The RenX Files: High idle problems

[Ecomike]

EcoMike> If you are experiencing any or all of these symptoms. If the idle on your 4.0L Jeep motor is higher than 750-800 rpm or the idle is high and changes between 1000 to 1500 RPM after you first start the motor or stays at this RPM even after engine has reached normal operating temperature . Or when you first start the engine and it revs up to 2000 to 3000 RPM then drops SLOWELY to 1000 to 1500 RPM after 5 to 10 seconds. One thing that will also affect the RPM is the ambient Air temperature change and engine temperature changes in this condition. ANYWAY, HERE IS YOUR PROBLEM. FIX THIS AND PROBLEM GOES AWAY FOR A VERY LONG TIME. YOU HAVE A INTAKE MANIFOLD GASKET VACUUM LEAK! YOU CAN TRY RE-TORQUEING THE INTAKE BOLTS BUT THIS WILL NOT FIX THE LEAK ENTIRELY...TRUST ME ON THIS ONE. SEE, THE INTAKE MANIFOLDS ON THE 4.0L ENGINES ARE VERY PRONE TO WARPING AT THE NUMBER 6, 5, AND HALF OF #4-3 INTAKE RUNNERS. SO IF YOU TIGHTEN THE INTAKE BOLTS, ENGINE WILL RUN BETTER BUT NOT 100 PERCENT. YOU HAVE TO BUY A NEW OR GOOD USED INTAKE MANIFOLD OR IF YOU HAVE A VERY COMPITENT MACHINE SHOP IN YOUR NEIGHBORHOOD THEY CAN RESURFACE THE INTAKE MACHINED GASKET SURFACE AREA TO MAKE IT TRUE FLAT AGAIN. ALSO IF YOU OPT FOR A USED INTAKE MANIFOLD ALWAYS TAKE IT TO YOUR LOCAL MACHINE SHOP AND HAVE THEM CHECK FOR WARPAGE WITH A MACHINIST straight EDGE. AFTER YOU HAVE SOLVED THE WARPAGE PROPLEM THEN INSTALL A "NEW GASKET" AND RE CONNECT EVERYTING AND FIRE IT UP AND GO. (INTAKE MANIFOLD GASKET LEAK DOES NOT ALWAYS SHOW IT SELF ON THE 4.0L JEEP ENGINE WHEN YOU TEST FOR IT) SO TRUST ME WHEN I SAY WITH OUT A DOUBT THIS IS YOUR PROBLEM . I hope this helps and good luck and happy trails.

I know this is a verrrrrry lonnnnng thread, but if you take the time to read most of it you will find that I already solved my problem, and it turned out to be combination of problems, some of which were intermitent, like a variable ground, loose ground connection near the oil dip stick, that was changing the TPS idle voltage by changing the ground quality of the TPS ground leg. I also had an IAT (Intake Air Temperature) sensor go south on me while I was solving the grounding problem. I also had minor CCV tubing and Valve cover gasket (vacuum) leaks that added to the diagnosis problem, and an IAC that died just about the same time the IAT sensor died.

Right now she purrrs like a kitten right nice and steady at 750 rpm in park at idle and 650 rpm in drive at idle, and she takes off like a scalled dog when you punch the gas, which is amasing since the odometer reads 248,000 miles and counting. Anthanks for the post, I am sure others will find your sugestion helpful. Just keep in mind that a vacuum leak at the intake manifold tends to cause a more steady, high idle. I had unpredictable idles running all the way from 250 rpm to 2500 rpm. The symptoms would change randomly after hours, days and weeks. Drove me nuts for almost 2 years finding and fixing all the little issues. Luckily, it was more like an occasional weekend hobby, and not a full time job figuring it out and fixing it.

 

[Ecomike]

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

The best I got before replacing the TPS was about 13.5 mpg on a single long highway run and about 12 mpg average city. So I got a good 4 to 6 mpg increase out of the new TPS only!

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, 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.

Oh, and my old idle problems, high, low and variable, are all still gone.

249,000 miles and counting! Long live the Renix XJ!

 

[Slo-Sho]

just out of curiousity can you post your ecu idle, wot voltages and the tcu, idle, and wot voltages?

 

[Ecomike]

just out of curiousity can you post your ecu idle, wot voltages and the tcu, idle, and wot voltages?

Its already way down below in this thread, , see post number 119 in this thread, and some older test data on the bad TPS is lower down in the thread here too, like post number 116?

 

[Slo-Sho]

Those are OHMS, which on a pot meter can vary quite a bit. Can you post the ecu voltages at idle, wot and tcu voltages at idle and wot?

 

[Ecomike]

Post 98 has the voltages for a good TPS, but the 7.5 V is a typo for the ECU idle voltage, should read 0.75 V.

