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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


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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

Mazda 6 , 2.3 engine, 2003 model, have a rough idle. Clean the MAF sensor an de the trhttle body intake, reste the computer. The engine betters the idle but in some times it goes back to rough idle....what i have to check? thanks


Sometimes if the throttle has been dirty for a while the engine will have been running rich due to lack of air intake. When this happens the O2 sensor on the exhaust manifold gets covered in soot and as a result is no longer able to measure oxygen levels in the exhaust accurately. If the O2 sensor sends a low voltage signal to the ECU (high level of oxygen in exhaust) the ECU tries to compensate by increasing injection times and this further enriches the intake mix. The end result is a lumpy idle and heavy fuel consumption. You may not yet have a check engine light but it might be a good idea to have your diagnostics scanned for any stored error codes. Failing that remove the O2 sensor and check it's condition if it's black with soot then you will know what is now causing it. Either change it or, since you have nothing to lose, use a plumbers burner to heat the O2 tip to red hot to burn off residues. Hope this gives you some avenues to think about.

Sep 15, 2011 | 2003 Mazda Mazda6

1 Answer

I have a 1998 bighorn, 3.5, V6.Of late the 'Check engine' indicator is always on when the engine is running.I have noted a more fuel consumption and some shaking of the car when on low speed.Is this to do with the Oxygen sensor?


If you have check engine light on you need to have the Diagnostic Trouble Codes read and this will give you a clearer idea of where the problem may lie. A faulty O2 sensor could certainly lead to an over -rich mixture but here is another possibility: Faulty coolant temperature sensor...leads to ECU not being told when engine has reached operating temperature ..... ECU instructs injectors to maintain open cycle condition and keep the mix rich as if engine is still cold. So the causes of rich mixture and lumpy idle can be varied. The DT codes really help in focussing area for examination and ultimately will save time and money. Autozone do DT code reading as a service.

Jun 16, 2011 | 1998 Isuzu Trooper

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

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