Recently just got a scan tool because i have been getting poor fuel mileage between 12-15 randomly. Also I get better city mileage than Highway.

Scan Tool says the fuel system is in Open Loop Drive. But it is getting good signals from all of the sensors.

I already replaced the O2 sensor, fuel filter, and thermostat. The operation temperature is a consistent 190-200 degrees.

What is causing the fuel system to stay in Open Loop Mode?

Thanks

 

When you say the engine is 190-200 is that according to the guage or the scan tool? If the computer sensor is bad the engine will stay in open loop. They're two separate sensors.

 

The gauge actually reports ~ 160F then a seperate thermometer at the thermostat outlet also indicates 160F, But the scan tool reports 190 to 200F

 

Now I have to some some reading and research to find out what else could be keeping you out of open loop. If the computer doesn't see the engine warming up that'll do it, but you have that covered. (I'm home, books are at work) Fuel mileage on these things suck, I average 13.5, and with the release of winter blend gas I lost about 1 MPG. Still, you should get better highway than city. Unless of course you live in a REALLY friendly state that lets you keep the gas pedal to the floorboard. Do you have all 4 forward gears? Does the convertor lock up if you hold a steady even speed on the highway? My truck is turning 1700 RPMS @ 60 MPH on the highway for reference. But that's a 4-door with 235/75/15's Check the CLSTR fuse in the IP fuseblock to see if it's blown. I'll check tomorrow on "open loop" thing.

 

Well my 89' TBI 4x4 got 24mpg highway 20 city, and 16ish towing. I figured I would at least get that...filled up today for a total of 12.4 mpg with 45-60mph driving and occasional stops. very conservative acceleration. I could stop and go 35mph all day and get 15-16mpg

I did find out that I am running pretty rich, Long Term Fuel Trim on the scanner at 45mph 1460rpm is -30.47%. I am still reporting Open Loop at 193 degrees F with the above cruising speed and rpms

My rpm's are around 2100ish I believe at around 70-75 its a 2 door with 235/75/15 4x4 which I understand can cut of 1-2 mpg...

I was reading somthing about leaky fuel injectors and open loop program. Can leaky injectors cause open loop conditions? I know a leaky injector system could cause low highway and possible higher city mileage.

 

Closed loop means the fuel is being controlled based on feedback from the O2 sensors, open loop means it's being controlled based on sensor input from the MAP, TPS and MAF. Going from open to closed is a function based on runtime and temperature rise. Pretty much what I always knew. It should never take longer than 8 minutes to change over. Usually in the 2 minute range. None of this addresses your problem though....... You don't have any codes or check engine light. (I'm sure if you did you would've mentioned it) I can only guess at two possibilities why it stays in open loop. You have an intermittent problem with the ECT sensor or connection to it. It's located on the drivers side head. I'm guessing the engine knows it's warming up but when the temperature glitches it resets the warmup "clock" so the VCM is always waiting for the engine to warm up a bit to go into closed mode.
Other possibility is your computer has lost its mind. (LOW probability)
Intermittent problems are frustrating because they won't set a code. The glitch doesn't fit within the parameters for setting any defined code and every trouble code has a set of well defined conditions that have to be met before it's active. I can only suggest replacing your ECT sensor or taking your scan tool, placing it in monitor mode, and watching that engine temperature a while for any jumps or wierd movements before throwing money at the problem. Gauges are damped and won't show small erratic changes.
Oh, and I'm wrong about there being two temperature sensors. Once upon a time there was a sensor for the computer (digital) and one for the analog gauge. Now, in the era of networked computers and dashboards, one sensor does it all.

 

Will a P0420 or a P0410 caused open look running conditions?

 

I can't find any info saying anything will knock a computer out of closed loop. However there's a logic to the idea if it doesn't believe your O2 sensors it would ignore them. (effectively open loop). Both of these codes require the engine to be in closed loop before setting so at some point you are in closed loop operation. I'm working from an Iphone so I can't provide links but if you search there's info for every code on this site. Start with the P0410. Only cause I see in common is an exhaust leak.

 

I replaced the fuel injections what a P-I-T-A. Replaced them with the MFI style injectors with the poppit at the injector instead of up top. Scan tool reports closed loop and fuel trims of 2-7%. Before the replacement I was getting like -30%

After the replacement I took a 200 mile roadtrip and I'm pleased to report a 25 city / 75 highway economy of 23 MPG.

The injector assembly was leaking i could see this by clean spots inside the intake manifold.

 

Sounds like success! Glad you fixed it!