I have a 1990 cherokee 4.0. It seems to be running too rich. Is there an adjustment on the throttle body or is it all electronically controlled?

 

First place to check is the 02 on that one, my 89 had the same issues and it was always the o2 sensor, make sure all your grounds are refreshed and don't replace it with a off the shelf sensor if its bad, do yourself a big favor and go to jeep and get one they ARE better quality sensors

 

Hows that vacuum line going from the TB to your MAP? (on the firewall). Needs to be perfect

The only adjustment on a Renix 4.0 TB I am aware of is calibrating the TPS after installation, and the TV cable back to the transmission.

 

I iwll check the O2 sensor.

 

How are you gonna do that?

Did you look at the MAP line? Very common for a small leak somewhere in there to cause rich running..........

 

So my 1992 was running rich too. So i took it to my buddies shop and hooked up his scanner. 27% over stock fuel rating. VERY rich. the cause? cracked exhaust manifold (like every jeep has/had/ or will have). the o2 sensor was picking up air coming IN thru the crack, reading it as running lean, and telling the comp. to dump more fuel in. fixed the crack, reset computer and she runs like a champ getting ~22 mpg

 

Kansas, your point is valid. Jtpton, as I've told Cruiser 100 times, check that map line. Salad, I hear your map line is really short!

O2 sensor check, for the brave, or stupid. >>www.cherokeeforum.com/f2/renix-02-sensor-test-impedance-rms-133153/

I was able to find mine was dead with a cheap meter.

But! Map is King. As I learned form Cruiser, and as Salad rightly points out, The map "horses" in the approximate mix, then the 02 "trims it". That line from the rubber plug on the throttle body, back and up to the map there on the firewall needs to be "splendid"

I ran TOO LONG! with my o2 stone dead. It ran pretty OK in "default pulse width". Flyen on the highway mileage was not awful. BUT normally mileage tanked. Check that brittle little map tube. My feed voltage from my ECU, (that's Electronic Control Unit, ), to my variable titanium resistor, (that's the 02 sensor), never varied from the 5V feed from the ECU,)

 

Also check your cat so it not is blocked, that can give the same sympthoms.
My ´89 cat had rattled for a long time then it stopped and the milage went down, replaced the cat and it went back to normal again from 12 mpg to 18 mpg.

 

Im am checking the cat and the o2 and the tb to map connections. thanks to everyone.