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2.0 petrol missfire when warm


MagicRMorris
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Hi

I have a c max 2.0 petrol zetec. 

Starts fine when cold, runs great until warmed up and then I get a missfire on idle. If I turn it off and try to start straight away, it will struggle to start, and still keep missfiring.

If I turn it off and leave it a few seconds it will satart fine and run as normal for a while.

I did get a eml on a couple of weeks ago with codes p1131 and p1132( running lean and rich}

I replaced the Lambda sensor thinking it may be that, but it is still the same. It has the individual pencil coils so I dont think it is coil pack and ht leads which seems to be common for this problem.

Any ideas anyone?

 

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9 hours ago, MagicRMorris said:

I replaced the Lambda sensor thinking it may be that

I think there will be two oxygen sensors, one each side of the cat. The downstream one is sometimes called CMS (Catalyst Monitoring Sensor). These codes are more likely to refer to the upstream sensor. I suggest you check which one you have changed, and whether they are interchangeable (for testing).

It seems P1131 & 2 can also be related to wiring faults in the wiring to the sensors, exhaust system leaks, leaks around the MAF, dirty or damaged MAF.

Can you monitor the fuel trim values? Systems like Forscan, & maybe Torque & other some OBD2 software can do this.

On cars which use the cat to remove NOX, as well as CO & HC, they need to rapidly switch between a little lean (excess air to oxidise HC & CO), and a little rich (excess fuel to reduce NOX). I think the P1131/2 codes are connected to this switching. If a fault (air leak or excess fuel) is pushing the mixture (fuel trim) too far one way, it could be that the switching can not then work, so both lean & rich errors can occur. The fuel trim may suggest whether it is being pushed rich or lean.

On the other hand, if these DTCs were a one off, and do not return, I think I would go back to investigating the ignition system further, as that is the more usual cause of misfires.

Also I have just seen that misfires will deliver unburnt fuel into the cat (past the O2 sensors), and that could upset the mixture switching above. So misfires due to ignition faults could cause the P1131/2 DTCs.

 

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Hi Peter, 

Thanks for the reply. It was the upstream sensor that I replaced. Unfortunately they are not interchangable to test.

I do not think it has a MAF . I do have scanner that can read faults and and fuel trim, but to be honest I wouldn,t know what to look for.

It does seem like a switch or a sensor at fault as it happens when hot, but clears when I turn off the engine and restart.

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35 minutes ago, MagicRMorris said:

I do have scanner that can read faults and and fuel trim, but to be honest I wouldn,t know what to look for.

If you can get some numbers out of it, then I, or someone else here, might be able to help interpret them, The more hard, accurate info, the better. It is important to keep monitoring for DTCs in cases like this. Sometimes it takes two errors to light the EML, and so there can be codes with the light out. Don't be afraid of them, there are intended to be helpful, even if it does not always work out like that!

On 26/10/2016 at 4:42 PM, MagicRMorris said:

If I turn it off and try to start straight away, it will struggle to start, and still keep missfiring.

If I turn it off and leave it a few seconds it will satart fine and run as normal for a while.

The internal parts of the plug, in particular, will have a cool down time in seconds, once the engine stops. And heat up pretty quick once running again. Also a breakover inside the coil will produce local heating with a similar recovery time, possibly. So I do not think ignition can be discounted, based on what you have said.

 

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Hi Peter

Just had my code reader on it these were  at tickover 740 rpm and 3000 rpm and warming up from cold

Fuel sys 1         CL           CL

calc load           29.8%    28.23%

stft B1           -3.91%       Between -3 and -8

ltft                  0               0

map              32Kpa       27Kpa

Spark Adv   -2-4          55TDC

IAT             30 c             41c

TPS      12.15%            14.9%

02sl       11 12             11 12

02s11    0.685v        0.650 -0.070

shrtft11    -4.69         -5.47%- - 7.43%

 

When the temp got to 85C it started missfiring. The main changes were

STFT B1  -19.71%

SHRTFT 11 -17.9%

Both on tickover. There are no codes stored at present.

 

Thanks

Stu

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5 hours ago, MagicRMorris said:

When the temp got to 85C it started missfiring. The main changes were

STFT B1  -19.71%

A negative Fuel trim means the injector pulse is being reduced from its nominal value, to cut down the amount of fuel. That means either not enough air is getting in, or too much fuel is getting in or passing through.

It would be informative to know if the STFT figure stayed near -4 up until the moment misfiring started, then fell quickly. If so it would indicate the misfire caused the excess unburnt fuel to get to the O2 sensor, and it had to cut back the fuel.

Or if the STFT rose steadily, then as it got to -19, the misfire started. This would indicate the mixture was wrong, and the trim was unable to correct it, causing the misfire.

Some causes for mix too rich & negative fuel trim that I can think of are:

TMAP calibration incorrect. But figures above (30kPa) look ok.

Fuel pump pressure too high.

Blocked air filter (But TMAP should compensate)

Low compression in cylinders

Faulty O2 sensor (changed)

Poor combustion, including misfire.

So I still suspect the misfire (due to ignition problem) is causing the fuel trim offset.

 

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the stft moved quickly as soon as it started missing.

I have some new plugs and a coil, sp will get them changed and see how it goes. I will let you know.

thanks for the help

Stu

 

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Changed the plugs today, Still the same unfortunately.

I have a new coil and have tried replacing each one individually and it still starts missing after about a mile of driving. I cant see any loose or damaged wires anywhere, any ideas on wheer to look next?

 

Thanks

Stu

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  • 1 month later...

I think that I have finally fixed it. Replaced EGR valve and has been running ok for 4 Days now.

I had already replaced this in April, so didn't really consider it. but ran out of ideas, so was not much left. The 1st EGR was from eBay, so I suppose that will teach me to buy decent parts in the future. It did not actually look too clogged up, but was obviously stuck open.

 

 

 

Stu

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