Fooling the Computer for Better MPG

[xjtrailrider]

sorry to hijack
while in the subject of overheating etc. tell me would a cracked outlet have reason to lower my mileage to 10.5 mpg or 4.5 km/l (as we use here). this running highway at around 65mph. i have had the vehicle at the agents and to no avail. my xj does seem to run slightly warm as well. it is a '96 4.0. the cat has been removed some time back and have done a barrel air filter conversion. i run on 30's so i can't see this having such an influence.
any help will be very much appreciated. where can i start looking. any sites will help.
thanx
steve

Yes, the influx of fresh air at the exhaust manifold causes the O2 to read lean which in turn will richen the air/fuel ratio to try to achieve what is required. I have had good luck welding the manifolds in the Jeep, you may have to remove a few things so you can get to the crack to put a bead on it though.

Are you getting a CEL after removing the cat. In my experience with OBD2 you will get a "catylist below threshold" code, PO420 or something like that.

 

[Ecomike]

This is really isn't the place to be discussing the overheating issues. I only posted it as a correction to my prior post regarding the oxygen sensor readings. There are several overheating threads that are active right now in the OEM section where we discuss these.

I just opened a new one about the best water pumps and fans clutches!

Here:

http://www.naxja.org/forum/showthread.php?p=244439193#post244439193

1bolt,

I have had few water pump problems over the years, and all but one of them was a visible leaker at the shaft seal when it died. I recently had an odd, very odd one on a diesel engine that actually let it suck air, at the rear gasket and shaft seal, but never leaked coolant, and it never overheated, but it did cause problems with the open system overflow bottle, as the trapped air would never let coolant get sucked back in overnight. I just have not had all the problems others have had with water pumps, so I tend to discount them. This time who knows, it may be a water pump impeller slipping on the pump shaft, like I have head stories about? Right now I banking on the thermostat being screwed up and or the fan clutch being old enough and week enough to be adding to my problem.

Lets move the discussion to my new thread on best water pumps and fan clutches!

 

[Ecomike]

Yes, the influx of fresh air at the exhaust manifold causes the O2 to read lean which in turn will richen the air/fuel ratio to try to achieve what is required. I have had good luck welding the manifolds in the Jeep, you may have to remove a few things so you can get to the crack to put a bead on it though.

Are you getting a CEL after removing the cat. In my experience with OBD2 you will get a "catylist below threshold" code, PO420 or something like that.

I had some carefully planned welding to one of mine where it was cracked at the old factory welds, but you need take great care not warp (assuming they are not already warped!!!) the flat sealing areas of the manifold!!!! Welding while mounted on the head may help avoid warping.

Be sure there is no GASOLINE, or fuel lines nearby while welding!!!

 

[stevieb]

xjtrailrider> that is all i wanted to know. then again Eco, over and under fueling has plenty to do with overheating together with timing. running turbos for years taught me that fueling and timing could overheat a car at idle. put the xj on a dyna, stick a thermometer in the exh and you will see why they sometimes tend to get hot. just one question i would like to know, nothing to do with overheating. how does a person know when the system is in open or closed loop. is there a way to measure this as i have installed a fan thermostat inline on the top hose and am just scared that the switching might just take the cooling to low (under closed loop temp). will this not affect the fueling and make it heavier on fuel?

 

[xjtrailrider]

just one question i would like to know, nothing to do with overheating. how does a person know when the system is in open or closed loop. is there a way to measure this as i have installed a fan thermostat inline on the top hose and am just scared that the switching might just take the cooling to low (under closed loop temp). will this not affect the fueling and make it heavier on fuel?

When you reach 205-215 you should be in closed loop providing that all sensors are functioning properly. For instance, the CTS could be faulty and holding the ECM in open loop when the actual coolant temp is running normal.

 

[Ecomike]

When you reach 205-215 you should be in closed loop providing that all sensors are functioning properly. For instance, the CTS could be faulty and holding the ECM in open loop when the actual coolant temp is running normal.

IIRC OBD-II systems are designed to switch into closed loop operation in less than 30 seconds of a cold start if the HEGO (Heated Exhaust Gas Oxygen Sensor) is working properly, regardless of the CTS operation.

