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Thread: IAC Creeps To 100%

  1. #1

    Default IAC Creeps To 100%

    Have the 2300 on a '79 F250, 351M. Has the stock Ford distributor/Duraspark system, MSD Blaster coil, MSD wires.
    Developed a problem, seemingly out of nowhere and I'm having trouble narrowing down a consistent cause or timing.
    Unit will be running fine and then the IAC will climb to 100% and then the RPM will drop off to 400 or 500. Seems to run OK once I start cruising, however, despite not feeling anything while cruising I will sometimes see the AFR stick at 15.7 and seemingly freeze there. Also notice the Closed Loop & Learn Limits will climb to 50 (that's where the max is set at) and then will seemingly kick back into a normal mode. This happens whether I run it on Open Loop or Closed Loop. I replaced the IAC as frankly a guess, made no difference. I set the IAC/throttle blades to 5% using the tape method and it seemed like it was cured for like half a day and then the problem returned. I had miniscule exhaust leak around my #5 exhaust port on the side the WBO2 sensor is on. I've since sealed up. Anyone else ran into this and knows what this is? Next steps to troubleshoot it?

    I did search though the forums here and haven't found the same problem, but that's where I got the exhaust leak and the throttle blades.

  2. #2
    Join Date
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    Default

    Show us a datalog of the issue and we'll take a look. Do you have a fuel psi gauge on it?

  3. #3
    Join Date
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    https://forums.holley.com/showthread...8356#post88356 (Holley Sniper EFI Fuel Pressure Tester/Gauge)
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS T/W 11R 205 heads, 232°-244° duration/.623" lift/114° LSA camshaft, 12:1 C/R, TFS R-Series FTI ported intake, BBK 80mm T/B, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 200A 3G alternator, Optima Red battery, R134a A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, -6AN fuel system plumbing, Walbro 255 LPH pump, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/3000 RPM converter, B&M Hammer shifter, FPP aluminum driveshaft, FPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 M/T Street Comp tires.

  4. #4

    Default

    How is your fuel system setup? Some of the symptoms seem to indicate inconsistent fuel pressure, but others seem more like EMI/RFI. What alternator are you running? Follow ABC's advice. Take a datalog and also upload the Config File in the Sniper. Through all of your "episodes" your Learn Table is likely corrupted which isn't helping. But that's an effect and not the cause.

  5. #5

    Default

    I do have a fuel pressure gauge in it, it never drops below 60-65 psi during the symptoms. I do have the IAC & ECU all wrapped in foil at the moment as I was curious about RFI. It's an original type alternator with an external voltage regulator. This system has been on for a couple years now. I'll try to capture a datalog while it's doing it tomorrow and post it up. I have the Learn turned off and I've reloaded the tune I have for it each time I made the adjustments above, and the symptom keeps returning.

  6. #6

    Default

    I think I did this right, I zipped a Config File & four datalogs in the attachment. Datalog 48 is cold startup, 49 is shortly thereafter before it gets to 160°F. I blipped the throttle in there dropping it temporarily and then it climbs again. 50 shows the AFR sticking at 15.7 as I was describing and coming to temperature. 4th is just cruising at 40 MPH, slowing down to stop sign and then accelerating back to 40 MPH. So after all that this morning, I drove it 100 miles over to Iowa, 55-60 MPH the whole time, not one hiccup there & back.
    Attached Files Attached Files

  7. #7
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    Your ECU is being told by the WBO2 that the exhaust stream is too lean, therefore adding fuel up to the CL Comp Limit. The question is, why? Based on the facts given, I suggest you run an operation test on the WBO2 sensor. I think there's a good chance you have a failing WBO2 sensor. Have you physically checked timing lately?

  8. #8

    Default

    A couple months back after I replaced the plugs, wires & coil I did. I have a spare WBO2 sensor. Be wise to retest the exhaust for leaks? It did have a very tiny one around the #5 port. I learned these engines don't use gaskets but graphite grease. I took the manifold off, cleaned it, sealed it with copper high temp, all new bolts torqued properly.

  9. #9
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    Quote Originally Posted by jmeritt View Post
    I have a spare WBO2 sensor.
    https://www.youtube.com/watch?v=rmnxKLjM25g (Wideband O2 Sensor Test/Diagnosis - it works!)
    If testing the WBO2 sensor key-on/engine-off, the sensor heater will time out, so you'll need to work fast.
    If you need more time, the WBO2 sensor test can be performed with the engine running in Open Loop mode.

    Be wise to retest the exhaust for leaks?
    Ensure there are no exhaust leaks.
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS T/W 11R 205 heads, 232°-244° duration/.623" lift/114° LSA camshaft, 12:1 C/R, TFS R-Series FTI ported intake, BBK 80mm T/B, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 200A 3G alternator, Optima Red battery, R134a A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, -6AN fuel system plumbing, Walbro 255 LPH pump, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/3000 RPM converter, B&M Hammer shifter, FPP aluminum driveshaft, FPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 M/T Street Comp tires.

  10. #10

    Default

    UPDATE: I finally got a chance to return to this today, and all four exhaust ports leak on that side again. Apparently I did a crap job. Anyways, Sunday or Monday project will be redoing that side again and seeing if I can get it sealed and I'll go from there.

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