intermittent poor acceleration/burning smell

[keeforelli]

hi guys


the megane i have has behaved itself recently- however tonight it lacked power again, feeling strained towards the top of the power band, almost like i didnt have the accelerator depressed far enough down, even though i did- this was also accompanied when i pressed on harder by a kind of burning smell.....could my brakes be sticking?...other symptoms include the car almost shunting on changing gear, so when i release the power to depress the clutch, the nose drops, when i put power back on, it lifts ...like its being held back..very weird...a kind of see saw effect

does the throttle have electronic settings that could be faulty?..quite often this fault disappears overnight...and the car feels instantly responsive again...

what would it feel like if the brakes were sticking?

any help would be great!!!

[AlexB]

It is very easy and cheap to check. Drive to the dealers, pay them £80, then a very experienced and trained man will touch each disk with his finger and tell you which one is sticking. Guess, how he will know?

[keeforelli]

hi!!


am i sensing a hint of sarcasm?...and nope, you got me....how would he know?

[AlexB]

Sorry, could not resist...

If a brake is binding then the disc must be abnormally hot after "gentle" driving at 30 mph without much braking. If the brakes are healthy then the temperatures of front discs and rear discs are pairwise identical. If something slows you down noticeably, then it should be equivalent to at least 2kW power dissipation in a form of heat. It is difficult to miss. You may also want to visually inspect the surfaces to make sure they are smooth and shiny.

On the other hand, it can be something with the engine. Check the obvious things and put your findings in the context of service and servicing.

[keeforelli]

thanks!


driving it today, it intermittently pulls then hesitates within the rev range/power band. im thinking now coils misfiring slightly..will go down that route...


Ta!

[keeforelli]

ps alexb

you could earn a rodrtune as a rolling megane consultancy!!...id hire ya!!!

[AlexB]

Thanks, I am already hired to diagnose things of 65 down to 45nm feature size and having many more components than a car...

If a coil is intermittently misfiring, its colour should be significantly different from the others -- it will be black. Misfire also causes the Toxic Fumes light and a specific sound having frequency components of half frequency of the normal sound. The smell may be coming from the Cat, who is working hard to burn the fuel unburnt in the misfiring cylinder.

There are other options as well. The temperature sensor may be playing up tricking the ECU into "thinking" that the engine is cold. Then a rich mixture is used (smell) and the top range is limited. MAF sensor may cause reduced performance as well.

Reading fault codes from the ECU would be useful. Many OBD readers can also record the engine parameters when driving and plot them as diagrams for further analysis. It would be helpful.

[keeforelli]

hi alexb


is the temperature sensor the TDC sensor or completely different?...i had this changed in march.....along with the coolant sensor.....?...had 4 new coils fitted in may...(and made sure the special grease was applied....)

i do get a strong smell of fuel sometimes....so links closely to what you describe.

whats the MAF sensor function?...not heard of this one.

[AlexB]

TDC is Top Dead Centre -- the sensor is at the crankshaft or flywheel. Its function is to get time reference for ignition and injection.

MAF is Mass Air Flow -- the sensor is at the exit of the air filter box. Its function is to measure the mass of air entering the intake in a unit of time. Then the ECU matches this amount with fuel going through injectors.

There are several temperature sensors in the car. The most important are an Air Intake Temperature (AIT) and Engine Coolant Temperature (ECT) sensors. An AIT sensor is often a part of MAF sensor and its function is to supply data to the ECU, so it could consult the maps (tables in its memory) and calculate the appropriate timing and injection charge. An ECT sensor tells the ECU when to switch into the warming-up mode (rich mixture, retarded timing and retarded VVT in the cars equipped with camshaft dephasers).

Basically, you will lose performance and fuel consumption/temperature will go up in case of MAF, ECT or AIT sensor failure. The car will refuse to start if the TDC sensor is dead.

I forgot to mention the O2 or oxygen sensor in the exhaust pipe. It detects when the mixture becomes too rich by direct measurement of O2 concentration in the exhaust gasses, which may happen, for example, due to imperfections of injectors. The ECU uses its readings to calibrate the whole system, so it is also very important. Now you see why one might want to read data directly from the ECU in order to diagnose a fault -- there are too many things that may cause similar effects. Most electronic faults in sensors are detected by the ECU and recorded in its memory. Mechanical faults and some ignition problems, however, are detected indirectly by observing abnormal combinations of readings. The ECU detects many of them, but not all, as some parametric faults may be equivalent to normal operation under some sort of peculiar circumstances. In this case you put the readings in the context of the complaints from the driver...

So, the approach to diagnosis is simple. First, you understand the system, then read fault codes from the ECU. If is does not help, then analyse the readings from all relevant sensors in the context of complaints. My impression that most garages fail the first point...

[keeforelli]

thankyou.....


i understand the principle now...and have a good garage....diagnostic the next step....

[AlexB]

The principle is al l know -- I have no experience with this engine... Anyway, please let us know how it goes.

Good luck.