I think buying an ALDL cable is a good plan. I purchased my ALDL cable from https://obd2allinone.com/products/aldlobd1u.aspOriginally Posted by randomname
I think buying an ALDL cable is a good plan. I purchased my ALDL cable from https://obd2allinone.com/products/aldlobd1u.asp
So I went ahead and bought that cable and get the same behavior. But as it turns out if I had just gone to data lists I would have seen some data trickle in. About once a minute some data comes in that seems correct (even with a ton of errors flashing at the bottom of the screen).
Does ERROR next to something in malfunction codes mean that the code is set? I replaced the distributor the other day and set timing which may explain why it says ERROR next to EST. But does the ERROR next to MAP High mean the MAP sensor is bad?
The van when cold actually seems to stay running but when it warms up and loop status closes the desired idle and engine speed are bouncing up and down. It comes pretty close to stalling when warm (and does once in a while).
Here are some screenshots of the data list:
Cold start:
92 van tunerpro.PNG
Hot and close to stalling (1) and (2):
92 van tunerpro close to stall.PNG
92 van closer to stall.PNG
BLM is indicating rich (taking away fuel). IAC says throttle sludged up (too many counts w/TPS being correct). O2 could be lying, unplug all oxygen sensors, perform hard reset (disconnect battery cables and short together w/fused 20 amp jumper lead, key on for 10 minutes), then try it. This will start w/128 fuel trims and force open loop. Then you will see if it mechanical (FPR, injector, plugged exhaust, EVAP purge) or electrical (oxygen sensor, etc.).
What value(s) are you looking at to see the counts with TPS being correct for IAC saying sludged up throttle? I recently rebuilt the entire tbi and it was a bit nasty.
Can you further explain how to do that hard reset? I ended up just disconnecting the battery and leaving the lights on to drain any power for about 30 mins and it seems to start with BLM at 128 after that.
But OH MAN I cannot believe I hadn't tried unplugging the O2 sensor yet... I just did that after the battery disconnect and guess what? After warming up it doesn't surge and try to stall while idling!!! When I plug the O2 sensor back and the loop status goes to CLOSED, it acts up again which is resolved by unplugging the O2 sensor again. So I think that the sensor is definitely bad and I'll buy a new one and replace it tomorrow to see if things are fixed.
I really wish I would have looked at this data before replacing the fuel sending unit and all the vacuum lines, rebuilding the TBI with new injectors, replacing the IAC, plugs, wires, and distributor. I think the new distributor did help fix what I believe was misses when idling (cold or warm) as that behavior went away. At least it's all good maintenance tasks for the future.
The van was stalling when idling, being shifted into gear (slowly coming to a stall), and when coming to a stop. Hopefully, after I put the new O2 sensor in, that a lot of these problems will disappear. The putting it into gear causing it to stall happened when cold too so I may have to investigate that further since I doubt the O2 sensor is affecting that.
Here are screenshots of the data output when the O2 sensor was disconnected:
van_cold_with_o2_unplugged.PNG van_warm_o2_unplugged.PNG
What you did is acceptable for a hard reset. The idea is to zero out all codes and adaptive memory. Your TPS voltage is good, but your IAC counts should be 25 to 30, hot idle with no AC in park. Your IAC of 75 generally indicates that not enough air is bypassing the throttle plates, usually caused by muck in the throttle bores. Is your TPS adjustable? If it is, search the archives for minimum idle speed, it will give you the procedure for adjusting that.
Screenshots do not help when you're looking at oxygen sensor data, BLM and integrator are useful but the oxygen sensor voltages should be swinging from 0.1 to 0.9 V, 2 or more times per second. Screenshots won't show this, only a lab scope or graphing multimeter. Do you understand the relationship between BLM and the integrator values?
There are many others on this site that are more knowledgeable on adjusting these OBD1 systems. Stay away from the cheap oxygen sensors. I have the best luck with Denso or Bosch, and buy the ones that are already set up with the connectors, not the universal ones that you have to attach your connector to.
Not sure of the condition of your wiring harness that it is 40 years old and should be inspected closely.
Keep up the good fight, we are all pulling for you.
Last edited by MO LS Noobie; 02-11-2025 at 04:04 PM. Reason: info
I'm not sure if the TPS is adjustable? Is that the hex bolt and twisting it left/right for adjustment? I'm learning here and see IAC position/count is definitely high throughout all of this. Maybe the ECM trying to compensate for something else?
I put in a new O2 sensor (the old one was rounded off so I needed to order an extractor socket) but the behavior is still the same when in closed loop. The BLM is around 128 when open loop and then when it gets closed loop the INT value drops significantly and the BLM follows indicating rich. Then the INT is around 130 and BLM is around like 98. Which causes the engine idle to surge up and down.
