The RENIX low Idle chronicles, long read!

The RENIX “low idle” chronicles!

The Jeep, 1990 Laredo 4.0/AW4/NP231 and swapped in non-disco D-30 and 29 spline 8.25.
This Jeep was bought brand new by my dad in late 89. He drove it until the week before he died of a rare liver disease. The XJ got away from me as my stepmother offered it to me but I couldn’t afford it. I found the XJ 3 years later abandoned in the back of a vacant lot with a blown engine and a homeless man living in it. I found the owner through some LEO friends of mine and I bought the Jeep for $450 dollars.

After getting its original 4.0 rebuilt and all new interior and paint I have a great running great looking Jeep but there has been one fly in the ointment, one thing that just bugs the crap out of me and that is the low idle.
When I installed the new engine I also started out with all new MOPAR sensors and outputs, a grand or so worth!
I do not remember if the idle was low in the XJ’s younger days, I only remember that dad had the piston slap issue with it from day one but the rebuild of the engine fixed those issues.
My idle is so low that it will almost stall when in gear, lights and A/C on. The TB is clean, the IAC is new, my MAP went out last week so I installed a new one and my mileage is back to 300+ a tank full again.
If you can think of it, its new on this Jeep. Mostly from just wanted to start out with all new stuff but also some things failed me like the fuel pump.
I need some solutions for this idle issue. I am of the thought now that my ECU may be bad since this Jeep sat for 3 years with the windows down. I think that the ECU may have moisture damage. The wire harness looks to be in A-1 condition, all of my grounds are good. I have upgraded the grounds with extra 4ga ground wires from the side post of the OPTIMA.

I want this fixed, give me some solutions!

I spent some time on it today. I cleaned the TB and IAC port. It was already fairly clean.

I have checked the cap/rotor/plugs, they look fine and have less than 1k on them.

I have hard wired the TPS as i was having some voltage drop across the connector. I hard wired both sides and installed some test ports on A-B-D wires. I rechecked the TPS and I have a solid 4.8v input.

Still the idle is low and sometimes erratic and sometimes smooth. I also have a raw gas smell at idle. The injectors and FPR are new, no leaks. The fuel pump has less than 10k on it and the filter was installed when the pump was. There are no gas leaks!

Could I have a bad charcoal canister? I have a canister from a 96 that I could put in it but they hook up differently and I'm not sure it will work.

I've almost ruled out everything except the wire harness to the IAC(my next check) and the ECU itself.

Will any ECU from a 90 4.0 work?

Does it idle higher when the engine is warming up?

Another question, when you cleaned the throttle body, did you clean the air passage on the opposite side of the iac air passage. It's a very small passage on top of the throttle body if I remember and can get dirty. There is a screw that adjusts the flow of air through that passage. The screw is right above the vacuum fitting that goes to the break booster. And I'm not talking about the screw that adjusts the throttle stop. This screw is usually covered up by a plug that you have to pop off. You probably are not supposed to adjust this, but I think it's there to fine tune your idle speed. I could be way wrong but I've done this to multiple renix rigs, and they idle just fine now. Try that before you replace the ecu.

IH392 said:

Does it idle higher when the engine is warming up?

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Yes, when it is in open loop it idles high and steps down as it warms up

IH392 said:

Another question, when you cleaned the throttle body, did you clean the air passage on the opposite side of the iac air passage. It's a very small passage on top of the throttle body if I remember and can get dirty. There is a screw that adjusts the flow of air through that passage. The screw is right above the vacuum fitting that goes to the break booster. And I'm not talking about the screw that adjusts the throttle stop. This screw is usually covered up by a plug that you have to pop off. You probably are not supposed to adjust this, but I think it's there to fine tune your idle speed. I could be way wrong but I've done this to multiple renix rigs, and they idle just fine now. Try that before you replace the ecu.

