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MALFUNCTIONING IAC???

Discussion in 'FAST Support Forum' started by 78 F150, Aug 9, 2018.

  1. fabr

    fabr Member

    Joined:
    Jan 23, 2017
    In general,things like ignition timing and target AFR's can influence vacuum/pressure sensor readings to where the AFR's can swing all over the place. Even a single miss will influence the ECU interpretation of what is actually happening and cause it to think it is lean and then it adds additional fuel to correct that but then it will see it is now rich and pull fuel. Another miss and it starts over, back and forth ,back and forth. This will create a wild swing in AFR's that could be misdiagnosed as a vac leak. Anyway,as AA said,wait till you get it back together but in the meantime very carefully inspect your plug wires both visually and with an ohm meter,cap (inside and out)for ANY signs of carbon tracking ,same for the rotor. Look extremely closely for even very faint evidence of tracking that may only look like a very faint pencil line. If there is ANY corrosion/rust on ANY of the metallic parts inside the dizzy,find the source and remedy.
     
  2. fabr

    fabr Member

    Joined:
    Jan 23, 2017
    AA brings up a good point-possible leaking or intermittent leaking valves. When you get the heads back I hope they have had a fresh valve job done. If not I would highly suggest you, old school style, lap the valves. I do all my heads even though the modern valve/seat machines are very , very good I still find a quick lap reveals the small amount of runout they leave and lapping guarantees a very good seal. After you get the truck running and cooled down,I would highly suggest you do a leak down test to verify all your intake and exhaust valves are not leaking. Of course ,the valve guides must be in good condition as well or perfect seating is at best hit and miss. Of course valve springs must also be in good condition especially seat pressure.
     
  3. fabr

    fabr Member

    Joined:
    Jan 23, 2017
    To your question on the regulator and why the vac line HAS to be used above idle. It doesn't. It CAN be used without it ever being used. The reg is a vac referenced style though. If you use the vac line(and I would) as manifold vac drops under hard acceleration it raises the fuel pressure in proportion to the vac drop. The more the vac drops,the higher the fuel pressure. Why is this a good thing you ask. The reason why is that under hard acceleration which is usually the highest fuel volume needed it is a means of trying to guard against low fuel supplied volume. It works quite well and the ecu adjusts the fuel calcs to the fuel pressure it sees instantly as the pressure raises/falls. Yes,you CAN do away with the vac line and it will run just fine unless your pump is on the ragged edge of capacity at WOT. Again,I suggest you use the vac line to it regardless.
     
  4. gremlinmt

    gremlinmt Member

    Joined:
    Mar 13, 2013
    I was always under the impression that vac reference was to keep pressure drop across the injectors the same regardless of engine vacuum.
     
  5. fabr

    fabr Member

    Joined:
    Jan 23, 2017
    nope.
     
  6. gremlinmt

    gremlinmt Member

    Joined:
    Mar 13, 2013
    Last edited: Dec 24, 2018
  7. J.K.

    J.K. Member

    Joined:
    Aug 7, 2016
    It confused me aswell. The way I understand it, in this particular system we set pressure to 43psi with vacume disconnected and capped or engine not running. Reconnect vacume after. From what I understood it is to "reduce" pressure at low engine speeds where vacume would be present, otherwise it could run richer than wanted at slower engine speeds. The only issue I can see you having is it running slightly richer at idle due to higher pressure at idle. The vacume would pull the pressure down at idle so the injectors can control fuel requirements better at slow speeds where there's a lower fuel demand.
     
    Last edited: Dec 24, 2018
  8. gremlinmt

    gremlinmt Member

    Joined:
    Mar 13, 2013
    Well, not exactly true. Engine vacuum is variable just about everywhere and even varies depending on the weather. Without a vacuum reference source, your ECU is constantly going to be fighting those variables. Running it for a period of time for testing vacuum leaks is one thing, but it's just making your ecu correct for non-linear changes that it can't measure with it's sensors.
     
