I heard on a regular car variation in RPM at idle time is an indication of a sticky plate. If this is true, how would I make a TCH "idle"? (I have a ScanGauge II that I can use to look at the RPM and such).

I do not think I can rely on the initial startup since it is for warm up and any RPM variation seen might be normal...

The highway MPG on my 2007 TCH has dropped about 2-3 MPG over about 1.5 years, now averaging about 35 MPG in Central Illinois when I cruise at 75MPH. I have already cleaned the MAF to no avail. I do maintain a high tire pressure ~ 38 psi and have used Gumout Regane Complete and RedLine SI.

 

To put the car into maint mode, do not touch the brake, turn the ignition on, press the accelerator pedal to the floor twice with the car in park, put your foot on the brake and put the transmission in neutral, let off of the brake, press the accelerator to the floor twice, put your foot back on the brake, put the tranny back into park, let off the brake, press the accelerator pedal to the floor twice again. You will now see the master caution light and the center display will say Maint mode.

Put your foot on the brake and press the start button. This will force the car to run without shutting off when it gets up to temp. This will force an idle condition on the car. To exit this mode, put your foot on the brake and press the start button. The next time you start your car, it will be back in normal mode.

 

These instructions have worked!!!

Perhaps a separate sticky thread for this would be very helpful!

What I have noticed with ScanGauge II is that after a few seconds of the RPM varying among 1300ish, 1100ish, etc., it started varying between 896 and 910, 911, etc., every 3 seconds or so. Every 5 seconds or so, the engine would vibrate which I could feel in my seat and steering as well. I would appreciate any advice on what these could mean. Anyone else has tried this? If you don't have the Scan Gauge II, can you verify the vibration part? Thanks.

EDIT: My car has ~87K miles. Bought it used about 1.5 years back at 76K miles.

 

What you are seeing as far as idle goes is normal. On first startup, the engine will idle at a higher rpm and a richer fuel/air ratio in what is known as "open loop". Once it begins to warm up, the idle speed will come back to normal and the car will go into closed loop once the cats come up to temp and the O2 sensors are working well. The small variation at that point is the ECU varying the fuel/air ratio, timing, throttle opening, etc to maintain the ideal engine idle.

Several things can make the idle speed response slow including old O2 sensors (their response time slows with age), fuel injectors, throttle body, temp sensors, air flow sensors, cam sensors, etc. I wouldn't worry about it though as more than likely, your scan gauge is set to sample every 2 or 3 seconds and that variation is what you are seeing.

As far as the "vibration" goes, you could have a loose or broken motor mount, low fluid level in the power split unit (transmission), fuel delivery problems, sensor issues, etc. You will more than likely need to take it to someone that can test everything out for you.

 

Is the car driveable in this mode, or does the engine just idle?

 

I read some place else that the car MUST NOT be driven in maintenance mode. Not sure how authentic that is, but I didn't put mine in drive since I was only after the idle RPM/behavior.

 

This is for shop use only, do not drive the car in this mode of operation as several safety checks are not performed while the car is maintenance mode.

 

Thanks guys.