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Discussion Starter · #1 ·
Hello All-

I am having an issue with the truck going out of closed loop and into open loop ~10 seconds after letting off the throttle.
Vehicle is a 1992 K3500 Dually 7.4L TBI 4l80E 4X4.
I have a snap on Ethos scan tool.
Information below is taken after a 20 mile drive on the highway:
Coolant temp is reading 190F
O2 sensor is jumping from .1 to .8 while in closed loop, then a steady .8 when it kicks back to open loop
O2 cross counts in closed loop ~200
New 3 wire heated o2 sensor (upgraded from single wire)
i. I installed the 3 wire heated O2 sensor to help keep the O2 sensor above 600F and to make sure O2 sensor cooling at idle was not the the cause of reverting to open loop

tps .54 at idle
Knock sensor counts 86

The hot idle is at ~850-900 RPM. The target idle is 750.
Idle is a little rough, but not bad.


What could cause it to jump out of open loop!
All I have to do is barely bump the throttle and it immediately goes back into closed loop, 10 seconds later it's back into open loop.

Any help appreciated!

New:
spark plugs (copper @ 0.35" gap)
spark plug wires
cap and rotor (brass contacts)
ignition module (Napa Echlin)
CTS (AC Delco)
O2 sensor (AC Delco AFS74)
New TBI (from Carb X) came with new IAC, TPS, EGR
TBI set correctly to 650rpm per procedure
New Cat

The "new" TBI I received frim Carb X is for a 1987 7.4l (older style).
Could this cause my open loop issue?
I know the TPS sensor is a different style, not sure about the IAC.
Would the sensors have a different response than the new style TBI?
Or should these TBI be interchangeable?
The TBI is not returnable, so hoping this is not the issue.....but if it is I will need to buy the newer style....:sick:
169810
TBI Comparison.PNG
 

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‘01 Silverado 2500HD 8.1/Allison 5sp xcab long bed
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Howdy and welcome!
The ‘87 style TBI unit should not matter.
Help me understand something: you mention three different idle numbers. Your current hot idle of 850, target of 750, and set point of 650.

What’s it supposed to be?

I’m wondering about the TPS. I see that it’s reading a good value of .54V at idle, but the second you tap it, it goes back to closed loop. Do you have an analog voltmeter to check the TPS for smooth operation to WOT?

The TBI usually relies solely on the CTS to get to closed loop and I really like that you used the heated O2 sensor. I moved mine to the header bung position on my ‘95. She loved that.
 

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What we need to do is figure out which is happening first.
You say the sensor reads .9v at idle,
And you say the system goes into open loop 10 secs after going to idle.
The .9v reading is probably pushing the CPU into open loop.
0.9 volts is a rich exhaust reading, usually caused by a vacuum leak.
But why would there be a vacuum leak only at idle.
I'm not sure on OBD, but on OBD2 the computer goes into open loop at wide open throttle.
Maybe the idle position causes the computer to think it's in WOT.
If that's the case, I'd suspect the accelerator position sensor.
 

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brings back bad memories... I had a similar problem which was causing me to fail emissions on my 93 5.7 TBI suburban. It would drop out of closed loop @ idle and immediately go back into closed loop when driving. Open loop @ idle defaults to a rich A/F so it failed emissions. Long story short - my O2 sensor was "lazy" - not changing fast enough/far enough @ idle in response to the ECM moving rich/lean. Dropping out of closed loop @ idle question

The ECM controls A/F during closed loop by driving engine rich then lean while watching that O2 reaches levels it expects within a certain amount of time. If it doesn't see O2 sensor react as it expects it drops out of closed loop. I spent a lot of time trying to find details on the criteria without a lot of success. It appears the criteria for getting into closed loop and/or dropping out of closed loop varies depending on situation - eg. it's different for a cold start and a warm start, different for idle vs various driving conditions - A/F cells. Generally, I understand it that the engine has to be to a certain temp within a certain time (in my case, by watching scan data on TunerPro - 79C). Then the O2 has to get above something like 700mv and below something like 400mV in response to rich/lean changes by ECM. I think there may be other criteria but lost interest in the search after getting mine to work.

