 sensor causes ECU to set longer injection cycle.


O2 sensor - 'high oxygen' sensor error causes ECU to set longer fuel injection cycle


MAF - 'over reads' in error the amount of air entering causing ECU to set longer fuel injection cycle


IAT - 'under reads' in error the temperature of incoming air causing ECU to set longer fuel injection cycle


Posted by on

Cars & Trucks Logo

Related Topics:

Related Questions:

1 Answer

po115


The Code P0115 is a coolant sensor error (measures temperature of engine coolant). First make sure that you have enough coolant in the engine. If there is enough coolant and you still get the error then the following explanation should help:-

What is it?
This is small electrical device for measuring the coolant temperature in the engine

Where is it located? It is usually located on the engine near to the thermostat housing. The ECT is sited on the 'hot' side of the thermostat so that it senses the coolant/engine temperature before the thermostat opens and allows coolant to flow through the radiator.

How does it work? Modern temperature sensors consist of a thermistor in a sealed unit. As the temperature rises the electrical resistance varies proportionately; some thermistors increase their resistance with temperature (PTC - positive temperature correlation) whilst others decrease their resistance (NTC - negative temperature correlation). When the engine is cold at start up the coolant sensor sends an appropriate signal to the ECU. The ECU responds by increasing the length of the injection cycles to enrich the combustion mix. This is an electronic equivalent of pulling the 'choke' out on a carburetor. As the engine warms up the signals from the coolant sensor cause the ECU to shorten the injection cycles making the fuel mix progressively leaner. The process of coolant sensor and ECU interaction explains why engines have a slightly faster idle when starting cold than when running hot.

Symptoms of faulty coolant sensor
Associated OBD2 error codes DTCs: P0115 - P119; P0125, P0126, P0128

  • 2.2a Poor starting - If the coolant sensor reports in error that the engine is warm the ECU (electronic control unit) will not enrich the fuel mix at ignition. The engine will falter at idle if it is not given additional help by the driver by pressing on the accelerator pedal to maintain speed. Once the engine has warmed up the engine will behave correctly.
  • 2.2b Fast/erratic idle, Poor fuel economy - conversely ifthe coolant sensor reports in error that the engine is permanently 'cold' the ECU (electronic control unit) will keep the fuel mix rich. This is OK at start up but will become more noticeable when the engine is hot; idle will be fast and lumpy. Fuel consumption will be high due the permanently rich fuel mix set by the ECU.
  • 2.2c Excessive emissions - the enriched fuel mix delivered in response to ECT (engine coolant temperature) signal error causes the exhaust to be heavy in un-burnt hydrocarbons. This often results in 'emission test' failure.

How to check? Most often the coolant sensor is quite separate to the temperature sender, so a correct read-out on the dash board does not necessarily indicate correct sensor function. Usinga voltmeter the resistance across the electrical terminals on the sensor can be measured. By removing the device from the car and putting the end of the sensor in a pan of hot water it should be possible to see an immediate change in resistance, it does not matter so much that the resistance goes up or down but that there is a discernable change with change in temperature. Generally high resistance equates to cold temperatures and vice versa. If there is no resistance change commensurate with temperature change then the sensor is at fault. If there is simply no resistance measurable (open circuit) then the sensor is at fault. If the sensor is working correctly check the connector, the wiring and the wiring insulation for faults and possible shorting.

How to fix? Replace if found faulty

May 25, 2014 | 2001 Hyundai XG300

1 Answer

My 2000 Grand Prix GTP runs great. However, the gas mileage has dropped to maybe 120-130 miles per tank and at idle there is a very strong odor of gas inside and around the car..noticed no drips on ground...any ideas?


Following list of possible faults should help:-

Coolant sensor - engine is signaled as being 'cold' all the time, not just at start up, causing ECU to set longer injection cycle

? Low coolant level can prevent sensor being able to detect coolant temperature

? Faulty 'open' thermostat allows coolant to circulate without regulation and perhaps prevents the engine from achieving normal running temperatures.


FPR - broken diaphragm allows fuel to enter the vacuum line and then into the inlet manifold. Additional source of fuel makes the fuel air mix richer causing a faster and lumpy idle.


Vacuum leak - 'high oxygen' signal from O2 sensor causes ECU to set longer injection cycle.


