New TPS solves MPG problem

Well the test results are in. The New TPS, Trottle Position Sensor, solved most, the rest of my low MPG problems and my low power, low torque problem.

It is the 87 renix 4.0, AW4 automatic, 4X4 Wagoneer. My old TPS tested good on the ECU side, but lacked the proper span on the TCU side. Both sides tested good at idle! The TCU side tested bad at WOT, not enough span. For more blow by blow details see the old thread titled "RenX Files" (or ReniX Files?).

Anyway my old 8 mpg local, 12 to 13 mpg highway gas mileage for the last 2 years, recently tested as high as 17.7 mpg, 119.8 miles, 6.77 gallons of 87 octane gas, with 10% 0-35 mph local travel, 20% 0-45 mph semi local travel, and 70% 55 - 65 mph highway travel all in the city. Other tests got 15.4 MPG mixed city stop and go with 70% city freeway travel, some in heavy traffic, and 14.68 mpg in 232.6 miles, 15.85 gallons of mixed city with some heavy stop and go traffic on the freeways at rush hour. Based on the test results I must be near 20 mpg for pure non stop 65 mph freeway runs. I have yet to get a pure 65 mph long miles run from a fresh fill up to confirm the best highway mileage, but based on the overall data it looks like I am at 20 mpg +/- 0.5 mpg for the 65 mph highway mileage now.

The best I got before replacing the TPS was about 14 mpg highway and about 12 mpg average city. So I got a good 4 to 6 mpg increase out of the new TPS.

The reason for the huge difference is the way the TPS signal handles the transmission operation via the TCU. The bad TPS was causing the Tranny to shift too early at too low of an engine RPM, like 2000 rpm at WOT, and I believe it was reducing the tranny fluid pump pressure to the torque converter thus reducing the power transfer to the rear end from the engine.

Anyway, the verdict is in, A bad TPS on the TCU side only can have drastic negative effects on gas mileage and power.

Oh, and for those hot heads here on (I say hot heads jokingly!, meaning those who think a 4.0 needs to run real hot to get good mileage), I am still running my 165 F thermostat!!!!!! and I have reached about 20 MPG highway mileage, and already measured a 17.7 MPG mixed city /traffic / mileage for a 119.8 mile test run. The worst mileage I got before replacing the bad TPS was all local neigborhood stuff, 0-35 mph , last winter with it parked for 10 minutes warming up each morning, got about 6.5 mpgs on one tank.

Ecomike said:

Oh, and for those hot heads here on (I say hot heads jokingly!, meaning those who think a 4.0 needs to run real hot to get good mileage), I am still running my 165 F thermostat!!!!!!

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I also think that the engine running hot is a joke--my t-stat is stuck open right now and I havn't gotten around to replacing it yet. On a highway trip of 500 miles I pulled in just over 25mpg (note difference in usa/canada with metric/imperial measurements though, so probably similiar to the 20mpg you are getting--I havn't worked it out yet). I think temps have a direct bearing on city driving, but not so much on highway driving.
Oh, and just to defy popular thoughts again, I get my best MPG from running the engine at 1750rpm on the highway.

clunk said:

Oh, and just to defy popular thoughts again, I get my best MPG from running the engine at 1750rpm on the highway.

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Comparing to what?????

Simple science will tell us that running sub-2k on the highway is going to yield better MPG than running above 2k on the highway.

As I explained it to my wife.... Less VROOOM VROOOM, means less SUCKY SUCKY.

w_howey said:

As I explained it to my wife.... Less VROOOM VROOOM, means less SUCKY SUCKY.

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Well said! I plan on using that one!

Where did you get the new TPS and at what price?

Glad to hear your problem is fixed...I was following the renix files and still need to do testing on mine. 1990 XJ.

w_howey said:

As I explained it to my wife.... Less VROOOM VROOOM, means less SUCKY SUCKY.

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now that is sig worthy

w_howey said:

Comparing to what?????

Simple science will tell us that running sub-2k on the highway is going to yield better MPG than running above 2k on the highway.

As I explained it to my wife.... Less VROOOM VROOOM, means less SUCKY SUCKY.

Click to expand...

