The RENIX low Idle chronicles, long read!

[xjtrailrider]

After one full tank of fuel I'm getting 15mpg in town. My idle is still low but much smoother.

 

[Ecomike]

15 mpg in town may be OK? Depends on how much start and stop, and parking lot time you spend in traffic?

"My harness shows some melting on the plastic loom but I uncovered all of it and looked it over good and checked the continuity across the suspected area, all was good"

I am wondering if you overlooked something here. A wire test while the wire is still connected to a circuit or grounds may fool you! If insulation is a little fried the carbon formed can cross connect wires leading to weak extra ground path that may be difficult to detect. Or humid days can create a mild ground path.

Is the O2 sensor (new one) still reading 5 V at idle, reading rich?

Usually a rich running engine at idle would idle high, not low, and be caused by leaking injectors, but yours sounds like the ECU is reading lean at idle and starving it for air.

Just for grins run it cold and hot with out the O2 sensor connected, and lets see what the idle is then?

Fill it up with the O2 sensor connected and do a 50 mile non-stop high way round trip run back the same pump, refill and re-figure MPGS for highway.

 

[xjtrailrider]

15 mpg in town may be OK? Depends on how much start and stop, and parking lot time you spend in traffic?

I have remote start and use it to warm up the jeep in the am for 10min before hitting the road. I also have 4.10 gearing, non-disconnect D-30 and I'm running 30x9.5's

I have found in the past that just running the tranny in the "comfort" mode nets me a 1-2 mpg increase over the "power mode". It goes into TC lock-up at a much lower mph. This bored/decked and milled engine is a torque monster so it can pull OD down to 20mph on level ground before it even considers dropping out(in comfort), it will drop out at 35mph in "power". The mileage I got above was in "power"

"My harness shows some melting on the plastic loom but I uncovered all of it and looked it over good and checked the continuity across the suspected area, all was good"

I am wondering if you overlooked something here. A wire test while the wire is still connected to a circuit or grounds may fool you! If insulation is a little fried the carbon formed can cross connect wires leading to weak extra ground path that may be difficult to detect. Or humid days can create a mild ground path.

Good point!

Is the O2 sensor (new one) still reading 5 V at idle, reading rich?

I was getting a neutral reading of 2.4 at idle after the install

Usually a rich running engine at idle would idle high, not low, and be caused by leaking injectors, but yours sounds like the ECU is reading lean at idle and starving it for air.

Just for grins run it cold and hot with out the O2 sensor connected, and lets see what the idle is then?

Fill it up with the O2 sensor connected and do a 50 mile non-stop high way round trip run back the same pump, refill and re-figure MPGS for highway.

I'll try it!

 

[xjtrailrider]

Mike, there is no change in the idle with the O2 unplugged:huh:

Whats going on with this Jeep? Is there a break in the wiring to the ECU?

 

[Ecomike]

Mike, there is no change in the idle with the O2 unplugged:huh:

Whats going on with this Jeep? Is there a break in the wiring to the ECU?

May be time to check the readings at the ECU.

At this point, I would just adjust the idle stop, then re-adjust the TPS to force a little higher idle now.

Then drive it and see what the MPGs are now. It sounds like the only problem left is the low idle, after re-reading the entire thread.

Then when you get the chance start testing the sensor signals at the ECU and compare them to the data at the sensor, or identify them wire by wire and ohm each wire end to end while disconnected.

 

[Ecomike]

I was getting a neutral reading of 2.4 at idle after the install

So it sounds like the O2 sensor is working at idle now, and the ECU is getting proper data at idle and it is no longer running rich at idle?

 

[xjtrailrider]

So it sounds like the O2 sensor is working at idle now, and the ECU is getting proper data at idle and it is no longer running rich at idle?

I'll check the new O2 again when I get a chance. The old O2 had a bad heater in it.

 

[xjtrailrider]

Okay, I'm going to clean this thread up some!

The Jeep;

1990 XJ Laredo, AW4, NP231, 4.10 gearing and "non-disco" D-30 front/29 spline 8.25 w/Detroit TT rear, 30x9.5 tires, Newly rebuilt engine w/15k on it, I replaced all sensors, including the distributor when i rebuilt the engine. The engine was bored 40 over, decked and the head milled w/3 angle valve job. 1996 valve cover w/1996 CCV system. New BOSCH injectors, Dual electric fans set to come on at 210* and off at 195* The EGR and all emissions are in place and working. Still on the closed cooling system, it is all new and works great.

The problem;

Low idle and occasional gas smell, poor fuel mileage

What I have found and tried so far;

I found the MAP to be out of range-replaced

I found the O2 to have a bad heater, replaced-I'm getting the "sweep" of the needle with a analog volt meter so it tests good

The fuel mileage went from 12mpg to 15mpg, not bad considering that is all in-town and using my remote start to warm it up.

The fuel smell is most likely the seals on top of the tank, they will get replaced soon.

I can only make the Jeep idle higher by doing 2 things, either by turning on the AC/Defrost(and nothing else) or by creating a vacuum leak. If I adjust the TB stop screw or the idle air bleed screw, the IAC readjusts the idle back to 300-400rpm!

When the Jeep is idling with the lights on, wipers on, AC on, electric fans on etc. it will almost stall. The idle is that bad!

