Lexus V6 burnt valve - advise please
Discussion
Afternoon all,
i have an issue with the wife's Lexus RX300. It's a 2002 model with 91,000 miles on the clock. My undersatnding is that this engine does service in plenty of applications around the world and I'm surprised it has issues at this milage, but there you go.
Symptom was the engine chugging at idle - it got worse when you engage Drive. The revs drop as normal but the chugging is then enough to make the car rock. Plus the dash lit up with fault lights.
It was due a service anyway so I took the car to my local mechanic who is a good guy and seems to know his stuff. My first thoughts were either spark plug issue or coil pack.
So the code reader went on and it came up as misfire in cyl 2. he had a coil pack but it made no difference. He then had a look and replaced all the plugs. All were good except number 2 - it was very black and burnt. He also did a compression test - cyl 2 compression is very low at idle but build to full compression beyond about 2,700 rpm.
His view is that it could be a burnt valve or maybe it's clogged with carbon and sticking. The only way to know was to start stripping the head and due to the labour involved and the car's market value, I told him not to bother for now and picked it up this afternoon. It is running a bit better, but it's still a bit sick.
Options as I see it are:
Keep running it and use V-power and maybe throw some injector cleaner stuff in and see if it can cure itself - is this possible?
Chop it in as faulty, accept the loss and trade into something else.
Find someone who want to take the head apart and recondition / reface the valves on cyl 2 - any idea on costs?
Up the RAC cover and run it as a smoker and see how long it lasts - not my usual approach but bangernomics may make sense in this case as it is still a runner.
Anyone esle have any thoughts? Anything I'm missing?
Cheers,
Matt
i have an issue with the wife's Lexus RX300. It's a 2002 model with 91,000 miles on the clock. My undersatnding is that this engine does service in plenty of applications around the world and I'm surprised it has issues at this milage, but there you go.
Symptom was the engine chugging at idle - it got worse when you engage Drive. The revs drop as normal but the chugging is then enough to make the car rock. Plus the dash lit up with fault lights.
It was due a service anyway so I took the car to my local mechanic who is a good guy and seems to know his stuff. My first thoughts were either spark plug issue or coil pack.
So the code reader went on and it came up as misfire in cyl 2. he had a coil pack but it made no difference. He then had a look and replaced all the plugs. All were good except number 2 - it was very black and burnt. He also did a compression test - cyl 2 compression is very low at idle but build to full compression beyond about 2,700 rpm.
His view is that it could be a burnt valve or maybe it's clogged with carbon and sticking. The only way to know was to start stripping the head and due to the labour involved and the car's market value, I told him not to bother for now and picked it up this afternoon. It is running a bit better, but it's still a bit sick.
Options as I see it are:
Keep running it and use V-power and maybe throw some injector cleaner stuff in and see if it can cure itself - is this possible?
Chop it in as faulty, accept the loss and trade into something else.
Find someone who want to take the head apart and recondition / reface the valves on cyl 2 - any idea on costs?
Up the RAC cover and run it as a smoker and see how long it lasts - not my usual approach but bangernomics may make sense in this case as it is still a runner.
Anyone esle have any thoughts? Anything I'm missing?
Cheers,
Matt
A proper burnt valve is a thing of the past these days.I would go for a sticking valve, not closing fully, which is more common on modern engines,caused by lacquer or varnish that builds up on the stem.The same can cause a hydraulic lifter to malfunction.I would try Forte engine flush in the old oil,Forte gas treatment and injector cleaner in the tank and give it a "good thrashing".Change the oil and filter and add Forte top end treatment to the new oil.That worked for me on another car with the same symptom as yours and was a lot cheaper than a full head job.
Thanks for the replies chaps.
I've put some Wynns injector cleaner into a full tank of V-power, we'll see how that goes. I always thought all this stuff was a bit snake-oil, but a tenner here and there seems small change right now...
I've put some Wynns injector cleaner into a full tank of V-power, we'll see how that goes. I always thought all this stuff was a bit snake-oil, but a tenner here and there seems small change right now...
stevieturbo said:
How is he doing a compression test at idle and 2700rpm ?
Erm, i don't know the answer to that. He just said that sompression at idle was low in chamber 2, but rose as the revs did so that by about 2.5-3k rpm it was as it should be. I've never seen / run a compression test before so no idea how it's done. Does that make sense?davepoth said:
chard said:
I always thought a compression test was run at cranking speed. So no it dosen't make sense to me.
Indeed - you need to take the sparkplug out to do it. I guess it would work at 2700rpm but I can't think of any sane reason to try it.Matt UK said:
It was due a service anyway so I took the car to my local mechanic who is a good guy and seems to know his stuff.
Hmmm, people who know their stuff do the basic diagnostic tests first rather than guessing or replacing components at random. Whatever he did clearly wasn't actually a compression test which is explained here.http://www.pumaracing.co.uk/comp.htm
Then read this.
http://www.pumaracing.co.uk/diagnose.htm
Only once you know for sure if there's really a mechanical fault inside that cylinder would you ever strip things down. Go and buy your own compression tester and for 20 quid you can find out. Anything that only manifests on one cylinder should be very simple to diagnose. If compression is down then a leak down test will establish whether it's an inlet valve, exhaust valve or gasket problem.
If compression is fine then it could just be a faulty fuel injector which could also give you a black spark plug. Anyone who can't within 5 minutes establish whether it's the former or the latter doesn't actually "know his stuff."
davepoth said:
chard said:
I always thought a compression test was run at cranking speed. So no it dosen't make sense to me.
Indeed - you need to take the sparkplug out to do it. I guess it would work at 2700rpm but I can't think of any sane reason to try it.Edited by oakdale on Wednesday 22 June 13:38
oakdale said:
Wnen the spark plugs and coil are known to be good, you can accurately assess the compression and change in compression with revs of one cylinder, by observing the plug firing voltage with an oscilloscope and comparing it to the other cylinders.
This may be the case, but if the mech can't use a compression tester, what chance does he have with an oscilloscope?!? Edited by oakdale on Wednesday 22 June 13:38
te fuel. Plain and simple. Either that or manufacturers are building heads and valvetrains out of putty.In my experience a compression test just tells you there is a problem ,to find out the cause needs a leakdown test,has this been done? If not, I would suspect the competence of the garage.I agree with the previous poster that a look inside with a borescope would be wise before stripping anything down.Incidentally I have recently replaced a burned/split valve in a Vauxhall X22XE engine,it had the same symptoms as the OPs' car.
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