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Username Post: Question about 02 sensor readings -93 burb 1500        (Topic#358412)
Mjricha 
Poster
Posts: 80

Reg: 11-30-17
10-11-19 07:47 AM - Post#2777651    

Hello all,

I've been having some issues with my 93 suburban 1500 350. Mainly bad gas mileage and bouncing rpms at idle (going high not low, 600-1200rpm at idle).

I didn't have any codes, so I hooked up the aldl scanner and noticed some things.

The oxygen sensor and closed/open loop readings seemed off to me, so I started researching. From what I found, it seems that the O2 sensor should bounce around between .1 and .9 during operation and this should close the loop. The only time the loop should be open is at wide open throttle. It seems that my 02 sensor is for lack of a better term "losing signal". As at random times during driving, the 02 will stop bouncing around, sit at about .35v and the loop goes open. And stays that way. Then sometimes it goes back to working normally, bouncing around. Of course, while I'm driving today I'm getting no bouncing rpms so I couldn't really see what was going on when that was occuring. So my questions are:

What could cause the losing of the signal of the 02 sensor? The 02 sensor is relatively new and is a GM part.

Could this contribute to the bouncing of the rpms at idle? The tps read normal, and it's voltages moved up and down slowly during the pressing and releasing of the throttle.

Am I correct in my thinking of how the 02 sensor should work with regards to the open/closed loop? I'm assuming this is where my gas milage issue is coming from.

Thanks!



 
Shepherd 
Frequent Contributor
Posts: 1863

Loc: Lake George, NY
Reg: 11-11-15
10-11-19 07:56 AM - Post#2777653    
    In response to Mjricha

Closed loop occurs when the engine reaches a pre-programmed temperature and the o2 sensor is heated. Prolonged idling can cause the sensor to go to open loop. .35 is probably the default open loop input. If the engine runs at the proper temp and there are no exhaust leaks, you either have a bad sensor or the wiring to it is the problem.



 
Mjricha 
Poster
Posts: 80

Reg: 11-30-17
10-11-19 01:06 PM - Post#2777678    
    In response to Shepherd

I was actually wondering about the wiring to it. It's one wire. But since the scanner is actually reading the voltages that makes me think that the wiring is fine. However I know that even with the 02 sensor unplugged it should still read .45 ish right? This leads me to think that maybe there's a grounding issue. Like it works fine but if it becomes ungrounded that's when it just acts like there is no 02 sensor and just goes into open loop. But there's no ground wire since it's just one wire. I'm going to look and see if there is a way to test the sensor itself.



 
Shepherd 
Frequent Contributor
Posts: 1863

Loc: Lake George, NY
Reg: 11-11-15
10-11-19 01:41 PM - Post#2777680    
    In response to Mjricha

The ground is the fitting in the exhaust, but make sure you have good engine grounds of course.



 
junkman104 
Frequent Contributor
Posts: 1424
junkman104
Age: 60
Loc: Murphy N.C.
Reg: 01-25-14
10-11-19 04:51 PM - Post#2777692    
    In response to Shepherd

Short and long term fuel trim numbers are helpful also.



 
Sabino 
Member
Posts: 105

Reg: 07-16-05
10-11-19 07:50 PM - Post#2777702    
    In response to junkman104

It uses CTS sensor to determine if temp is high enough to go into closed loop. I believe criteria is 79C. If you are data logging you can see the CTS and ensure it’s good. CTS is also used in engine mapping so if it’s bad it can mess up a lot of things.
Once closed loop is enabled it drives engine rich & lean and expects to see O2 vary accordingly. If O2 doesn’t get above 700mV and below 400mV, it drops out of closed loop. I had a problem once where my 93 was dropping out of closed loop at idle and it was due to a “lazy” O2 sensor which would vary, but not quick enough & far enough. Replacing O2 fixed it.

That said, I wouldn’t think a bad O2 sensor will cause idle problems in terms of rpm varying. Some people disable closed loop. I think it’s mostly emissions, but not sure.

