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Misfire on idle B230FK 940

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Old Jan 14th, 2018, 14:33   #1
ie940
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Default Misfire on idle B230FK 940

Volvo 940 estate B230FK 2.3 1996

Developed a misfire on idle only. Drives and accelerates fine. Cylinder 1 has the problem.

If it's idling, very lumpily below 1000rpm, then I can pull the #1 HT lead or injector plug off and the idle doesn't change. It'll pop and fart on the exhaust, but unplugging the injector stops this.

I've changed the injector, checked it's ohms, changed the FPR, cleaned and gapped the plugs, fitted new Bosch leads, cleaned the recently replaced Beru cap and rotor arm.

Compression is slightly down on #1, but not to the point it wouldn't run? Approx 120psi opposed to 135psi on the other cylinders, which equates to low 8 bar instead of the Haynes specified 9 minimum.

Thoughts?
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Old Jan 14th, 2018, 14:52   #2
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Thoughts?
I think you will find it is number one cylinder low on compression which does show up at idle, this one wears more than the others as it is nearer the water pump so does run cooler.....although in theory the difference between cylinders should be no more that 29psi but in practice who know`s?

Why don`t you remove the plug and put 2 teaspoons of oil in that cylinder , leave for a while to flow around the rings then replace the plug and fire it up.....forget the smoke but the misfire should go thus confirming the low compression?

When doing the compression test do it when almost cold and also when quite hot?.........it is all relative between each cylinder so temp. does not really matter but it may show up and prove more to you the information that you need.





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Old Jan 14th, 2018, 16:49   #3
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Just to add to that useful info there, when you do the compression test make sure the fuel pump is disabled (or all injectors unplugged - easiest to pull the fuel pump fuse, start it up and wait until it runs out of pressure) and that you hold the throttle wide open during the test.

Also if it appears #1 is still low, check your valve clearances.

Out of interest, what plug gap are you running? Should be 0.65-0.70mm which may help if you've set them larger.

What oil are you using and when did it last have an oil/filter change? I'd also suggest adding between 0.5 - 1.0L of synthetic ATF to the engine oil to help free the piston rings and also generally clean inside the engine and revive the various seals throughout. You may find a slightly sticky valve is causing the problem and the ATF will help with this.
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Old Jan 14th, 2018, 17:50   #4
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Cheers chaps. The comp test was done when warm but not totally hot, the fuel pump fuse was out and the throttle was WOT.

Plugs were gapped to 0.7mm, Haynes said 0.7-0.8.

Oil change was recent to 10/40 semi with a new filter.

I'll try a wet #1 test to see if it idles any better, cheers.
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Old Jan 14th, 2018, 19:11   #5
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If the wet #1 test improves things, i'd definitely add some synth ATF to the engine and give it a mobile decoke.

It's pointing to either a ring problem or possibly sticky valve. Have you had any head gasket trouble with it at all?

On one of mine the head gasket had been done about a year or so before i bought it.

It developed a slight misfire and it turned out the head hadn't been tightened properly, 2 bolts were only finger tight, three of the rest were only just "nipped up" and the rest weren't even as tight as the first stage of tightening should be (44lb/ft if memory serves without checking the torque settings) so i slackened them all one by one when cold and tightened them as per the Haynes recommendation, misfire gone!

Had to retension the cam belt after as it became a bit slack as a result but i replaced it soon after that anyway.
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Old Jan 15th, 2018, 09:35   #6
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Quick update, it idled quite nicely below 1000rpm and gradually got worse as the car warmed up. Worst when up to operating temperature. Popping a lot on idle now but still drives and boosts absolutely fine.

HG not been touched under my ownership (2 years)
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Old Jan 15th, 2018, 10:22   #7
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Quick update, it idled quite nicely below 1000rpm and gradually got worse as the car warmed up. Worst when up to operating temperature. Popping a lot on idle now but still drives and boosts absolutely fine.

HG not been touched under my ownership (2 years)
Is that after doing the wet #1 test?
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Old Jan 15th, 2018, 12:11   #8
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Sorry no wet test yet, that was driving it to work as my other car decided not to start!

Wet test will be done tonight hopefully after doing a leakdown test.
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Old Jan 15th, 2018, 13:15   #9
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The reason i asked about the wet test is because it's actually beginning to sound like either a MAF or Lambda problem.
However the fact it seems to run ok under boost conditions suggests the MAF is ok, they usually give trouble all the way through the rev range but idle reasonably.

A dodgy Lambda can cause very poor idle as well, with a weakness anywhere this would show up even more with a dodgy Lambda sensor.

See what the wet compression test brings first.
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Old Jan 15th, 2018, 22:38   #10
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I did a wet idle test and it didn’t improve. Not tried a wet compression test.

Did a leak down test but got 100% on every cylinder so i think I’ve messed something up.

Changed TPS, dizzy cap, leads, plugs and checked timing marks on the belt. At a loss now.

You can still pull #1 lead or injector and not affect the poor idle.
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