  9. fabr

    fabr Member

    Joined:
    Jan 23, 2017
    It has nothing to do with pressure drop across the injectors. Yes it does lower pressure at low engine speeds and then as vac drops it raises it to the max of static pressure setting. It is to lower the load on the pump at low demand . The system does not demand that it use a vac referenced regulator. The only thing that cares about lowering the pressure at low flow demands is the pump that is happier not running at full pressure when not needing to. Soooo,at low vac/WOT it raises pressure as manifold vac drops. The ECU is doing as I described above and recalculating pulse widths based on the actual fuel pressure. The ECU doesn't care if it is run at higher or lower than 43psi as it can easily compensate for the actual pressure it detects within reason. Therefore he CAN run without the vac reference but ,as I said,I would not.

    The reverse is true when running piggyback injectors on a boosted engine and running a boost referenced rising rate regulator.

    To be clear,a vac referenced regulator lowers pressure below the set point at higher idle vac map areas and the ecu compensates for the lower pressure than the set point.

    A rising rate boost referenced regulator on the other hand will not lower the pressure when vac rises but will raise the pressure as boost pressure rises.

    Two completely different applications and 2 completely different regulators but both types adjust the pressure to lower the pressure/load on the pump when not needed.

    Again,vac or boost referenced regulators are not mandatory but are used on many systems to save pump wear AND to lower noise levels of the pump when idling around. The ecu doesn't care. Yes,if tuned with either type of regulator and the regulator malfunctions or the vac/boost reference line fails it will not run right but if tuned without a vac/boost referenced regulator it will run quite happily.

    Many years ago,must be about 30 years ago, when Holley first came out with their 2D primitive injection no vac referenced regulator was used and the high pitched noise would drive a person nuts. Hence,the use of vac or boost referenced regulators became almost mandatory if the ecu doesn't control pump run speed to accomplish the same lower load/noise from the pump result.
     
  10. fabr

    fabr Member

    Joined:
    Jan 23, 2017
    Gremlin,the ecu is constantly recalculating for different fuel pressures with or without a vac referenced regulator. There is no such thing as constant fuel pressure no matter the system. The only reason for a vac referenced regulator on non boosted systems is to lower the demand on the pump when not needed(idle/cruising) and in turn making for a longer lasting/quieter pump. The ecu doesn't care if there is a rising/lowering or a constant rate regulator used. All it knows is it was tuned at a given fuel pressure base line and any reductions/raises in the pressure. One qualifier for this would be when using huge injectors and as JK said it would make job of tuning/controlling the huge injectors much easier for the ecu with a vac referenced regulator that lowers the pressure and in turn lowers the fuel flow through the injector for a given pulse width. We must remember that flow is proportional to pressure.

    We are aware that the 8 injectors used in the FAST throttle body do not all fire at all rpm's right? They are staged. This is how FAST deals with the fuel flow requirements of 1200 hp capable claims yet can be used on much lower hp engines with no injector changes. 4 injectors do primary duty and then the other 4 kick in when the first 4 cannot supply enough fuel.
     
  11. fabr

    fabr Member

    Joined:
    Jan 23, 2017
    From FAST: (8) hidden injectors; Uses (4) at idle & phases in additional (4) as needed
     
  12. gremlinmt

    gremlinmt Member

    Joined:
    Mar 13, 2013
    Well, I'm sure it's just coincidental that vac referenced regulator drops the pressure at a rate that would keep pressure drop across the injectors the same. Probably pulled that rate right out of the ether, no engineering involved.
     
  13. fabr

    fabr Member

    Joined:
    Jan 23, 2017
  14. 78 F150

    78 F150 Member

    Joined:
    Feb 6, 2013
    Got my torque wrench today. Also got the headers, block-to-head surface, and intake rails cleaned up and ready to go. Heads should arrive at the machine shop on 1/2/19.
     
  15. A A

    A A Well-Known Member

    Joined:
    Jun 22, 2014
    That's a quick turn around. When they come, just be sure to support the heads upside down and use a small amount of gas to make sure the valves aren't leaking on the seat. If no fuel passes the seats, the heads should be ready for installation. A quick visual inspection should also insure no valve stems we're damaged in shipment.
     
