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Matt
Matt, Mechanical Engineer
Category: Car
Satisfied Customers: 21627
Experience:  BEng hons Mech engineering, in auto industry 22 years
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VW polo 1.4 16v. No lights on, but when running at 30mph in

Customer Question

VW polo 1.4 16v. No lights on, but when running at 30mph in 4th / 5th gear at 1500rpm it judders / hesitates.
Submitted: 1 year ago.
Category: Car
Customer: replied 1 year ago.
When should I expect an answer please.
Expert:  Matt replied 1 year ago.
Hello This could be an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and can lean the engine out and can also allow the engine to rev up when not desired causing rough running.As its a mechanical fault it tends not to turn on the fault light and you can sometimes hear a 'hissing' noise with the engine running. Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts. The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak.Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements. It’s also worth getting the fuel pressure checked as if this is low due to a blocked filter or faulty regulator or even a poorly pump will all result in insufficient fuel being delivered to the engine Might also be worth checking the wiring and connector to the airflow meter for any signs of corrosion or damage. you can do a quick fault find if you unplug the meter and run the engine without it.if the engine condition is the same then chances are the meter or the connection to it is faulty Air leaks are very temperature dependent as gaps can open or close up as things expand with heat, so the weather and engine temperature can effect them. This leads them to be quite intermittent in the case of mild leaks its also worth cleaning the airflow meter by removing it from the car and spraying the exposed sensor wires inside the tube with a brake or switch cleaner - ensure that the cleaner is one of the old fashioned, non Eco type that does not leave a residue On no account touch the sensor wires with anything physical as they are extremely fragile
Customer: replied 1 year ago.
Shown your reply to my mechanic, but he says it does not have an air leak, and it doesn't have an airflow meter. Obviously I don't know.
He didn't rule out the poorly fuel pump.
Even though he has never changed an accelerator pedal, we are wondering whether a fault there could cause the problem. The car seems ok until it gets to temperature.
Expert:  Matt replied 1 year ago.
Ok thanks for the extra information and yes a faulty throttle pedal could be the issuethe best way to diagnose this is to monitor the throttle pedal signal with an diagnostic tool that can read in real time, so you can see the quality of the pedal sensor signalalso if you were to remove the intake so you can see the throttle then have your assistant slowly push down on the pedal as the throttel should open smoothly with no jerking or steps another possibilty is the MAP sensor itself is faulty as its the main load sensor for the engine this can also create a juddering as you describe again the best way to diagnose this (without just fitting a new sensor) is to monitor the pressure signal with a diagnostic tool to check that its reading steadily with no sudden jumps in the reading
Customer: replied 1 year ago.
Purchased a MAP sensor but won't be fitting until Monday. Hope it cures the problem. Will be in touch.
Expert:  Matt replied 1 year ago.
Hello OK good luckjust get back to me on this post
Matt and other Car Specialists are ready to help you
Customer: replied 1 year ago.
Put new MAP sensor on. Car is a bit better but not totally cured.
I'll use car and see if it gets better with time.
Customer: replied 1 year ago.
Map sensor fit, and after a couple of days car seemed to settle down. It may also have been down to the Shell Nitro petrol I filled up with. It did however throw the EPC and EML on a couple of times. EPC goes out at restart and first time EML went out after about an hour. When they came on last I got the EML code. It was something to do with crankshaft B position actuator intermittent?? I got light put out and will see what happens. This morning car is running fine and no lights.
Expert:  Matt replied 1 year ago.
Hello Ok to get a crank position sensor error message then it would be prudent to consider replacing that sensor as if its fails it will stop the engine also an EPC fault can be a faulty wheel speed sensor ?Ideally if you can get a fault code read off to confirm this However with ABS systems 99% of the time its a problem with one of the wheel speed sensors and I'd first check the connections at all four corners first for any signs of corrosion or chafed wiring.The sensor is located in the wheel hub so you'll have to remove each wheel in turn to inspect them all.Also check that the toothed wheel that the sensor points to is clean and free of any debris.you can check each sensor's function by plugging the two output wires into an AC voltmeter and spinning the wheel quite quickly, you should get a voltage signal that increases with rev's. Its possible that the fault is only a bad connection to one of the wheel sensors.This may be remedied by removing each wheel in turn and unplugging each sensor and cleaning the contacts with a squirt of electrical contact cleaner. Then refitting.
Expert:  Matt replied 1 year ago.
Hido you still need help?Bear in mind that the site takes a deposit from you at the beginning and this is held by the site until you rate my answer at which point the cash is split between the site and the expert. I am only paid for my work on this question if you rate my answer, using the star system at the top of the screen. Please do not forget! Thank you

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