I have always read people reporting thier best mileage from running at 2200-2250rpm because that is when "peak efficency/power occur". I find running at that higher rpm gives a bit more power but the excess noise and fuel consumption isn't worth it.

clunk said:

I also think that the engine running hot is a joke--my t-stat is stuck open right now and I havn't gotten around to replacing it yet. On a highway trip of 500 miles I pulled in just over 25mpg (note difference in usa/canada with metric/imperial measurements though, so probably similiar to the 20mpg you are getting--I havn't worked it out yet). I think temps have a direct bearing on city driving, but not so much on highway driving.
Oh, and just to defy popular thoughts again, I get my best MPG from running the engine at 1750rpm on the highway.

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We must have different gearing and tires....I am at 2400 rpm in 4th gear at 65 US MPHs using 235/75R15 tires, which is my best mileage. Gearing, vehicle speed and tire size seem to affect peak torque and therefore optimal engine RPM, I think?

I am actually using a good, used TPS pull from a parted jeep, but the new one I bought was 34.95 at Teamcherokee.com, and it tested the same as the used one I am currently using.

clunk said:

I have always read people reporting thier best mileage from running at 2200-2250rpm because that is when "peak efficency/power occur". I find running at that higher rpm gives a bit more power but the excess noise and fuel consumption isn't worth it.

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My auto XJ isn't a good comparison, cause of the 33 inchers on it. But on my MJ,4.0l 5-spd, 3.07, NP231 and 235/75/15's it sits at about 1650rpm at 55 in 5th gear. I get about 20-21 mph highway. I drive so slow in it, old people pass me.

w_howey said:

I drive so slow in it, old people pass me.

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lol, that's the same as me. It's hard to drink coffee, keep a constant weary eye on the oil pressure gauge (I'm still getting past the "trust" stage of my jeep where I can just drive it on 500 mile long highway trips with no worries) and keep constant speed so I just take it slow. Plus, I like to see how much MPG I can pull from it.

Ecomike said:

I am actually using a good, used TPS pull from a parted jeep, but the new one I bought was 34.95 at Teamcherokee.com, and it tested the same as the used one I am currently using.

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wow, what I good deal on that TPS! When I replaced the TPS on my 1990 last year I couldn't find one for less than $100 locally for renix era jeeps. The 1991+ TPS's where dirt cheep, but for the Renix they were really expensive.

Well I confirmed my highway only mileage today on a city freeway, with frequent overpasses, not a straight flat road so it should be even better on a straight flat road, but there was no traffic so I was able to drive 48.2 miles, 99% freeway at an average of 60 mph, with a 55 mph min, 65 mph max speeds.

I topped of the tank first, until it spilled over, and topped it back off three times at the same pump, same station after the freeway trip. If I use the first gas shut off value I got it calculates to 26.34 mpgs, if I use the second pump shut off value I got, it calculates as 20.51 mpg, and the third pump shut off value which overflowed again calcualted at 18.54 mpg.

1st = 1,83 gallons
2nd = 2.35 gallons
3rd = 2.6 gallons (overflowed before shut off)

If I estimate the full tank at 2.45 gallons, just before it overflowed, I get 48.3 / 2.45 = 19.67 mpg, then times 1.02 for the tire size speedometer error, I get = 20.07 MPG!!!party1:

So I have officially joined the 20 mpg freeway/highway mileage XJ club!!!party1: :yelclap:

I am still only averaging 14 mpg due to the traffic and stop sign congestion in the local city DD routine.

Ecomike said:

Well the test results are in. The New TPS, Trottle Position Sensor, solved most, the rest of my low MPG problems and my low power, low torque problem.

It is the 87 renix 4.0, AW4 automatic, 4X4 Wagoneer. My old TPS tested good on the ECU side, but lacked the proper span on the TCU side. Both sides tested good at idle! The TCU side tested bad at WOT, not enough span. For more blow by blow details see the old thread titled "RenX Files" (or ReniX Files?).

Anyway my old 8 mpg local, 12 to 13 mpg highway gas mileage for the last 2 years, recently tested as high as 17.7 mpg, 119.8 miles, 6.77 gallons of 87 octane gas, with 10% 0-35 mph local travel, 20% 0-45 mph semi local travel, and 70% 55 - 65 mph highway travel all in the city. Other tests got 15.4 MPG mixed city stop and go with 70% city freeway travel, some in heavy traffic, and 14.68 mpg in 232.6 miles, 15.85 gallons of mixed city with some heavy stop and go traffic on the freeways at rush hour. Based on the test results I must be near 20 mpg for pure non stop 65 mph freeway runs. I have yet to get a pure 65 mph long miles run from a fresh fill up to confirm the best highway mileage, but based on the overall data it looks like I am at 20 mpg +/- 0.5 mpg for the 65 mph highway mileage now.

