CheapXJ said:
210 is the correct operating temperature. if it doesn't get hot enough, you will get crap mileage.
when you say "adjusted" the TPS, are you measuring the contact voltages down at the ECU or are you just guessing?
there's no reason to ever dick with any screws on the throttle body unless it's worn out.
there's some good info here
http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm
as far as getting better fuel economy, make sure you're running a 195 degree thermostat. so many people run 180s and I don't know why. it's just plain wrong!
Where do you get this 210 from? Never mind, rhetorical question, I already know the answer...have you ever tried a 180 F thermostat?
What year is your jeep?
I don't have first hand thermostat experience with the 91 and newer jeeps, but my experieince says a 180 F thermostat is best in warmer climates for the renix, 87-90 years. 195 F T-stat might be useful in Canada on a Renix.
IMHO a properly working, new cooling system with a 195 F thermostat should hold 195 F, not 210, unless there is additional heat load beyond the cooling systems normal operating design capacity, in which case the temperature may rise until the secondary electric cooling fan cuts in, in which case the temperature may cycle a bit... of course the location of the Gauge Temp sensor versus the thermostat can show a temperature differential, if located in a remote area. This comes into play in our discussion since the older jeeps have a Temp sensor in the rear of the head, while the newer(est) XJ's have a single sensor in the thermostat head.
Now for the so called crap mileage, I feel like the lone ranger here on this. I have run a 160 F thermostat on my Renix 87, 4.0 L, 4x4, with no problem for 2.5 years. I am currently running a 180 F thermosat, but only to help with keeping warm on the cold morning start ups. Now that I have a properly working and calibrated TPS, properly working new O2 sensor, good working ground wires, and a solid new parts tune up, I am getting a solid 20 mpg highway with a 180 F thermosat. I run the cooler thermostats because heat is our enemy down here, Houston during the summer. In the summer we see 100 F ambient, 100% humidity, and it loads bageebers out of the AC and cooling system, so a 195 F thermostat is overkill for us.
On the D*cking with the throttle body screws, some of us have inherited throttle bodys that POs adjusted (D*cked with as you put it) and so after exhausting all other possibilities via vacuum leak repairs, calibrations, etc, , some of us have had to d*ck with ours to get them set back to OEM settings.
Mitsumotors,
Yes, a high idle will hurt mileage. So will a bad O2 sensor or bad O2 sensor wiring. So will a bad TPS if the TCU side is out of wack, as I discovered.
The idle increasing as the engine warms up is a dead give away that you probably have an intake manifold leak somewhere that increases as the engine warms up. Any vacuum leaks in the valve cover, CCV vacuum lines, other vacuum haoses, the intake manifold, will cause a high idle. I also discovered that as the engine warms up, a dirty, loose engine ground wire on the block will change ground resistance, thus changing the TPS idle voltage, thus making the ECU think there is a foot on the gas pedel, thus raising the idle!!! So pull and clean all those engine block and head, firewall, and battery grounds!!!! 87-90 renix jeeps are notorious for ground problems causing high idles, or worse.
On the MAP sensor pulling the vac line, it is the vacuum that is changing, the IAC is already probably already closed to it's lower limit, so the ECU gets confused by the additional air, and incorect MAP sensor data (since you just unpluged it, telling it the MAP is now ambient, and not 18 inches of vacuum, so in essence you have lied to the ECU since the engine vac is 18 inches and not 0, which makes it run rough, too lean at first, until the ECU figures out the MAP is giving it bad data!!!!
John Mitchell,
That valve is the fuel regulator. If the fuel regulator diaphram starts leaking you get unmetered raw fuel into the intake manifold, and you may get poorly controlled fuel pressure, too high or too low, that adds to the engine control and MPG problem.
