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Mk4 fiesta misfire - I'm stuck!


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Hi, 

 

hoping someone can help as I'm stumped! 

Bought a fiesta with a misfire hoping it was a simple ignition problem but fairly sure it isn't. 

The misfire on occurs once the car has warmed up for approximately 3 minutes.

I've checked for spark and they're all firing (3 is a little weak but sparks) 

I've run the diagnostics on it & have codes P0113 & P0102 which led me to air intake.

Ive changed the air sensor on the side of the air box, air filter wasn't to dirty nor was air box but cleaned anyway. Cleaned MAF with brake cleaner - no change at all.  

Run car with MAF disconnected and no change. I did this by clearing codes, ignition on for approximately 2 minutes then started. 

 

Anyone know the answer? 

Cheers

Rob

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8 hours ago, Dorey1989 said:

I've checked for spark and they're all firing (3 is a little weak but sparks) 

A spark in air, outside the engine, is a rather poor guide. Under compression in the cylinder, it takes a lot more voltage to jump the gap. Two or three times, I would guess.

One check is to open up an old plug to double the normal gap, or more. Wire the plug body to the engine, & test each lead in turn.

It is important to never run the engine with a disconnected plug. It could damage the coil pack, or even the ECU.

I still would put plugs, leads & coil packs at the top of the suspect list, for a misfire.

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11 hours ago, Dorey1989 said:

I've run the diagnostics on it & have codes P0113 & P0102

I have just had a look at those code definitions.

The MAF one I think you have checked. MAF errors are a bit prone to popping up due to other faults, so should be ignored for now.

The IAT one is more positive: Intake Air Temperature Circuit High Input.

This may be the "air sensor" you changed. But if not:

This temperature sensor (IAT) is usually in or near the inlet manifold. I am not convinced it is likely to cause a misfire, as the mixture is controlled by the MAF and then trimmed accurately by the oxygen sensor(s). But maybe it could cause a misfire.somehow.

The IAT is just a thermistor, which is a resistor whose resistance reduces as temperature increases. At 15C, I would expect it to be between 5k and 100k ohms. Open circuit or short circuit would be wrong.

If the IAT error keeps coming back after being cleared, then maybe it is suspect. But ignition is still a prime suspect.

 

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  • 3 weeks later...

Hello,

 

update with this problem. I've changed lambda sensor, plugs, ht leads, coil pack & the air sensor as I've previously said. The problem still remains and showing me the P0102 code. Any ideas? 

 

Cheers

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On 25/10/2016 at 11:44 AM, Dorey1989 said:

Run car with MAF disconnected and no change.

 

5 hours ago, Dorey1989 said:

I've changed lambda sensor, plugs, ht leads, coil pack & the air sensor as I've previously said. The problem still remains and showing me the P0102 code.

P0102 is Mass or Volume Air Flow Circuit Low Input . This could be a hard electrical fault in the MAF. If the MAF was giving a badly out of range reading , then disconnecting it would make no difference, as it was being ignored anyway.

I guess the air sensor on the air box is the IAT (Air temperature). So about the only thing left is the MAF. You said you have cleaned it, but if broken, that would not help either.

If it is the MAF, my post on Oct 25 was a bit wrong blush.png, I should have read your info a bit more carefully, perhaps! I think I assumed it had been changed.

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Thank you for your response Peter. So next is a MAF. I know I should go genuine but as the car is for dog walking, tip runs etc I don't want it owing me to much money. Would an after market replacement suffice? 

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1 hour ago, Dorey1989 said:

should go genuine but as the car is for dog walking, tip runs etc I don't want it owing me to much money.

On a 17 year old car, I certainly would not buy a "genuine" one, at about £200! Let's be honest, if it lasts a couple of years, there is a fair chance it will outlast the car.

I would go for a cheap one that comes with a 60 day return, from a seller with a good reputation (according to the listing, at least). Like this:

http://www.ebay.co.uk/itm/ACP-Air-Mass-Sensor-MAF-Trade-Price-Genuine-Direct-Replacement-/142133276814?fits=Car+Make%3AFord|Model%3AFiesta|Cars+Year%3A1999|Plat_Gen%3AMK+IV|Cars+Type%3A1.4i+16V&hash=item2117cdb08e:g:5pUAAOSwzaJX7W2w

(Check your car details are correct before buying).

£28, and if it does have to go back, 2nd class postage is not very expensive. The biggest risk is that having got it, it makes no difference, then it will be hard to tell if the new MAF is faulty, or the problem still lies elsewhere. But you will only have lost a few quid for the return postage. And there is always some risk the new MAF may not work, being damaged in transit or storage, or something. Nothing is ever 100% in real life!

 

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