Technical Annoying intermittent problem!

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Technical Annoying intermittent problem!

bandido41

Bandido A Maneira!!
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My car is very temperamental, when it is cold its fine but as soon as she warms there are times where she struggles to pull of and she has a flat spotuntil about 3500 rpm and then she will start pulling like crazy there are other times where even after she has warmed up she is perfect there is nothing wrong with her tis problem is very intermittent any suggestions as to what this might be, i have already changed the lambda sensor and the mechanic put the car on the computer and the computer said it was the knock sensor and the map sensor is this posible and/or likely if so whereabout is the knock sensor situated? She also becomes very heavy on petrol when this problem occurs, thye engine is a !.6 16v HLX im going crazy cos i dont want to be changing sensors unnecessarily.
 
the mechanic put the car on the computer and the computer said it was the knock sensor and the map sensor is this posible and/or likely

both are possible, but if the diagnostic wasnt done on a fiat examiner smart machine then you can't have any confidence in the results, plus you dont know if either of the fault codes are simply old codes left in the ecu from previous faults.

a knock sensor will only send a voltage signal when it detects knock. if knock is occuring and the sensor is faulty, then the ecu will not be aware that knock is occuring. this will mean you will suffer a loss of performacne and fuel economy. since knock would not happen at all times, the problem could seem to be intermittent even though the sensor is permanently faulty. to test a knock sensor you need to use an oscilloscope, which isnt really something a DIYer can usually do.

try using super unleaded for a while, this will make knock less likely. if you notice the fault then seems less common, it suggests the knock sensor is the problem.

the map sensor similarly will also cause of loss of performance and fuel economy. it is less likely to show intermittent problems since the sensor will be in use at all times, so a faulty sensor will not be sending a signal at all times, causing a constant fault. however it is not impossible for intermittent map faults to occur, especially when a wiring issue is the cause. personally i think this is less likely than the knock sensor.
 
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Thanks jug i will try using super unleaded for a while. If i were to replace the sensors would the codes have to be reset from the ecu or would they correct themselves once they were replaced? The mechanic said he was going to reset the ecu and i should drive it for a while and then take it back forhim to check the only thing is it costs me £75 just for him to put the car on the machine without actually doing anything.
 
yes there is a code clearing procedure for the ecu. you dont need to clear codes from the ecu (once fault is fixed ecu will operate as normal without the need to clear codes), but it is a good idea to clear them because it means you wont get confused by old codes next time you do a diagnostic.

the main reason few people bother clearing codes is because it costs a lot, you pay once to clear the codes, then a second time to read them, and then a third time to clear the codes once the fault is fixed. at a fiat dealer those 3 sessions would cost a total of £195. its easier to pay £65 once for a set of codes and then try to work out which ones are likely suspects for the current fault. the money you save will usually pay for the repair.
 
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