He may have multiple problems, including a dead O2 sensor set up. A dead O2 sensor has a huge effect on gas mileage.

Using an analog volt meter tap the 2 black wires to the O2 sensor (Not the larger diameter orange wire, it feeds 12 volts to the HEGO sensor's internal heater) and read the live voltage (do not disconnect the O2 sensor) from the O2 sensor. It should swing back and forth from 0-1 volts at a steady idle, and at 2000 rpm should close in at 0.45 volts with a mild swing from about .3 to .6 volts max. If it does not oscillate it is bad, or it may be missing the 12 volts on the orange (Mine is orange, 87, I am guessing yours is too, but the large third wire, what ever color it is) wire (bad relay?), or the computer or wiring harness could have a problem (short?).

 

[gradon]

On my 96 obd2 and monitored with the lm2(wideband o2/scanner/datalogger)
Open loop: cold start, ~75%-->wide open throttle any rpms, and on my 96, part throttle above 3100rpms
Closed loop: warmed up and part throttle up to 3100rpms. pcm maintains 14.7:1 afr

I don't know if all scanners have the capability(never used one for an obd1), but mine has an open loop on/off indicator as well as short and long term fuel trim %s. My dad's scanner has the cl/ol designations and in cl it'll give the fuel trims.
I've learned to love the obd2 setups based on the real time data you can get--very helpful in diagnosing problems. However, the obd1s and renixs are easier to manipulate/mod without them interfering.

 

[Ecomike]

On my 96 obd2 and monitored with the lm2(wideband o2/scanner/datalogger)
Open loop: cold start, ~75%-->wide open throttle any rpms, and on my 96, part throttle above 3100rpms
Closed loop: warmed up and part throttle up to 3100rpms. pcm maintains 14.7:1 afr

I don't know if all scanners have the capability(never used one for an obd1), but mine has an open loop on/off indicator as well as short and long term fuel trim %s. My dad's scanner has the cl/ol designations and in cl it'll give the fuel trims.

Have you ever tried running with CTS disconnected to see if it ever goes closed loop with out a CTS signal?

 

[Ecomike]

When you reach 205-215 you should be in closed loop providing that all sensors are functioning properly. For instance, the CTS could be faulty and holding the ECM in open loop when the actual coolant temp is running normal.

Now that think more on this, if the CTS gave a real cold all the time state to the PCM it might hold it in open loop.

IIRC my Renix swithes to closed loop with out a CTS even connected.

 

[xjtrailrider]

IIRC my Renix swithes to closed loop with out a CTS even connected.

Thats good to know, I've never tried it.

 

[gradon]

I'll check it out for you

 

[kidarkitect]

Hey guys, this is a really interesting thread. I honestly don't know a whole lot about, the electrical end (fooling the ecm) however, I drive a 89 renix 5spd and the two biggest improvements in my gas mileage (12-15 or so when I bought to 18-20 in the city and 25 on highway now) were replacing my 02 sensor (the old one was only finger tight, may have been a problem there???) and six months later replacing the exhaust from the manifold back w/ a 2.25 dp 2.5 inch cat and cat back. (it had leaks throughout) i've always driven like a grandpa, so there's no placebo.

Another idea completely ECM UNrelated is installing old fashions hubs up front if you have a non disco dana 30. It should decrease overall resistance since less stuff will be moving. as you drive. Am I way off base on this? I cant really test it since I still have a vacuum disconnect front end. ( I tend to attribute some of my gas mileage to it and it is where I got the idea from) I would like to know if anyone has tried this and what results they've have.

 

[gradon]

Unplugging the CTS does keep my 96 in open loop mode and it started with 12.5:1afr and crept up to 13:1. Dash gauge got up to the normal 215*(old gauge read 195* until I broke it, so I know not all gauges read the same--just a guide. The moment I plugged the cts back in it went to CL and 14.7:1.

 

[gradon]

Before the stroker, I blew the pcm's 12v output circuit in my 96 by letting the o2 sensor harness kiss the Borla(the 12v heater wire to be specific) and the temp 98 replacement pcm wouldn't activate the e-fan. I just had the cts unplugged, keeping the aux fan on constantly(I didn't have the lm2 then). You could leave it in OL and make your own fuel map using a psc1 or apexi totally eliminating having to stay at 14.7:1--you can have your >15 and I can have my <14(esp in that 2.5-3K ping prone area). The only place you really want the two different afrs is at idle for those cold starts.

