Misfire And Low Idle When Engine Warm

[Jaris]

Hi all, hope somebody could shed some light on this problem I am having recently with my 03 Yaris T Sport please.

At the start of the week my Yaris started misfiring slightly, was only really noticeable when I put my foot down and the engine would jump slightly when accelerating, engine warning light came on also. Then on the way to work in the middle of the week the power almost completely went when I was going down the motorway. I went from doing 70 to just about having enough power to pull over the side of the road :(

I got towed home then I limped to the local garage, doing about 20 mph, although strangely the power returned for a brief period just before I got there. The mechanic ran a diagnostic on my car, which turned up the error code P0172 (system too rich) which the mechanic said in his experience was down to the mass air flow sensor. He spent the day tweaking my car here and there, new spark plugs etc. and cleaning out the existing sensor which he said had helped as the power returned to normal for the most part when he took it for a spin, but then would drop intermittently. He advised to order a replacement Air Flow Sensor.

I ordered a replacement off eBay (genuine Toyota one but second hand) which I fitted today. The car seemed fine to start with but as soon as the engine warms up the revs drop from about 2 to 0.5 and the engine will start juddering around the point where it sounds like it is about to stall but won't actually stall. So I guess unless I've been unlucky enough to buy another MAF that isn't working then it isn't this. It is the part of the engine where the MAF sits that judders the most, also when it broke down by the side of the road there seemed to be a lot of heat coming off the engine.

My next idea was to check the ignition coils but I was wondering if anyone had any other suggestions to try?

Many thanks!

[Madasafish]

Oh dear.. where to start?

Then on the way to work in the middle of the week the power almost completely went when I was going down the motorway. I went from doing 70 to just about having enough power to pull over the side of the road :(

That sounds like a lack of fuel.. So either a fuel pump failure or a pipe blockage.

Your mechanic is an incompetent. He'll replace everything at vast expense.

Mileage and when last serviced and what has been done at the last service and any MOT advisories would help.

I'm North of Stoke. Are you near me? I have a code reader..

(I am recovering from lunch so no sense for a while:-)

[Adam C]

Have you thought about a Terraclean service?

It could be an excessive carbon build up inside the engine or even the catalytic converter. Terraclean can sort that out so may be worth a try?

[Jaris]

Thanks for the replies, especially Madasafish for the code reader offer and Adam C for the suggestion (not heard of this but will keep in mind for the future).

I put the part in again the next day but disconnected the Battery this time (forgot to do this first time around) and it seems to have done the trick. Car was misfiring slightly that day but now seems back to normal so I guess it was the MAF. Checked fuel injectors at the same time and they sounded like they were working fine so hopefully it was nothing to do with fuel getting to the engine that might rear it's head again in the future.

Cheers!

[Jaris]

So...the misfires have started creeping in again :(

When I put my foot down when going up through the gears my car feels like it is struggling to accelerate, then all of a sudden something kicks in and it will shoot forward at normal speed.

No engine light yet but does feel like it's idling quite low (around 500rpm) when it's warmed up, to the point where there is some 'judder' coming from the engine. No noticeable mpg drop, maybe 2mpg lower than norm.

Will look at testing the coil pack this weekend as a start

[bathtub tom]

Try unplugging the MAF. If the running improves, then I'd suspect it.

[Jaris]

^Unplugged the MAF and it didn't even start lol! But when I replugged it in the engine warning light came on

Didn't get chance to look at it properly this weekend but will tomorrow.

[Jaris]

Still having lots of fun trying to sort this. Misfires got progressively worse so I took the replacement MAF out and tried cleaning it but it just went worse again! :/

Checked the coils but they seem fine and the plugs under them sound like they're firing also. Dismantled the air intake all the way up to the throttle body but no noticeable problems. Thought I'd maybe been unlucky and ordered a 2nd hand MAF that was on it's way out so ordered a new after market one. There was a considerable improvement but was still misfiring and seemed to go worse today and the engine light came on :( Tempted to order another MAF to rule that out as the chance of ordering 3 dodgy ones in a row are surely minuscule!