Since you have a new O2 3-wire, it would seem less likely my problem is your problem. You mention that O2 moves between 0.8 and 0.1 - is that while driving/with throttle or at idle or both? As mentioned, I think the default condition @ idle when in open loop is rich, so not sure a 0.8 in open loop at idle is unexpected. Did this just start out of the blue after working normally for a long time? or is this a problem which started after making mods/updates? Just curious - do you have stock cast iron exhaust manifolds or headers?

Since it appears to be an idle problem. I'd watch your temp, O2, and memory is there are some condition flags like O2 ready, closed loop, warm start, etc. You should see temp go up and at some point O2 start varying. Maybe you can see if it's solidly moving between limits or no. if you are using headers, I've heard of problems where they don't keep O2 hot enough at idle.

I am no expert on this, just fumbled around enough to fix my problem. As it appears you are diving into the scan data, you might find someone to help over on gearhead-efi where the tuners hang out who are very knowledgeable on this stuff. Good luck!
 

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Discussion Starter · #5 ·
brings back bad memories... I had a similar problem which was causing me to fail emissions on my 93 5.7 TBI suburban. It would drop out of closed loop @ idle and immediately go back into closed loop when driving. Open loop @ idle defaults to a rich A/F so it failed emissions. Long story short - my O2 sensor was "lazy" - not changing fast enough/far enough @ idle in response to the ECM moving rich/lean. Dropping out of closed loop @ idle question

The ECM controls A/F during closed loop by driving engine rich then lean while watching that O2 reaches levels it expects within a certain amount of time. If it doesn't see O2 sensor react as it expects it drops out of closed loop. I spent a lot of time trying to find details on the criteria without a lot of success. It appears the criteria for getting into closed loop and/or dropping out of closed loop varies depending on situation - eg. it's different for a cold start and a warm start, different for idle vs various driving conditions - A/F cells. Generally, I understand it that the engine has to be to a certain temp within a certain time (in my case, by watching scan data on TunerPro - 79C). Then the O2 has to get above something like 700mv and below something like 400mV in response to rich/lean changes by ECM. I think there may be other criteria but lost interest in the search after getting mine to work.

Since you have a new O2 3-wire, it would seem less likely my problem is your problem. You mention that O2 moves between 0.8 and 0.1 - is that while driving/with throttle or at idle or both? As mentioned, I think the default condition @ idle when in open loop is rich, so not sure a 0.8 in open loop at idle is unexpected. Did this just start out of the blue after working normally for a long time? or is this a problem which started after making mods/updates? Just curious - do you have stock cast iron exhaust manifolds or headers?

Since it appears to be an idle problem. I'd watch your temp, O2, and memory is there are some condition flags like O2 ready, closed loop, warm start, etc. You should see temp go up and at some point O2 start varying. Maybe you can see if it's solidly moving between limits or no. if you are using headers, I've heard of problems where they don't keep O2 hot enough at idle.

I am no expert on this, just fumbled around enough to fix my problem. As it appears you are diving into the scan data, you might find someone to help over on gearhead-efi where the tuners hang out who are very knowledgeable on this stuff. Good luck!
Your story of failing emmisions due to open loop @ idle are what I am currently experiencing.........


***I installed the heated oxygen sensor to keep the O2 sensor above 600F to make sure the O2 sensor cooling at idle is not the cause of the open loop issue.
The O2 heater circut wires are both Black and are connected with 12 gauge wire to a relay that provides battery power to the heater circuit.
The relay is powered by the key in ON position.
The original ECM O2 wire is connected to the Blue O2 sensor wire.
I am using the AC Delco AFS74 O2 sensor and have tried 2 of the these, along with another brand.

The # wire O2 sensor is for debugging the open loop problem only, I am sure the heater would burn out in short order if left continuously on.

I drove the truck into work this morning ~45 miles and over a mountain pass.
Here is the hot Idle data from the snap on scan tool after the drive:

RPM 875
STFT 128
LTFT 131
Base PW 1.3
Spark Advance 20 deg
O2 voltage in closed loop .1-.9V (as soon as it kicks into open loop after ~10 seconds the O2 V is .08V+)
O2 Counts 242 (during driving the cross counts go from 0 and count up to 255 and then repeat, the reading at hot idle was 242)
Map 31 (kPa) (or 9Hg) snap on scanner outputs kPa
Map V 1.1
Barometer 0.99
Barometer V 4.82

The truck will initially be in closed loop @ idle after coming to a complete stop or putting it into park.
After ~10 seconds it kicks back into open loop.
During the closed loop 10 second time period the O2 voltage is swinging normally .1-.9V (as I am watching the scan tool).
Then ~10 second later it kicks into open loop and O2 V stays above .8V.

