Idle Issue; not a very unique title I know

[willincalgary]

I'm perplexed by an idle issue. Currently have a very persistent miss at idle (read this virtually ever cycle it misses). I've pulled spark plug wires with the engine idling to test which cylinder is missing. Pulling wires 1-3 causes an immediate drop and deterioration of idle. Pulling wire 4 makes almost no difference to idle indicating it is cylinder 4 which is missing. What I have done:

• Spark Plugs - All new plugs; gaps checked; all the same and an even light tan/grey colour; no deposits; no burning/melting; perfect appearance according to charts from manufacturers; swapped plugs from cylinder to cylinder with no change; see picture, the number 4 plug is on the left
• Wires - All new wires; problem identical before and after changing wires; swapped wires from cylinder to cylinder with no change
• Cap - new cap on distributor; no change from old cap to new cap
• Rotor - new rotor; no change from old to new rotor
• Points/Condensor - don't have them, Hall effect electronic pickup
• Coil - newish; maybe 6 months of use
• Carburettor - brand new; adjusted according to Weber procedure
• Compression Test - within 2 lbs on all cylinders; within spec when corrected for altitude (test run at 6700 feet and 80 F)
• Fuel pump - new electric pump
• Valve clearances - checked and reset 1000 miles ago
• Spark - pulled wire 4 and grounded it to the valve cover; huge strong spark jumping a big gap; indicating the coil is healthy and the #4 plug is getting spark

I've tested for vacuum leaks by spraying carb cleaner on intake manifolds, carburettor base plate and adapter plates and the vacuum hose for the distributor advance. I've actually done this several times because I couldn't believe it wasn't a vacuum leak. No change in idle no matter how much I spray. Coincidentally I have a very clean intake manifold at the moment.

I've been flogging the truck through the mountains of Colorado the last few days. Averaging 16 MPG driving at 55'ish MPH at Colorado altitudes with a truck half full of rocks with the other half full of camping gear and tools. So the fuel economy is quite good. It's running a bit hot but that could be an issue of my expectations (climb rate).

What have I missed?????

www.cambrianadventures.blogspot.ca

[jonnyc]

Considering everything you have already done, I would also be confused. My next areas to look at, in this order, would be:
1. Readjust valves, taking great care towards the back end.
2. Any issues around the back end of the exhaust manifold and front pipe.
3. Any head-gasket issue around the back cylinder.
These things can be VERY frustrating.

[Howsomever]

Curious to what happens if you open up the idle mix and run richer?

[willincalgary]

Considering everything you have already done, I would also be confused. My next areas to look at, in this order, would be:
1. Readjust valves, taking great care towards the back end.
2. Any issues around the back end of the exhaust manifold and front pipe.
3. Any head-gasket issue around the back cylinder.
These things can be VERY frustrating.

You wonder about some sort carbon buildup on a valve not allowing it to seat properly. That should show up in the compression test, as should a head gasket issue. When I did the compression test all the plugs were out so if the head gasket were blown you'd see it. What were you thinking to look for with the exhause manifold?

[willincalgary]

Curious to what happens if you open up the idle mix and run richer?

I fiddled somewhat with the idle mixture. Running richer seemed to make the idle worse, probably because the front three cylinders were now unhappy. I'll try again tomorrow.

[disco2hse]

If you are not using points, should you be using resistive plugs and leads?

[willincalgary]

If you are not using points, should you be using resistive plugs and leads?

I have to admit I have no idea what that means. Could you elaborate please? Some cylinders are firing fine so if it can't be a major issue.

[disco2hse]

This really http://www.ngksparkplugs.com/techinf...aqresistor.asp

Realising of course, that you are not using an onboard computer

However, it may be that the magnetic pickup in the distributor could be affected by spark leakage (admittedly unlikely with your new leads, but can happen if they have a coating of dust that has got damp or oily) or else electro-magnatic noise from the leads or plugs.

Actually, have you checked that the earth to the chassis is well connected, and that the distributor is properly earthed?

I'm not sure if you mentioned it, but I assume it fires correctly when under load, accelerating, decelerating, or with constant throttle. That is, it is only playing up at idle. I take it too, that it doesn't backfire.

[willincalgary]

I actually do run an onboard computer, gps and satellite phone. I can remove them and see what happens. I still don't see how this would effect only a single cylinder though.

The engine still misses somewhat under load but not nearly as much. If one cylinder were not firing under load I would not expect reasonable fuel economy numbers and the even spark plug coloring and condition I see.

[thixon]

So it looks like you've replaced your entire ignition system, checked for vacuum leaks everywhere (even the brake servo?), pulled the plugs, and tuned it. Damn. If you hadn't switched back and forth on your wires, I'd of immediately said it was a bad wire that was arcing somewhere.

How many miles are on the motor? Carbon buildup on the valves can cause hot spots and pre-combustion, but that doesn't sound like what you're describing. I know you did a compression test, but a leak down would be a better test to perform in this case. Since it doesn't sound like an ignition or vacuum problem, I have to head toward the mechanical side. Rocker shaft issue , bent push rod, or cam lobe, maybe? The push rod/cam lobe is highly unlikely based on what you describe but I'm just throwing it out there.

[milhouse]

Take anything I say with a grain of salt, as I'm not extremely familiar with a Hall effect distributor...

You say that when you ground the #4 wire you get a good, strong spark... but is it occurring at the correct time? Meaning when the #4 cylinder is supposed to fire? Is there something dirty or accidentally jumped inside your distributor, causing the #4 cylinder to receive spark, but at the wrong time?

[thixon]

I completely missed the Hall effect distributor. Which one do you have? It could be that the ignition module is going bad. Does it start right up, or does it take a while?

[thixon]

A couple more things I thought of: Weak valve spring, or slightly bent valve? This ones bugging me. Post the results.

[Howsomever]

If I understand this correctly...............only at idle does it miss and it is number 4 cylinder. Off idle and everything is fine? Let me guess Weber 34ICH? Besides the adapter piece that turns it 90 degrees on the manifold are you using the phenolic spacer piece? As mentioned before I hope you checked the valves. If you park with the front of the vehicle high like on a slope do you still have the miss? Did you have the miss at idle with the previous carburetor?? Since you are using an electric fuel pump what is the fuel pressure at idle? Rovers I just love'em!

[disco2hse]

I actually do run an onboard computer, gps and satellite phone. I can remove them and see what happens. I still don't see how this would effect only a single cylinder though.

LOL

I meant "onboard computer" as in ECU. My TD5 Disco has 7 of them.

You might want to take a run over to a local workshop where they have diagnostic equipment and trace what is happening with the spark. As suggested, I suspect the spark is out of sync. This may be caused by a number of things, failing ignition module, leakage. You will see fairly clearly on the oscilloscope what the plugs are doing and if there are issues there. Takes about half an hour.