2004 Mazda 6, 2.0 petrol, same engine that Ford use.
Engine light comes on, initially no symptoms, then the day after its lurching at constant throttle or after coming back onto the gas from a trailing throttle. Like it’s missing. It’s not massive and it’s not constant but it’s a pain in the ass. On full throttle it’s fine, and it’s worse at steady 40-60mph, better at 70.

I did some research and the diagnosis was possibly coil or throttle position sensor (TPS) to I did some more research and the general opinion was to disconnect the TPS and see what happens.
So I did.
Lurching goes away entirely, but the car ticks over at 1200-1500 rpm, obviously it doesn’t know where the throttle is so it compensates with higher idle.
So I buy a new TPS.
Problem is still there.
Yes I’ve reset the engine light.

Normally I’d turn this over to a mechanic I trust but he’s away for a week or two. The car only gets used 1-2 times a week, but If I can fix it it’ll save some money. TPS was only £10, coil pack is £37, but I don’t reckon it’s that – Surely if it was coil related it would get worse under full load/high revs and disconnecting the TPS wouldn’t affect it…. Same logic for plugs/leads…?
Vacuum leak? I can’t see where you’d get one of those, but I’m open to suggestions.
Any ideas?
Cheers in advance :o)

Can you plug in a cheap fault code reader and see what it suggests?
I bought one off eBay for about £10, and whilst not infallible they can pointnyouninnthe right direction.

It sounds ignitiony related.. mine did similar and turned out to be ht leads. Thankfully it was the first thing I checked.

Do you have a code reader if you’ve reset the engine warning light? What was the code that was present?

No i don’t have a code reader. The light is reset by disconnecting the battery then dumping the electrical system by pressing the brake or turning the lights on. Old trick….

Surely if it was ignition then disconnecting the TPS would make no difference? Not arguing, just want to understand your logic….. 🙂

2004 – have a look closely at all the vacuum hoses, you may have a leak causing the engine to get confuses with the air fuel mix. My old 2003 polo had a split hose just after a sensor, it was a PITA to find as you couldn’t see it when looking down in the engine bay.

Beware, when I began to take things apart to replace that section of hose other bits of hose began to fall apart.

Does it have an idle air control valve?

Symptoms sound similar to my old Fiesta that had a sticking idle air control valve.

Ford wanted silly money for a new one, so I took the old one off & cleaned it out with GT85 before sticking it back together and it was fine for the remainder of the time I had the car; about another 3 years.

no one saves money playing parts darts.

anyone near pp with a diag set up willing to plug it in and read the codes before he chases his issue all round the engine bay :d

Mass airflow sensor just after the air box. Be very careful. It’s a very fine wire that has a set resistance. The resistance changes as air passes over it. They get clogged with crap from the egr system. If it is clogged get some spray solvent cleaner and give it a blast. You can use a cotton bud but be very gentle. Worth a try and won’t cost a fortune.

Code reader would be a good start. Torque app and a Bluetooth obd socket will do.

If that recognises nothing, most likely air/vacuum issue.

I’ll have a pint on crank position sensor.

I’d start simple and put some injector cleaner through it first.

I find these issues rather depressing as cars have become increasingly complicated and controlled by solid state electronics. All in the name of emission control. There will come a time soon where motoring will go full circle and only the wealthy will be able to afford to maintain new cars, as keeping older cars going will require a shed full of replacement sensors and no time to **** about fault finding. The rest of us will still run new cars courtesy of finance agreements, where, as we all know, there is only one winner.

Put the parts cannon away and either get a cheap code reader or get it read by a garage.

And the TPS sensor at £10 – chuck it in the bin 😀

Marko

My reckoning from experience playing ‘parts darts’ it will be one of these (in order):

Crank position sensor
MAF
coil/Leads

Also From experience playing parts darts, get it plugged in to a code reader.

Marko makes a very fair point ….. at 10 pounds theres every reason to think your CPS is duff …and so may be your new one at that price.

thing is you can test almost all these sensors with a multimeter if you get the expected characteristics from the tech manual theres no need to play parts darts even if you havnt got a code reader (although a u340 is only 9.99 and pays for its self in one hit) its not 1987 anymore

By lurching, I take it you mean the power gradually comes and goes, almost giving the sensation the cars rolling forwards/backwards like a boat rocking on waves?
I’ve seen drivers complain of motion sickness because of it, as it really does feel like being on a boat rolling about.

If it is, the official term is surging at constant load.
Possibly a sticking EGR valve, or something else in the induction system sticking, which the ECU can’t quite compensate for. Getting the fault codes read is the first step, but the codes are likely to be a symptom, rather the problem.

Marko do you peruse BBA reman forum 😉

Marko makes a very fair point ….. at 10 pounds theres every reason to think your CPS is duff …and so may be your new one at that price.
thing is you can test almost all these sensors with a multimeter if you get the expected characteristics from the tech manual theres no need to play parts darts even if you havnt got a code reader

There was a thread on BBA where a clued up tech scoped for faults and found the waveform on the crank sensor a mile out, replaced the sensor with a Eurocarparts sensor, and the fault remained, even though the wave was well within spec. Other pico users provided waveforms of known good contemporary engines, yup, waves matched.
This went on for days until the guy went to a main dealer…
Fault cured. Wave form on main dealer and ecp part was identical.

There’s a moral here somewhere

When you reset the engine light you performed a reset to ecu, did you follow the “relearn” process for the ecu by letting it idle for half hour without touching the throttle? Did this with my MAZDA 3 2006 sport 2.0 petrol.

