Intermittent O2 reading

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Dont fake it break it, May 10, 2020.

  1. dyeager535

    dyeager535 1 ton status Premium Member

    Joined:
    Dec 13, 2000
    Posts:
    29,234
    Likes Received:
    1,104
    Location:
    Not Roy WA
    Not in my experience.

    I use cruise control, and I've noticed that the throttle sometimes will be jerky (off/on/off/on) during cruise. I dont have the vaccum reservoir, but if I did, I'd use it. I'm certain the direct, fluctuating vaccum is what the issue is.
     
  2. Dont fake it break it

    Dont fake it break it Registered Member Premium Member

    Joined:
    May 21, 2019
    Posts:
    56
    Likes Received:
    0
    Location:
    Visalia
    So the latest for anyone interested is that based on the repeated suggestions from BP, Big Block, Yeager, and everyone else, I looked up a thread here on adjusting the fuel pressure at the regulator. It was 13 in line after the filter but after rebuilding the regulator with no affect PLUS two completely different springs, I modified the regulator cap to increase the spring pressure. Sadly I had taken back the pressure tester I borrowed but there is a very noticeable decrease in code 44 lean condition errors. I still got a couple but nothing like before.Thinking about giving that screw a couple more turns to see if it goes away completely.
     
  3. dyeager535

    dyeager535 1 ton status Premium Member

    Joined:
    Dec 13, 2000
    Posts:
    29,234
    Likes Received:
    1,104
    Location:
    Not Roy WA
    13 is the upper spec of TBI, it's supposed to run 9-13 PSI, but most at 9 say it's noticeably worse.

    If this "fixes" the problem, if the pressure gauge is to be trusted, I'm inclined to think something else is amiss.
     
  4. bp71k5

    bp71k5 3/4 ton status Premium Member

    Joined:
    Mar 31, 2006
    Posts:
    7,698
    Likes Received:
    1,716
    Location:
    Knoxville, TN 37922
    Build(s):
    1
    I dunno. I had to bump mine up over 16 on a stock-ish motor but also had a chip burned for it. For sure check the scanner to see what it says though.
     
  5. Dont fake it break it

    Dont fake it break it Registered Member Premium Member

    Joined:
    May 21, 2019
    Posts:
    56
    Likes Received:
    0
    Location:
    Visalia
    Maybe I’m missing something simple on the pressure test but how do you test what the regulator is doing? I’m hooking up the pressure tester after the filter because I can’t even get the connection up against the throttlebody. Not sure what would change between there and the filter anyway unless there’s a noticeable constriction. I’m thinking the spring in the regulator was too soft to begin with or worn out. The standard rebuild kit doesn’t exactly say what pressure the spring is. Either way, it’s nearly achieved the desired result. Only two check engine lights in the last couple days were previously I would’ve gotten eight or more
     
  6. dyeager535

    dyeager535 1 ton status Premium Member

    Joined:
    Dec 13, 2000
    Posts:
    29,234
    Likes Received:
    1,104
    Location:
    Not Roy WA
    No, pressure after the pump up to the regulator should be 13PSI (or whatever the pump is capable of pushing, plus regulator "setting"). There is nothing wrong with measuring where you are. As a matter of fact, on my TPI setup, I decided to measure before and after the fuel filter to see if there was any drop across the 10 year old filter. There wasn't FYI.

    The entire system is "tuned" for that 9-13PSI spec. The ECM is assuming the injectors are being presented with 13PSI (I believe, it's got to be something between 9 and 13), so it calculates how long an injector needs to be open based on how much fuel the injector can flow at a specific PSI. If you bump the PSI up beyond what the ECM thinks the injector is actually getting, under closed loop conditions this is not an issue...the O2 will see rich, and dial back the injector pulsewidth to get the mixture to where it needs to be.

    But under open loop conditions (acceleration, cold engine, idle if programmed that way) then the engine will simply run rich. It's ignoring the O2 sensor, and the injectors are adding too much fuel (again because the ECM calculates the injectors need to be open X time, as the PSI is coded as spec'd by GM). This is definitely a rabbit hole, but this is why using fuel pressure adjustment alone can be problematic. Upper spec is 13PSI. The coding in the ECM is certainly not calculating based on some higher number. So if it won't run at 13PSI without issue, there IS something else going on. I mean it could be as simple as a mis-calibrated fuel pressure gauge, which could mean nothing is wrong. If it solved the problem and you see no adverse affects, it really doesn't matter lol.
     
