2.0i Hesitation / Surge on Light Throttle

[Mazda6Driver]

Hi all,

I'm after some suggestions as to what could be wrong with my new 2.0 petrol Mazda 6.  It's only done 31k miles had 2 owners (before me), and has a full Mazda service history.  It's due it's 4th (50k) service in December or in 3k miles time.

Under light throttle, the car seems to hesitate and surge randomly very slightly when cruising at a steady speed (usually at around 2k rpm).  It is very slight - Initially I put it down to the slightly nervous ride quality, but recently I'm either noticing it more, or it's becoming worse.  It also seems to the same on over-run - coasting down hills in gear with no throttle. 

Under full throttle, the car is smoother, but seems a little down on power - it struggles up steep hill on the way to work in 5th that I wouldn't expect it too (my wifes 1.6 peugeot 308 copes fine).  I can't be sure (since I've not had the car for long enough) but I suspect the economy's down too - I'm averaging 37mpg on my 20 mile commute to work (mainly open country roads mostly 50 - 60 limits).

Has anyone had anything similar?  I'm not hugely experienced in DIY car maintenance, but this is the first car I've had a ages that's not still under warranty so if I can diagnose or repair the car myself it'd be a bonus.

Reading around, it sounds like it could be almost anything.  The ignition system - plugs or coils, the maf sensor, the throttle body, the EGR....

So far I've checked the air filter which doesn't look too bad.  I was going to check the condition of the plugs tonight, but got the engine cover off, saw the coils then got scared  .  I also checked the oil - which looks a little high - a bit over max but I assume it was overfilled at it's last service / by the previous owner.  There's no mayo under the filler cap, and the coolant is just under max.  What would you check next?

Am I likely to be doing any damage driving while it's running like this?

Any help or suggestions much appreciated.

Thanks in advance.

Dave

[Steve_c]

I don't have any direct experience of problems with Mazda petrol engines, but what you're describing doesn't sound like a plugs / coils problem.  37 mpg doesn't sound too bad for a petrol engine, particularly if you're having to contend with steep hills on your daily commute.

Hesitancy / surging sounds like a fluctuating input to the engine management - maybe a throttle position sensor or an idle control valve (if there is one).  If it was the MAF sensor, I'd have expected it to behave consistently as there are no moving parts, i.e. always reading low or high but not fluctuating.  It might be worth checking to see if you've got any splits in the air intake hose, as that might be letting in unmetered air, but I'd expect that to be worse under full throttle unless the extra suction is pulling the split closed.

I'm no mechanic, so I could be miles wide of the mark.

[Mazda6Driver]

Thanks for your input Steve - will certainly check my intake hose.

I've just ordered a cheapy ODB2 bluetooth adapter from amazon, will hopefully be able to do some live data logging which should help.

Just to add, the car idles ok - perhaps a little lower than I'm used to in the range of 650 - 750 rpm, but doesn't sound lumpy and doesn't hunt.  The idle speed does change when the cooling fan comes on - can't remember if it goes up or down and the cooling fan does come on a lot (engine temp gauge never moves from normal) which I believe is just a characteristic of this car? 

Thanks

Dave

[Steve_c]

I mentioned the idle valve as the ones on my Vauxhalls (Cavaliers and Vectras) were prone to a variety of problems.  In particular, the one on the Cavalier was fine at idle, but when crawling in traffic with very little throttle it would open and close the valve constantly.  On the Vectra, I [also!] had a problem with the throttle position sensor which I cured simply by unplugging and refitting the connector a few times to clean up the contacts - might be worth a go.

Wifey has a '58 plate Mazda 3 2.0 which I assume is the same engine as yours.  I'm pretty sure the cooling fan doesn't come on much in hers.

[bazzamf]

I agree with Steve_C's points. However, it might be a silly question, but do you keep the A/C on all the time? I've found that it does have an affect on the power available and can also slightly affect the running when it turns on and off automatically.

37 mpg sounds OK for the petrol engine.

[zigzag]

I don't know if this is a characteristic of all Mazda 6s but the hesitation/surging is very noticeable when the AC compressor clutch is engaging/disengaging as well as the loss in power. But as bazzamf suggests, try turning off the A/C to see if the problem goes away.

[Mazda6Driver]

I agree with Steve_C's points. However, it might be a silly question, but do you keep the A/C on all the time? I've found that it does have an affect on the power available and can also slightly affect the running when it turns on and off automatically.

37 mpg sounds OK for the petrol engine.

Cheers, now the weather's a bit cooler I'll try turning off the a/c to see if that makes a difference. I'm less worried about the mpg now, on a 90mile run I just recorded 41.5, doing between 70 and 80 so not much wrong there.

