A little advice. Low idle. Code 21 code 28 And ECU won't clear (1 Viewer)

[ampline]

1994 FZJ80, 215k.

I am experiencing a low idle (400rpms in D with my foot on brake at stop light and engine vibration). If I put in N, it bumps up to 550rpm and it immediately smooths out. Otherwise it runs great and I don't notice any lack of power.

And have a code 21 and code 28.

So. I figured I'd throw money at the codes and I replaced both O2 sensors with new Denso sensors today. Yet I still have a code 21 and code 28 even after "clearing" the codes. Both codes were present immediately after resetting ECU, even without starting the vehicle. The codes will not go away, I've pulled battery, and EFI fuse numerous times. Any suggestions?

 

[Desert Dino]

It sounds like ECU or wiring malfunction

Flowing

 

[Malleus]

It's (too late now) always a good idea to troubleshoot (resistance check) the sensors before replacing them. There's a lengthy troubleshooting flowchart in the FSM.

 

[ampline]

It's (too late now) always a good idea to troubleshoot (resistance check) the sensors before replacing them. There's a lengthy troubleshooting flowchart in the FSM.

Yeah, I should have tested them, however they were the original sensors at 215k miles. So I figured it wouldn't hurt anything to have new.. was hoping it may help with the low idle issue.

 

[Malleus]

I wasn't picking. My point was that even if the sensors were bad, you might find the root cause easier by tracing the logic circuit, which is what the troubleshooting flowcharts are designed to do.

My newest 890 has old sensors that need to be replaced, just by the looks of them. When I replaced them on my oldest, the code persisted. Still haven't gotten around to tracing it.

 

[cruiserdan]

The low idle is a result of interrupting power to the ECM. With 93/94 models the ECM needs to re-learn curb idle. this process takes several startups to complete.

An easy way to force the process is start the truck and let it warm up for a minute or so and shut it off. Leave it off for around 15 seconds and then restart it and let it run til the idle stabilizes. Shut it off again and repeat the process. It will take several cycles and the idle will gradually creep back up.

As far as the codes are concerned it is possible that the aftermarket sensors are not functioning properly.

 

[ampline]

I wasn't picking. My point was that even if the sensors were bad, you might find the root cause easier by tracing the logic circuit, which is what the troubleshooting flowcharts are designed to do.

My newest 890 has old sensors that need to be replaced, just by the looks of them. When I replaced them on my oldest, the code persisted. Still haven't gotten around to tracing it.

The low idle is a result of interrupting power to the ECM. With 93/94 models the ECM needs to re-learn curb idle. this process takes several startups to complete.

An easy way to force the process is start the truck and let it warm up for a minute or so and shut it off. Leave it off for around 15 seconds and then restart it and let it run til the idle stabilizes. Shut it off again and repeat the process. It will take several cycles and the idle will gradually creep back up.

As far as the codes are concerned it is possible that the aftermarket sensors are not functioning properly.

Hey thanks for info.. ive definitely experienced the low idle after an ECU reset. But this low idle has persisted for a few weeks with no real improvement. The only codes I have are the 21/28 codes. I figured I would start there. I'm assuming it's time to start tracing some wires. I have an intermittent speedo too (swapped clusters and installed new VS1 to no avail).

I'm thinking I have some wiring issues. Other than these problems it runs and drives amazing.

I'm more confused on why the codes aren't clearing after a reset? They persist even before starting the vehicle (after reset). Unless the wiring or new sensors are so fubarred the computer immediately finds the fault?

I'm used to clearing the codes and them disappearing for a bit before ECU detects them.

 

[Malleus]

I'm no ECU expert, but that has been my experience.

I usually clear mine by pulling the 15A ECU fuse. I don't have any faith in the WiFi dongle; I don't believe it if I can't see it.

 

[cruiserdan]

Re the persistent low idle, how often have you interrupted power to the ECM? Perhaps you are not getting sufficient start cycles in between attempts to clear the codes.

 

[ampline]

Re the persistent low idle, how often have you interrupted power to the ECM? Perhaps you are not getting sufficient start cycles in between attempts to clear the codes.

