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Matt
Matt, Mechanical Engineer
Category: UK Car
Satisfied Customers: 20966
Experience:  BEng hons Mech engineering, in auto industry 22 years
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Wefwef My Renault Clio Grande 1.2 8 valve misfires on

Customer Question

Wefwef
My Renault Clio Grande 1.2 8 valve misfires on tickover(lumpy idle).
Otherwise it runs well.
I have changed spark plugs and leads, tried 2 coil packs, tried swapping injector and fuel rail, cleaned throttle body and tested compression(all good).
Please help me Sir --it's driving me crazy!
Oswald Hall
Submitted: 1 year ago.
Category: UK Car
Customer: replied 1 year ago.
WefwefHow are you doing?Any thoughts?Oswald Hall
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 cleaning out the throttle body 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 1 year ago.
MattThank you. This seems good sound fault finding advice plus clear instructions and I will give them to my mechanic in the morning for him to action accordinglyI should have mentioned that no faults showed up on the computer plug in testing and that the mechanic sprayed the engine with WD40 to check on any air leaksWill communicate a progress update tomorrowThank you againBest regardsOswald Hall
Expert:  Matt replied 1 year ago.

no problem Oswald

good luck

Customer: replied 1 year ago.
MattOK--progress report SirOn start up the engine runs smoothly for approx 20 seconds and then reverts to its lumpy idle. The rev counter goes up and down slightly and you can then feel the shake through the driving seat.When you then switch off the warm engine and restart it, the engine runs smoothly again for approx 20 seconds and then starts its lumpy idle again, as described above!Incidentally when pushing the brake pedal up and down quite quickly the rev counter climbs and the engine speed increases?-never known this before—related or not?The jobs we have done are as follows:Fitted:
New coil pack, leads and Spark Plugs
New Crankshaft Sensor
2nd Hand Throttle Body
New Cylinder Head Gasket—all vaves ground in and Tappetts set
Engine to Inlet Manifold pipe replaced new
Changed Injectors and all electrical connectors on Injectors
Cleaned Earth Straps
Fitted new Cam Belt(so timing is correct)Sprayed lighter fuel as per your instructions --no engine noise changeAll cylinders are 185 psi on comp testHelp help help!!!What on earth can it be???Best regardsOssie
Customer: replied 1 year ago.
Posted by JustAnswer at customer's request) Hello. I would like to request the following Expert Service(s) from you: Live Phone Call. Let me know if you need more information, or send me the service offer(s) so we can proceed.
Expert:  Matt replied 1 year ago.

Hello Oswald

thanks for the extra information

as pushing the brake pedal effects things I'm wondering if you have an leak through the brake servo?

what I'd suggest is to remove the hose to the servo and plug its end so the engine isn't pulling air in and see if that helps at all

if its does then check over the hose for any signs of leaks and if thats OK then servo may need replacing

Customer: replied 1 year ago.
MattThank youWe will try that on MondayBest regardsOssie
Expert:  Matt replied 1 year ago.

understood

good luck

Expert:  Matt replied 1 year ago.

Hi

do you still need help?

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