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Thread: Intermittent Rough Running Diagnosis

  1. #16
    Fuel Injected!
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    Terrific guide, thank you mecanius!

  2. #17
    Fuel Injected!
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    Finding the root cause continues to elude me...let me share what I've done in a couple checks this evening and last.

    Replaced the TBI base gasket, torqued to specs 12 ft lbs.
    Verified the hose to the MAP holds vacuum
    Verified the hose and vacuum canister hold pressure (though they slowly leak, not enough to make this idle so high)
    Verified the PCV valve and hose don't have any cracks, and is in place
    Verified the hose from the TBI to the EGR control unit is in place.
    Verified the hose from the EGR to the EGR control unit holds vacuum

    Found that the charcoal canister hose, that runs from a port on the front of the TBI, does NOT hold vacuum. I do not know if it is supposed to!

    Then I tried setting the IAC.
    Warmed engine up and in closed loop mode.
    Turned key off, jumped the 1 & 2 terminals on the ALDL port, put key to ON for ten seconds. I have read the IAC should be buzzing at this point, but there was no noise. Unplugged IAC. Key off, jumper out. Plugged in PC, and started vehicle, IAC showed zero, and idle was still 1900. Is it possible my IAC is not working and holding it wide open? Any other checks I should try?

    Pressed on the underside of the EGR valve, which dropped the idle while I held the valve open. So that doesn't appear to be a leak...

    I also tried using Wd40 spray on the ports and intake gaskets but it didn't seem to have any effect. I then tried spraying it directly into the TBI, and it didn't have any effect there either. The fan is so strong at 2000 rpms that it's just dispersing the wd40 too quickly.

  3. #18
    Fuel Injected!
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    That's because that procedure to adjust the minimum air doesn't work for PCMs. Middle of post #9 mentions how to adjust it.

  4. #19
    Fuel Injected!
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    Good morning all. Here's a recent log file, after I swapped the memcal. When running with this one, I'm not seeing the massive number of knocks. I didn't drive it, just was idling.

    IAC shows zero at idle, which I think means it's fully closed based on the fact the idle speed is so high.
    BLM and INT both are below 128, which I believe means the engine is running rich and the PCM is trying to pull fuel out. BLM goes as low as 96, while INT is rising back up towards 128.

    This is a bit counter intuitive. I've been assuming that the idle is high due to a vacuum leak, which would indicate a lean condition, and the system would be adding fuel. The PCM should know it's supposed to be at idle since the TPS shows a zero or near zero throttle position. What do you all think about that?

    Re: IAC
    I don't follow the steps in post 9 to set IAC. It says to adjust the idle screw until you see a certain count, but that's already backed off all the way. So opening the throttle blades further would increase the engine speed further, and keep the IAC at zero, right?
    It also talks about turning off the key for 10s, which I do every time I stop troubleshooting. So according to my interpretation, my IAC should be set. I'm just uncertain....and trying to confirm everything.


    I'm going to have to take a break as I'm headed out of town tomorrow AM. I might be able to read the stock chip and post it here tonight as I ordered a part from Moates to allow me to do that.
    Attached Files Attached Files

  5. #20
    Fuel Injected!
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    Tonight I did my best to confirm no vacuum leak. Sprayed starting fluid everywhere...no indication of any leak.

    Did the battery off reset of IAC.

    I confirmed it appears my vehicle doesn't have a solonoid valve on the EVAP canister, so that acts lke a constant vacuum leak....but I confirmed with a short plugged off vacuum line that it had no effect anyways on the idle.

    I don't know what to check next, so unless the group has an idea it's going to have to go to my mechanic..what a bummer! I hope he can use a scan tool to confrm the IAC is responding. Perhaps it's stuck wide open and that's causing the problem...

  6. #21
    Fuel Injected!
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    Watching one of your logs^^^10-13-19 your IAC almost never ventures from 0. Depending on what's programmed into the PCM typically 145, Key On Engine Off, then 145 is what you should see and you should be able to see this in TPRT. IDK...going back and reading your 60 posts...I'd be suspect of the SPR bored out TBI you have. Was that ever changed to perhaps a stock unit? It appears that this engine should run on a stock tune based on the information that you provided.
    Last edited by stew86MCSS396; 3 Weeks Ago at 09:39 PM.

  7. #22
    Fuel Injected!
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    I sent in my TBI to SPR as a core, and he transferred the throttle brackets over, so I haven't tried a stock one on this engine. I do not have a spare. I agree that I didn't need to perform that upgrade....perhaps I will try a stock burn chip. I was hoping to get it back to running as well as possible with the current chip and setup, before changing things further.

  8. #23
    Fuel Injected!
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    I burned a chip with the stock bin, and validated it with Moates burn2. Chip is from Moates.

    Installed it, and the thing started up. Set the base timing back to zero.
    Went through the IAC reset procedure.

    The service engine light is now flashing on/off just over once per second. TP5 shows an error at the same rate in the bottom status bar. It also doesn't show many of the sensor values, and shows every error code has been set.

    I removed the chip and adaptor, reset it to make sure it was seated properly, tried again, same result.

    I will have to do some more searching to see what might be causing this one...

  9. #24
    Fuel Injected!
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    Last time I've seen anything like this, cable used for emulating was dirty. Blew it out with compressed air problem solved, but I think maybe an offset issue. With the wrong offset you can verify the chip all day long and it'll report back good but won't work in the 'puter. http://www.gearhead-efi.com/Fuel-Inj...Flash-and-Burn
    Also you're probably running off limp mode hence no sensor readings in TPRT. For some reason with PCMs, after resolving whatever problem it is, I could never get the CEL to go away without having to disconnect the battery.
    Last edited by stew86MCSS396; 3 Weeks Ago at 12:02 AM.

  10. #25
    Fuel Injected!
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    Good morning! Thanks Stew for the tip, this community has a wealth of knowledge. I'd read that page at some point in the past, but didn't remember it when I was burning a chip. The offsets were correct though, since my size bin was the default Moates used in his SW.

    This led me on a full search for Moates and chip burning posts. Using the moates guide, and realized I needed to use erase chip and verify blank steps before programming. Since the chip had been verified, this was a bit surprising, but that appears to be the only thing that changed which allowed it to work. Or there is something else happening that I haven't found yet.

    I now have the stock chip burned and the vehicle running, with the codes cleared. It still has the problematic high idle and shaking. However, the INT and BLM values with the stock chip at idle and part throttle are much closer to 128. Knock counts are lower.
    Attached Files Attached Files

  11. #26
    Fuel Injected! vilefly's Avatar
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    Has anyone checked if the IAC valve is even moving in the real world? It should seat itself and open back up once upon KOEO conditions. Another problem is mismatched pintle styles that do not seal properly against the throttle body idle passage. They all swap. Try plugging off the inlet port to the IAC, and see if the rpm drops.....use a rubbermaid glove or something, whatever works. If no change, then remove and plug off all non-critical vacuum ports, remove the pcv valve from the valve cover, start engine, and block off the pcv vent on the other valve cover. Now block off the pcv valve grommet in the remaining valve cover with your hand, and see if the crankcase still has vacuum. If it does, then you have an intake gasket problem.

    The only other thing is setting the tps to exactly .50v at closed throttle, and have the timing set to stock timing at idle (on the chip, too), but I suspect this has already been done.
    Last edited by vilefly; 3 Weeks Ago at 08:02 PM.

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