Rough idle

[GrahamG]

Can anyone offer some possible causes of an intermittent rough idle?

It is very rough, possibly half the cylinders out. On most journeys there is no problem.

When it occurs there is missing as the car accelerates away from stationary up to about 15mph and then it clears and all is smooth. In a car park today it cleared up as I played with P, N and D. I returned after an hour and drove home normally.

At speed acceleration is smooth and powerful.

Car is fully Lexus serviced with 88,000 miles.

There are no warning lights on dash.

I am asking for a list of possible suspects because the intermittent nature of the fault may be difficult for a garage to diagnose.

[Mr Ponkerson]

first thing I'd suggest checking would be vacuum leaks and spark - plugs, leads, coils, dizzy caps.  There are loads of things it could be, but these are probably the simplest and most obvious places to start.  I had a similar issue with a mk3 ls400 and went through lots of things - turned out to be dizzy caps and a coil.  It started intermittent for me, but ended up constant.

[GrahamG]

Just connected an OBD scanner and just one pending code PO133 which points to a faulty O2 sensor and a number of other possible causes.

If you know these causes does any fit with the intermittent nature of my problem?

Two years ago this happened to me at the lights, so bad I thought the engine would stall but it pulled away and for a year no repeat. In the last few months it has happened three times with increasing frequency.

[Mr Ponkerson]

Might be worth clearing  the code and run for a bit and see.  Could well be a duff sensor or bad connectors.

[BachelorDays]

Haven't done anything with the air filter recently?

O2 pre-CAT sensor means it doesn't like the air/fuel mixture going in.  I'd check the MAF connectors and the air filter housing first.  As stated earlier, could really be anything and vacuum leads et al should always be examined.

Interesting that it clears once it picks up speed.  I wonder how the MAF fares in your cars.  Any other noticeable traits - is it worse when engine cold, etc?  Notice any difference in the way the car cranks/fires up?

[GrahamG]

Thanks for the suggestions.

Have the car into Lexus Guildford next Thursday for a Techstream  code analysis.

It has only occurred when engine warm. Car starts perfectly.

Will report findings then.

Very strange to me how it can be so bad at just one set of traffic lights and then disappear for months on end.

 

[The-Acre]

I think the advice given so far is good because it could be so many things. Checking with Techstream will save much head scratching and hit and miss remedies.  Shame it isn't the following Thursday, I'd have met you there for coffee!

[Tinonline]

Not sure if it works on Lexus but does on a Merc: disconnect the MAF If mid or rough running goes you have your culprit. 

[The-Acre]

2 hours ago, Tinonline said:

Not sure if it works on Lexus but does on a Merc: disconnect the MAF If mid or rough running goes you have your culprit. 

Yes, I was going to suggest that as well.

[messi]

I had this issue on a sc430, it's funny but happened once and then it never happened again for months, but when it did happen again, the fault code was for 1 of cylinders misfiring, when we checked it, it was the coil pack, it had a crack in it, replaced it, and all was well after that, the fault only occurred when the engine was warm

[brendangeorge]

On 11/15/2018 at 1:46 PM, GrahamG said:

Just connected an OBD scanner and just one pending code PO133 which points to a faulty O2 sensor and a number of other possible causes.

If you know these causes does any fit with the intermittent nature of my problem?

Two years ago this happened to me at the lights, so bad I thought the engine would stall but it pulled away and for a year no repeat. In the last few months it has happened three times with increasing frequency.

Hi Graham

This is a common problem - I had is happen several times.  A Lambda sensor will be failing - if the ECU thinks the engine is running rich it put it in limp home mode.  The sensor that has failed will be in the very hot part of the exhaust (if I remember there are 4 and 2 are in the down pipes) which is why it takes time to fail i.e. when the car has warmed up.  I used to get replacement in the US (Rock Auto) or Alibaba and have an independant fit them.  One of them is a pain TO FIT because you have to take a seat out to access the connector.

Good luck

Bren

[GrahamG]

Thanks for everyone's contribution. Lexus Guildford were reluctant to fit a new Lamda sensor because they could not guarantee it was the problem but after waiting over an hour for a diagnosis I gave the go ahead and took a taxi home. 

Next day they delivered the car home to me [35min] for £360 total, car cleaned inside and out.

Interesting they charged £135 plus VAT for 1.5 hours labour [ actual time, at least 2.5 hours] an hourly rate therefore £108 inc VAT.

[The-Acre]

36 minutes ago, GrahamG said:

Thanks for everyone's contribution. Lexus Guildford were reluctant to fit a new Lamda sensor because they could not guarantee it was the problem but after waiting over an hour for a diagnosis I gave the go ahead and took a taxi home. 

Next day they delivered the car home to me [35min] for £360 total, car cleaned inside and out.

Interesting they charged £135 plus VAT for 1.5 hours labour [ actual time, at least 2.5 hours] an hourly rate therefore £108 inc VAT.

So is it all OK now or too early to tell?

[GrahamG]

The problem had been occurring after 25min driving and the driver who delivered the car home to me confirmed there was no misfire after a 35min trip.

So no guarantee now but I live in hope.

[The-Acre]

1 hour ago, GrahamG said:

The problem had been occurring after 25min driving and the driver who delivered the car home to me confirmed there was no misfire after a 35min trip.

So no guarantee now but I live in hope.

Keep us posted won't you, hopefully the problem is sorted!