O2 sensor - 'high oxygen' sensor error causes ECU to set longer fuel injection cycle


MAF - 'over reads' in error the amount of air entering causing ECU to set longer fuel injection cycle


IAT - 'under reads' in error the temperature of incoming air causing ECU to set longer fuel injection cycle

May 10, 2011 | Pontiac Grand Prix Cars & Trucks

2 Answers

PO103 and 116 code on a 1997 jetta 2.0


P0103 - Mass or volume air flow circuit high input.

Symptoms: Engine stalls, running rough, excessive fuel consumption, excessive smoke from the tail pipe, check engine light on.

Possible Causes: Dirty mass air flow sensor, dirty mass air filter, mass air flow sensor harness is open or shorted, mass air flow sensor electrical circuit poor connection, intake air leaks, faulty mass air flow sensor.

P0116 - Engine coolant temperature circuit range/performance.

Symptoms: Check engine light on.

Possible Causes: Low engine coolant level, ECT sensor harness open or shorted, ECT electrical circuit poor connection, faulty engine coolant temperature sensor (ECT), faulty engine coolant thermostat.

Apr 27, 2011 | Volkswagen Jetta Cars & Trucks

1 Answer

My car Nissan 2.4 SE d/c year 2006, petrol, fuel injection has been giving me problems lately. a couple of years ago it started by smoking (black smoke), fuel consumption was too much, revs were inconsistent sometimes to a point of stalling. I took it to Nissan dealers and they replace the computer box and MAF sensors. It was okay for a while it has now started the same problem again. I am an offroad 25% of the time. It concerns me what could be the underlying cause or if at all that was the real problem. Please help


Possible faults:-


Coolant sensor - engine is signaled as being 'cold' all the time, not just at start up, causing ECU to set longer injection cycle

? Low coolant level can prevent sensor being able to detect coolant temperature

? Faulty 'open' thermostat allows coolant to circulate without regulation and perhaps prevents the engine from achieving normal running temperatures.

To check put a voltmeter across the coolant sensor connections and there should be a difference between the engine being cold and hot. If not the sensor is faulty. Check sensor connections and wiring.



FPR - broken diaphragm allows fuel to enter the vacuum line and then into the inlet manifold. Additional source of fuel makes the fuel air mix richer causing a faster and lumpy idle. To check the FPR have the engine at idle and pull the vacuum line off it. If the FPR is healthy the engine will pick up in revs for a few seconds and then settle back to a very lumpy idle. If fuel dribbles out of the vacuum line then replace the FPR.


Vacuum leak - 'high oxygen' signal from O2 sensor causes ECU to set longer injection cycle. Check all hoses and connections with the inlet manifold. Check the inlet manifold to block junction and around injector ports. A small squirt of WD40 on suspect areas will cause the engine to momentarily pick up in revs if there is a leak.


O2 sensor - 'high oxygen' sensor error causes ECU to set longer fuel injection cycle. A voltmeter across the sensor pins should show above 200mV (note this is AC voltage). Check again when hot as the sensor can fail progressively and lose voltage as it gets hotter.


MAF - 'over reads' in error the amount of air entering causing ECU to set longer fuel injection cycle. Located in the collar next to the air filter box on the air ducting. Make sure the MAF is clean, do not touch, just spray it with electrical cleaner when it is off the car. The MAF has three wires (0volts; 5 volts supply; signal). Check that the signal voltage varies according to engine speed (air being sucked in). If not change it


IAT - 'under reads' in error the temperature of incoming air causing ECU to set longer fuel injection cycle. often co-located with the MAF but can be nearer the throttle on the air ducting. Has two wires. Unplug the IAT from the ducting and check that its resistance changes when you blow on it. Give it clean with electrical cleaner spray. If not change it



Hope the above check list helps.

Apr 12, 2011 | Nissan Hardbody Cars & Trucks

Not finding what you are looking for?

3,090 people viewed this tip

Ask a Question

Usually answered in minutes!

Top Mercedes-Benz Experts

Colin Stickland
Colin Stickland

Level 3 Expert

22156 Answers

yadayada
yadayada

Level 3 Expert

75822 Answers

Michael Contreras
Michael Contreras

Level 2 Expert

166 Answers

Are you a Mercedes-Benz Expert? Answer questions, earn points and help others

Answer questions

Loading...