I replaced the old battery about 2 weeks ago, and I hadn't cleared codes since. I've been daily driving it since then. 2-3 trips a day. I've only owned this 80 for about 2 months. The first month it was at a friend's shop for a front axle rebuild. So I'm just trying to baseline it and get it reliable. Thank you for your time and your knowledge.

 

[ampline]

I wanted to eliminate possibility of an ecu issue, so I swapped ecu with another 94. Code 21 and 28 are still present before starting. Code 42 comes up until the "no speedometer" issue starts operating after about 10 minutes of driving. Once speedo starts working, my OD off light stops flashing.

Am I correct in thinking that this could mean I have a bad ground or broken/shorted wires for these codes.

So after digging a little deeper I've found that the 3 codes I'm chasing 21,28,42 all utilize the same ground, and the wiring for these three separate issues are contained in the same section of wiring harness (up firewall, near EGR and across top of cowl).

I guess it's time to start tracing and ringing out wires or start unwrapping the harness?

 

[Desert Dino]

Flowing

 

[Tools R Us]

...
I'm more confused on why the codes aren't clearing after a reset? They persist even before starting the vehicle (after reset). Unless the wiring or new sensors are so fubarred the computer immediately finds the fault?

I'm used to clearing the codes and them disappearing for a bit before ECU detects them.

Haven't seen this exact issue, but have seen codes set as soon as the ECU is powered. Takes a full failure on the offending circuit, like short or open, the ECU cant "see" the sensor, or whatever it's looking for. Best to do the meter checks on the offending circuit(s) from the ECU connections. On some of the early rigs the O2 harness is poorly routed, known to rub, short on the drive shaft, etc.

 

[gummycarbs]

My '94 had high cold idle, low hot idle, and intermittent stalling at stops. Plus O2 codes. I eventually fixed it with NTK O2 sensors, thoroughly cleaning the IAC valve, replacing all the vacuum hoses (including the brake booster vacuum hose) and PCV hoses, bypassing the EGR (temporarily) and EGR modulator, and clearing carbon that had made it past the bad modulator. It was a pain, but my idle is absolutely spot-on now.

 

[Desert Dino]

My '94 had high cold idle, low hot idle, and intermittent stalling at stops. Plus O2 codes. I eventually fixed it with NTK O2 sensors, thoroughly cleaning the IAC valve, replacing all the vacuum hoses (including the brake booster vacuum hose) and PCV hoses, bypassing the EGR (temporarily) and EGR modulator, and clearing carbon that had made it past the bad modulator. It was a pain, but my idle is absolutely spot-on now.

IAC valve??

 

[Malleus]

IAC valve??

 

[ampline]

I just removed and cleaned my IAC, it didn't remedy the low idle in D with foot on brake. My idle on start up and in P is spot on... it only idles low 390-400rpm in driving situations once fully warm..

I'm guessing no signal from the O2 sensors could cause this.. anyone have any thoughts? Thanks in advance.

 

[gummycarbs]

I just removed and cleaned my IAC

Did you test it while it was out? You apply voltage and it just moves a little bit in each direction. And did you clean the intake where the IAC goes? I used carb cleaner and a toothbrush to clean the intake as much as I could without removing it. I had quite a bit if gunk in there, which was probably affecting the airflow.

I think there's also an IAC reset where you do something like switch the ignition on and off again a certain number of times in 15 seconds or something like that. Cruiserdan explained it somewhere. I'd search and link, but I'm on mobile.

 

[ampline]

Did you test it while it was out? You apply voltage and it just moves a little bit in each direction. And did you clean the intake where the IAC goes? I used carb cleaner and a toothbrush to clean the intake as much as I could without removing it. I had quite a bit if gunk in there, which was probably affecting the airflow.

I think there's also an IAC reset where you do something like switch the ignition on and off again a certain number of times in 15 seconds or something like that. Cruiserdan explained it somewhere. I'd search and link, but I'm on mobile.

Hey thanks for responding. I did clean out throttle body as much as I could from IAC opening, and I did do the reset.. I'm going to keep driving it a bit to see if idle stabilizes. Regardless I made an appointment for an auto electrical specialist on wed. If I can't trace the bad circuits by then, I guess I'm going to have them do it. As much as that blows up my DIY ethic. I need some dependable transportation. Thanks again.

 

[sp4]

Hey did you happen to resolve this issue?