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K Series misfire


Foggy

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I am seeking the guidance and wisdom of all on blatchat.  

My 1.8 K Series Roadsport is suffering from a misfire at or around 2k rpm.  Under load its fine, just pottering around through town etc produces rough running/misfire as I say, at approx 1800 - 2k rpm.  Connections on the ecu, injectors, coil, oxygen sensor, distributor and ht leads have all been checked and the TPS switch and plugs have been changed.

Any ideas what over avenues we could look at?

Thank you

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By checking the leads, have you just checked connections, or done a test on resistance with a multimeter? If leads are of different lengths you do a simple calculation to check all is well.

(I did this a few months ago and found one lead was shot) 

youtube: 

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Hi Stridey, the connections were all checked by my local friendly mechanic without my presence so I have no idea how they were tested but I have a multimeter so I'll follow your suggestion.  

Many thanks

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Not sure how to read your post... Were the connections to the oxygen sensor checked, or was the sensor itself tested? Might be worth temporarily unplugging the lambda sensor and seeing if it drives better. Safe to drive with it unplugged for a bit to test. If that helps, replace it.
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Thank you for that, removed lambda sensor connection and out for a nip down the road to test.  This time I had the other half with me who noticed "the exhaust sounds funny" and sure enough...... crack in the exhaust where the tailpipe joins the silencer! *rolleyes*

Thank you all for your contributions. 

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Hi John,  the Lambda sensor would appear to be fine and the exhaust leak the most likely culprit.

The exhaust has been removed for repair during the process of which, we noticed further evidence of cracking along the weld where the pipe meets the cat.  I am assured it's all perfectly salvageable hand has gone to a recommended TIG welding specialist locally.  Once the exhaust is back on the car I will report back but the general consensus of opinion is the exhaust leaking would be the most likely cause of the misfire at low rpm...... unless others don't agree?

 

 

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Hi Revilla, not quite sure what you mean by the leak being ahead of the sensor? I have the bog standard k-series single pipe exhaust and the lambda sensor is just after the primaries. 

The exhaust has had a very neat repair and is now back on the car but the original fault is still there.  I did notice that once fully up to temperature the misfire is a lot less evident.  On that vein, the fan is cutting in sooner than prior to the misfire, anything on the coolant side liable to cause a similar misfire?  I have to stress that once under load and accelerating, the car behaves absolutely fine. Just refuses to trundle along at around 2k rpm without the misfire. *scratchchin*

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If you have the 4-into-1 manifold under the bonnet with the lambda sensor just below where they meet and then a single pipe out through the side skin, a crack in the exhaust pipe beyond that point won't affect the engine and won't be the cause of your misfire. It will just make a noise. It's hard to know what to suggest next. The EU2 engine won't support ODBII diagnostics. It still sounds ignition-y to me. What tests were done on the coil? My wife's StreetKa had a very similar misfire, usually as soon as you tried to give it a little bit of throttle at low RPM, went fine once you got the revs up. That was a coil pack breaking down. Is it better or worse once it's hot?
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Not sure what tests were done on the coil just that “it had been checked”.  Once warmed up then yes, the problem is a lot more manageable.  It was the warming up aspect that I noticed the fan cutting in sooner than I would have expected from past experience.  When cold, it’s constantly misfiring at around the 2k mark but if you then apply a generous amount of throttle, absolutely fine! 

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Hi foggy, I had exactly the same problem with my 1.8 Supersport that took me nearly a year to cure. The car was virtually impossible to drive slowly due to a constant misfire but ran fine when your foot was down. I changed virtually everything before I stumbled on the cause. The plugs, distributor cap, leads and rota we’re all replaced with no effect as were the water temperature sensor, the air temperature sensor and lambda sensor. Again with no effect. I reset the throttle position sensor several times and cleaned the idle speed air control valve twice, I also checked the ECU manifold pressure sensor all again with no effect. So with only the ECU and injectors to change, I took the cheapest option and replaced the idle speed air control valve and ureka success and the car has run beautifully ever since. This might not be your problem buts it’s worth a shot. Buy one from eBay for an MGF at about £10 from the people’s republic of China. 

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So, in light of everyone's kind suggestions as to what the fault may be, I'll try the following when I next get the time;

Lambda sensor (again)

Rotor arm

idle speed air control valve

on another post along a similar vein fingers were pointed at the water temperature sender so I'll try that theory as well.

Many thanks for all your contributions and I'll report back once I've had the chance.

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Another thing to check: does the HT lead from the coil to the distributor run very close to the crank sensor or its wiring? The crank sensor signal is quite weak and it has been known to pick up interference from the HT lead causing a misfire. If it is close, try routing it away, maybe using a loose plastic wire tie if necessary.
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Good evening all.  Having read further re the symptoms a lambda sensor failure would produce I am coming down on the side of that sounding like a likely culprit for my misfire.  Having read how to test with a multi meter a thought then occurred to me...... could I just not unplug it?  Or am I being a tad naive?!

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Good afternoon all. 

Well I have checked the lambda sensor again and it makes no difference at all to the running so I’m ruling that out. The dizzy retaining bolt is tight and the HT lead is still sat in its clip away from the crank case sensor so I’m ruling that one out as well. The water temperature sensor has been unplugged and that makes no difference either.  The IACV I removed and cleaned which was a bit sooty, is that normal? Either way cleaning and replacing/disconnecting made no difference. I have had a good look over various electrical connections that may cause problems but nothing of note.

One thing I did notice today and that operating the throttle via the throttle body as opposed to the accelerator pedal, the misfire seems to be evident higher up the Rev range as well as the previously mentioned 2k (although 2k is still the worst!).  Would this be because the engine is not under driven load or another symptom of the problem getting worse?

Think I’m rapidly getting to the stage where the car needs the attention of a professional with more knowledge than me now.  *confused*

I really am very grateful for everyone’s input though, thank you all.

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One other thing to look at would be the carbon pick-up in the dizzy cap.  That needs to be smooth and clean at the point of contact, and be pressing firmly against the rotor arm.

Re the coil, do you have anyone nearby with an EU2 who would be prepared to let you swap for testing purposes?

JV

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Just re-read the whole thread and I can't see any mention of the plugs being checked or changed? Have you taken a look at them? They could be wrongly gapped or crudded up (to use the technical term). Even if not at fault, their condition might give you some clues as to what is going on.
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