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Matt
Matt, Mechanical Engineer
Category: Car
Satisfied Customers: 21997
Experience:  BEng hons Mech engineering, in auto industry 22 years
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Good evening, I own a Mazda MX5 which started to judder some

Customer Question

Good evening,
I own a Mazda MX5 which started to judder some months ago. Pressing the accelerator would lead to no immediate response, then the car would jerking forward as though the fuel/engine was just kicking in. Does not seem to happen when accelerating hard or at high speed, but the constant juddering can be bad when travelling in slow moving traffic in second or third gear.
Our local family mechanic could not diagnose the issue. Mazda dealers diagnosed a problem with the air throttle control valve. This was replaced and I saw an improvement in the car which only lasted a few weeks. Another mechanic who has examined the car said he felt there might be an issue with the fuel injection system, but that there may be another underlying problem...

Any thoughts much appreciated! Am at a loss as to what to do.
Submitted: 3 years ago.
Category: Car
Expert:  Matt replied 3 years 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 cleaning out the idle speed control motor / valve as these get clogged up with carbon and some brake cleaner washed through helps free things off.

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 3 years ago.

Hello again,


 


My thanks for your answer and time thus far - much appreciated. One thing I failed to mention - it may have a bearing or may not - when the vehicle is full of fuel it seems like it judders less?


Would this be indicative of what the fault might be?


 


Kind regards,

Expert:  Matt replied 3 years ago.
Hello

Ok
one other thing to try then is loosen off the fuel filler cap to see if this helps at all as there's a valve inside to let air in gradually as the tank empties and so prevent a vacuum developing