I deliberately went to direct resistance measurements of the mounted, and unmounted TPS with out the wiring harness conected to elliminate outside wire and ECU/TCU influences, in other words to isolate the TPS and determine what it should read by itself in ohms. If you have a good meter and a good battery in the meter the ohms readings should vary less than 1-2% between good meters, or should be accurate to within 2 significant figures if the measurement is taken in the middle of the scale (use the proper scale or range if it is not automatic), which is plenty for what we are testing. It also eliminates the variations in wiring and terminal connection resistance between vehicles and variations in the ECU output voltages.

Keep in mind those volatges you asked for will vary considerably based on the ECU and TCU output voltage and system grounds (or resistance to ground).

 

[Slo-Sho]

I know this, I too also have a renix and that is why I wanted to know what the voltages were in YOUR rig. Never mind then.

 

[Ecomike]

I know this, I too also have a renix and that is why I wanted to know what the voltages were in YOUR rig. Never mind then.

My mistake, it was post 99 I was reading and those are Mud8's numbers, not mine. Don't have time to pull those readings, got two head jobs for my kids wheels waiting on me, but I will pull them and post them when I get caught up over the holidays. In the mean time, from memory the engine off, power on TPS values were finally Text book, with 4.85 volts input at the TPS from the ECU/TCUs. I do know I had .82 V at idle on the ECU side. I finally get your point, we need to test the entire system as a last measure to see where it ended up as a system. I know I ran the tests, and as I recall they were so text book, right on that I did not record or post them. Good point

 

[Ecomike]

Thought I would add this to my favorite thread:

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, but even that beats what I was getting.

 

[Ecomike]

Re: The RenX Files: High idle problems & low mileage

OK, I rechecked everthing this time on my O2 sensor, this data is correct, some of the prior data I posted may be off.

Power and engine off. Sensor disconnected. All readings were taken with the meter ground wire at the battery negative post. Cold O2 Sensor read:

Infinite resistance to ground on all three wires!!!!
__________________________________
O2 harness connection wires read:

#1 (black) 1.3 ohms
#2 (black) 4,000 ohms
#3 (orange) 53.5 ohms!!!!!!
_________________________________________
Then with all three wires connected to the O2 sensor they read:

#1 (black) 1.3 ohms
#2 (black) 4,000 ohms
#3 (orange) 8.3 ohms!!!!!!!!!
___________________________________________
Power on, engine off, all three wires connected to the O2 sensor read:

#1 (black) 150 MV ( I may need to clean and tighten my gounds again)
#2 (black) 5.00 V
#3 (orange) 11.87 V
___________________________________
Engine on, and hot:

Running 2500 rpm, then rapid decel to idle stop, I got under 1 volt for about 8 seconds after a brief rise to 3 to 4 volts for a split second!!!!!

So if renix is shutting off the fuel and running it lean the Renix FSM is wrong and the Bosch web site is right about the low voltage being lean. If the renix FSM is wrong (or mine is acting up) and the fuel is staying on during decel to 1200 rpm, then the Renix FSM is right and low volts is rich.

I got 1.5 to 4.0 volts at idle, about 1 second between swings across the 2.45 volt sweet spot at idle. Idle rpm seemed to be a bit higher (like 25 rpm higher) at 4 volts and lower at 1.5 volts!!!!

Got a rapid 2.5 to 3.5 volt swing across the 2.45 volt setpoint at 1600 to 2500 rpm steady engine speed, about 2 swings side to side per second. It finally got to a tight 3 volts at 2,400 rpm steady engine speed once the engine was good and hot (took about 10 minutes of run time). Last time I ran this test I was getting 2.45 volts almost dead on, +/- 0.1 volts, this time it was reading closer to 3 volts. :huh:

The voltage readings were taken with a high quality high impedence FET analog meter that was zeroed before the ohm tests were started. The ohms readings were taken with a high quality, high impedence digital ohm meter.

 

[OverTheHillsATTW]

This whole thread should be a sticky for Renix users. I went a year or two without knowing how to solve my idle problems. Here it is. If people see this they get frustrated knowing that there are more than 10 pages to view. Really its the most helpfull topic tool for Renix era engine owners and would stop alot of idle questions.

 

[OverTheHillsATTW]

I'm having a problem and am calling all Renix owners. I swapped TB's with my 90 renix. My shift pattern went back to normal. I took my Jeep to a friends garage to help me read and adjust anything that needed it because I still had highs and lows for an idle. We plugged in a scanner and got nothing for the throttle %. The tps isn't showing any voltage. I cleaned all my ground connections prior to this also. He printed out some computer diagrams and such and said that It seems C7 off the computer isn't sending any power to the TPS. Should I be swapping my computer also? My jeep is riding like a dog only maxing out at around 2,500 rpms and shifting no matter what. I can't get a reading out of the tps.