I read the PCV valve could be the problem so have taken that out and cleaned it, went for a quick spin and the misfires were possibly less but too early to tell really.

I read on another site that the O2 sensors and also Throttle Position Sensor may be the problem.

Does anyone know if an '03 Yaris has a Throttle Position Sensor and where it is located please?

[bathtub tom]

Try an 'Italian tune up'.

Make sure your oil and coolant levels are adequate first!

Take it onto a dual carriageway or motorway and hold it at the red line (or close to) for a minute or two. This means 60 -70 MPH in second or third!

[Jaris]

^Thanks, gave it a try but no joy. Could still feel the misfires when at the red line.

Seems the car struggles when under load when going up through the gears but if I put my foot down it regains power suddenly. If I try to drive at a constant speed with my foot just touching the peddle it judders which makes me think it's throttle related.

Anyone know if a Throttle Position Sensor exists on an 03 Yaris please?

[bathtub tom]

Have you tried Googling P0172 and reading through all the results? The main suspect is usually the MAF and I'm concerned you replaced it with a secondhand one, but I'm not suggesting you go out and buy a new one.

Have you checked all the sensor pipes for leaks?

If you've a modern 'phone or laptop, it may be worth spending a few quid on something like this: http://www.ebay.co.uk/itm/HHOBD-Torque-Android-Bluetooth-OBD2-OBDII-Check-Engine-Auto-Code-Reader-Adapter-/251941430742?pt=LH_DefaultDomain_3&hash=item3aa8e125d6

You can then read live data like fuel trim, which may help diagnose the problem.

If you're not confident, then you may well have to bite the bullet and pay an experienced mechanic to diagnose it.

Don't waste your money on buying parts and throwing them at the problem!

[Madasafish]

I suspect a failed ignition coil.. not uncommon at this age...

[Jaris]

^Thanks for the suggestion, I did suspect that originally. Did a quick check though (took each coil out whilst engine running) and they all seemed to be working to an extent, could even hear them sparking the plugs underneath when I placed them back in.

I'm on my 3rd replacement MAF at the moment (2nd hand but a Denso one). Car is almost back to normal now but does misfire slightly when accelerating. This points to the MAF being the problem but I don't understand why when I replace the MAF each time I get some degree of misfire still

I'm hoping it is just the to do with the ECU relearning but I'll see. Don't really have time to go down the route of cleaning the throttle body etc. at this time but might clean the MAF and check the plugs as a quick fix just to see if that brings it back to 100%.

[Madasafish]

The problem with the test you did is that it showed they were working.... BUT the misfire happens when the engine is HOT.. A clear indication of a failing coil..

You need to test using a known working coil and substituting it for each coil in turn...And running it when hot..

[Jaris]

Ok, I see what you're saying. I have cleaned the MAF using electric contact cleaner and it has improved things again but that's not to say the root of the cause isn't related to another part I guess. I will try purchasing a single coil, then at least I can rule that out. Thanks.

[Jaris]

Wow can't believe it's nearly a year since I started having these problems! So I fixed the main problem around Christmas time. Turns out it was one of the O2 sensors (pre cat/manifold one). However my car worked fine for about a month then started having some high idle problems. As I originally had suspected the throttle position sensor as the culprit for the original problem, I replaced it with a cheap after market one which lasted about 6 months then failed! Popped my old one back in and car seems back to normal for now.

Never a dull moment with this one!  :)

[Matt Hunter]

I need help....i have relaced, coils, plugs, air filters, fuel filter, serviced, tried 2nd hand throttle bodies, still missing reving the same as Jaris, this is my daughters 1st car. I need help...please someone tell me what to do, i've now booked into auto elect to checkout. 

[scannerman]

On 7/2/2015 at 9:35 PM, Jaris said:

Ok, I see what you're saying. I have cleaned the MAF using electric contact cleaner and it has improved things again but that's not to say the root of the cause isn't related to another part I guess. I will try purchasing a single coil, then at least I can rule that out. Thanks.

I am led to believe that MAF sensors need to be cleaned with, guess what? MAF sensor cleaner. Other cleaners may leave residue on the sensor when dry. I have two Yaris and have cleaned both during services.