Rough idle and one-time misfire

[Brunoandrade98]

Hi all,

My 197 has developed a rough idle. When idling, the car sorta shakes/vibrates a bit.

It does seem to drive okay, with no issues, but the other day, I turned the car on after not driving it for only 3 days and it had a very obvious misfire and low on power. It did sort itself out after it warmed up a bit, but the rough idle remains whether the engine is cold or at temperature.

I replaced two coils at the end of last year with used ones after I (accidentally) ruined the rubber seals on them with used (but working) ones. I needed the car for a 1000-mile road trip the next day and it was all I could find locally - It was a miracle that I could find them at all...

All 4x spark plugs have been replaced at the same time with the correct NGK ones. The car did very well on the 2500 miles I drove in total during the trip and gave me no trouble whatsoever. With that said, the sparks are only ~6 months and 3000 miles old, so still quite new.

Any ideas on what I should check? My CAN Clip and OBD reader show me no faults whatsoever, but something is definitely wrong.

I grabbed a multi-meter and measured the Ohm resistance on every coil and got consistent results of 0.6 (can't remember if Ohms, kOhms or whatever else...)

Last but not least, am I safe to drive the car, even with the rough idle, or does it cause more engine wear than normal? I change the oil every 6 months for a (hopefully) longer life of the engine, so wouldn't want to undo that by driving it in its current state.

Any input appreciated

 

[RSRowe]

Mine was lambda sensor when it started running rough on idle. There were no codes.

 

[Frank Perry]

Check inside your coils - see if there's any corrosion. Usually you'll see some corrosion around the plug which helps diagnose the potential of a bad coil.

Failing that, Lambda/Oxygen sensor(s) could be bad as suggested above.

Also check your exhaust flexis aren't blown, you'd usually notice as the car would get louder but this would have an effect on the air/fuel ratio and can cause poor/lumpy idles etc.

Good luck and keep us posted!

 

[Brunoandrade98]

Mine was lambda sensor when it started running rough on idle. There were no codes.

Apologies, I read the reply yesterday, but couldn't reply.

Do you know if they can be tested with a multi-meter? If I remember correctly, they're not cheap, so I wouldn't want to buy one unless I really had to.

 

[RSRowe]

You can check to see if the voltage is pulsing, but that doesn’t really tell you anything.

 

[Brunoandrade98]

Check inside your coils - see if there's any corrosion. Usually you'll see some corrosion around the plug which helps diagnose the potential of a bad coil.

Failing that, Lambda/Oxygen sensor(s) could be bad as suggested above.

Also check your exhaust flexis aren't blown, you'd usually notice as the car would get louder but this would have an effect on the air/fuel ratio and can cause poor/lumpy idles etc.

Good luck and keep us posted!

One of the coils is a bit brown from previous corrosion, but I can't see any corrosion on the coil or plug.

Before the road trip, the car had a terrible misfire, and it turned out to be that the windscreen wiper hose had split and was dumping water into the spark plug wells.

I cleaned it up as best as possible, replaced spark plugs and greased the hex nuts on them and the metal contact at the top. I also greased the coil's contact as well and seal - I used Superlube Synthetic Grease, which works for electrics

Flexis appear to be fine. I actually upgraded to a PowerFlex top engine mount, which reduces a lot of the back and forth movement of the engine when driving, reducing wear (in theory)

 

[Frank Perry]

Sounds like you've done a fair amount of maintenance but given one of your coils is brown, it could well be corroded inside... I thought cleaning and putting electrical grease on would fix but it didn't and ended up replacing all 4. I have a set of coils if you're local to Bristol, welcome to have one to test whether your coil is the culprit!