 

[Ecomike]

I'm having a problem and am calling all Renix owners. I swapped TB's with my 90 renix. My shift pattern went back to normal. I took my Jeep to a friends garage to help me read and adjust anything that needed it because I still had highs and lows for an idle. We plugged in a scanner and got nothing for the throttle %. The tps isn't showing any voltage. I cleaned all my ground connections prior to this also. He printed out some computer diagrams and such and said that It seems C7 off the computer isn't sending any power to the TPS. Should I be swapping my computer also? My jeep is riding like a dog only maxing out at around 2,500 rpms and shifting no matter what. I can't get a reading out of the tps.

Most scanners can not read the 87-90 ECU electronics. What scanner was he using? Is it actually set up for Renix? It could have been reading a problem at the scanner connector itself that is not really there in the rest of the renix circuit.

Best thing to do is get a volt-ohm meter and go take live readings right at the TPS. You might try disconnecting the battery for 30 minutes to clear the old TPS settings from the ECU memory. Once you do solve the problem with a wire patch wiring replacement, TPS calibration, and or a new TPS the renix will still act odd, idle wise for about 3 restarts till it learns the new TPS idle voltages. Most problems are caused by worn TPS internals, solved by replacing the the TPS followed by TPS calibration, and or ground wire clean up and replacements, folowed by TPS recalibration, or frequently all of the above.

 

[OverTheHillsATTW]

I told him about the scanner. I'm not sure what he was using but did infact have some sort of "old" jeep connector. It plugged right into my fuse relay box under the hood. I plugged my old TPS in and used my volt meter and pulled a reading of 5. I think im on the right track now. I'll have to wait a day to see how it rides. The idle went down from 2k to 1k. I just hope im not stalling out in drive. Right now im on jack stands redoing brake lines and such.

 

[Bent]

Not sure where to put this link but this seemed like a good spot.

Renix sensor diagnostics.

 

[letank]

SNIP

I've never seen a Renix with 5.0 volt input voltage, it's usually somewhere between 4.6 and 4.8.

SNIP

The input voltage is rarely (if ever) 5.0 volts. The closer you set one side of the TPS to the ideal index voltage (of 4.2 volts for the A/T) the lower the other side gets ( away from the ideal 0.8 volts for the ECU).
Manual tranny is much easier, you only have one voltage to contend with, 0.8-.82 volts, the ideal setting.

Reviving the eternal High idle, 1990 w 2.5L 193Kmiles and 5 speed (AX5)

started to Idle high 1100-1500 but comes dow slowly to 900-1000.

Unplugged the TPS, no changes, the IAC is fully retracted.

Tested w ignition ON, engine off
CB (input-ground) 5.14V
AB (output-ground) 0.27V

Funny, everyone has too much feedback, and I am stuck w not enough.

Should I adjust to 0.8 volts or lower to 0.22 or... I am using Chryco diagnostics for 2.5 89 and 90 XJ.

How do you disco the ECU cable, I can only see one screw facing the driver

Michel
90 wranwran 2.5
74 wagoneer 5.9

 

[Ecomike]

Reviving the eternal High idle, 1990 w 2.5L 193Kmiles and 5 speed (AX5)

started to Idle high 1100-1500 but comes dow slowly to 900-1000.

Unplugged the TPS, no changes, the IAC is fully retracted.

Tested w ignition ON, engine off
CB (input-ground) 5.14V
AB (output-ground) 0.27V

Funny, everyone has too much feedback, and I am stuck w not enough.

Should I adjust to 0.8 volts or lower to 0.22 or... I am using Chryco diagnostics for 2.5 89 and 90 XJ.

How do you disco the ECU cable, I can only see one screw facing the driver

Michel
90 wranwran 2.5
74 wagoneer 5.9

0.83 volts is optimal for the TPS idle setting according to the last '90 FSM. Older FSM has 0.80 and 0.82 depending on year.

 

[letank]

0.83 volts is optimal for the TPS idle setting according to the last '90 FSM. Older FSM has 0.80 and 0.82 depending on year.

Thanks Mike, I'll play w it manana.

BTW ground tested according to specs < 1ohm, I have .5 and .3 ohm. Reading low values is always a challenge.

The diagnostic procedure (Chrysler Powertrain diagnostic procedures) that i have does not list any specific voltage, if it is below 1 volt, then test ground resistance from B terminal of TPS to vehicle ground to be less than 5 ohms, then adjust voltage to attain idle speed.

The few other thinks that i gather from web searches:
PCV, EGR, the usual vacuum leak at intake, and brake booster grommet

Michel
90 wranwran 2.5, AX15 193Kmiles
74 wagoneer 340Kmiles

 

[slowjeep]

my 90 4.0 is hard at startup when cold and extremely hard when warm, if I manage to start it when warm as soon as I go to gear rpms fall to 0!!!!, what 's going on?!?!?!?!?!

 

[88 Wagonman]

my 90 4.0 is hard at startup when cold and extremely hard when warm, if I manage to start it when warm as soon as I go to gear rpms fall to 0!!!!, what 's going on?!?!?!?!?!

Try giving it a little throttle when starting and see if it helps, if it does then go back to the begining of this thread and start reading.