It seems that all the sensors are in the correct value range for closed loop operation and the O2 heater circuit is working correctly, but I cannot figure out why it will consistently go into open loop @ idle after a 10 time interval.

If I slightly bump the gas pedal, then it immediately kicks back into closed loop.
If I am in gear and at a complete stop, it will kick to open loop in ~10 seconds.
If I let off the brake (no gas pedal applied, it will immediately go back into closed loop.

So tapping the throttle or taking the foot of the brake in gear (with no throttle applied) will kick it back into closed loop.

I am stumped....

Is it possible that the factory programming is set to idle in open loop?
This is the 1 ton with 10,000 GVWR.

Any replies or help are truly appreciated!
 

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Just to clarify, you used 0.8 volts and 0.08 volts in your last post, I assume you mean 0.8v
 

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Yeah, very confusing. What did the scan show as CTS/ engine temp @ idle? Curious if temp is dropping as it idles?
 

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Discussion Starter · #12 ·
Howdy and welcome!
The ‘87 style TBI unit should not matter.
Help me understand something: you mention three different idle numbers. Your current hot idle of 850, target of 750, and set point of 650.

What’s it supposed to be?

I’m wondering about the TPS. I see that it’s reading a good value of .54V at idle, but the second you tap it, it goes back to closed loop. Do you have an analog voltmeter to check the TPS for smooth operation to WOT?

The TBI usually relies solely on the CTS to get to closed loop and I really like that you used the heated O2 sensor. I moved mine to the header bung position on my ‘95. She loved that.
Glad to hear the TBI should be ok!
I have a digital volt meter and will check this tomorrow.
I am hoping to solve the open idle this weekend!
 

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One other general question - I assume it never goes into closed loop @ idle. That is, on a cold start it never gets into closed loop also. It's not just a problem after driving.

This is one of those things where you can go down the rabbit hole of trying to understand the scan data and just get more and more confused. If you look into all the flags/conditions - there are a ton of things which are potential qualifiers for closed loop - ie. there's a closed loop @ idle flag, closed loop timer flag, rpm closed loop conditions flag, etc.

I think I'd check all the basics like Cowboy/Rayvoy suggest first. Look for air leaks, bad IAC, EGR? - maybe a rich open loop condition @ idle with an air leak is reason for a little high idle and enough to screw up the rich/lean control of ECM at idle but not enough to mess up things with throttle/more airflow .

If you want to look into the rabbit hole of scan data - you need to look at the traces, not just the numbers on the scanner changing. The key is that the O2 is changing in response to the int (short term trim) and that the ECM detects that as a rich/lean condition within whatever time it's expecting. You should be able to see the int count changing in an attempt to drive the A/F rich or lean and the O2 responding. If the O2 signal is noisy - it could be causing problems on the rich/lean detection. Its not a perfect, clean system. Here's an example of going into closed loop @ idle after I finally found my problem. good luck
 

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Is it possible that the factory programming is set to idle in open loop?
Yes, it's possible. The older ECMs (like the 1227747) did that, I don't know about your 92. There's a parameter which tells it to go into CL when TPS goes above 1.92v so, that would probably explain why you go into CL when you blip the gas pedal. In the calibration I just looked at it has a 20 second delay before going into OL at idle.

Did it ever stay in CL since you owned it or is this a new issue? I remember seeing a thread over on GearHead-EFI.com about this, he had the same issue passing smog in OL but, I can't remember what the fix was.
 

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Discussion Starter · #15 ·
Yes, it's possible. The older ECMs (like the 1227747) did that, I don't know about your 92. There's a parameter which tells it to go into CL when TPS goes above 1.92v so, that would probably explain why you go into CL when you blip the gas pedal. In the calibration I just looked at it has a 20 second delay before going into OL at idle.