The best I got before replacing the TPS was about 14 mpg highway and about 12 mpg average city. So I got a good 4 to 6 mpg increase out of the new TPS.

The reason for the huge difference is the way the TPS signal handles the transmission operation via the TCU. The bad TPS was causing the Tranny to shift too early at too low of an engine RPM, like 2000 rpm at WOT, and I believe it was reducing the tranny fluid pump pressure to the torque converter thus reducing the power transfer to the rear end from the engine.

Anyway, the verdict is in, A bad TPS on the TCU side only can have drastic negative effects on gas mileage and power.

Oh, and for those hot heads here on (I say hot heads jokingly!, meaning those who think a 4.0 needs to run real hot to get good mileage), I am still running my 165 F thermostat!!!!!! and I have reached about 20 MPG highway mileage, and already measured a 17.7 MPG mixed city /traffic / mileage for a 119.8 mile test run. The worst mileage I got before replacing the bad TPS was all local neigborhood stuff, 0-35 mph , last winter with it parked for 10 minutes warming up each morning, got about 6.5 mpgs on one tank.

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Two questions and a statement. First question, the engineers designed the motor to run hot, why in the world would they do this if it is unnecessary or undesireable? I find it hard to swallow, that they are that incompetent.
Second question, what is the 165 degree thermo doing to your flow characteristics? Specifically, does the temperature gage stay stable or fluctuate (radically)? I sometimes run a 180 in the summer, I've noticed gage temperature fluctuations become more pronounced and RPM dependent.
Statement, if the engine temperature sensor gets below around 140 degrees, which is very possible in a northern climate, the ECU is falling out of closed loop. A typical heat rise between the inlet and outlet of the motor is 40 degrees (average). The aux fan comes on when the temp. gets above 190-200 degrees at the inlet. The math is simple, a 165 degree thermo may work out for you, depending on the ambient temperature and the flow characteristics of the system at certain RPM's. But the chance of the system defaulting to open loop because the engine temperature sensor is too cold have to increase, especially in northern climates.
What works in Texas may not work in Minnesota.

8Mud said:

Two questions and a statement. First question, the engineers designed the motor to run hot, why in the world would they do this if it is unnecessary or undesireable? I find it hard to swallow, that they are that incompetent.
Second question, what is the 165 degree thermo doing to your flow characteristics? Specifically, does the temperature gage stay stable or fluctuate (radically)? I sometimes run a 180 in the summer, I've noticed gage temperature fluctuations become more pronounced and RPM dependent.
Statement, if the engine temperature sensor gets below around 140 degrees, which is very possible in a northern climate, the ECU is falling out of closed loop. A typical heat rise between the inlet and outlet of the motor is 40 degrees (average). The aux fan comes on when the temp. gets above 190-200 degrees at the inlet. The math is simple, a 165 degree thermo may work out for you, depending on the ambient temperature and the flow characteristics of the system at certain RPM's. But the chance of the system defaulting to open loop because the engine temperature sensor is too cold have to increase, especially in northern climates.
What works in Texas may not work in Minnesota.

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Good questions! I see at least some one here is willing to try and keep me honest, LOL.

Some engineering modifications are done to meet beaurocratic, political and legal requirements to meet sometimes redundant, even obsolete but still enforcable environmental regulations. It takes 10 times longer to get rid of an obsolete emmission hardware requirement as it does to add a new one. Thus some stuff is done not for sound engineering, but to meet regulatory, beaurocratic code requirments.

Let me try and answer these one at a time. First off, regarding incompetent engineers, although I am one who would frequently question the competence of Detroit Automotive engineers, that is another subject for another day, on this topic it has more to do with compromises that they must make to meet frequently conflicting goals and design objectives. If they could install cheap, variable flow rate, smart thermostats that increased fuel economy they probably would be competent enough to do so, if it did not cost them bottom line profits. I think the hot thermostat usuage has more to do with winter climate passenger comfort than most anything else, and second, possibly a little to do with emissions controls, but nothing to do with closed loop operation requirments.