 

[fool4wheelin]

I googled stoich meter and came up with this:
http://www.egauges.com/pdf/AutoMeter/837j.pdf
they even have a matching O2 kit with the weld in bung for the pipe.
I don't believe that the renix O2 sensor would be compatible with this guage.

I got through page 7 in this thread and decided it was time post. This is a great thread, but its not easy to read this whole thing.

I think 87manche has the best idea, put an O2 sensor on first to see where you're even at. I wouldn't want to be messin with my O2 sensor's signal until I know where I'm starting at, then having the gauge to show where you're adjusting too is crucial. I'm hoping to buy one of those gauges when I get the time and $$$.

What kind of A/F ratio are you hoping to obtain? We all pretty much agree that "leaner" is better for MPG's, but what else is affected (other than emissions) when you go lean?

I have some other general Economy comments to keep in mind:
- A stock Jeep Cherokee has a CdxA (drag coeffecient) of 11.63 ft^2. This isn't that good. Lifted jeeps are even worse with drag.
- The 4.0 has 7 main bearings (my 401 V8 only has 5).
- The 8.8:1 compression ratio Sucks. When cruising (and the throttle is <25%), the cylinder pressures are even lower. This leads to poor thermal efficiency.
- The stock ignition has MUCH room for improvement.
- The AW-4 Automatic is a little outdated for Automobile Efficiency. The lockup is good, but more gear ratios would be nice.
- The Front Axle doesn't have a disconnect like they used to.

Most XJ's have the 4.0 and the Automatic. IMHO, I don't think its possible to get much over 20 MPG with this setup...

 

[Ecomike]

Unplugging the CTS does keep my 96 in open loop mode and it started with 12.5:1afr and crept up to 13:1. Dash gauge got up to the normal 215*(old gauge read 195* until I broke it, so I know not all gauges read the same--just a guide. The moment I plugged the cts back in it went to CL and 14.7:1.

Nice!

Now the only remaining question is what temperature does the CTS show when it switches to closed loop!

 

[stevieb]

Eco- just found this little gadget surfing the net looking at O2 sensors.

http://www.amazon.com/dp/B001PCOP44...e=380337&creativeASIN=B001PCOP44&linkCode=asn

any thoughts as to what they have done? why would they want to adjust the heater and the out put voltage?
steve

 

[stevieb]

just read thru a couple pages i missed out on.
i have 2x O2 sensor as usually fitted, both have wires black, grey, 2x white.
Eco you have got me worried on your post. how would i test the 4x wire sensor?
How is it possible to test/check to see if she is open/closed loop once she has warmed up?
i do not even ride her at the moment since she has become so heavy on fuel. i can't afford paying $3.40 per gallon right now. salaries here in SA ain't so good anyway. one good thing her mileage is staying low.

 

[Ecomike]

just read thru a couple pages i missed out on.
i have 2x O2 sensor as usually fitted, both have wires black, grey, 2x white.
Eco you have got me worried on your post. how would i test the 4x wire sensor?
How is it possible to test/check to see if she is open/closed loop once she has warmed up?
i do not even ride her at the moment since she has become so heavy on fuel. i can't afford paying $3.40 per gallon right now. salaries here in SA ain't so good anyway. one good thing her mileage is staying low.

4 wire system has 2 separate grounds. A third wire is battery voltage, 4th wire is 0-1 volt out with engine hot and running. If the 4th wire voltage on the pre-cat O2 sensor (to ground) oscillates back and forth across 0.45 volts it is in closed loop and is OK.

 

[gradon]

I tried yesterday and today to monitor what temp CL starts, but the lm2 showed it staying in OL(even though it had went to CL and 14.7). I'll try resetting the lm2 and have the afr, cts, and OL on the same screen so I can at least tell by the afrs the exact temp it does this(I can record too, but should be able to catch it live). The lm2 does have it's limitations: I usually can monitor 4 obd2 parameters at once in addition to the afr(you can set priorities, etc)--any more and then it's too much for it to process and then you don't get accurate live data.