Hesitation and rough running at low rpm - carbon buildup?

[sram]

Hi all

Happy owner of a 2008 IS250 here. Had it two and a half years and it's been a joyous ownership experience so far compared to previous German cars. There's one issue I'm having and after a lot of research on here and some Googling, I think it may be carbon buildup related. I wondered if anyone had any thoughts?

For the last few months I've noticed that low speed running is getting rougher. If I'm cruising along at about 1200-1500 RPM the car will gently shake and hesitate. It feels like a misfire. In addition, if I've been driving at anything other than slow town speeds and then come to a stop at a roundabout or traffic lights, etc the engine speed will sometimes drop below normal idle, become lumpy and then something presumably within the ECU will kick in and cause the throttle to open up a little to prevent it stalling. The engine then settles back to a normal idle and is fine until the next time it happens, perhaps the next time I come to a stop or it could be a few days later. 

The symptoms have gradually been getting worse over the last couple of months. 

I looked for the names of some recommended Lexus/Toyota independents via this site and Google and I called two of them this afternoon. Both of them told me that they haven't ever heard of carbon buildup on the intake valves being problematic for the 2.5 V6 engine in this car. One even told me that carbon buildup on the valves wouldn't cause running issues! Needless to say I don't really have any confidence in either garage after talking to mechanics there.

I know that intake valve carbon buildup seems to be an inherit problem with DFI engines from this era and you only have to spend 15 seconds online to find plenty of posts about VW group engines suffering the same symptoms.

Could anyone recommend someone that could help me out? I've found the DIY guides for stripping and manually cleaning with brushes and carb cleaner, etc, but because this is my daily driver I can't really afford to do the work myself as, although I am a reasonable home mechanic, anything I tackle always seems to turn into a weekend or two's worth of work!

Car has a full main dealer service history (until I bought it) and has 114,000 miles. I've done one oil change since having it, approx 5000 miles ago last year. The plugs were done by a main dealer approx four years ago.

Many thanks for any advice or pointers you can offer. 

 

[J Henderson]

Have a read in this topic

A faulty coil pack was responsible in this case.

[ColinBarber]

It could be carbon, or dirty throttle body etc. Ideally you need to use an inspection camera down the intake to inspect the valves.

[Spock66]

I've experienced these symptoms with a vacuum leak (not a Lexus though).

[Linas.P]

Carbon build-up is nowhere near as big issue in UK as it is in US, I really don't know why, but in UK (and Europe) IS250 rarely have this issue, at least not to the point where it becomes a problem. 

I have previously assumed my car has carbon build-up (at ~180k miles) and I did hydrogen cleaning which actually made absolutely no difference. 

Rough idle could be result of many reasons, throttle body, lambda, o2, gearbox solenoids (especially if as you said it happens when you coming to stop engine almost wants to stall), it could be spark plugs (if you say it seems like it is misfiring - maybe it actually is misfiring?). Many different things can be causing it... for example seizing idlers, or seizing A/C compressor, failing A/C compressor clutch or failing A/C compressor clutch solenoid can cause it as well, there is literally few dozen things to consider. So just assuming you have issue with carbon, because this seems to be common issue in US and generally DI engines is kind of premature.

Now there is some good info - 114k miles and full dealer service is good start. Spark-plugs 4 years ago checks out as well. 5k miles since last service is good, but at the same time it needs oil soon. 10k miles service schedule is BS, these cars needs oil much more often (same cars in Europe gets serviced every 10k kilometres, so 6k miles)- I would say at least once a year and no more than 3-6k miles intervals, especially if driven a lot in city traffic. Lexus own service schedule says that 10k miles are "extended service interval" and there are certain things that one has to comply with to qualify e.g. if car "often" is driven for long periods of time at "high-speed" (e.g. 2h on motorway at 70MPH), if car is used for towing, if car is "often" used in city traffic then they do NOT qualify for extended intervals and thus should receive service every 3-6k miles. Other things to consider - O2 and Lambda are rated for 75k miles, if they are getting "lazy" which would be expected by 114k, then you can get rough idling. Same for throttle body and for MAF, they can get dirty and cause rough idling, both can be cleaned. 