While I agree that a 160 F thermostat is too low for northern climates especially in the winter, I disagree with what I call open-loop legends (similar to urban legends) that the engine coolant temperature has any effect on entering closed loop operation, and the so called need for hot thermostats to get the ECU or PCM to enter closed loop operation.

Everything I have read other than disscusion group folklore says the closed loop operation starts in 5 to 60 seconds after engine start up for those engines that have HEGO O2 sensors, as these sensors have built in heaters to speed getting the O2 sensor to the proper operating temperature needed to allow closed loop operation. HEGOs have been used since at least 1987 on the Renix and later HO jeep engines. Some O2 sensors were at one time, non heated, and took considerable time to get hot enough to allow closed loop operation, and during idle would re-enter open loop operation. O2 sensors have a critical operation temperature window that they need to reach before they start yielding O2 data. That is why the HEGO, Heated Exhaust Gas Oxygen sensor was invented and used to replace the early non-heated O2 sensors. At one time, and only on certain vehicles with non internally heated O2 sensors it may have been important to use a very hot thermostat.

So what I am saying is my reaching a 20 mpg value, which others said was impossible with a 165 F, or even a 180 F thermostat (actually it turns out mine was 160 F), proves what I have been saying, that closed loop operation does not need an engine to run hot to enable closed loop operation, but in fact closed loop operation is possible with a 160 F thermostat, if you have a HEGO O2 sensor, which all OEM jeeps came with, except possibly certain oversees jeeps which would never run closed loop anyway if they came without O2 sensors.

I will agree that the 160 ( or 165 F) thermostat is way too cold for northern climates to comfortably heat the passengers!!!!!! It is almost too cold for a 40 degree day here, which is real cold for us. In fact now that I have finished my MPG / 160 F Thermostat tests and just completed the heater hose and bottle replacement, I have finally installed a 180 F thermostat for those occasional 30 F mornings here just to warm me up faster. I had originally installed the 160 F thermosat when I first bought the jeep as it had multiple overheating problem sources and problems when I bought it, like a clogged radiator, cloged CAT, bad fan clutch, no thermostat....

Many here at had said I needed a 195 F thermostat to reach closed loop operation to get my MPGs up where they belonged, but as it turned out that was not true, and my real power and MPG problem was a bad TCU to TPS ground, combined with a bad TPS on the TCU side only and a bad O2 sensor. Fixing those three alone nearly doubled my highway mileage, even with a 160 thermostat.

In the winter my system sets at 160 to 165 F steady state all the time. Once it hits about 80 F ambient here and when I use the A/C the engine CT would climb to a max of 200 F, after a long highway drive if I pulled off the freeway and sat at a light or drive through food place for a while, on a 98 F day it would hit 200 to 202 F. The A/C operation added about 10 to 15 degrees to the CT. But the temperature was always very stable, very slow to climb and very slow to drop back down, but in all that time the electric fan was never triggered, at least not until a 100 F day with the A/C running at idle for 60 minutes in the driveway, then when the Electric fan did cycle on and off a few times I noticed frequent, rapid temperature swings from about 185 to 205 and back to 185 F.

Between winter and summer, with the 160 F thermostat, and with the engine running at 160 F in the winter and 190 F in the summer, the MPGs stayed miserably low, so did the WOT power. Once again, the O2 sensor and a new TPS and a new TPS to TCU ground signal wire solved the MPG problem, not a 195 F thermostat.

Did answer all your questions?

Thanks for the question! It helped me refine what I was trying to point out here from my test results and from my reading research on closed loop versus open loop operation and thermostat temperatures.

On another topic, I think the on off operation of the electric fan, and some high flow rate thermostats, and high temperature thermostats are the cause of some people having rapid temperature oscilations in their cooling systems, and not always trapped air at the sensor.

There is always the possibility, that there is a difference between what it's supposed to do and what it actually does.

8Mud said:

There is always the possibility, that there is a difference between what it's supposed to do and what it actually does.

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So the key words there are sufficient, what ever that is, and calibrated. The CT and MAT sensors are calibrated down to -40 F according to the same manual. I see no claim there as to what sufficent is? I suspect that they meant to refer to the O2 sensor being at a sufficient temperature.

