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Matt
Matt, Mechanical Engineer
Category: Car
Satisfied Customers: 22153
Experience:  BEng hons Mech engineering, in auto industry 22 years
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Assistant: How can we help? Car is fine when accelerating

Resolved Question:

Hi there
Assistant: Hello. How can we help?
Customer: Car is fine when accelerating but when load is taken off engine feels like car is missing or something juddering in front bottom of gearbox area
Assistant: How old is the transmission? Have you checked the fuel filters?
Customer: 2006 car with 156000 miles, fully serviced last service last May
Assistant: What is the model of your '06 Kia?
Customer: Sorento
Assistant: Are you fixing your Sorento yourself? What have you tried so far?
Customer: Not tried yet, need to know what it is first?
Assistant: Anything else you want the mechanic to know before I connect you?
Customer: How much will it cost me
Assistant: I'm not sure of the exact price, but there's only a $5 deposit. The rest of the price information will be on the page I send you to.
Customer: I'm in uk is that ok
Submitted: 1 month ago.
Category: Car
Expert:  Matt replied 1 month 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, so 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 or brake cleaner 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.

Air leaks are very temperature dependent as gaps can open up as things expand with temperature so depending on how the leak occurs ( on a hose joint with a clamp expansion will help seal and on a vacuum hose expansion ill make a leak worse) so leaks can be better or worse as the engine heats up.

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

It’s also worth cleaning the MAF sensor wires, as they can get coated with dirt over time which then offsets the reading
Use some contact cleaner or brake cleaner to spray onto the wires to remove the dirt – on no account touch the wires with anything as they are very fragile

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