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Spi Misfire On Cylinder 3 When Warm.


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#1 MatP

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Posted 07 February 2020 - 12:25 PM

Hi all,

           my 1991 SPI Cooper failed its MOT on emissions: hydrocarbons too high. The MOT tester told me it is misfiring, and he is correct.

 

Once the engine is warm and idling, cylinder 3 doesn't fire. But it does when the car is first started up. The other cylinders all seem to be firing OK.

 

I've changed distributor cap, leads, plugs, and it hasn't fixed the problem. 

So that led me to think that its not the ignition electrics.

 

I checked the valve clearances. I've done a compression test, and all 4 cylinders give the same reading at 160PSI.

So that leads me to think that it's not the engine or cylinder head.

 

I've gone through the usual checks in Sprocket's super useful SPI posts. Vacuum lines look OK. I've changed the lambda sensor (didn't make any difference).

 

I've put a Gunson colour tune plug on cylinder number 3 to watch what happens. Ignition fine when the engine first starts up from cold, but when the engine is warm cylinder 3 stops firing. I can still see the sparks, just no explosion. So I reckon that rules out the ignition electrics.

 

Now I'm stumped. If the compression is the same in cylinder 3 as in 1,2 and 4, is there any way this can be caused by a problem in the cylinder head, head gasket, or engine?

 

Can any of the (many) SPI issues that produce misfire and bad idling lead to a systematic problem in one cylinder only?

 

Any advice much appreciated on which direction I should be going with this.



#2 Homersimpson

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Posted 08 February 2020 - 08:54 AM

It would seem unlikely that it's fuel related, have you checked the valve clearance? Although they would have to be really bad for the engine heating up to stop fuel getting in to the cylinder.

Have you tried changing the distributor car, rotor arm and leads? You may have a spark but is it strong enough to ignite the fuel?

#3 DavieW

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Posted 08 February 2020 - 09:59 AM

Can you swap plug leads from say cylinders 3 and 2 and see if the misfire "moves" from 3 to 2? Similar for the plugs if that doesn't change to eliminate each component? One item might be breaking down when it gets hot.



#4 MatP

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Posted 09 February 2020 - 12:09 AM

Thanks guys.

                 Yes, I checked and adjusted the valve clearances (only tiny adjustments at that). They're fine I think.

 

I have also changed the distributor cap, rotor arm, and leads. I've also tried swapping the plug leads between cylinders. And I've swapped the plugs between cylinders. Problem stays in cylinder 3.

 

I have tried a HT lead tester, that goes between the plug and the lead, and lights up if there's a circuit. I presume that means that everything leading up to the plug is OK. Further test, I took the tester off the plug and observed that a spark would jump about 1.5cm between the tester and the top of the plug. So it really does look like there is a strong enough spark to me.



#5 genpop

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Posted 09 February 2020 - 07:38 AM

You may check the lift of the rockers.if the cams of the camshaft are worn or one of the valve springs is broken you may have this problem.



#6 GraemeC

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Posted 09 February 2020 - 08:22 AM

Have you done 5he. Impression test when the car is warm enough that the misfire happens?

have you done a leak down test?



#7 MacGyver

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Posted 09 February 2020 - 09:30 AM

Have you checked the coil? I had a similar problem a few years ago (15...), swaped for a different coil and it was cured. You probably did this already tho...

Ben

#8 MatP

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Posted 11 February 2020 - 05:12 PM

Thanks all. 

 

            they're very good suggestions. I did have the engine up to temperature when I did the compression tests. I haven't done a leak down test, or checked the lift of the rockers yet, but I should do both if I can't find a resolution any other way. I think I already checked the coil, but likewise, I will do again if the approach below doesn't sort the problem. 

 

My latest thinking is that the problem may be a leak in the inlet manifold gasket that feeds cylinders 3 and 4. I sprayed wd40 over that area while the car was warmed up last night, and observed the engine rpm go up and the misfire temporarily disappear. Ordered new manifold gasket. Fingers crossed.






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