I have tested my O2 sensor at ambient start up, at temperatures as low as 70 F ambient, cold start up on my 87 Renix and found the ECU to be in closed loop operation in just a matter of seconds, maybe 10 to 15 seconds after start up based on the O2 sensor data output which indicated that the engine was already operating in closed loop at idle and at a constant RPM at temperatures that low. I have read similar details at the Bosch O2 sensor web site IIRC, indicated that HEGO was and is used to get the engines into closed loop operation in seconds versus minutes and to keep them in closed loop operation at extended idle. Maybe I can find the site page and post it here later.

At rapid deceleration it ran rich for about 10 to 12 seconds and then settled back into the closed loop status. Not sure if it's just the ECU taking that long to cut back the fuel rate, or if it deliberately runs rich on rapid decel. maybe a bit of both, or maybe it takes that long to use up the excess gas from the last injection, but it is obvious from the meter (analog, high impedence, FET Multi meter ) when it is in closed loop operation and when it is not. I also noticed it ran a little lean for just a second during rapid acceleration, but for only 1 second.

Personally I think that what that says is if one of the temperature sensors is giving invalid data, out of range, then it defaults to open loop!

Looking at the rest of that section in the FSM I see it says the engine runs in open loop during deceleration period.

Good thing I printed this one out 6 months ago, liked to never found it again.
It was here(1), and again at several other www.forparts.com URLs, as I have several copies I printed out but none of them will load tonight. Perhaps its temporary so here is one of the urls on one of my printouts.
(1) http://www.forparts.com//Bos02update2.htm

I really can't remember where I read the engine needs a temperature of 140 degrees at the engine temperature sensor to go closed loop. That little gem may be a leftover from my YJ days and carburetor/O2 regulated air fuel ratios and totally irrelevant for Renix. But there is often some continuity in engineering principles.
Just a thought, but it isn't necessary for the ECU to use the O2 sensor signal, even if it's being sent.
One sure way to find out is to hook up a simple indicator light from a power source to the EGR solenoid ground and when it lights up you know your in closed loop (at least at in cruise mode, though likely not at idle, decel or WOT).
Thanks for the links. By the time I finally figure out the Renix completely they will likely be fossils. I've reverse engineered some fairly complex and oddly engineered systems in my life, the Renix is a real head scratcher. I usually manage to get inside the engineers heads eventually and figure out the why, with the Renix the why has often proved more elusive than most.

8Mud said:

I really can't remember where I read the engine needs a temperature of 140 degrees at the engine temperature sensor to go closed loop. That little gem may be a leftover from my YJ days and carburetor/O2 regulated air fuel ratios and totally irrelevant for Renix. But there is often some continuity in engineering principles.
Just a thought, but it isn't necessary for the ECU to use the O2 sensor signal, even if it's being sent.
One sure way to find out is to hook up a simple indicator light from a power source to the EGR solenoid ground and when it lights up you know your in closed loop (at least at in cruise mode, though likely not at idle, decel or WOT).
Thanks for the links. By the time I finally figure out the Renix completely they will likely be fossils. I've reverse engineered some fairly complex and oddly engineered systems in my life, the Renix is a real head scratcher. I usually manage to get inside the engineers heads eventually and figure out the why, with the Renix the why has often proved more elusive than most.

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Well I have seen guys right here preaching that it needs to be up at 195 F to go closed loop, which I am convinced is BS now. I agree, and suspect that it is a leftover concept from the early O2 sensor days when the first O2 sensors did not have built in heaters. The print out Bosch history I have here suports that as well. Also some cheap one wire universal O2 sensors are still available over the counter, and those would have issues, even at 195 F hot idle from what I read, as the extended idle is not even hot enough to keep the O2 sensor working properly.

Regarding your comment: " Just a thought, but it isn't necessary for the ECU to use the O2 sensor signal, even if it's being sent."

I considered this, but if the ECU was not in closed loop my O2 sensor would read rich, instead it oscillates quite nicely, and frequently across the 2.45 Volt middle, 14.7:1 A/F ratio, set point which can only happen if the ECU is in closed loop mode and forcing the A/F ratio to the 2.45 volt setpoint. The oscillation is the typical over shoot and under shoot of Proportional control loop hard at work. I did have a nice long 1/2 hour discussion with an old timer O2 sensor engineer at Bosh about all this about 6 months ago, partly how I got smarter on the topic. 5-90 was my first coach on it, then the Bosch engineer filled in the gaps for me.