[Wombat]

Perhaps it may be worth giving the throttle body a quick inspection? I think they slowly accumulate a carbon residue around the 'butterfly' which may cause a lack of precision in the opening and closing of the valve. To view this, just pop the concertina rubber pipe between the air cleaner box and the manifold. It's held on by two large jubilee clips (philips screw). The valve is brass coloured is sprung-loaded, so press it open on it's upper half, whilst you clean the lower. The spring is quite strong, so mind your fingers. Clean the carbon off with a clean rag and a tiny amount of wd40 sprayed onto the cloth. It's normal for it to be a bit dicolored around the edges with black residue. Be careful not to lose/leave anything in there, as it'll get drawn ino the engine on start-up.

[Mazda6Driver]

Well, drove home tonight without the a/c and didn't notice a single blip. The only other difference was that the car was fully loaded which might be a factor. Will experiment some more this week on my commute.

If it does turn out to be just the a/c kicking in I'll be a bit gutted since it looks like something I'll just have to learn to live with. On the positive side at least I might be able to stop worrying!

Thanks for the TB tips wombat, will certainly take a look at some point.

[richyrich]

Mazda6Driver, was it the A/C after all?

I'm having the same issue whether the A/C is on or off.

Car is a 2007/57 and has done 29k miles. Very low mileage so I'm stumped as to why a car of this 'age' is acting this way.

Things I've tried:

1. Cleaned the TB
2. Cleaned the EGR
3. Replaced pre-cat O2
4. Cleaned inside the accelerator pedal (it sticks a little)
5. Replaced MAF sensor

I've also removed the resonator in the air box as well.

All have delivered zero results and I am still suffering with the tip-in stumble, part-throttle/cruise stumble (you can feel the car kind of shaking at cruising speed/revs. Using the cruise control also exhibits this), and it also stumbles/bucks when I let off the accelerator.

Also, if I gradually let off the throttle, the deceleration is very, very jerky. It feels stepped even. At low speeds - near idle - the surging/stumble is absolutely appalling: I'll let the car decelerate down to near idle speed to come into a car park in 2nd, touch the throttle very gently the car will buck and the revs will bouncing/search.

I bought an ODBII scanner and have masses of data to play around with. Some of the graphs I've produced look extremely nice as well. Shame I have no idea what I'm looking for! What I am seeing via the data though is, when I release the throttle pedal, the absolute throttle position (is this at the throttle body??) bounces. Is this normal?

I really need some help with this guys. As you can see, I have tried various things that have been mentioned across the world and in many forums, but to no avail. I have also spend quite a bit of money on it as well, an approach which I am now loath to continue, hence the OBDII scanner. Does anyone have any reference data which I can maybe trawl through or know what to look for and wouldn't mind looking at the data my motor has produced? Oh, there are also no fault codes being thrown. I've had a couple for a dead battery and when I removed the MAF, but that's it.

Thanks guys,
Rich

[Mazda6Driver]

Nope, it's not the AC - though having it on definitely makes it worse.

Your symptoms sound very similar, trying to follow a queue of slow moving traffic is very difficult to do smoothly in 1st / 2nd gear at low RPM.

There seems to be a very pronouced jerk in the transition between coasting in gear then applying the throttle to accelerate.  I've started to put it down to a certain amount of slack in the transmission.  My wifes Peugeot does a very similar thing, she says she doesn't notice it, but it really annoys me.

I've learnt to live with it to be honest, the car was serviced last month and I asked them to check a few things - the plugs were OK, leads tested OK, the only thing they noticed were slightly high emissions (HCO), but I think that may have just been that the car wasn't fully up to temperature when tested.

I've not done much investigation myself, but would like to check the throttle body at some point.

Let me know if you ever get to the bottom of this - It does spoil an otherwise excellent car to drive.

Dave

 

[richyrich]

Thanks for replying mate.

I experience the exact same thing in traffic. The car kangaroos at its worst, and is very jerky at best.

I am inclined to agree with you regarding slack in the drive train/transmission. I think I've read something somewhere regarding dual-mass clutches and/or flywheels. Not sure if our cars have those though.

The last thing I'm going to do is actually completely replace the TB for a new one. After that, I'm done trying to figure out whati is!

I'll certainly keep you posted with my progress.

Cheers,
Rich

[meddylad]

I had the exact same problem - turned out it was the lambda sensor that was on its way out - the OBDII reader gave a code P0133 & P2195

Had it fixed but cost £250+ VAT - not a cheap part thats for sure

[richyrich]

Thanks for replying, meddylad.

I had that done a month or 2 ago but to no avail.

The behaviour continues to get worse. I just can't drive this car with anything approaching smoothness. Constantly bucks and kangaroos at any throttle and any speed. In traffic, it looks like I'm an amateur manual driver.

Just waiting for it to throw a code out or breakdown; I can't afford to just keep replacing parts in hope.

Sad because otherwise it's a very nice car!

[meddylad]

which sensor did you replace?? there are two - A pre cat & post cat.  The pre catalytic converter sensor was the one causing my issues