Extremely Low Idle

[RKTJeep]

The MJ has had a low idle since i bought it. So here's the chain of event that got me to where I am now.

 

Last night i replaced o-rings on my fuel injectors and the gasket on the valve cover. Everything went well and i went for a 15-20 minute drive to let it get up to temp and see how everything was running. I also put some seafoam in the oil with intentions of changing oil today. When i got back from the drive i pulled into the driveway and my idle was only at 200-300 rpm, which was very low for it being up to temp. I was able to get it into the garage and while looking it over i realized the MAP vacuum line was broken at the TB. I left it at that with intentions of repairing the line this morning.

 

This morning I picked up my replacement vacuum harness and TB gasket. I pulled the TB off and, made the grave mistake of not marking the TPS. I decided to continue with cleaning the TB and IAC. I repaired the MAP vacuum line and reinstalled everything including the new vacuum harness. I set the TPS by aligning the sensor with "dirt" witness marks it left on the TB. I reconnected the negative terminal on the battery and measured my ground resistance at the TPS connector pin, the reading was 0.01. I decided to try and adjust the TPS by checking the voltage between wires A&B then B&C i would then manually adjust the sensor to get them to the 17% difference. However, I am not able to get my probes into the back of the connector. I'm worried about damaging the green rubber seal at the terminals.

 

Since i couldn't get the TPS set i figured i could atleast give it a shot to see how it runs. I figured with a fresh TB and vacuum lines it should have improved slightly. Unfortunately that's not the case. I was able to get it to start by giving it a little bit of gas, but i would have to hold my foot on the pedal to get it to idle at 800 rpm, otherwise it drops down to 200-300 rpm. 

 

It looks like I'm worse off than i was before since I now have a mis-clocked TPS. I'd like to start by at least getting it adjusted properly, but i'm not really sure how to get my probes in there. Any suggestions, recommendations on where to begin tackling this?

 

I'm going to clean up the engine block ground since i have not done that yet, I will update the results of that. Any help would be appreciated.

 

[buckwheat]

Bend some paperclips and backprobe with them.

[cruiser54]

Click on the link below and do Tips 1 through 5. Skip 2 if you have an 89 or 90. 

 

TPS instructions are tip 8.

 

Look VERY closely at the TB to MAP line. 

[RKTJeep]

I was able to get the TPS set properly. I found out that it's easier to get my probes around the rubber grommet if i push towards the connectors plastic, not the wire its self.

 

Before getting the TPS set i decided to perform the ground wire modification for the sensors. The TPS ground was giving a reading of 1 ohm, right at the limit, (After the mod it read 0.2 ohm's) so i figured it was worth doing. I also put new terminals on my battery leads and i cleaned up the ground connection on the dipstick stud.

 

After all this, the idle has improved slightly. I was able to start it without giving it any gas, and the idle improved as it started to warm up.

 

I'm not really sure where to go from here. Is there a diagnostic test for the idle air control to verify that it's functioning properly?

[cruiser54]

A bad oxygen sensor can cause a low idle also. 

[RKTJeep]

Click on the link below and do Tips 1 through 5. Skip 2 if you have an 89 or 90. 

 

TPS instructions are tip 8.

 

Look VERY closely at the TB to MAP line. 

 

I forgot about some of the connector cleaning. Simple and it's promising since the ground path improvements appeared to help the idle.

 

Thank you!

[cruiser54]

 

Click on the link below and do Tips 1 through 5. Skip 2 if you have an 89 or 90. 

 

TPS instructions are tip 8.

 

Look VERY closely at the TB to MAP line. 

 

I forgot about some of the connector cleaning. Simple and it's promising since the ground path improvements appeared to help the idle.

 

Thank you!

 

I like simple. 

 

I see now you have an 88. Do Tip 27........Try 2 first if you want to. 

[RKTJeep]

A bad oxygen sensor can cause a low idle also.

I tried searching for the answer without much luck. Is it possible to test the o2 sensor?

[cruiser54]

 

A bad oxygen sensor can cause a low idle also.

I tried searching for the answer without much luck. Is it possible to test the o2 sensor?

 

For the Renix years, 87-90, the O2 sensor has 3 wires, 2 black and 1 orange. The orange wire (largest gauge of the 3) is the 12-14 volt power that comes from the O2 sensor heater relay on the passenger side firewall, and that powers the internal heater in the sensor so that the sensor can work at idle, and almost immediately after start up. Loss of that power will hurt gas mileage even with a good O2 sensor. 

