Odd 'popping' Sound Coming From One Cylinder At Very Low Revs

[SweetBadger 94]

Hi, bit of an odd one this, and hopefully nothing to worry about... but when my engine is at idle and the revs drop to below 900 rpm there's an odd 'popping' sound coming from cylinder 1.

 

It's especially apparent when the engine is cold, and completely goes away at anything above 900 rpm.

 

Compression is good and even across all cylinders and the engine runs well and smoothly. There's no tapping from the head as far as I can tell, and it all sounds nice and tight.

 

The popping noise is getting on my nerves a bit though!

 

Here's a quick video:

 

You can clearly hear the 'popping' through the number 1 inlet runner after I make it run on 3 cylinders at 19 seconds, and then on 2 cylinders at 30 seconds - i.e. when it's idling below 900 rpm.

 

http://vid16.photobucket.com/albums/b49/sweetbadger/6D297646-962F-4CFB-B520-8994F839674B.mp4

 

Any thoughts? Anything to be concerned about?

Edited January 29, 2015 by SweetBadger

[acox99 74 1 Cars]

Could be a bit lean at idle.

[SweetBadger 94]

I thought it might be something like that but it still happens after I unplug the cylinder 1 injector, and with the injector not operating I guess there'd be no fuel to cause a pop or backfire?

[acox99 74 1 Cars]

What does the plug colour look like on that cylinder. Seems like an odd one as without fuel I can't think what would cause a pop, and if it was a valve that would show up in a compression or leak down test.

[SweetBadger 94]

Had the plugs out last week, have done about 600 miles in it since it was mapped and they all looked fine and a uniform colour across all cylinders.

 

If it was a bent or leaky valve I'd have thought it'd get louder as revs increased, so it's a bit of a mystery!

 

If anything it sounds like a valve is opening against a slight vacuum, but for it to be doing that it'd have to be opening late which wouldn't happen on one cylinder only...

Edited January 29, 2015 by SweetBadger

[acox99 74 1 Cars]

If it was cam timing you would have it popping through more than one cylinder, and a lumpy uneven Idle or reluctant to rev out. I was thinking more a sticky or slightly burnt valve. But that would cause a hot spot and it would pink.

 

If it had a vacuum it would cause a misfire a bit like if a split servo pipe pulls air on 1 cylinder.

[SweetBadger 94]

It is the only cylinder providing the vacuum for the servo, so guess it could be something to do with that, will have to block it off and see if it stops the noise.

 

Have bigger problems at the mo though, started idling badly and seems down on power. Pulling the injector plug off cylinder 3 does very little to how the engine runs, so that cylinder is not firing properly.

 

Plus all the plugs came out black so something's amiss...

 

Compression test still shows good compression on all cylinders though (2, 3, & 4 @ 200psi, cyl 1 slightly down @190psi)...

[welshpug 1,657]

that's not much at all, I'd change the plugs and give it a good run.

[j_turnell 137 3 Cars]

I'd be tempted to have another take off for the servo on another runner, replace the plugs and check everything is balanced.

[SweetBadger 94]

I have a spare threaded barb and I think I might tap into another inlet runner for vacuum as you suggest - I've noticed a lack of servo assistance when getting on the brakes very quickly after closing the throttle which would be helped with vacuum from another cylinder.

 

I popped a spare set of plugs in it this eve but it made no difference to the crappy idle, and still cyl 3 seems to be doing very little on idle - if I pull an injector plug off any of the other cylinders you really notice the the drop in revs and misfire, but pulling cyl 3 injector hardly makes a difference. Seems the same off idle (to a lessor extent) too.

 

I'm going to eliminate a few simple things first - like swapping over an injector and plug lead from another cylinder to see if it moves the issue.

[acox99 74 1 Cars]

Easiest way to test the servo is to just clamp the pipe off, or remove the barb and blank it off with a bung and see if your idle improves. Try one thing at a time so you can eliminate rather than several things. Because if it changes you won't know what caused the change.

[SweetBadger 94]

Had a quick look at lunch today:

 

- Swapped an injector over - no difference

- Swapped HT leads around - no difference

 

Adjusted the air bypass screws on cylinders 2 & 3 and hey presto idle quality much better! Now removing an injector plug has the same effect on the idle quality for each cylinder.

 

So they were out of balance, although I'm not sure how seeing as I've not touched the throttle stop or air bypass bleed screws since it was mapped...

 

So guessing sitting there on idle with unbalanced bodies is what's sooted up the spark plugs.

Edited February 3, 2015 by SweetBadger