  16. 78 F150

    78 F150 Member

    Joined:
    Feb 6, 2013
    Quick turnaround? Uumm, by the time it gets to the machine shop it will be 1 1/2 weeks just to get back to Michigan (will arrive 1/2/19). Then about 2 weeks (maybe a little less) to get the work done, then I need to mail him a check to pay for the work he did. Once he receives my check then he will ship the heads back to me, which will take at least a week (or more) to get back to me, so we're looking at the end of January before I get them back. I don't think that's a quick turnaround. But I'm ok with that because I won't have the money to pay him until 1/10/19. So it will work out.

    I will check the valves aren't leaking as you suggested.

    So check this out....I ordered that adjustable vacuum advance canister from Summit on 10/27/18, which they had to get from the manufacturer. It was supposed to ship on 12/24/18, but I haven't received an e-mail yet that it's been shipped. So I just logged onto my account to check the status and now it says it won't be shipped until 2/18/19!!!! What the hell!!! I've already got the new one I got from Autozone installed on the truck, but it's non adjustable. So not sure if I should just cancel the order or just wait for it to arrive. I'm leaning toward just waiting for it to arrive. I can get the engine tuned pretty close (with your help) in the mean time while I'm waiting for it to arrive. So now it's just a waiting game for the heads and vacuum advance canister.

    Hope y'all had a Merry Christmas!!!!
     
  17. A A

    A A Well-Known Member

    Joined:
    Jun 22, 2014
    Lol, I misread your post. I thought you were already getting them back.

    Thanks! Is it over yet? I haven't lifted the lid on the bomb shelter to look outside yet.;)

    The weather has been so lousy here I haven't gotten to finish excavating for my new underground garage. I only like going a few inches deeper for the floor, and a few feet further on one end. At least I have the drainage in and have no flooding in the hole.

    I have parts supposedly coming on the 1st and 2nd. I'm going to install new NGK plugs and the Vibrant 17930 resonators to the GMC. A video like the first handheld video will follow to show the new lower volume. The current spark plugs are the same UR4 stock heat range plugs I was using when I first started tuning the GMC, and they were overheated some a few times before I got the timing slope were it needed to be. I used another set of UR6 for only a few weeks that were too cold. So, these same UR4 plugs have been running for about two years. I'm going to replace with UR5 plugs, which should be dead on the money in this engine. The sound should be all that will change - possibly closer in volume to my factory Silverado exhaust.
     
  18. 78 F150

    78 F150 Member

    Joined:
    Feb 6, 2013
    Well it hasn't got bad here....yet. I'm sure it's coming soon. I think it's the calm before the storm. Where are you at? I'm in southeast Washington State.
    Underground garage? I'm so jealous!!! You have all the outbuildings and toys I dream of having someday. I guess I better start playing the lottery....LOL

    I'm really curious to hear your exhaust with the resonators!!!! When are you planning on installing them? Can't wait for the video. I'm still torn on which mufflers to go with. I absolutely love the sound of the Thrush glasspacks my wife has on her truck, but your mufflers sound good too.

    Question.....if there's a hole in my muffler, would it affect the O2 sensor reading, even if the muffler is at least 4' downstream from the O2 sensor? My mufflers are old and on one of the mufflers there's a small area (about the size of a quarter) on the outside casing that has rusted away, but it's not rusted all the way through to the inside of the muffler. I have no idea if it's actually leaking or not because there's several layers to the metal on the muffler, but I was just brainstorming on the cause of my crazy a/f ratio. I need to inspect the muffler a lot closer. The a/f ratio reading acts like a vacuum leak, but I wasn't sure if a hole in the muffler that far downstream of the O2 sensor would affect the sensor or not.
     
  19. 78 F150

    78 F150 Member

    Joined:
    Feb 6, 2013
    The heads arrived at the machine shop today. Phase 1.....complete. Hope y'all had a great New Year!!
     
  20. A A

    A A Well-Known Member

    Joined:
    Jun 22, 2014
    Yup. But the weather is miserable - raining daily with soggy ground everywhere - probably 48-60 inches of rain in the last two weeks. When it's not raining it's drizzling. Got the new resonators today.
     

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