I’d check your CTS. It’s not same sensor as is used for gage so even if gage looks right, you can still have drivability problems due to bad CTS

65 Belair 350
68 Camaro 327
93 Suburban 5.7L TBI


 
Mjricha 
Poster
Posts: 80

Reg: 11-30-17
10-12-19 09:07 AM - Post#2777726    
    In response to junkman104

So I'm looking at my scan app (aldl Droid) and I don't see any options for showing short term/long term fuel trim. Would they happen to be called something else? I tried googling it and didn't come up with anything.



 
Shepherd 
Frequent Contributor
Posts: 1863

Loc: Lake George, NY
Reg: 11-11-15
10-12-19 09:08 AM - Post#2777727    
    In response to Mjricha

Back in the day it was called block learn.



 
Mjricha 
Poster
Posts: 80

Reg: 11-30-17
10-12-19 09:18 AM - Post#2777728    
    In response to Shepherd

There is one called the block learn Address change. Also there's rich/lean and slow rich lean. Could any of those be it?



 
Mjricha 
Poster
Posts: 80

Reg: 11-30-17
10-12-19 09:23 AM - Post#2777731    
    In response to Shepherd

Also block learn multiplier and block Learn cell



 
65_Impala 
Very Senior Member
Posts: 4326

Reg: 12-29-02
10-13-19 06:03 AM - Post#2777770    
    In response to Mjricha

The last two. BLM and BLM cell. BLM is the trim adjust value. The numbers make up a small rpm vs vacuum table so BLM cell is which cell it's currently using.

I'm not sure you have a problem since it is switching to open loop and the O2 quits changing at the same time. That's what happen when the engine operates in certain modes. Your sources were wrong on open loop only happening during wide open throttle. It will run in open loop while EGR is active, which might be what you are seeing.



 
Mjricha 
Poster
Posts: 80

Reg: 11-30-17
10-13-19 11:38 AM - Post#2777788    
    In response to 65_Impala

Ok so here are the numbers when the truck is running properly (or seems to be)

Idle:
coolant temp: 93-95
02: .1-.9
Block learn multiplier: 95
Block Learn cell: 19

Driving:
Coolant temp: 90-94
02: .1-.9
Block learn multiplier:105
Block learn cell: 5-9

Of course the truck has been driving semi decent the whole time I've been data logging. The rpm did start to surge once, but nowhere near as bad as it does sometimes. It was just a second but I noticed that the tps voltage was bouncing between .6 and 1.1v and the 02 was high, between .8 and .9v.

Just in case it matters, the pcm (gm brand) is new and the transmission has been rebuilt as of 6 months ago. The trans always shifts hard into 2st gear after the truck warms up. Transmission shop told me that the computer was making the hard shift into second. Once I replaced the computer it was doing the same thing. I don't know if these are independent problems or if they are all connected. I know the tps can effect the transmission that's why I thought it might be related.





 
Chevytech 
Subject Matter Expert - Senior Member
Posts: 3837
Chevytech
Loc: Twin Cities, Minnesota, U...
Reg: 04-25-04
10-13-19 12:40 PM - Post#2777795    
    In response to Mjricha

The readings you posted show the computer has determined there is to much fuel getting in the engine and is leaning out the mixture.

For those of you that are wondering why you are not getting replies to your thread:

Did you give the model, year, engine, fuel system type, and transmission information?

If your vehicle has been altered from stock, let us know about that too.


 
Mjricha 
Poster
Posts: 80

Reg: 11-30-17
10-13-19 12:58 PM - Post#2777796    
    In response to Chevytech

Ok sorry for the dumb questions, I'm just trying to learn! After you mentioned those, I went and started reading. So basically if my BLM is less than 128 the truck is running rich. So since I have 90 -105 the ecm is trying to lean the mixture out, correct?

So if that's the case, then it is the mix actually getting leaned out? Because the rich flag on the scanner and my mpg say otherwise. Also my exhaust smells like gas. From my reading I also understand the block learn cell as well. To me this is still pointing at the 02 sensor right? Because the BLM uses the 02 sensor to make adjustments.



