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Matt, Mechanical Engineer
Category: Car
Satisfied Customers: 24224
Experience:  BEng hons Mech engineering, in auto industry 22 years
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My car is stuttering a lot both while accelerating and while

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Hello, my car is stuttering a lot both while accelerating and while maintaining certain speeds. I’ve had a look under the bonnet and can’t see anything obvious, however took it for a run to listen to it and while heavily accelerating I can hear air (it sounds like when you deflate a blow up mattress). Its a ford Mondeo turbo diesel if that helps.. normally I’d take it to a garage but I’m planning on scrapping it when the mot runs out in November and saved up to replace it, so I don’t really want to pay out if I don’t have to.


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 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 will make a leak worse) so leaks can be better or 1 vacuum leak path that you won't find with the above test is if the brake servo is leaking
so try clamping off servo hose to see if this has any effect

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

Customer: replied 10 months ago.
Wow, thankyou for such a detailed reply.. very impressive! Had a feeling it could be one of many different areas, i’ll Try some of your easier recommendations tomorrow during daylight although I’m very limited knowledge wise. An opinion based question for you if you don’t mind - the car is pretty worthless value wise now, In your opinion would it be be better to;1) source extra funding and get a new car now
2) take it to a garage and have it looked at professionally. If so, what sort of ballpark cost would you guess at?Thankyou so much!


well while the airleak test might look a bit long-winded its quite simple to do and air / vacuum leaks are often easily rectified, from what you describe I don't its too serious so I'd say its worth paying for a few hours of a small garage's labour to put it right.

don't go a main dealer as the labour charge will be double

so depending on your location labour rate should be £50 / hour to £80/hour

Customer: replied 10 months ago.
Hello again, managed to check under the hood today and can’t find any obvious air leaks although the oil was very low despite being topped up recently. Had to obviously drive the car to get some fresh oil and noticed that when the engine stutters during acceleration it’s kicking a fair bit of smoke out. Didn’t go on any roads over 30 so couldn’t check if it’s doing The same when stuttering at a higher speed, so I’m wondering if it could be the EGR valve? I replaced it (and the turbo hose?) around 3 years ago which solved a similar problem then, although that was more lack of power and clouds of smoke then rather than the seemingly air leak problem this time around. Could it just be a build up inside this causing the issue do you think? Planning on taking it out to have a look but not convinced this fits the issue from my limited knowledge.Also, this is my first time using this site so I haven’t rated yet which credits you the money, so I wait until finished or can I still ask you questions once I’ve rated? Sorry, complete bonehead here! Really appreciate the help though!


don't worry about asking lots of questions and I can continue to help after rating anyway

but yes if its using oil its quite possible the EGR valve may be a bit gummed up and sticking open

often a good test / modification is to fit a blanking plate between the EGR valve and the inlet manifold with just a 3mm bleed hole in the plate

so only a tiny bit of EGR can ever make its way though

this will show if EGR is the issue and some folks leave the plate in for the power gains it usually brings

Matt and 2 other Car Specialists are ready to help you
Customer: replied 10 months ago.
Sounds like we might be onto a winner here then hopefully! Just googled a blanking plate and found one for £3.99 which can be delivered early next week, so if I order one, clean out the valve tomorrow (hoping it’s quite dirty) and then fit this plate when it arrives next week hopefully that’s the problem solved.I only have to do a few small 5-10mile journeys for the next few days, as long as I clean the valve and keep an eye on the oil levels I assume I’m not putting the engine under any risk am I?Will rate you now so you get paid, really appreciate the level of expertise you’ve shown Thankyou!

sounds like its quite mild effect so yes you should be fine to carry on using the car

certainly top the oil up and keep an eye on the level

Customer: replied 10 months ago.
Hello again Matt, finally got round to fitting the new blanking plate today as I had a rounded nut and couldn’t do it on Wednesday as I didn’t have time/tools.I’ve attatched an image (if it’s worked) of a bolt that somehow managed to snap while taking it out.. the bottom end is still in the groove and there isn’t enough space to get it out from the bottom. It’s also snapped too high up to get any grip with the remaining bolt (or a replacement) to connect the bits back together, so there’s basically a loose connection above the EGR valve.I took it for a test drive and there’s no smoking now (yay!) and the turbo works fine in lower gears, but going up a hill acceleration beyond 50mph was tragically slow. I’ve refitted the turbo pump this evening in case I hadn’t tightened it properly but haven’t taken it back out to test it yet.I did get my partner to rev the engine for me while i looked at the engine and there doesn’t appear to be anything untoward going on, nothing coming out of the loose connection etc.Any thoughts or ideas? I’m working tomorrow so not going to be able to ask my local mechanic until Monday as he only works Saturday mornings! Any advice you could give would be hugely appreciated!


OK so it sounds like the EGR was faulty so thats one aspect

its possible that your loose connection is the reason for a lack of power though as you will struggle to seal the valve against boost pressure so you'll be losing air through that joint

so I'd say you have to remove the EGR valve again so you can hopefully can get onto the remains of the bolt with some vice grips to undo it

and if you can see the end of the bolt where its snapped off then sometimes you can knock it round with a small screwdriver / chisel to unwind it out of the hole - a bit of WD40 soaked into the joint beforehand will help with this

Customer: replied 10 months ago.
Hello again,Removed the EGR valve but the broken bolt is immovable. It actually looks like it’s been welded so I’m just going to have to replace the whole valve, found one that’s not too expensive which isn’t too bad. Only problem I have now is that I need two replacement flange bolts to connect it to the inlet manifold. This is where I’m stuck.. any idea what ones I need, size etc, and where the best place to get them is? I’m thinking it’s probably best just to go to my mechanic and ask!Thankyou so much!


I'm about 90% sure you need some M8 and some M6 bolts

m8 = 8mm diameter with a metric thread and they come in varying lengths

if you have one to take with you to the shop then you can match it up and I'd go to a motor factors to find the bolt

engine bolts have to be at least an 8.8 grade and anything that's 'self' coloured or black is usually a good quality bolt

Customer: replied 10 months ago.
Absolute legend, Thankyou so much!