Idle fluctuations caused by a loose spark plug AND a bad ignition coil (Fixed)

anglotron

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My LR3 recently ticked over 150k miles and suddenly developed an idle perturbation, not really a lumpy idle, but more of a brief 50 rpm or so dropout that was intermittent. It didn’t seem to manifest at higher speeds. I cleaned the throttle body, PCV, etc. and checked for obvious vacuum leaks. No obvious problems and I never had any codes. I did replace the MAF as preventative maintenance and the oil filler neck as the oring was deformed. No change.

So, I suspected a clogged injector due to my LR3 also feeling really sluggish around 30-50mph range in traffic and also the relatively high mileage. Ran a can of BG44K through the tank which seemed to immediately improve the issue, but the problem was back when I refilled the tank. Looked at the price of new injectors (wow!) and decided to run another can of BG44K through again, just to be sure. Again, seemed to improve the issue but the problem remained after a fresh tank of gas.

By some small miracle I managed to find a full set of genuine NOS Denso injectors on eBay from Taiwan for a great price. Old injectors physically didn’t seem in too bad a shape, but the new ones have definitely improved the drivability and seem to have restored the torque I was missing in traffic. I’m very happy with the result and I’ve also gained 1MPG on my commute! However, the idle problem still remained.

Engine runs great and sounds like a sewing machine aside from the perturbations and I began to suspect the EGR valve, but didn’t want to drop $400 without being somewhat sure. Pulled it off this morning with the intention of cleaning out the carbon and inspecting the pintle seat. Didn’t look too bad and the pintle seat looked fine. What I did notice when I filled the internal chamber with carb cleaner was an air bubble coming from the actuator shaft. I’m pretty sure the EGR valve is working, but the actuator seal/bush is worn and creating a vacuum leak into the inlet manifold when the valve is closed at idle. Not sure how it’s designed internally, but I did notice quite a bit of side play in the actuator shaft.

The one thing that still confuses me is why the BG44K seemed to improve the idle issue, but perhaps the extra air introduced was offset by it somehow.

At this point it seems reasonable to suspect the EGR valve and see if this finally resolves the issue.
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anglotron

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I replaced the EGR valve last Wednesday and it did nothing to fix the lumpy idle issue, although the old valve had tons of play in the shaft compared to the new. The problem actually got much worse on Friday, to the point I was worried about making it home from work and so the only thing left was either ignition coils or spark plugs. I decided to pull all plugs this morning, which is what I should have done in the first place. Ignition coils superficially all looked ok and again, I never had any misfire codes.

When I went to undo #5 spark plug however, it was essentially slack and took no effort to unscrew! The others were all tight. The crush washer on #5 plug was also dirty compared to the rest, indicating it hadn't been sealing to the head properly. I was sort of expecting the plug electrodes to look worse than they did, but they all look fine for the 25k they have on them. I fitted them 5 years ago and I've never experienced a plug come loose before.

I cleaned and put everything back together and from a cold start the problem was completely gone. Rock solid idle and no exhaust pops. Took it for a spin around the block to warm it up, drove great, but the problem now only manifested itself when the engine was hot and still 95% better than it was before. I suspect the crush washers on the spark plugs aren't sealing 100% when things expand and I'll have to fit new ones.

I guess the lesson here is to not overlook the basics!

(Edited: Cylinder numbering is wrong in the photos. Cylinders #1 and #5 are at the front of the engine. #1 is left bank viewed from FRONT of engine)

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Plugs that look wet is just WD40 I used to lubricate threads when removing. #5 crush washer dirty.

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remember5

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With Landrover as with everything else I have to force myself to step back and think about what the simplest issue could be and go from there. And most of the time keeping it simple is the fix. Great info, thanks!
 

anglotron

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After the engine cooled down last night I pulled all the ignition coils and re-torqued all the plugs. One tightened a little bit more, but nothing crazy. Put everything back together and the exhaust was popping like mad, but the idle was relatively stable. I was pretty sure it couldn't be the plugs at this point.
I thought it actually might be the O2 sensors gone bad, pulled both front and rear fuses and it didn't change anything. Also couldn't see anything obvious on my Gap tool - although I really need to upgrade to the BT version.

I then went about swapping each ignition coil out sequentially with a new one, starting it up and seeing if it made a difference. This finally fixed the problem! Took it for a 30 minute drive and it all looks good.

I think I had two concurrent issues, one was a loose spark plug (#5) possibly lowering the compression in that cylinder and the other was a lazy if-not-quite-dead ignition coil (#8). This could explain why I felt it drove so sluggishly. Tightening the spark plug seemed to fix the gross idle fluctuations and replacing the ignition coil fixed the exhaust pops. I never had any error codes which made diagnosis harder and it's somewhat ironic that a Land Rover didn't throw a code when one would have REALLY been useful!

Clogged injectors and an EGR valve leak were never the problem, but they were probably not functioning optimally and I should be good for another 100k now.
 

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