 
Mjricha 
Poster
Posts: 80

Reg: 11-30-17
10-13-19 01:20 PM - Post#2777798    
    In response to Chevytech

Could it be fuel pressure being too high as well? I don't have a way to test it because there is no where to put a pressure gauge. I'd have to buy a kit and install a shrader valve on the fuel line.



 
Mjricha 
Poster
Posts: 80

Reg: 11-30-17
10-13-19 02:07 PM - Post#2777803    
    In response to Mjricha

So just to start with something simple I checked all my grounds and everything is fine. I had previous electrical issues and I made all new grounds then. I took the 02 sensor out and sanded the crap out of the hole where it goes into the exhaust. Then I screwed it back in. So just for fun I left it unplugged and started the truck. The 02 reads .45 (because it's not reading anything) and the BLM was Pretty perfect at 126.

Then when I plugged the 02 sensor back in, it all went downhill from there. Rpms surging, tps between .5 and 1.0, 02 sensor going between .8 and .9 and staying there, BLM back down to 100-105, staying in rich condition.

So at this point it has to be the oxygen sensor right?



 
Chevytech 
Subject Matter Expert - Senior Member
Posts: 3837
Chevytech
Loc: Twin Cities, Minnesota, U...
Reg: 04-25-04
10-13-19 02:41 PM - Post#2777805    
    In response to Mjricha

  • Quote:
Ok sorry for the dumb questions, I'm just trying to learn! After you mentioned those, I went and started reading. So basically if my BLM is less than 128 the truck is running rich. So since I have 90 -105 the ecm is trying to lean the mixture out, correct?

Correct

  • Quote:
Could it be fuel pressure being too high as well?

Fuel pressure that is to high will make it so more fuel is injected.

  • Quote:
So just for fun I left it unplugged and started the truck. The 02 reads .45 (because it's not reading anything) and the BLM was Pretty perfect at 126.

This is normal.

  • Quote:
02 sensor going between .8 and .9 and staying there, BLM back down to 100-105, staying in rich condition.

If the oxygen sensor voltage stays between .8 and .9 volts the system should keep trimming fuel in an attempt to get the oxygen sensor voltage down, and the block learn number should go low.

If I were working on the truck the first thing I would do is test for voltage with the truck running with the negative meter lead on the battery negative post and the positive meter lead on the engine block. There should not be more then 0.1 volt.

Does this have a Bosch oxygen sensor or is it OEM?

For those of you that are wondering why you are not getting replies to your thread:

Did you give the model, year, engine, fuel system type, and transmission information?

If your vehicle has been altered from stock, let us know about that too.


 
Shepherd 
Frequent Contributor
Posts: 1863

Loc: Lake George, NY
Reg: 11-11-15
10-13-19 02:55 PM - Post#2777810    
    In response to Chevytech

Cheap enough, just replace it.



 
Sabino 
Member
Posts: 105

Reg: 07-16-05
10-13-19 05:39 PM - Post#2777822    
    In response to Shepherd

I would agree that replacing O2 just to eliminate that as a variable would be worth it given it’s easy to replace and about $12.

I’m confused on BLM cell @ idle x 19? I thought there are 16 cells and @ idle you should be in BLM cell 0,4,8, or 12.
Typo?
What is TPS reading @ idle?

65 Belair 350
68 Camaro 327
93 Suburban 5.7L TBI


 
Mjricha 
Poster
Posts: 80

Reg: 11-30-17
10-13-19 09:40 PM - Post#2777835    
    In response to Chevytech

Ok I will check this. Just to make sure I'm understanding correctly you want me to check and see if the block has a charge due to a positive short somewhere to the engine block?

The 02 sensor doesn't seem to have any markings on it so I don't think I can tell what brand it is. This makes me think I must have ordered a crap brand one. I usually try to get all GM stuff. I'll just go buy a GM one and also test the other stuff too.

Should I get the Bosch or the acdelco one?

Thank you for the info!



 
Mjricha 
Poster
Posts: 80

Reg: 11-30-17
10-13-19 09:42 PM - Post#2777836    
    In response to Sabino

Sorry yes that was a typo it was supposed to be 16.

