Unsteady rpm

[Dritto]

Hi, I've had unsteady rpm since I bought the car. I have since done a complete tune up, vacuum hose, TB, IAC and PCV clean, intake gasket and alot more. But I'm still having unsteady rpm. As you can see in the attached video, it's not only at idle.
Any suggestions?

https://www.youtube.com/watch?v=2mVT...dex=16&list=WL

[Steve940estate]

That doesn't look too bad to me, as fuel injection goes the stuff on 940 is pretty old tech now so I don't know if they were ever much better.
How many miles has the car done, the only thing I could suggest is doing a compression check to see if there are any discrepancies.
Another thing that might have an effect is the condition of the battery/alternator output.

[Dritto]

150.000km on the odometer. I know that cylinder #2 exhaust valve is out of spec by 0,05mm loose. The battery doesn't seem healthy, so it could be that. I will check alternator.

[Jimsiss]

Wow, wish mine was that stable on idle. Mine regularly needs revving to get it to idle correctly on start up. It looks relatively stable for the type and age of car.

[Laird Scooby]

When you say you did a complete tune-up, did that include new dizzy cap, rotor arm, plugs and HT leads?

That's a very regular "semi-misfire" you have there. It's firing on all 4 pots but one of them is weak. This could be down to the exhaust valve clearance on #2 but i doubt it, after all there is a window of 0 - 0.05mm tolerance on the valve clearances anyway.

It's much more likely to be a worn, dirty, faulty injector or an air leak round an injector, dodgy HT lead, dodgy plug, dirt/tracking on the dizzy cap or a corroded contact inside the dizzy cap or possibly a sticking valve or compression ring on one piston.

Try the following :

Add some injector cleaner additive to the fuel
Add some ATF (preferably synthetic) to the engine oil, about 0.5L should do it.

Take it for a long, fast run then see if things have improved - perhaps take another video for comparison.

If there is no improvement, try squirting WD40 (engine running) round each injector where it enters the inlet manifold - if there is a bad seal on one of them, this will show up by changing the engine note.
If this doesn't show any problems, use some insulated pliers to remove and replace each plug lead in turn from the spark plugs. When you find the cylinder that has the partial misfire, there will be little or no difference to how the engine runs, conversely removing the plug lead from the other "good" plugs will make it worse.
Once you've worked out which cylinder has the partial misfire, switch off and remove the plug. Now swap it with one of the others, run the engine again and do the remove the plugs leads thing again to see if the fault moves. If it does, the plug is faulty.

If the fault doesn't follow the plug, remove the dizzy cap and check for cracking, tracking, dirt, corrosion on or near the contact associated with that cylinder. If all looks good, swap the HT lead for that cylinder with one of the others - don't forget to swap them at the plug end as well!

Run the engine again and see if the fault has moved, if so the HT lead is faulty. If it hasn't then the HT lead is ok so swap them back to how they should be.

Hopefully after all that you should have a solution to the problem, if not then a compression check may show up some more. Don't forget to remove the fuel pump fuse(s) before doing the test which should be done hot with a wide open throttle and all plugs removed.

[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.

[Laird Scooby]

Turn your volume control up Ash and have another listen - seriously!

[aardvarkash10]

Originally Posted by Laird Scooby

Turn your volume control up Ash and have another listen - seriously!

turn the volume up??? Then I wouldn't be able to concentrate on the last episode of Stranger Things...

I was going to do a vid of Moomin in reply. One tick-y tappet and uncertain state of tune, he wobbles around a bit at idle. Meh. Put your foot in and he still frightens JDMs on the motorway on-ramp. If he idles a bit odd, I can pass that off as a 3/4 cam....

[Laird Scooby]

Think there's a subtle difference Ash and as you put it, it scares the JDMs on the on-slip.
I settle for scaring anything German, regardless of what i'm driving!

[Dritto]

Originally Posted by Laird Scooby

When you say you did a complete tune-up, did that include new dizzy cap, rotor arm, plugs and HT leads?

That's a very regular "semi-misfire" you have there. It's firing on all 4 pots but one of them is weak. This could be down to the exhaust valve clearance on #2 but i doubt it, after all there is a window of 0 - 0.05mm tolerance on the valve clearances anyway.

It's much more likely to be a worn, dirty, faulty injector or an air leak round an injector, dodgy HT lead, dodgy plug, dirt/tracking on the dizzy cap or a corroded contact inside the dizzy cap or possibly a sticking valve or compression ring on one piston.

Try the following :

Add some injector cleaner additive to the fuel
Add some ATF (preferably synthetic) to the engine oil, about 0.5L should do it.

Take it for a long, fast run then see if things have improved - perhaps take another video for comparison.

If there is no improvement, try squirting WD40 (engine running) round each injector where it enters the inlet manifold - if there is a bad seal on one of them, this will show up by changing the engine note.
If this doesn't show any problems, use some insulated pliers to remove and replace each plug lead in turn from the spark plugs. When you find the cylinder that has the partial misfire, there will be little or no difference to how the engine runs, conversely removing the plug lead from the other "good" plugs will make it worse.
Once you've worked out which cylinder has the partial misfire, switch off and remove the plug. Now swap it with one of the others, run the engine again and do the remove the plugs leads thing again to see if the fault moves. If it does, the plug is faulty.

If the fault doesn't follow the plug, remove the dizzy cap and check for cracking, tracking, dirt, corrosion on or near the contact associated with that cylinder. If all looks good, swap the HT lead for that cylinder with one of the others - don't forget to swap them at the plug end as well!

Run the engine again and see if the fault has moved, if so the HT lead is faulty. If it hasn't then the HT lead is ok so swap them back to how they should be.

Hopefully after all that you should have a solution to the problem, if not then a compression check may show up some more. Don't forget to remove the fuel pump fuse(s) before doing the test which should be done hot with a wide open throttle and all plugs removed.

Thanks for your post The clearance is .45mm, according to Haynes the cold checking numbers are = 0.30-0.40mm for all valves.
The cap, rotor and plugs have been changed this year, while the HT leads looked good, and had proper resistance. The injector seals don't seem to be faulty either.
Fuel additive has been used multiple times, and I changed the fuel filter after. 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?