Curious to see how this turns out. I have a ZR2 which I seem to be averaging 10-12 MPG which is MUCH lower than the '97 2dr I replaced. That being said, I've got a 3.73 gearing and 33x10.5 tires, and the catalytic converter/exhaust is rattling around. I am hoping to get back to 15-16 MPG, as my buddy has a ranger 4.0L with Dana 60s front and rear and 38" TSLs and he gets 15MPG. Given, I have a few known issues, but what else should be looked at?

 

Captain, the 180-190 is an estimate based off a thermometer in the vent reading around 170 and the gauge on the dash just below 210. The tires are factory 235 70s with even wear. Odometer is spot on, but I did notice my revs to be about 100-200 higher at 70 mph than my 98 was with the GU6 (my 98 was at 2000 even at 70 in lockup, whereas this is between 21 and 2200 in lockup) but I don't remember if the 98 had 70 or 75 series tires on the door sticker. I'm going to try a set of upstreams next, but I am hesitant as there is no MIL. I also read on another post about possible coolant temp sensor, but don't know the effect that will have as I am reading good temp on the dash and IIRC, these go into closed loop aroun 105. I will keep updating daily until I figure this out. I appreciate the serious responses. Any other help will be greatly appreciated because I am burning through an 18 gallon tank in less than 200 miles

 

Can't remember the tire size for sure, but 235 70 sounds right.

Always best to check coolant temp with a scan tool. After the engine sets for 10 or 12 hours, before you start it, connect a scan tool and compare ECT & IAT with ambient temperature. They should all be within ~2 degrees of each other. If one is off too far, chances are it needs replacement.

With a GU6 3.42:1 axle, 70mph in lockup is spot on 2000 RPM. If you're at ~2150 RPM, the converter is not locking up, and there goes your mileage. If you take off "civilized" from a stop you should feel 4 distinct shifts.

Upstream oxygen sensors are the ones you're concerned with. Downstream only monitor the converter(s) and have no affect on engine performance. Oxygen sensors react to changes in fuel mixture both lean to rich, and rich to lean. The faster they can react, the better fuel mileage you can expect. During the life of a sensor, reaction time (normally)slows down. At around 100K miles, they have slowed down enough to make a noticeable difference in fuel mileage, (that's why they recommend replacing them at 100K). It's also very common for a sensor to develope a slow reaction time in only one direction, which will also affect fuel mileage.

Several things determine when the PCM will switch to closed loop. The PCM watches more than just coolant temp. ECT, IAT, a calculated coolant warm up time depending on what start up temperature and IAT was, oxygen sensor heater on time, etc etc. The lowest temperature it will switch to closed loop is ~96F at the ECT sensor, but, again, everything has to meet the criteria. I forget the DTC, but there is one that says something like "coolant took too long to reach operating temp", that's the time frame thing I was referring to.

 

I will have to check the coolant temp with a scan tool then. When I drive and the truck is at temperature, I get 4 shifts. 3 for gears and the 4th for lockup. I know it definitely is locking up as I get a "5th" gear and the revs do not change with throttle position as they would out of lockup. The last time I had a bad TCC, I was constantly throwing P1870 slipping code. I had a major problem earlier in the year where it would go into lockup as soon as it shifted into 4th no matter what, causing the truck to bog and downshift back to 3rd and back to 4th. Found out I had a weird spike from the TPS and changed that and now it shifts and locks up normal. I know of the 3 other GU6 trucks I've had, 2 were spot on 2k at 70 and the other one (not this truck, this would be 4) was a tad above 2k. I never really question things like that as the accuracy of GMs gauges has never been their strong point. I just read another thread that you played a major role in, about the vacuum switch on the Tcase. That is my first order of business in the morning, as every now and then, I get the 4wd front end hop on tight turns when in 2wd. I will take everything into account as you have stated a few things I haven't thought of. Really glad you decided to stop by my thread, Captain haha. I will keep you updated.

 

So I checked the vacuum switch on the T-case, and everything seems to be normal. its not gummed up and moves freely. There was no fluid in the lines when I disconnected the Tri-connector, but I did notice that it was under vacuum when the truck had been off for a few hours. IIRC, Captain, you had mentioned that in 2WD, the switch should vent to atmosphere. If that is the case, I should not be getting vacuum on the connector. I filled up the truck this morning and averaged 12.2 (194 miles on 15.9 gallons). I have gone about 25 miles and I'm already down an eighth of a tank, so things are staying exactly the same.

 

brakes dragging? how good mpg u expecting?

 

No dragging brakes. I'm expecting 18 like I was getting 3 weeks ago...

 

The vacuum switch has 3 lines: One is the source line from manifold vacuum: provided there are no leaks in the circuit, it should have manifold vacuum at all times, engine running or not, 2WD or 4WD. The second line runs from the vacuum switch to the axle actuator under the battery tray. The third line runs straight up, a couple of inches, to a vacuum tee. One side of the tee goes to the transfer case vent tube. The other side of the tee goes to a filter attached near the top of transmission dipstick tube. When 2WD is selected, manifold vacuum is capped off inside the switch, and the axle actuator line is connected to the vent circuit inside the switch. When 4WD is selected, manifold vacuum is connected to the axle actuator line inside the switch. Shifting back to 2WD, the axle actuator is connected to the vent circuit, the vacuum is vented to atmosphere, the diaphram relaxes which disengages the right front axle from the differential.

 

great diagram, maybe the back of the tcase is where my mystery hose is from! i didnt know the vent hose was in the tailsection.

 

That is it a great diagram I know I will need it soon.I have it saved to my documents.
Blown 462 I have ask, did this problem just occur all in the suden or say over a period of a couple to a few weeks?I have to admit I dont know all the ends and outs of the o2 sensors and other related electronics.I can understand if this occured over a period of time that you have sensor or vacume issue or something going on maybe even a possible trans issue but if It just started out of the blue I still think you have a gas leak somewhere.If not externaly internaly like inside the planum like leaking fuel injectors.If it was me I would be looking at the simple things first just a thought.