Single cylinder misfire - 3.5 on SU

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z4cAtS

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Near M4 J13
Hello, I am after a bit of help.

I have a 10A SD1 3.5 on SUs installed in my 88".

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
 
125 seems fair enough, even bordering on good and if you have a good spark that leaves the valves - is cam in good condition. One end of the cam (can;t remember which) suffers from lack of oil and can wear - maybe not lifting the exhaust valve enough? Other option could be valve guide has gone - does it smoke on startup?
 
I have no idea about the cam condition, I have yet to open it up that far.

There is no noticeable smoke, the fresh plug I tried today came out spotless after a couple of minutes running, other ones have been somewhat carboned/ lightly oiled up.

Have also swapped the leads around and tried an old one, nothing seems to make it fire.

Was getting a 10mm spark from the lead/plug when held to earth.
 
I will give it a look tomorrow, it looked / felt the same as the others when it was running with the rocker covers off the other weekend.


Cheers.
 
You don't say how the compressions were checked - hot/cold & throttle open/closed. If it was the former I'd say that reading of 125 was low & as for the 'range' do you mean there was that much variation over all all eight readings?
 
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.
 
z4cAtS said:
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
Well at least they are pretty even, which is good but (assuming this is a HC engine) I still think they are low. My last 3.5efi was giving an average of 160 +/- 10% after 150k. That said it doesn't look like the compressions themselves are causing your problem, so it's over to our learn'd friends on here ;)
Click to expand...
 
I have not noticed it using any oil, the exhaust gas is generally clear, however I 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?
 
z4cAtS said:
I have not noticed it using any oil, the exhaust gas is generally clear, however I 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?
Click to expand...

Just re-read your OP & realise we are talking about an ex SDI lump, so my question about the CR was a bit pointless :eek: Reading your post I'd say yes #2 certainly has it's problems, though I'm only speaking as someone who has owned four of these vehicles over the last 14 years, experienced the problems (& paid all the bills) not a mechanic.
 
Were the compression readings with throttle full open?
I guess you need to be thinking along the lines of valve guide wear, valve stem oil seal and cma but its odd for uneven can wear to give issues on the one cylinder. Would be tempted by a new plug and lead first.
Would not be the first valve stem seal I had seen fail
 
I don't think a 10D engine even has valve stem seals, and if they do it's the pointless flat rubber washers that break and fall off in no time. It sounds like it should be a worn cam, but the compression test suggests otherwise. So before digging that deep, first:
New dizzy cap, Rotor arm, HT leads, Plugs. It's much easier for a spark to jump in an open environment than when in a cylinder under compression.
Are colour tune clear spark plugs still available? Could be the perfect diagnostic aid here.
 
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
 
I am getting more confused as I go along. :wtf:

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?
 
Well, I have found the source of the problem :eek:
A bit of head scratching got me to look at what could be different for No. 2, and there it was in plain sight, the brake vacuum take off.

When I removed the hose to the brake booster and put my finger over the hole, I got ignition, on removal, just spark.

The eventual reason was that the servo was leaking air out of a fitting on the lower front half of the unit. I expect on one of the last times out, something broke the front part off, leaving an air hole.

Once this was plugged with a spare propshaft bolt, the cylinder fires correctly.

At least is was a cheap and simple fix.

Many thanks to all who contributed suggestions.

:5bcheers2::5bcheers2:
 
Well done for finding the problem before you went down the road of a rebuild! You must have a great sense of self satisfaction & be a happy bunny.
 
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