To be clear I really have no idea of what this can be, but I'm going to explain.

​​​​​​So about 2 weeks ago my car started to jump back and forth on the RPM. Like if it suppose to be on 2000 it jumps down to 1800rpm and back to 2000, and keeps jumping. That applies to any RPM I'm in. Now, the thing is that this happens every time I'm out for a drive. But not until the car is fully warm and I have driven for a while. It does not happen when the car is on neutral or park. Only in drive and reverse. And the most annoying thing is that it doesn't do that all the time. Tbh the car drives normally, but I'm worried that if I don't take care of that problem it's gonna become a bigger problem. I have ECU tuned the car before, but removed the tune for 3-4 months ago. Some people said it was the tunes fault, but don't really see how. Plz help me to find what the problem is. Has anyone had this problem before?

 

Check for vacuum leaks.
Disconnect the - negative - battery for 10-15 min to reset the ECU.
Clean the throttle body and MAF sensor.

 

I have checked for leaks, cleaned the throttlebody too. But how do I clean the maf sensor?
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MAF sensor cleaner. Remove the two tiny Phillips screws from the air box, unplug it and hold it in the air. Spray the cleaner drenching the inside and outside. Repeat like 5x. Reinstall sensor.

 

Yeah I did that today, it dint solve the problem tho. So I think it's something else.

 

When is the last time the plugs were out?

If this is a city car doing lots of short runs, you may be suffering from carbon buildup. Have you read up on that?

 

I changed the plugs when the car was at 131000km, the car is at 138000km now. I'm fully aware of the carbon build problem in the IS250. This is my everywhere car, I do drive the car hard now and then in hope to clean some of the carbon.

My friend meant that the problem was from an early ECU tune I had on my car. I removed the tune for 3 months ago so I don't understand how the tune is the problem. But he said that there is still some of the tune in the ECU? I thought that was weird since they resented the car to original ECU.

 

I have the RR tune and no issues like that. Mines a 350 tho. Regardless, tune maps should not induce that if done correctly.

My suggestion is this. Get either torque Pro and a bluetooth adapter OR OBD Fusion with the Toyota Lexus Scion add on pack. IMO, Fusion is the better app with more features but it's a PITA to log data from. That said, torque logs data and you need fuel trim info. Look at both banks long and short term fuel trims as well load. and timing ignition advance. Then you can see what the engine is doing. Do a search for OBD Fusion APP on this site, you will find some info. A good bit of it from me.

Last little bit. Take an empty paper towel tube and place it to your ear. Scan the engine at idle listening for vacuum leaks. It is very directional. Oh and when is the last time you changed the oil? Cam timing is done through a controlled oil leak at the 4 camshafts VVTi. Crappy oil can lead to variable timing which could work against you causing this.

And as another option, buy two bottle of Yamaha Ring-Free and run them back to back and then change the oil. It is amazing stuff.

 

I change the oil every 10000km always a little before that. But right now I'm at 11000km since I changed, I'm a little broke so no mony for oil. Well I can't lie to you, I don't use the best quality oil for my car, and that's because I'm cheap. But I'm fully aware that good oil makes a difference.

I have in my opinion a really good Obd2 tool. It's from carsoft? Can't remember, but it allows me to see a lot of thing while the car runs. And I dint know about that VVTi thing with the oil, really cool, helps me a lot 👍

Ps: oil in Norway is expensive AF! That's why I buy cheaper oil.

 

I would suggest taking a look at your PCV it could be sticking. Malfunctioning evap purge valve maybe as well. I had this happen to me. With engine on disconnect the connector. It should close the valve. Next disconnect the vacuum hose under the intake that goes to it. It should not have any vacuum.

You can also cap the port and go for a drive and see if it surges.

Only thing I can think of for now. It seems like a vacuum related issue.

 

I will try that out. I have changed the pcv valve at 131000km along with the spark plugs.

 

Sorry I forgot to answer. I tryed this out, and there was no vacuum after disconnecting the sensor. I still have no idea what the problem can be.

 

Is the Check Engine Light illuminated? If so, try and get some OBD codes on your car. Could be something like a bad oxygen sensor, faulty Exhaust Gas Recycle (EGR) valve or other emissions fault. I had similar problems with my 2001 Ford Ranger pickup truck and they found a faulty coil pack and oxygen sensor.

Keep looking. Find a code reader or pay for some diagnostics by a mechanic or dealer.

 

No check engine light. I have a good diagnose tool, but there is no code in the engine or anywhere else. But maybe it could be the egr, because it only happens when the car is fully warm.

But do the IS250 have a egr? Possibly Im being stupid now haha but I feel like egr is a diesel car thing.

 

I'm wondering if you have a failing ECT. Engine coolant temp sensor. On your scan tool what temp is the engine sensor reading when the RPMs start to jump?

Had an old Honda that would do that. Though it would throw a code.