Cylinder roughness without misfire

[hedges88]

I had a breakdown about 2 months ago in my W169 A class due to a cylinder 3 misfire. AA guy checked the spark plug heat shield-coil pack cables were all secure and the problem went away. His diagnostic equipment however showed continued "rough running" of cylinder 3, I did not know that was possible to measure, but apparently the crankshaft position sensor knows if the cylinder that last fired rotated the crankshaft more or less than the others.

I have had a problem with hesitation around 2000 RPM but only when the engine is cold. There are no misfires detected from cylinder 3 and my basic ELM scanner shows no misfires detected at all during a drive cycle. AA man said to renew coil pack or coil pack leads (car is nearly 13 years old and 93k) but said an injector fault could not be ruled out.

If absolutely no misfires are detected is it possible that combustion has taken place but has been poor compared to the other cylinders?

As the car is very difficult to work on due to the engine placement I just wanted a heads up if I can do anything to tell if it's the coil pack, coil leads or an injector before I start stripping the "engine bay" and get out the mirrors on sticks. I can't swap a lead and see if the problem moves to another cylinder as I don't have the diagnostic equipment to measure rough running like the AA patrolman. Given the engine is sweet as a nut once warmed up is it likely to be the rubber of the coil pack leads? Coil pack leads are cheap enough, new coil packs (2x each driving 2x cylinders) are not so cheap I wondered if I can do anything to diagnose the problem for myself. Is a fuel injector problem likely to remedy itself once the engine is warm?

I'm about to part with over £500 for a new timing chain and thermostat (stuck open) so anything I can do myself to save money would be a massive benefit

[stevieturbo]

Without test equipment.....it's the parts cannon other than a basic visual inspection. You can get basic spark jumpers which can be a decent test for a visual on any spark, although it stuff is inaccessible that may pose a challenge, but they are cheap.
And if a problem is intermittent...might be hard to catch it.

With a basic generic OBD reader.....may as well be the parts cannon. Proper diagnostic tools will always give you more information. Generic OBD give very little info, and sometimes it can even be misleading with wrong codes and of course no codes as it does not have the ability to communicate properly

This is a good tool specifically for checking ignition in-situ

https://gtc.ca/product/gtc505-engine-ignition-anal...


Although if someone messed with the wiring before and a problem improved, it might imply there are still issues with that wiring that need investigation and they have now re-appeared.

And there are many ways the ecu can determine a misfire, although how that system works can also depend how reliable that determination is as different manufacturers use different methods ( and some none at all )

[hedges88]

stevieturbo said:

Without test equipment.....it's the parts cannon other than a basic visual inspection. You can get basic spark jumpers which can be a decent test for a visual on any spark, although it stuff is inaccessible that may pose a challenge, but they are cheap.
And if a problem is intermittent...might be hard to catch it.

With a basic generic OBD reader.....may as well be the parts cannon. Proper diagnostic tools will always give you more information. Generic OBD give very little info, and sometimes it can even be misleading with wrong codes and of course no codes as it does not have the ability to communicate properly

This is a good tool specifically for checking ignition in-situ

https://gtc.ca/product/gtc505-engine-ignition-anal...


Although if someone messed with the wiring before and a problem improved, it might imply there are still issues with that wiring that need investigation and they have now re-appeared.

And there are many ways the ecu can determine a misfire, although how that system works can also depend how reliable that determination is as different manufacturers use different methods ( and some none at all )

Saw a YT video that suggests spraying water from a mister bottle over the leads in the dark to see if any spark becomes visible, don't know if that would be helpful.

I think I'll resign myself to having new coil pack wires installed during the timing chain replacement and then if it's the coil pack I can manage that okay myself. iCarsoft would be the cheapest way to get more advanced diagnostics I think, they seem pretty good and relatively inexpensive

[stevieturbo]

Depends whether it is an ignition problem I guess and in an area where water might cause it to get worse. It is a crude test and might only be worthwhile for breakdown in the HT leads.

Rather than asking whoever is changing the chain to throw other new parts at it...why not ask them to diagnose the problem ? Presumably they have some knowledge and equipment ?

And might be worth doing it before changing the chain in case there is some other major issue that might require taking things apart, which might have just been put back together swapping the chain lol

This alleges it is a brand specific tool and appears to have decent coverage.. Although W169 seems to be an oldish model, it may or may not have a misfire detection/reporting strategy.


https://www.gendan.co.uk/product_MAXIECUMBB.html#

[hedges88]

Just wanted to update my post to say that it WAS the leads from the coil packs to the spark plugs/heat shields. The difference in the engine is breathtaking. Most notably it sounds a hell of a lot better, there was an almost exhaust leak like blow from the engine before very slightly which is gone. Performance and economy massively improved. Temperature of coolant has risen several degrees (despite the stuck open thermostat) and catalyst temps are up a good bit, which leads me to believe cylinder 3 was firing but barely. It probably had a flame front like a bic lighter due to poor electric delivery to the spark plug.

Moral of the story, change your spark plug wires especially on an old car because it is very much worth it!!

[Krikkit]

And the thermostat too presumably! Good result mind.

[hedges88]

Krikkit said:

And the thermostat too presumably! Good result mind.

I'm kind of irritated by the thermostat actually. You have an ECU complex enough to understand many parameters including ambient air and intake air temperature as well as the coolant temperature, but countless drive cycles of long lengths never causing the coolant temperature to normalise should really be something that triggers a DTC. It knows if the coolant temperature sensor is faulty but a stuck open thermostat it's oblivious to

[stevieturbo]

hedges88 said:

It knows if the coolant temperature sensor is faulty but a stuck open thermostat it's oblivious to

No it doesn't. It will only know if the sensor is open circuit, or short circuit. Anything in between is always a variable.

[Boosted LS1]

You can often see duff plug leads in the dark. Sparks will jump all over the place.

[hedges88]

stevieturbo said:

No it doesn't. It will only know if the sensor is open circuit, or short circuit. Anything in between is always a variable.

Point taken, the sensor could deliver information that's wrong and the ECU wouldn't know unless it's output was determined to be "implausible". Then as you say you do at least have a DTC thrown from electrical issues

I guess I'm expecting a little too much of an ECU to work out the fact that something like a thermostat stuck open is amiss when it has to so quickly and constantly deal with the many many things it has to do. It's just that one of the main areas of focus of an ECU is to make the engine as environmentally friendly as possible and having a thermostat stuck open is not very good for the motor itself or its emmisions.