The lurching issue with mine was similar and traced to clogged throttle body. Resolved about 6yrs ago and fine since

Good luck!

They get clogged with crap from the egr system.

Didn’t think petrol engines have an EGR system..?
To be honest I’m probably just gonna ignore it until my mobile mechanic comes back from hols
My thought is that it’s vacuum leak somewhere and when I disconnect the TPS it overfuels and this masks the issue, either that or a blocked injector.

Marko do you peruse BBA reman forum

I know of it and have looked at it in the past but I don’t contribute.

I do have a Picoscope and my own library of good and bad waveforms, but I don’t work on the frontline anymore, so my diagnostic skills are getting a bit rusty these days.

Having said that I helped a friend out the other month. Garage said they needed a new timing chain and tensioners etc, because of a camshaft incoherence fault code – they assumed it meant the chain had stretched and the camshafts were out of phase slightly, hence the code. Quick check showed nice square wave form on one camshaft sensor, garbage out of the other. I removed the sensor and sure enough somebody had replaced the OE Bosch one with a ‘Fazet’ branded one. New Bosch sensor and bingo no fault codes and EML out. Total cost around £65 including the part (mates rates) and not the £600 the garage wanted to replace the chain.

Moral of the story is don’t fit ‘cheap’ engine management components.

Hth
Marko

Sounds a bit like a MAF sensor issue to me – I had very similar experiences albeit on my diesel bmw which resulted in a new MAF. You could try locating and cleaning yours with some carb cleaning spray (don’t touch or break the wire in it). Normally located after the air filter but before the inlet manifold.

Parts darts 😆

I find these issues rather depressing as cars have become increasingly complicated and controlled by solid state electronics. All in the name of emission control.

a) emissions control is pretty important, so this is a good thing

b) They aren’t that complicated, really. Just different. It’s pretty straightforward what these sensors do, it’s just their effects that are difficult to figure out. Well, in the old days they were also difficult to figure out, you had to know. Fortunately these days you can plug in a computer and most of the time it’ll tell you what’s wrong. Bad sensor, change sensor, done.

I’m with molgrips on this one.

1970’s – adjust the points every 1500 miles

2010’s – you can go almost far in some cars without even filling with diesel

I own a car from the 1970’s that’s done about 100,000 miles, it’s on it’s 2nd chassis and body, 3rd engine or rebuild, and 3rd (or more, it’s a known weak link*) gearbox rebuild and his noodly appendage knows what else.

*even compared to the rest of the car.

If I were faced with a 70s car that was surging or idling irregularly, I’d have no idea what to do.

it’s on it’s 2nd chassis and body, 3rd engine or rebuild, and 3rd (or more, it’s a known weak link*) gearbox rebuild and his noodly appendage knows what else.

Is it even the same car any more?

it’s on it’s 2nd chassis and body, 3rd engine or rebuild, and 3rd (or more, it’s a known weak link*) gearbox rebuild and his noodly appendage knows what else.

My car’s got bad wiring to the MAP sensor. I know this because I plugged it in, and I can watch a graph of the sensor output compared to the real atmospheric pressure, and I can see it jump about as I wiggle the wire. I love diagnostic information 🙂

Trigger’s broom?

If I were faced with a 70s car that was surging or idling irregularly, I’d have no idea what to do.

Dig out the Haynes manual.

First thing I’d do there is replace the HT circuit. I used to do that as a matter of course in the late 80s, “it’s on the original plug leads and done 30,000? Madness.”

these days you can plug in a computer and most of the time it’ll tell you what’s wrong. Bad sensor, change sensor, done.

Newp, dtc’s just give you a ball park idea of where to look, you need to know how to interpret the data the diagnostics are telling you

Take Marcos scenario up there, but swap it with a common failure in a Nissan – The dct code will say cam sensor outside of parameters, and most people swap the cam sensor, and wonder why it still won’t start.

So £30 down the pan before you even start…

The fault most times will be timing chain stretch or variable valve timing unit gone west because someone tried saving money on a timing chain job… A horrible task involving driveshaft and sump out, so it’s prudent to buy a full kit rather than just a chain and tensioner.

The number of these I’ve seen as non runners and in the boot or cab you’ll find a cardboard sensor box

I had similar symptoms on mine. Turned out to be a bad earth. Took the earth straps off, cleaned and put some grease on. Hey presto.

Diagnostics said it was various sensors, I did replace one or two before realizing it was not helping.

The best bit is that it won’t cost you anything to try.

Good luck

UPDATE

Mechanic came (He’s good, I trust him) and plugged it in – A few error codes including a misfire on no1 cylinder, so we replaced coil, leads and plugs. I was going to do plugs anyway. I was at work and he said it was OK….
I got home and it was just the same, maybe a little worse.So that was most of £204 wasted. (Coil was over £100)
We didn’t have time to look at it as we were of to the IoM TT, he’s been back today and replaced the EGR valve. That fixed it.
When he originally plugged it in there was all sorts of codes including EGR, so he cleared them and drove it, then only got the misfire code, hence replacing the ignition. Fair enough I guess.

Didn’t think petrol engines have an EGR system..?

Well, you live and learn….
🙂

By lurching, I take it you mean the power gradually comes and goes

NO, it was properly lurching under mild/medium acceleration or constant throttle, like it was cutting in and out. Almost impossible to drive, kangerooing all over the place!

The only way to dive it was absolutely flat out. Which I did, hoping an ‘Italian tune-up’ might work….. 😈