 

One of the black wires is a common ground for the heater power and O2 signal to the ECU, so a poor ground will give a voltage feedback from the heater power input, to the ECU causing poor mileage even with a good O2 sensor. 

 

The third wire, also black is a voltage feed wire, 5 volts, from the ECU to the O2 sensor. The O2 sensor is an O2 concentration sensitive variable resistor. At optimal O2 concentration the 5 volt input feed to the O2 sensor drops to 2.45 volts due to losses across the O2 sensor to ground. That same wire if disconnected from the O2 sensor will read 5 volts constant to ground. 

 

At idle that voltage should read 1-4 volts oscillating quickly back and forth roughly once every second. At 2000 rpm it should run between 2 and 3 volts max, and is optimally running between 2.3 and 2.6 volts at 2000 rpm (in park). A digital meter can NOT be used for reading the O2 sensor voltage, but it can be used to test the ground and the 12-14 volts to the heater and the 5 volt feed from the ECU with power on and engine off. You must use an old style analog meter with the needle gauge on the display to see the voltage swing back and forth with the engine runing. 

 

If the O2 sensor readings are not right, say they read 4 volts or 1 volt steady, you have a problem. BUT before you blame the O2 sensor make sure it has good wiring, and make sure the proper voltage is feeding it, by turning power on, engine off to read the engine off voltage feeds (12-14 volts on the orange wire, and 5 volts on one of the two black wires), and ensure the ground wire (power off) reads less than 1 ohm to the battery negative post.

 

A leaky exhaust system or leaky fuel injector(s), or bad compression, bad rings or leaky valves, bad plugs, wires, cap, rotor, HV coil, and so on, or combination of these, can also cause a lean or rich condition that gives you high or low O2 sensor readings that are not the O2 sensors fault, so try and verify those other items also before buying parts like an O2 sensor to fix your problem. 

[RKTJeep]

Just to give an update.

 

I decided to replace the O2 sensor without checking the output the electrical circuits. There was no noticeable difference in idle upon startup. Unfortunately i did not have enough time to rewire the engine harness by removing the connector. I was in the process of moving from Alabama to Colorado and i needed  the truck running so it could be loaded on a car hauler, and at the time it was running enough to get it loaded.

 

Luckily it started up and had no problems getting it unloaded out in CO. My only problem now is that i don't have a garage to work on it our here.

 

I've decided to break down and take it to a shop tomorrow, Something i've never done before. I'm going to have them trouble shoot the idle as well as what i presume to be a lifter tick/chatter. There was a slight tick when i first bought it, but it got worse after i changed the oil.

 

I have a feeling the previous owner was running a higher weight oil. I switched to 10-30 since it would be seeing colder temps on Colorado Springs. At start up the oil pressure gauge takes about 2-3 seconds to shoot up to 60psi, once up to operating temp, pressure is around 20psi at idle, and does not exceed 40psi at higher rpms. I have a feeling the lower oil pressure is the reason for the lifter noise, but i'm concerned it is part of a deep underlying issue.

 

If it's something major i hope it will make it through winter or at least until i can get access to a garage, where i can swap in a long block. I have no intentions of selling the truck so putting money into is not a concern, my goal is to make it a reliable vehicle so i can depend on it for my 50 mile commute as well as taking it on some trails up in the mountains.

[cruiser54]

A shop will be totally lost and it's nothing major. Save your money. 

[RKTJeep]

Cruiser, this is the first time i've taken a vehicle to a shop. It's unfortunate  that i don't have a place to work on the truck, other than outside, where we just got 16" of snow this week.

 

So the low idle was diagnosed as a faulty IAC. After replacing the IAC the engine idles right where it should on startup.

 

The shop looked into the lifter engine noise further. They pulled the valve cover off and cranked the engine, they said there was plenty of oil making it to the top end. After that they removed the rockers and inspected the seating surfaces and said everything checked out.

 

It's sounding like this may be a deeper problem. I'm going to keep driving it until late spring (or catastrophic failure) since i cannot leave my road with my 2wd car after any snowfall.

 

In the meantime I am deciding what kind of rebuild or swap I want to do. I'm wary of the long block strokers available, I'm thinking a refreshed block working with a local builder will fit my needs better. I will also swap in an AX-15 while the engine is out. The auto shop i took it to quoted me $1,600 to perform an Engine, Transmission, and T-Case swap.