  7. Dont fake it break it

    Dont fake it break it Registered Member Premium Member

    Joined:
    May 21, 2019
    Posts:
    56
    Likes Received:
    0
    Location:
    Visalia
    That’s a very solid response, thank you. I get what you’re saying about the vehicle being tuned for 9 to 13 PSI. The fuel pump in it, can’t remember what brand, advertised a higher rate but pretty much 13 to 14 is what it’s measuring. It’s a loaner gauge from AutoZone so questionable accuracy. I hear what you’re saying about running rich in an open loop but thankfully that doesn’t last too long anyway and certainly won’t be in that condition during the smog test. It makes sense that in a closed loop it will just see it as running rich if that condition exists now and make the adjustment. Sure does sound like adding fuel pressure is compensating for something like you suggest. Fact is, I do know that a problem still exist because I do still get an occasional light. Generally it’s at low speeds, either when I’m pulling out of the driveway or when I get off the highway onto a residential road. But that’s the funny thing about this gremlin, it doesn’t happen all the time even under identical conditions. Per your last line, I’ll take what I can get.
     
  8. dyeager535

    dyeager535 1 ton status Premium Member

    Joined:
    Dec 13, 2000
    Posts:
    29,234
    Likes Received:
    1,104
    Location:
    Not Roy WA
    Unfortunately my luck with the Check engine light has been that if it comes on and comes off, sooner or later down the road, something is going to bite me in the ass lol.

    You might be lucky though. It could be as simple as a bad connection somewhere, and it's so infrequent an issue, and non-detrimental, that if it stays like that, it's not really hurting anything.

    When the light gets more frequent, or other problems develop, at least then you'll have more to go off of. It's frustrating when a problem is not repeatable or consistent. It took me a year to figure out my problem was related to the fuel pump. Of course, the truck is only driven a few times a year, but still...intermittent problems, even when serious, can be exceptionally difficult to diagnose.
     
  9. Dont fake it break it

    Dont fake it break it Registered Member Premium Member

    Joined:
    May 21, 2019
    Posts:
    56
    Likes Received:
    0
    Location:
    Visalia
    Yeah, this is my daily so it’s really been stressing me out. Even though it had the occasional problem before I did the rebuild, the real kick to the jewells is that I somehow made it worse. I was reading another thread, I think the high idle stall thread, and it was suggested there was a potential vacuum leak due to a cracked intake.
    I seem to recall a set screw on the underside of the intake with a hairline crack coming from the threaded hole. I also read that if enough material is removed when heads are machined, that could change the alignment and angle of the intake seal to the head.
     
  10. Wes Harden

    Wes Harden 1/2 ton status Premium Member

    Joined:
    May 28, 2005
    Posts:
    1,294
    Likes Received:
    1,454
    Location:
    Southern Califorina
    Adjusting the fuel pressure regulator at the tbi will not change the fuel pressure the pump can deliver. The fuel pressure regulator is on the tbi so it regulates pressure at the injectors. I think this is the reason gm never offered a test port for fuel pressure on tbi. If adjusting the regulator gets you the results you after go for it.
    If your eyes start burning in open loop or the milage drops off you went to far.
    1 question did the Bosch o2 come with a blank wire and you needed to add the end?
    Good luck.
     
  11. Dont fake it break it

    Dont fake it break it Registered Member Premium Member

    Joined:
    May 21, 2019
    Posts:
    56
    Likes Received:
    0
    Location:
    Visalia
    “If your eyes start burning..you went too far”:rotfl:
    Yeah the 02 came with the connector. I actually removed it and wire nutted it to the ECM wire because that connector was questionable. I get what you’re saying about the regulator. I’m just questioning the spring that comes with a rebuild kit. It’s longer than the previous one So kind of hard to compare the pressure within the space of the regular cap. I didn’t really have anything that I knew was 14 pounds to sit on the cap to push the spring down to a flat surface in an effort to determine the difference in resistance between the two springs. I do shit like that tho. No wonder everything takes so long. :dunno:
     

Share This Page