Also another note, I can get another 1-2mpg if I run the tranny in the "comfort" mode. I normally run the tranny in the "power" mode to save wear and tear on the clutch pack.

My next step is to start looking for wire harness issues to the ECU

 

[Ecomike]

Running rich for a while tends to burn out the Cat converter, so if the gas smell is only while running, the Cat may dead.

I never got past 15 mpg city with 87, and finally after three winters of cold AW4 slip in D that went away with in minutes of warm up, the AW4 finally lost too much pump pressure, and is now being swapped. The 89 XJ has been getting 18 MPG city easy, 20 to 22 mpg highway. Your gearing and tires may combined with city may never get much higher than 15-17 mpg, unless you were getting better MPG recently, and then it suddenly fell off?

There may be something wrong with the throttle body. FWIW, At some point the throttle body can be set to where even if the IAC is 100% closed, the engine gets enough air to run at 700 rpm!!!! Uncle Bob may be the way to go at some point. But be sure to rule out wiring loses from the sensors to ECU before you give up and let Uncle Bob loose, LOL.

Also check the battery voltage at idle, and with all the junk running, AC on max, blower on max, head lights and see if there is not a loose serp belt or dying alternator involved here? It should be able to hold at least 13.5 V with everything running at idle. If one side of the alternator diode set is bad the voltage may be dropping, and the alternator may be dragging it down. Or a loose belt?

 

[xjtrailrider]

Before the axle swap I did get close to 17 in town before i started having issues.

What could be wrong with the TB?

It woudn't be hard to find another one, our local yard is full of C4C RENIX's

I'll check the alternator, it was new with the engine and it is a 130 amp unit(my next door neighbor owns a starter/alternator rebuild shop) so he built one up for me.

 

[Ecomike]

I have read reports of problems that were solved with a different throttle body, What was wrong with them I do not know.

So if you are at 15 mpg, plus 1-2 in comfort mode, you are already at 16 to 17 mpg, after the axle swap. Were you running 30s before?

Oh, and if the Cat did die from excess rich running fuel overdose, it could be partially blocked now (melted), reducing MPGs now. Just depends on how rich it was and for how long and how hot it got. Its the domino effect, one problem leads to another.

 

[xjtrailrider]

So if you are at 15 mpg, plus 1-2 in comfort mode, you are already at 16 to 17 mpg, after the axle swap. Were you running 30s before?

The 17mpg was on 30's, 3.55 gearing, disco front end and comfort mode, so maybe my mileage isn't too bad right now. I'm running a tank through with it in comfort mode right now to see what I get.

 

[xjtrailrider]

Okay Mike, Uncle Bob(LOL!) came by today and adjusted that throttle stop, I then re-adjusted the TPS to a dead nuts 83%

I now have a solid 600rpm idle in drive, 700 in park, 500rpm with it in drive and the electrical system maxed out and AC on.

I also repaired the charcoal canister orifice that had slipped into the hose from the tank and turned sideways, it was mostly blocked. This may help with the gas smell I was occasionally getting.

I'll drive a tank of gold through it and post the results.

 

[Ecomike]

Say hello to Uncle Bob for me!!!:wave:


LOL

 

[xjtrailrider]

Say hello to Uncle Bob for me!!!:wave:


LOL

Will do! I still have plans to check the wiring thoroughly to the ECU from each sensor but this will suffice for now.

 

[Ecomike]

I frequently forget to mention that people need to check for exhaust leaks, cracked exhaust pipes. The O2 sensor compares the exhaust gas O2 concentration to out door ambient air O2 concentrations, and if exhaust gas is blowing on the O2 sensor from a crack or gasket leak, it can throw off the measurements, and the computer will get miss led and waist fuel!!!!!!!!

 

[xjtrailrider]

I frequently forget to mention that people need to check for exhaust leaks, cracked exhaust pipes. The O2 sensor compares the exhaust gas O2 concentration to out door ambient air O2 concentrations, and if exhaust gas is blowing on the O2 sensor from a crack or gasket leak, it can throw off the measurements, and the computer will get miss led and waist fuel!!!!!!!!

This is a very good point! I had that issue on my 94 and on one of my 96's. On the 94 I swapped in a new manifold, on the 96 (my trail rig) i just welded it up myself to save money. They have both held up for several years now.

Fortunately my 90 has a solid exhaust system.

 

[xjtrailrider]

Quick update;

After adjusting the throttle stop and then readjusting the TPS, this Jeep has a whole new personality! Its nice to have it idling high enough to run some electronics at stoplights.

I have not checked mileage yet but it just started to drop below the "full" line at 75 miles

 

[Super mud]

Okay Mike, Uncle Bob(LOL!) came by today and adjusted that throttle stop, I then re-adjusted the TPS to a dead nuts 83%

I now have a solid 600rpm idle in drive, 700 in park, 500rpm with it in drive and the electrical system maxed out and AC on.

So what fixed it? And who is Uncle Bob?

 

[xjtrailrider]

So what fixed it? And who is Uncle Bob?

It unfixed itself....again. I have a new ECU that I'm going to try next.

Uncle Bob was just an imaginary person to blame the improper throttle stop adjustment on haha!