And tps sits at .5 at idle if the engine is not surging/rpms bouncing. It goes between .6 and 1.1 when it decides to surge. It freaks you out if you come to a stop it feels like you're pressing the gas and the brake at the same time. It doesn't do it all the time but when it does this is what it does.



 
65_Impala 
Very Senior Member
Posts: 4326

Reg: 12-29-02
10-14-19 06:34 AM - Post#2777850    
    In response to Mjricha

The TPS voltage should not change when the throttle pedal is not moving.

Besides checking wiring and sensor, check the throttle body for a worn housing where the shaft passes through and the cable setup.



 
Chevytech 
Subject Matter Expert - Senior Member
Posts: 3837
Chevytech
Loc: Twin Cities, Minnesota, U...
Reg: 04-25-04
10-14-19 09:59 AM - Post#2777866    
    In response to Mjricha

  • Mjricha Said:
Ok I will check this. Just to make sure I'm understanding correctly you want me to check and see if the block has a charge due to a positive short somewhere to the engine block?

The tes will show if there is a bad ground causing a positive charge on the block that would mess with the oxygen sensor reading.

  • Quote:


Should I get the Bosch or the acdelco one?



I would stay away from Bosch. AC Delco would be good.


For those of you that are wondering why you are not getting replies to your thread:

Did you give the model, year, engine, fuel system type, and transmission information?

If your vehicle has been altered from stock, let us know about that too.


 
Sabino 
Member
Posts: 105

Reg: 07-16-05
10-14-19 09:03 PM - Post#2777917    
    In response to 65_Impala

Agree with 65-Impala. If TPS is moving around when you have your foot off the throttle that should be fixed.
If you do have TPS jumping around, if it moves enough the ECM will bump into another BLM cell because it thinks you are moving the throttle and another engine map is needed for that condition. Does BLM cell change when the TPS jumps around?

Still confused on a BLM cell of 16. I think this is correct for our years. https://www.gnttype.org/techarea/faq/BLMINT.html

Perhaps it’s mapping a 0 into 16. What datalogger are you using?

65 Belair 350
68 Camaro 327
93 Suburban 5.7L TBI


 
65_Impala 
Very Senior Member
Posts: 4326

Reg: 12-29-02
10-15-19 03:19 AM - Post#2777922    
    In response to Sabino

19 is likely correct. Some PCM had specific idle cells.



 
Mjricha 
Poster
Posts: 80

Reg: 11-30-17
10-15-19 05:17 PM - Post#2777976    
    In response to Sabino

Ok so I double checked the log and it was actually 19. Sorry about that.

I ordered a acdelco 02 sensor it should be here Friday or Saturday.

I voltage checked between the negative battery terminal and the engine block. It was 0.00 - 0.01.

Let me ask this I guess. Is there anything electronically that could move the tps? other than physically like the throttle body and the cable? The cable seems taught but not too tight. The pin that goes through the throttlebody doesnt wiggle or seem loose. The throttlebody is clean as well. I went ahead and voltage tested the tps with the multimeter and it moves smoothly through the voltages when with no gaps in voltage so it really doesn't seem like the tps is bad. Anything else I can check?

I am using the data logger that comes with the Aldldroid application.

What is making this really difficult is that I can't do anything to make the voltages on the tps jump around. It does it seemingly completely at random. I drove the truck around for about an hour today and it didn't do it at all. When it does do it it's only for a minute or two at a stop light then the next stoplight it's fine again. I'm just going to start logging every time I drive that way I don't miss it.



 
CowboyTrukr 
"8th Year" Silver Supporting Member
Posts: 4273
CowboyTrukr
Loc: Salt Lake City
Reg: 06-20-09
10-15-19 10:12 PM - Post#2777993    
    In response to Mjricha

No, the TPS is strictly a position transducer. Nothing moves it except the throttle plate shaft.

Greg

'95 K1500 Z71 EC Short Step 5.7L+0.040/NV3500
'00 Explorer XLT 4.0 V6 Auto
'94 K2500 5.7 NV4500 ECLB - SOLD
‘87 GMC S15 SCLB 4.3 Auto - SOLD

"The only thing necessary for evil to triumph is for good men to do nothing" Sir Edmund Burke


 
Sabino 
Member
Posts: 105

Reg: 07-16-05
10-16-19 08:27 AM - Post#2778026    
    In response to 65_Impala

  • 65_Impala Said:
19 is likely correct. Some PCM had specific idle cells.