In short - there are several things that you can check, but carbon build-up would not be even in top 10 of my list of the issues to worry about. 

[H3XME]

16 hours ago, sram said:

Hi all

Happy owner of a 2008 IS250 here. Had it two and a half years and it's been a joyous ownership experience so far compared to previous German cars. There's one issue I'm having and after a lot of research on here and some Googling, I think it may be carbon buildup related. I wondered if anyone had any thoughts?

For the last few months I've noticed that low speed running is getting rougher. If I'm cruising along at about 1200-1500 RPM the car will gently shake and hesitate. It feels like a misfire. In addition, if I've been driving at anything other than slow town speeds and then come to a stop at a roundabout or traffic lights, etc the engine speed will sometimes drop below normal idle, become lumpy and then something presumably within the ECU will kick in and cause the throttle to open up a little to prevent it stalling. The engine then settles back to a normal idle and is fine until the next time it happens, perhaps the next time I come to a stop or it could be a few days later. 

The symptoms have gradually been getting worse over the last couple of months. 

I looked for the names of some recommended Lexus/Toyota independents via this site and Google and I called two of them this afternoon. Both of them told me that they haven't ever heard of carbon buildup on the intake valves being problematic for the 2.5 V6 engine in this car. One even told me that carbon buildup on the valves wouldn't cause running issues! Needless to say I don't really have any confidence in either garage after talking to mechanics there.

I know that intake valve carbon buildup seems to be an inherit problem with DFI engines from this era and you only have to spend 15 seconds online to find plenty of posts about VW group engines suffering the same symptoms.

Could anyone recommend someone that could help me out? I've found the DIY guides for stripping and manually cleaning with brushes and carb cleaner, etc, but because this is my daily driver I can't really afford to do the work myself as, although I am a reasonable home mechanic, anything I tackle always seems to turn into a weekend or two's worth of work!

Car has a full main dealer service history (until I bought it) and has 114,000 miles. I've done one oil change since having it, approx 5000 miles ago last year. The plugs were done by a main dealer approx four years ago.

Many thanks for any advice or pointers you can offer. 

 

You know, I had the same symptoms. The car would even sometimes stall if I went from 6th to neutral coasting towards a junction, but then it was also shaking at the lights and was doing like 500rpm and you could just feel it was struggling so I had to give it some revs. Almost as if the oil pressure was low. This happened only sometimes, and maybe 2-3 weeks later I changed my oil, cleaned the maf & air filter, started using Tesco 99Momentum more and poured one can of Liqui Moly injector cleaner into the fresh tank of good fuel. I forgot about this issue now but come to think of it I had nothing like that ever since. 

Like Linas already mentioned, Carbon build up to a point where it's causing issues is very rare in the UK, but if your car lived in London all it's life, it may never get a proper chance to stretch it's legs often enough so it would kind of make sense, but still, I don't think it's that given your mileage. Service it, do what I did and see what happens, if that doesn't do it I would be inclined towards a faulty coil / spark plugs etc. Do the free or cheap stuff first before moving onto spending money on replacement parts that may have no effect!

[WMeek]

I've noticed this recently on mine when I use accessories, for example I'll have the AC and with today being sunny chilled seats on, if it is at idle and I select a single wipe the revs dip from about 700 to close to 500 but then come back. Similar when I put it into reverse, the camera comes on and it briefly dips but comes back. It's recently had the Battery and alternator replaced. It's a car that has lived most it's life in rural Northern Ireland so has had plenty of chance to stretch it's legs, 80k miles.

[Linas.P]

My first suggestion would have been - alternator or Battery.  But as you said it was just replaced, then I would look into general ground issues, could be some crusty contacts somewhere. 

Basically, it seems like there is resistance to the ground, so when you turn something on it puts extra load on alternator. As well it is quite surprising because thing you mentioned are all very low power, I would expect it for heated seats/mirrors/windscreen/rear window, but not for cooled seats. As well when reversing it may be more to do with gear change than it is with reverse camera. 

In short - car should not be doing it and I would speculate it is electrical.