Unsteady rpm

[Dritto]

Originally Posted by aardvarkash10

some considerations:

Thats pretty stable imo. Its getting a bit nitpicky to worry about such minor fluctuations at idle, but if its bugging you Dave's suggestions around vacuum leaks are consistent.

I would also consider engine loads. Yes, its not moving, but the alternator, power steering pump, air con (if fitted) etc are all presenting a load that could potentially cause these variations. Your electric fuel pump, for example, can provide the sort of load that would lead to the amount of variation you are seeing.

At higher engine speeds you run into two often overlooked issues. First is, your foot is not stable. Even if you try hard (actually, especially if you try hard), you are unable to maintain steady pressure on the throttle - our bodies actually use a reference feedback system that REQUIRES slight variations to maintain what we perceive as a steady state. Try holding your hand out absolutely still in front of you. You can't.

Second is, at no load the engine's fuel system automatically moves to as lean a mixture as it can sustain or is tuned for. Lean is harder to fire, and can lead to varying degrees of "lean surge". Even when you use a throttle screw to maintain the high idle speed to overcome you foot wobbling, the engine will tend to vary as it moves from "just lean enough" to "too lean" and back again. Even if you don't see a change in rpm, you often get slight changes in engine tone. Its normal. Don't sweat it. How often are you sitting idling at 2,200RPM anyway? The majority of the time the engine is under a load and the mixture is slightly richer.

JM2CW.

I know it's pretty nitpicky, but I just want something to do, and that is to keep it in top shape :P I can feel the change in vibrations through the chassis, so that is also one reason. As for the accessory load, aren't they constant? They shouldn't change in amount of load in a standstill, except for the AC compressor when the clutch kicks in, but I have it off almost all the time. Maybe it wasn't much smoother from the factory, as you say.

[Laird Scooby]

Originally Posted by Dritto

What is the idea or trick behind adding ATF to the engine oil?
I tried to remove leads as you instructed, but it behaved the same for all the cylinders. I have yet to do a compression test. BTW I noticed a spark through a gap in the distributor cap - but I'm thinking it is the rotor contacting the contact point, correct?

The idea behind the ATF is it gently cleans the inside of the engine and is safe to leave in. My line of thought is the valves are a bit dirty on their stems and sticking slightly causing them to not quite close properly. This could be just the one valve or possibly all of them.
It will also help clean the piston rings allowing them to do what they should and improve compression and general running.

There are a few other benefits but those aren't really relevant at the moment such as "Seal-Swell" which is an additive in most, if not all, ATFs that helps to revive and condition the various seals throughout the engine - you rarely if ever get an oil leak on an automatic gearbox unless there has been a problem internally.

On cars with hydraulic tappets/lifters, it cleans and revives the hydraulic tappets as well. For example, the car i had between my last 740GLE and my current 760GLE was a Jeep XJ Cherokee 4.0 and when i first got it, that had at least 3 noisy lifters.
When i say they were noisy, they sounded like i had at least three pile-drivers running under the bonnet when it was cold, not much better when it was hot either!
First oil change a week or so into ownership and i used about 1.5L of ATF in the engine on top of the 5L of oil it had already taken - total capacity was just under 7L so it wasn't over-filled either!
After a few hundred miles, that engine was quiet, cold or hot and pulled much more smoothly, quietly and more powerfully.

Obviously you don't have hydraulic tappets so you won't get that benefit but when i got my 760, that was also noisy, particularly when cold, not as smooth as it should be either. That has had the ATF in the engine oil as well and it's like a different car now.

That spark in the dizzy cap needs investigating! First there shouldn't be a gap that you can see through and secondly you need to try and find out where it's going - it's possible you've lost the carbon brush from the centre of the cap and you saw the spark jumping, if that is the case, i'm pretty sure you've found the problem.

[Dritto]

Originally Posted by Laird Scooby

The idea behind the ATF is it gently cleans the inside of the engine and is safe to leave in. My line of thought is the valves are a bit dirty on their stems and sticking slightly causing them to not quite close properly. This could be just the one valve or possibly all of them.
It will also help clean the piston rings allowing them to do what they should and improve compression and general running.

There are a few other benefits but those aren't really relevant at the moment such as "Seal-Swell" which is an additive in most, if not all, ATFs that helps to revive and condition the various seals throughout the engine - you rarely if ever get an oil leak on an automatic gearbox unless there has been a problem internally.

On cars with hydraulic tappets/lifters, it cleans and revives the hydraulic tappets as well. For example, the car i had between my last 740GLE and my current 760GLE was a Jeep XJ Cherokee 4.0 and when i first got it, that had at least 3 noisy lifters.
When i say they were noisy, they sounded like i had at least three pile-drivers running under the bonnet when it was cold, not much better when it was hot either!
First oil change a week or so into ownership and i used about 1.5L of ATF in the engine on top of the 5L of oil it had already taken - total capacity was just under 7L so it wasn't over-filled either!
After a few hundred miles, that engine was quiet, cold or hot and pulled much more smoothly, quietly and more powerfully.

Obviously you don't have hydraulic tappets so you won't get that benefit but when i got my 760, that was also noisy, particularly when cold, not as smooth as it should be either. That has had the ATF in the engine oil as well and it's like a different car now.

That spark in the dizzy cap needs investigating! First there shouldn't be a gap that you can see through and secondly you need to try and find out where it's going - it's possible you've lost the carbon brush from the centre of the cap and you saw the spark jumping, if that is the case, i'm pretty sure you've found the problem.

I've used CRC Motor Flush two times on this engine, it's supposed to clean the engine internally, by leaving it in for 5 mins with the engine running and then draining. Good to know that ATF can also be used.

The original cap was replaced by a Facet cap, but they looked exactly the same. On the picture I've marked the gap where I can see the spark through.

[aardvarkash10]

the blue spark you can see is to be expected - there is an airgap between the rotor terminal and the plug lead terminal and a spark generates across the gap.

If you are really worried about it, pull the cap off and look for thin, spiderweb like black lines on the inside of the cap. These are carbon tracking lines and indicate the spark is being lost to ground by tracking down these lines.

The best technical running test is to get on a 'scope set to read the secondary ignition voltage and short each spark plug lead to ground while the engine is running. On the scope, you will see the voltage generated across the rotor/cap gap. It should be less than 5kv - typically 2-3kv.

[Laird Scooby]

I forgot about that dimple in the cap, isn't there a flash-guard fitted on those dizzys?

[Steve940estate]

Originally Posted by Laird Scooby

I forgot about that dimple in the cap, isn't there a flash-guard fitted on those dizzys?

Only the early cars had them I think. I fitted one on mine to keep the oil that leaks through the distributor getting on the contacts so easily.