Interesting, thanks I didn’t know that.
@Mjricha - do you happen to know what ECM you have and bin? If not - don’t go looking, it’s just a curiosity for me.
Can you confirm ECM is original to your truck? And what trans do your have - 4l60e, 700r4?

As to problem - if TPS is jumping up to 1.1v randomly at idle and when that happens the engine surges it sure seems to me that that could be the problem. If you see BLM cell change when the TPS jumps then that would confirm a change in engine mapping. It’s always easy to spend other people’s money but perhaps replacing TPS is next step after O2.

On cable, I would back it off until slack, verify TPS doesn’t change. Then tighten just until slack is out and no slop in pedal.


65 Belair 350
68 Camaro 327
93 Suburban 5.7L TBI


 
Mjricha 
Poster
Posts: 80

Reg: 11-30-17
10-17-19 07:41 AM - Post#2778102    
    In response to Sabino

So when we got the trans rebuilt and it still shifted hard into 2nd, the mechanic said that this means that the trans is getting high pressure and that's controlled by the computer. It's a 4l60e. So we got a new computer from rockauto and it is the acdelco one listed for the truck acdelco # 21691.

I'll go ahead and adjust the cable and check the voltages. Its not a bid deal if I have to order another tps they're not too expensive.

Edit: ordered acdelco original gm equipment tps. Itll take forever to get here (next Thursday) but that'll give me time to test everything else.

But this is what's really confusing me. So today, the engine was surging so I plugged the scanner in real quick and this is what I got:

Tps: .5-.8
BLM : 158
BLC : 17
02: .38-.42

This seems like the opposite problem from before. The BLM had been really low for a long time, never seen it higher than 110 and all of a sudden it's up to almost 160 and stayed there the whole rest of the time I was driving.

I really want to try to avoid just being a parts changer and actually figure the problem out but other than swapping out the 02 and the tps I don't know what else I can do/test at this point



Edited by Mjricha on 10-17-19 07:47 AM. Reason for edit: No reason given.

 
Sabino 
Member
Posts: 105

Reg: 07-16-05
10-18-19 07:07 AM - Post#2778179    
    In response to Mjricha

it can be confusing trying to figure out what’s going on from a snapshot of a few data points. There are many more variables the ECM uses and people tuning engines capture traces of a lot more variables to figure out what’s going on. Check out gearhead-efi for a flavor of the complexity.

I would focus on fixing the obvious bad first. In your case, the TPS should not jump around sitting @ idle with foot off throttle.

I would also recommend verifying the basics - fuel pressure and timing are correct. There is great link by someotherguy above on how. https://www.chevytalk.org/fusionbb/showtopic.php?t...
The ECM has no direct feedback on timing or fuel pressure and if they are not right it can cause all sorts of weird problems. For example, I had a problem once where it would surge at idle as well as hard/late shifting. Turns out it was low fuel pressure. At idle the low fuel pressure would cause engine to almost die, then idle enrichment would kick in and run rpm up, kick out and repeat. Driving, because of low fuel pressure I had more throttle at any speed than normal and ECM thought I was trying to accelerate so delayed shift. Replaced fuel pump and everything was resolved. I’m not suggesting fuel pressure is your problem - just that it’s good principle when chasing problems to verify the basics are right to prevent chasing your tail.

As to your question on lean instead of rich - looks like you’re in cell 17 where you were in cell 19 before? Mappings are different for each cell so not sure you can make comparisons. Ideally, you’d like to verify you are in an idle cell when idling. If you knew the bin in your ECM someone on gearhead-efi could probably tell you - but i dont think it’s really needed for your case with flakey TPS. btw- did you put prom from your old ECM into your new?

If you decide to go down the rabbit hole of trying to understand how ECM works in more detail. TunerPro is a good tool, but it’s a time sink.

65 Belair 350
68 Camaro 327
93 Suburban 5.7L TBI


 
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