Did it ever stay in CL since you owned it or is this a new issue? I remember seeing a thread over on GearHead-EFI.com about this, he had the same issue passing smog in OL but, I can't remember what the fix was.
scratching my head.....I don't know if it ever went into closed loop at idle......
I had passed smog test for the last 16 years.....then failed.......
The factory pellet CAT was ~3X larger compared to the new CAT (all the new allowed CATS for this truck are small)..
Don't know if that makes any difference....
I purchased a Snap On scan tool and noticed the open loop at idle.....It failed smog with the new CAT, considered a gross polluter.......
So I started with a tune up, check for vacuum leaks (found none)....and then proceeded to change all the emissions related parts since they were all stock and nearly 30 years old.....
List of items changed out.....
New:
spark plugs (copper @ 0.35" gap) (AC Delco)
Heavy duty plug wires with ceramic ends (Napa Echlin)
cap and rotor (brass contacts) (Napa Echlin)
ignition module (Napa Echlin)
CTS (AC Delco)
O2 sensor (AC Delco AFS74)
New TBI (from Carb X) came with new IAC, TPS (Set TPS to 0.54V)
New EGR (AC Delco)
TBI base idle set to 650 (this was set by Carb X with live engine testing) I checked this per the TBI base idle procedure and it was spot on at 650rpm. (this procedure @ hot idle temp calls for jumping the A/B pins on the obd1 port, turning the key on for 30 seconds to drive the IAC to fully closed, unplug the IAC, turn off ignition, then restart the ignition and check base idle with scan tool, adjust base idle screw to achieve 650rpm +/-50rpm, turn off engine, reconnect IAC plug, disconnect NEG battery wire (to clear computer), reconnect NEG battery, start vehicle......

If the base idle is 650 with the IAC fully closed, then I suspect I do not have any vacuum leaks.......

I have been driving for a few days with the scan tool on and monitoring.....
It seems the o2 sensor is switching .1-.9 V at idle.....then creeps up to .7-.8V, then kicks into open loop....
But, the scan data is so slow I am not sure if it kicks into open loop first and then O2 is @.7-.8V or if it's the inverse....

If it is running rich then kicking into open loop I would suspect too much fuel or misfire (unburnt fuel) is driving the o2 .7-.8V
Injectors are spraying perfectly.....and have been leak tested and flow checked (within 2%).....
Plugs are new, spark plug wires are new, ignition module new, cap and rotor are new.....fuel pressure is 12-13PSI...
TBI is remanufactured.....so fuel pressure regulator should be new.....no vacuum leak......

What else could it be.....distributor.......spark [email protected] hot idle is holding steady @ 20 degree on the scan tool.....
But, could it still be a distributor issue that is causing a random misfire and driving up the O2 V and kicking into open loop....it seems to idle smooth.....

Or, is this truck just programmed from the factory to idle in open loop and the new CAT cannot clean up the emissions as well as the stock cat?

It is frustrating......

I need this truck to pass smog soon.......

Is there a way to determine if the factory set open loop at idle..?

Is there a way to program the computer to maintain closed loop at idle? or any other way to trick it to stay in closed loop at idle?


Thanks for any and all responses!
 

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Or, is this truck just programmed from the factory to idle in open loop and the new CAT cannot clean up the emissions as well as the stock cat?
The newer Cats should do a better job than the old pellet style ones. Especially in NOx, they are a 3-way instead of a 2-way.

Is there a way to determine if the factory set open loop at idle..?

Is there a way to program the computer to maintain closed loop at idle? or any other way to trick it to stay in closed loop at idle?
Yes to both. You can find your BCC (.bin) and PCM service #, then look at it with TunerPro RT to see the parameters for Open Loop and Closed Loop. If, as in my example, it has a minimum TPS for closed loop, you may be able to change that to below your .54V to keep it in CL. The only issue is "burning" your stock EPROM unless you were to get an Emulator or modify your PCM to accept a flash EEPROM.

You mentioned you have a new EGR, is the intake manifold plugged with carbon deposits from the exhaust port in the head to the EGR valve mount? If you manually open the EGR, while idling, does your engine almost die?

What part of emissions are you failing?
 

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Discussion Starter · #17 ·
Here is a photo of the ECM label:
169846



I looked up the service number and this is what I found from Tuner Pro website:


169847



I opened the file in note pad:

169848


TunerPro and TunerPro RT - Professional Automobile Tuning Software

I have no idea how to interpret the data.........

Is there anyone on here that could look at this .bin file at determine if this is factory default to open loop @ idle??

