I own a RX400H of 2005. Since a couple of months my engine (at least I think) is sometimes acting strange at speeds of around 80kmh. It runs a bit distorted and I haven't got all the horsepower when accelerating. When I slow down or speed up the issue dissapears again after a while. Then the next day or a couple of days later it can return (or not).

I suspect myself one of the sparplugs or the coils. How can I check if this is the issue, because I saw that changing the sparkplugs is a half a day job ?

Sparkplugs were changed at 86000km. Current km is 156000k. So it overall doesn't seem bad to change them.

Maybe any other ideas what it could be or what I can check?

 

Obvious questions:
What does a code reader show? most readers and definitely Techstream will tel you if it is a plug or coil pack it will store errors
When was your last tune-up?
Have you had any recent engine or electrical work done?
Did this just happen on ths last tank of gas? Does this malfunction coordinate wtih your last fill-up?
Has there been any changes recently to the car?

You really need to give a lot more information if you want accurate/reliable suggestions

Yes spark plugs are @4 hour job for the rear ones but there is a lot more to do while back there especially at the mileage you're at now.

Lots of people "suspect" stuff, then throw money and time at their car, and are not close to resolving the problem we see it here all the time nechanics/dealerships feast upon this mentality

 

At 132000km the waterpump and timing belt was renewed incl the other belts.
After this only normal maintenance was done, like oil/oilfilter. But also the disc brakes.

I connected techstream which I use for my Prius, but I don't see any error codes when selecting engine. I also can only select the lexus rx400h from 2006 in Techstream. Do maybe this techstream is useless for me. Is there an other way to read the codes.

The issues has be present for a few months now and becoming more frequent.

 

You're due for:
Plugs - go with NGK or OEM never go cheap here. IF the went cheap here this could be an issue but not likely since no code was seen by Techstream. Did you do the work? Do you know which plugs they used?
Plug tubes,
EGR valve (Mine ran $7) - clogged/stuck valve causes poor gas mileage mine was completely clogged
(I recommend replacing the coil electrical plugs (I think mine ran maybe $5 for all of them they get very brittle it was very quick/easy to replace just the plastic end)
Many recommend replacing the valve cover gasket while back there
Engine air cleaner - easy and cheap DIY - may help. Take a good look at the small vacuum lines going to your air cleaner box these are notorious for getting cracked/dry/no longer sealing causing air leaks/vacuum leaks and poor gas mileage or engine running poorly. Very cheap and easy should be replaced every 60K they cost me under $10 usd
Coolant change - no effect on your current issue
Brake fluid change - no affect on your current issue
oil changes in tranny, rear diff and front ransfer cases - likely zero effect on your current issue

Brakes, waterpump, timing belt are not going to cause this issue so you're good there
There should ot be any difference with 2005 or 2006 model year in techstream. If it were a spark pluk or coil Techstream would show an error you can rule those out

Aside from doing the above even just for preventative/necessary interval work... try this - if your tank is low put in about 5 gals of ethanol-free fuel, a bottle of Techron or Seafoam, go out and run the engine hard, beat it like it owes you money... redline it preferably going up a hill under load (they call this an "Italian tune-up). It can blow out carbon, bad gas, all kinds of things out. It is a cheap thing to do unless you get pulled-over then you may get a free cavity-search so it'll make it all worthwhile. Constantly lugging engines is not good for them at all they do far better when they get pushed every so often.

How does it idle? Is it smooth, does it transition from idle to higher RPM's pretty quickly when parked or on the road? No engine coughing at higher speeds just 84 kph?

Still could be a vacuum leak, clogged or bad fuel injector (likey seen by Techstream), bad gas not sure

 

Wow, thanks for the extensive answer and great advice!!
I bought the car second hand a couple of years ago from the 1st owner. The car was always maintained at the offcial Lexus dealer and the previous owner gave me all the invoices of the car. So I got the complete maintenance history.
So I can be sure that original spark plugs were used.

I first want to make sure that my Techstream is working properly, because when I connect my 2010 Prius it automatically finds the car. When I connect my lexus I have to select the car myself with the dropdown boxes. So although it connects, I am not sure if it working properly and gives me the fault codes.

 

Techstream should be working fine though you're having to input your RX, mine does the same perhaps it is the age of the ECMS's who knows. When in TS go to "health" I believe it will read all codes in all computers take a look there. Then go to Engine ECU specifically and look over at history that is where you might find former faults. If your speak plugs or coil packs were a problem they would certainly appear in history as would fuel injector issues. Get good gas in it, perhaps a fuel system vleaner like mentioned above, go run it hard for a few miles, clear any current or former codes (be sure to write down what they were though) and see what happens. Good luck. Report back whether this resolves things or if anything continues. If you are doing this yourself and feel like a decent DIY'er while workng on the rear plugs I cleaned my fuel injectors and replaced the seals (<$20)... not needed but cheap and easy while you're back there.

Congrats on having the history of maintenance that is big, go look to see when the plugs were last changed, At 100K miles it is due for a major tune-up as mentioned and at a dealership this will be quite costly, a tune-up may well resolve your issue on it's own. but also add several things mentioned:

Replace vent lines (cost me @$10 USD mine were shot) - very easy requires no tools only a razor or knife to cut them
Replace the EGR valve on back of motor under intake ($7) - mine was completely clogged do this when doing the rear plugs it is right on top of the engine under the intake

Take a very good look at all the rubber bits in the suspension as they are likely either shot or about to be... these are not expensive but labor will be if you don't DIY (front and rear sway bar links, lower front control arms, ball joints, steering rack and the bellows mine had holes in them). Doing LCA's will require a wheel alignment. Lastly look good at the struts front and rear, are the bellows torn? (mine are), any fluid leakng from any of them (highly likely at 100K miles), it is probably time to change them out (front and rear KYB's run @ $500 for all 4 mounts are another $211 labor is pretty easy maybe 2 hours for a shop 3-5 hours for a decent DIY'er)

These sounds like a lot of "niggles" or costly bits but these are things any/every car needs at these miles/this age. These are very good, reliable cars but like anything they do require wear items to get replaced. Once done you'll be good for many miles

 

It actually takes a lot of misfires for Techstream to log a dtc fault. In many cases the misfires can be felt many months before the problem becomes severe enough to set the misfire code.

There is an option within Techstream to view the misfires during a drive cycle even if they are not often enough to set a dtc. You need to check for these misfires after the engine has run rough but before you shut the car off. Once you shut the car off and restart these go back to zero. The current monitor and the cumulative monitor both reset to zero misfires after you shut the car off. ( At least on my version of TS)




This is inside Techstream under the Engine tab ----> Monitor. Select Monitor tab on the left. Select current or cumulative tab. Then on the misfires line select details or summary ( I don't remember which it is) One just opens a useless pop up window. Then you can scroll up and down to view all cylinders.



This is under Engine ---> Data list. Then scroll to the right. Note there are no misfires reported on the cylinders. This is not the best place to check. Unless you have a hard misfire that happens for an extended period you likely won't show any misfires here. Data list doesn't display minor misfires like the Monitor tab.


This check mode under Utility is another option that can be helpful. The criteria to set codes is reduced on some items allowing them to set dtc's when they normally wouldn't.