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I have cleaned that port in the past and the plug is missing from mine but the screw is still there so it had been adjusted sometime in the past. I removed the screw while the intake was off during the rebuild and cleaned the port. I also replaced the "O" ring on the screw. When I adjust this screw it will change the idle for a minuet or so then the IAC readjusts it back to 300ish RPM.

One other thing that could also cause a bad idle, is an EGR valve isn't completely closed. Pull it out and make sure it's sealing well. I once had a plugged vacuum line to the EGR valve that caused it to stay open, but that is probably not your problem.

IH392 said:

One other thing that could also cause a bad idle, is an EGR valve isn't completely closed. Pull it out and make sure it's sealing well. I once had a plugged vacuum line to the EGR valve that caused it to stay open, but that is probably not your problem.

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I have looked at the EGR as well. I made a block-off plate and tested the Jeep without it for a month or so, I really saw no difference in how it ran either way. I have a new EGR and solenoid on there now.

I have no drive-ability issues what so ever with this XJ, just a really low and poor idle.

I did some more checks today on the O2 sensor. I'm getting 5v at idle and I can make that voltage drop if I create a vacuum leak or un-plug a fuel injector.

So the O2 seems to be operating properly, correct?

I also adjusted the set screw or bleeder screw on the side of the throttle body, (not the throttle stop screw) I see some difference in idle smoothness but not in idle speed.

I'm still digging, give me some more idea's

Low idle:

IAC; mixture is too rich; ECU THINKS the mixture is too lean and makes the mixture too rich; CTS is bad so the ECU thinks the engine is cold and enriches the mixture; the engine is too cold.

ACT sees colder than normal air charge and ECU richens mixture? Have you moved the ACT sensor to the intake tube? It could be seeing colder temps and "fooling" the ECU into a rich mixture.

joe_peters said:

Low idle:

IAC; mixture is too rich; ECU THINKS the mixture is too lean and makes the mixture too rich; CTS is bad so the ECU thinks the engine is cold and enriches the mixture; the engine is too cold.

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I checked the CTS with the engine cold and hot and I'm getting good numbers on it

Shorty said:

ACT sees colder than normal air charge and ECU richens mixture? Have you moved the ACT sensor to the intake tube? It could be seeing colder temps and "fooling" the ECU into a rich mixture.

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My ACT is in the stock location

I have pulled the ACT(MAT) and checked it in front of the garage heater, its numbers followed the chart in the RENIX manual as well although I didn't note the actual temp that the sensor was at but it increased and decreased ohms when heat was applied and removed.

Keep them coming, I know that I have missed something!

I checked my mileage today, I'm getting 14mpg and that is not compensating for the 4.10 gearing, 30"tires and stock speedo gear.

I adjusted the throttle stop a little better today, I had the throttle plate closing into the TB some so I adjusted it out some and reset the TPS.

My idle has picked up to around 400rpm

I also adjusted my e-fan controller so that the fans do not cut on until 215f and cut off at 190f. I had them running too cool (on at 195, off at 175) that may have lead to at least some of my running rich and loss of mileage.

I added some Lucas FI cleaner to a full tank today and I plan on running out 2 tanks then check my mileage again.

Now to figure out how to compensate for the tires/gear ratio and calculate actual mileage.

Is your O2 heating element working, check to see that you have 12 volts to the O2 sensor. It's definitely rich, the O2 sensor is supposed to fluctuate hi and low, and 2.5 volts being the midpoint of those fluctations

IH392 said:

Is your O2 heating element working, check to see that you have 12 volts to the O2 sensor. It's definitely rich, the O2 sensor is supposed to fluctuate hi and low, and 2.5 volts being the midpoint of those fluctations

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I checked it and got 8.5 ohms resistance. I went to Advance Auto and pulled a new Bosch like mine off the shelf and measured it at 8.4 ohms resistance. If I remember correctly the RENIX manual calls for 5-7ohms

Ive got 12v in and a good ground. So the 5v i'm getting is a "rich" reading. I got it backwards somewhere:looney: Makes sense now though as the sensor should be in the mid range when all is normal.