The MAP high code can probably be ignored in the screenshots as I ran out of fuel and tried to run it a few times before realizing that along with some other ones.
close_loop_idle_issues.PNG
in_drive.PNG
warm_open_loop_idle.PNG
close_loop_int_drops.PNG
Also thanks for bearing with me as I fumble through this and trying to teach me how to fish. Good learning experience to understand these things.
Best guess without having vehicle here in front of me is the wiring or the PCM. The tune seems to be close because when you start cold it runs OK until it goes into closed-loop, then it starts to go rich because of the low oxygen sensor voltages. Is this a stock exhaust system and have you double checked it for leaks? An exhaust leak upstream of the O2 will cause it to run rich.
I once had a truck in the shop that exhibited these same symptoms but it went lean because the harness was contaminated with battery acid shorting 12 V to the oxygen sensor wire.
The TPS is adjustable by loosening the two or three small bolts that hold it to the throttlebody, then rotating it slightly. Some of them are slotted and some of them not. Your TPS voltage is okay but if you need to address the high IAC counts by opening the throttle then you will need to readjust the TPS voltage to 0.5 to 0.6 V. Once again there is a detailed procedure on this, do a search on this site.
Last edited by MO LS Noobie; 4 Weeks Ago at 03:44 PM.
Sorry about the delay had to go out of state for work. So noticed the O2 pigtail was stripped and replaced that, helicoil repaired the O2 sensor threads and have the new O2 sensor in. Testing it with my multimeter and adding a display in TunerPro RT shows that the O2 voltage is consistently high until the BLM is around 98 and then it starts crossing. That causes all sorts of idle surging in close loop as it tries to correct rich and then lean etc. I did inspect for leaks and didn't find any and I did replace the map sensor (new one reads about the same value). Played with the TPS as well and no change really. Also did the reset IAC procedure with no real change. It came down from 140 to around the 70s. Something is going on and I'm pretty stumped.
I am assuming that this is a stock truck with a stock tune in it. If so clear the memory again and leave the oxygen sensors disconnected. Drive the truck. If it runs good, then you can assume your base tune is correct, and only when it goes into closed-loop, as you have stated, things go wrong. If that is the case either the computer is getting incorrect information, or the computer itself has a bad component in it.
With the oxygen sensors unplugged, what is the voltage reading on the harness side, should be 0.45 VDC, this is the bias voltage that the PCM outputs on the oxygen sensor wires to check integrity. As I stated in post 22 the harness could be contaminated. If it tests bad you can jumper around it by making a wire from the oxygen sensor straight to the computer, and try that.
Otherwise the computer is getting the correct data and just misinterpreting the results, which would indicate a bad computer. This assumes that all of your body and engine grounds are good, especially the one to the thermostat housing, and maybe the rear of the heads.
Okay so I just used my multimeter to measure that bias voltage and it reads consistently at 0.34 VDC at the pigtail and at the pin of the computer itself. Checked resistance between the two and it is negligible. So I believe that the wire is not bad. But the computer is not outputting the correct bias voltage which indicates it may be bad?
Also, checked the base tune and that seemed to perform just fine with BLM staying around 121-128 the entire time.
Correct. Bias voltage low. Do you have another PCM you could program with this tune and try?
On another topic, if you zeroed your fuel trims and left the O2s disconnected, all fuel trims should stay at 128. There is no feedback from the O2 to adjust trims. BTW 121-128 is good if you were getting O2 signals.
Sorry should have clarified that it was 128 disconnected and then a second test drive reconnected was in that range. Then when idling in closed loop it works its way down to a BLM in the 90s.
Could just drive around with O2 disconnected and be a lot better off but I'd like to figure out what is going on?
I see the junkyard near me has a 92 and a 93 G20 van and I'm gonna try to grab a ECM out of the 92 tomorrow. Hoping it will be the same kind. Do I have to put in the exact same one or would a similar one from a 89 (they have one in there too) or the 93 work?
A search on here would tell what interchange numbers might be, others here are far more knowledgeable on the OBD1 stuff.
Oh for sure and I did that and saw where $42 is plug and play for the $99. So I'll try to grab an ECM out of the boneyard from a 1987-1991 GM vehicle for the $42 or a 1992 one for the $99. Going to definitely try to get a $42 if I can as it has a lot more support and info out there on it. Spent most of today replacing leaking coolant and transmission cooler lines so I can flush the system tomorrow while hopefully testing the new to me ECM.
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