If it is, then how do I proceed.....
 

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I feel for you having gone thru similar in past. As I mentioned I went down the rabbit hole of trying to understand the data stream to debug the problem and did eventually figure it out - but as you mention it can be very confusing as it's not easy to separate cause from effect. People like PlaywithTBI and others on gearhead-efi know this stuff but I imagine it didn't come easy or quick. I only got as far as I did because I'm anal retentive and an engineer - if not the suburban I'd had been obsessed with some other puzzle.

My advice based on my experience. If you are going to try to use scan data as a tool to figure out what's going on you need to be able to capture the data stream and dump to something like excel to plot the data of interest. Looking at values on a screen wasn't helpful to me. You need to see what flags are being set, what cell you are in, how the O2 is responding in response to changes in Int/trim, etc. I'd focus on cold start idle and hot idle as they are simplest/easiest to understand. I'd watch the O2, temp, Int, closed loop flags and see if the O2 is appearing to respond to changes in trim and or trim just rails without O2 responding, trim never changes, etc. btw - I did not find this path to be instant gratification - eventually was helpful but took a while for me.

That said, at end of day the ECM/BIN isn't changing and if it used to work something has changed in engine. An input/sensor to ECM is different than it was or the engine isn't responding to directions as it was.
A few thoughts
- check and clean your grounds - there are something like 3 or 4. Firewall, by CTS (I think this one is for ECM), battery, and? Sensor inputs to ECM could be noisy and ECM having problems with getting good inputs.
  • seems unlikely but I didn't notice that you checked your fuel pressure. I had a problem once where marginal fuel pressure (it was ~ 11psi) caused idle enrichment to kick in. Mine was idling poorly so doesn't sound like your problem but marginal fuel pressure can result in weird problems as ECM has no feedback on fuel pressure.
  • I assume you checked timing with light on distributor install. There's a connector which has to be disconnected or connected, I can't remember.
  • check that the throttle adjustment on cable is not holding open the blades a bit, I assume you likely did this when new TBI install
  • perhaps try another O2 sensor, it's not impossible that a new part is not good. maybe even a 1 wire to see if you have original manifolds as they aren't too expensive - always easy to spend someone else's money...
Good luck
 

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Discussion Starter · #19 ·
"You mentioned you have a new EGR, is the intake manifold plugged with carbon deposits from the exhaust port in the head to the EGR valve mount? If you manually open the EGR, while idling, does your engine almost die?

What part of emissions are you failing? "

When I replaced the TBI/EGR I took off the intake manifold and thoroughly cleaned the manifold and the EGR port.

I don't recall the emission test fail data...... I will have to dig up the report.....

When it goes into open loop, the gas smell is strong from the tail pipe.....
 

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Here is a photo of the ECM label:
169846



I looked up the service number and this is what I found from Tuner Pro website:


169847
OK, you have a 16147060 PCM and a BCC (.bin) AXRD. In TunerPro RT you use a mask file to make sense of what's there. The file is written in Hexadecimal, that's what the $ stands for. So, you download your .bin file and the corresponding .xdf (mask). In your case it's the $85. Then you can look at your calibration to see how it's programmed. Your AXRD .bin has these parameters, including "TPS To Enable Open Loop Idle". It's set for 2.34% (red arrow) which means you'll go into OL when your TPS is below that and you're at idle speed.

169851


I'm guessing something else is causing your emissions issue since, as you said, it passed before. YMMV

FYI, I found these files on GearHead-EFI.COM

My advice based on my experience. If you are going to try to use scan data as a tool to figure out what's going on you need to be able to capture the data stream and dump to something like excel to plot the data of interest. Looking at values on a screen wasn't helpful to me. You need to see what flags are being set, what cell you are in, how the O2 is responding in response to changes in Int/trim, etc.
You can also download the corresponding definition .adx file so you can data log in TunerPro RT with an ALDL to USB cable on your laptop to see exactly what's happening when you have issues. It has playback and graphing capabilities for this.
Here's an example of when I data logged to find out why the engine was surging at around 2100 RPM while cruising. It turned out the ICM was throwing random pulses to the ECM which, in turn, changed SA and VE based on the RPMs it thought it was running. Note the RPM 4775 in red.

ICM Surging.JPG
 
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