Okay, I'll pick up another O2 tomorrow, thanks!

The new O2 is in. It still idles low and a little erratic. I still have a gas smell at idle. It runs no different:dunno:

I checked the heater on the old O2 and it has 12.5ohms resistance. The new one has 5 ohms resistance so the old one was on its way out.

Here is a video of the tach at idle and it shows how erratic the idle is.

No numbers to prove it yet but I do believe the new O2 is giving me worse mileage than the old one did and the idle is worse now as well.

This thing is driving me nuts!

I had that problem once when I had either the TPS or O2 sensor harness connector connected backwards. Mine had lost the tabs and plastic that normally only allow assembly one way.

I would suspect a loss of signal from the O2 sensor to the ECU. ECU may only be seeing 2.5 of the 5 volts due to poor contacts, or a bad wire. I have had O2 wires get too close to the exhaust and short out too!

In the past, when I have been unable to isolate the low idle problem, I adjusted the idle stop and then the TPS to force a minimum idle so it would not die. It set a minimum air flow beyond which the IAC could not reduce further. Later when I found and fixed all the other issues, I adjusted it back down.

I also had a bad MAT sensor giving me a high idle and cold start problems, so a reverse MAT problem (resistance bad in the other direction) I guess could cause a low idle, but I think you said all those parts are new? Have you checked every single sensor wire, the ground and signal at the ECU?

Ecomike said:

I had that problem once when I had either the TPS or O2 sensor harness connector connected backwards. Mine had lost the tabs and plastic that normally only allow assembly one way.

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I checked this a week or so ago and then made double sure the new O2 is connected correctly.

Ecomike said:

I would suspect a loss of signal from the O2 sensor to the ECU. ECU may only be seeing 2.5 of the 5 volts due to poor contacts, or a bad wire. I have had O2 wires get too close to the exhaust and short out too!

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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

Ecomike said:

In the past, when I have been unable to isolate the low idle problem, I adjusted the idle stop and then the TPS to force a minimum idle so it would not die. It set a minimum air flow beyond which the IAC could not reduce further. Later when I found and fixed all the other issues, I adjusted it back down.

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I adjusted my stop screw to spec but I opened the bleed screw all the way up, that seemed to help for a little, maybe gave me 50-100 more rpm at idle

Ecomike said:

I also had a bad MAT sensor giving me a high idle and cold start problems, so a reverse MAT problem (resistance bad in the other direction) I guess could cause a low idle, but I think you said all those parts are new?

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My MAT is 2 years old but I pulled it out last weekend and tested it in front of the garage heater and it was ranging up/down as it should. I did not note actual temp readings as I measured the resistance but it was functioning properly. I have a spare new MAT(I bought 2 while I was at it because those are getting hard to find) and its function matched the one that is in there. The CTS is 4 years old and i checked it with a cold engine and then a hot engine, its showing proper function as well.

Ecomike said:

Have you checked every single sensor wire, the ground and signal at the ECU?

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I'm about ready to try this, the ECU is really a pain to get to.

Thanks for looking over this thread for me Mike, I would love to fix this idle issue. The Jeep runs great otherwise, my mileage is not so great but I am running 4.10 gearing (and a non disco D-30) on 30" tires so i know that is at least some of the mileage drop but at one time I was getting 24 mpg highway on this jeep and for some reason it just dropped off and that was before the axle swap.

I have tightened up/cleaned all of the connectors(again) and decided to give the battery a good charge while I was at it and it was a little weak. It may be time to ditch the Optima and go with a traditional battery.

Part of the problem is that I have 2 daily drivers, this XJ and my Hemi Commander, I alternate weeks between them and the Optima seems to loose some charge sitting.

I have noticed since I charged it last night that; 1) it starts much quicker 2) it idles much smoother and higher.

I also replaced the positive terminal, I found a small crack in it that was causing a loose connection, this was also a contributing factor.

I'll post a report back on mileage/idle soon