Single cylinder misfire - 3.5 on SU

z4cats · Post by **z4cats** » Mon Nov 17, 2014 12:57 pm

Hello, I am after a bit of help.

I have a 10A SD1 3.5 on SUs installed in my Land Rover.

The problem I have is that No. 2 cylinder is not firing, using a infrared temperature probe shows the exhaust not keeping up with the other cylinders as the engine warms up.

I have checked the spark and it is healthy enough to jump 5-10mm.
The compression gauge showed around 125psi (range of 115-140)
The other plugs are a nice tan colour, this one was a bit wet and carboned.
A brand new plug also went this way.

With the rocker cover off, everything looked and felt ok.

So... Any ideas

Thanks, Paul

DaveEFI · Post by **DaveEFI** » Mon Nov 17, 2014 3:14 pm

Have you measured the resistance of the plug lead?

z4cats · Post by **z4cats** » Mon Nov 17, 2014 3:53 pm

Ok, just done a bit more testing.

No 2 is 13k ohms
No 4 & 6 are 12k ohms

Old plug lead of 10k ohms put on, with a brand new plug and still not firing.
This spark was jumping up to 10mm or so when held to earth.

Spongo · Post by **Spongo** » Mon Nov 17, 2014 4:39 pm

Is the "wetness" on the plug defineatly fuel? Sometimes a small coolant leak will cause a misfire.

Just a thought

DEVONMAN · Post by **DEVONMAN** » Mon Nov 17, 2014 4:39 pm

What's the history?

Did the fault suddenly happen or is it a newly fitted engine etc.,

Denis

DaveEFI · Post by **DaveEFI** » Mon Nov 17, 2014 5:14 pm

z4cats wrote:Ok, just done a bit more testing.

No 2 is 13k ohms
No 4 & 6 are 12k ohms

Old plug lead of 10k ohms put on, with a brand new plug and still not firing.
This spark was jumping up to 10mm or so when held to earth.

That resistance is fine. When a lead is faulty it reads several times the average.

Not sure how good the spark is in open air is much of a test, though.

Can you look at it in total darkness so see if there's any sign of the spark tracking?

It does sound like a spark problem - assuming the cam is opening the valves.

z4cats · Post by **z4cats** » Mon Nov 17, 2014 5:22 pm

The plug has smelt of fuel, and have been a bit oily wet.

I don't think it is losing water

The engine has been in for years afaik, I have had the car for a couple of years but have not had much use out of it so far.

I have rewired it and fixed some chassis rust, so it has not had much running this year at all - just 130 or so miles over the last two weeks.

It does get up and go if given the beans, and behaves itself pretty much

I will try and get it out for a few miles tomorrow to see if anything has improved.

z4cats · Post by **z4cats** » Tue Nov 18, 2014 1:05 pm

I have just redone the compression test - engine warm - WOT

8 - 132 = 128 - 7
6 - 125 = 123 - 5
4 - 130 = 135 - 3
2 - 130 = 135 - 1

This time No. 2 got hot quickly, but didn't get as hot as the 4, 6, 8

The sparks on removal were now quite sooty, I haven't been for a drive since the weekend though.

I will clean them all up and try to get out later on for a quick blast.

stevieturbo · Post by **stevieturbo** » Wed Nov 19, 2014 5:03 pm

If the plug is wet, replace it.

A good strong spark via a visual check should give a good indication if the system is adequate.

Or slightly better are the various spark testers where you can open up a large gap and check for ignition power

eg, this sort of thing

http://www.toolsource.com/ignition-spar ... 54392.html

http://www.autopartshop.com.au/ignition ... ester.html

Gunsons used to do one years ago, but havent seen them for a long time

UK sellers of similar ?

z4cats · Post by **z4cats** » Wed Nov 19, 2014 8:31 pm

Thanks for the replies.

I have removed the N/S carb and checked the setup.

One thing I found was that the vacuum advance port was gummed up, whether this makes any real difference I will see.

The float setup looks ok, although have not done many miles since it was last off.

Ran out of time today to get much further.

Will put it all back together tomorrow if possible.

unstable load · Post by **unstable load** » Thu Nov 20, 2014 7:26 am

How is the oil consumption on the engine? You could have an oil control ring problem that is gumming up that particular cylinder.
Can you get your hands on a bore-scope to take a lokk-see down the plug hole?
I am not convinced that the carb is at fault because it would be all the cylinders fed by that carb experiencing problems, not just one.

z4cats · Post by **z4cats** » Thu Nov 20, 2014 3:03 pm

I have not noticed it using any oil, the exhaust gas is generally clear.

Have just run it for 15mins or so to set up the carbs, and have got some oil on the No. 2 plug tip.

Not a good sign I guess?

DaveEFI · Post by **DaveEFI** » Thu Nov 20, 2014 3:48 pm

Still seems to me like an ignition fault. Have you tried another dizzy cap and rotor arm? Some of the modern replacements are of poor quality. Also, how are your plugs leads laid out? The BL manual is quite specific about this to avoid cross firing.

z4cats · Post by **z4cats** » Wed Nov 26, 2014 7:12 pm

Thanks for the replies.
I now have a brand new dizzy cap / rotor arm and two colortunes.

When I can get at it in some daylight I will give it a go.

Cheers

z4cats · Post by **z4cats** » Mon Dec 01, 2014 8:44 pm

I am getting more confused as I go along.

With 2 Colortunes at idle, no. 8 works fine, but no. 2 just sparks.
Give it some throttle and no. 2 starts to work but very intermittently.

One thing though is that with the even rocker cover off, there can be seen a large volume of oil flowing up into the top. Would overflow the head if left running, it is also flooding over the valve / spring. This would explain the oil on the plug if it is dripping down.
The odd rocker cover has almost no oil compared to the even side, there is just a little flowing.

I am starting to think I should just get the engine out to check and rebuild it.

Got a spare 3.5 anyone?