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  1. #1

    Default 85 GT 5.0 Carb 5 spd Idle Issue

    Last October I bought this 85 Red GT convertible with a little under 36k. It has an all stock 5.0 with the 4180 carb and 5 spd. It seemed to run fine when I got it home, then put away for the winter, then started up this spring. Again, seemed to run fine but I noticed some backfiring. Next time I tried to start, it choked and coughed and barely held idle. Definitely not roadworthy.

    I started pouring over previous posted threads to see what was shared on similar issues. Recommendations ranged from replacing the EGR valve to the power valve in the carb. So I did both. I replaced the original 2-stage PV with the single stage that comes in the carb rebuild kit. The engine ran much better so I thought I had addressed the issue. Not so lucky.

    Once I drove it for about 15-20 minutes, I could begin to detect a missing either at speed or accelerating. Then when stopped, it would idle at about 1k to 1200, then drop to almost a stall, sometimes recover, sometimes stall completely. My next suspect was the ignition system.

    I've now replaced distributor, coil, spark plugs and wires, and the ignition module; Motorcraft where possible; the dizzy from LMR. While it does run somewhat better--accelerates well, holds speed, etc.--I still have the idle-drop issues and an occasional misfire on acceleration (e.g., taking a right turn and starting up from 2nd gear).

    Any advice or guidance would be appreciated. Just not sure where next to turn. Thanks!

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  2. #2
    FEP Super Member xctasy's Avatar
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    Jun 2012
    Dunedin 9011, New Zealand, South Pacific


    The ignition system and fuel system need checking in turn.

    I prefer to check the fuel system first.

    A little back ground on the 85 Ford 5.0 HO fuel system, and why it is the way it is, and what is in there.

    Ford used a Hot Idle Compensator valve (HICV), starting from 1959 with the two barrel Carter ABD in 383-430's in Edsels, Lincolns, Mercurys and Fords in the US and some South American cars and trucks. Due to major changes with power accessories, but also climate and elevation for its destination state.

    Back in 59, those cars actually had three crutches to hold air fuel ratios right. The HIC valve, a Dash Pot for idle drop off on over run, and very special idle speed-up control solenoid.

    Importantly, Ford was responding to the need to create stable idle in cars with power steering, air conditioning, and varying transmission types, sometimes automatic, sometimes Top Loaders or T10's, sometimes 10.5 compression with Hi Octane needs, sometimes Regular or Standard grade low compression plodders. Running altitude could be the Andie's, or Death Valley.

    Your Holley 4180C is the culmination of all that jazz, but Ford outsourced the carbs to Holley to make the car idle right, and meet emissions and performance requirements as well. Vehicle Emissions weren't much of an issue in 1959, but they were a major by 1962, and sure were 26 years later. What I'm saying is the 4180C has 12 crutches that require your attention, before we add the 13 th complication, our oxygenated fuel and its ability to form slag when the engine is off.

    So you have to go through a lot of little things first. All the settings Ford made OEM in 1985 get ruined when modern gas is used.

    First thing is to ensure there is no build up of internal crud and precipitates caused by ethanol gas. The Holley 4180C is the carb most effected by build up in not only the gasoline well tubes, but also the air bleed well tubes, which was a specific work-around Holley did for Ford and Chevrolet who used this carb OEM.

    This carb is not a Carter ACF or Rochester Reproductions heavy duty Zinc Oxide, its Holley's typical low rent alloy. So modern oxygenated gas really cruds it up, not just with slag, but by forming an oxide bond with the fuels precipitate. That forms always in the many very small holes. They need physical cleaning out with a combination of ultra sound and steaming with a very small diameter polypropylene brush and air. Simple Green and a combination of other thinners or Mutraic acid wont do it all.

    WalkingTall (Mike) explains this here. Link 1

    If your carb is clean, then its just a matter of looking at these 12 things.

    1. Don't ever adjust the HIC valve first. But here is how to do it Link 2

    2. Don't reset the tamper proof, 5.0 HO only four corner idle first. But here is how to do it Link 3 Post #17 at

    3. Don't reset the Choke Pull Down setting. It must be re-established, but do not do it first. But here is how to do it Link 4

    4. The right gaskets and EGR insulator and HEAT SHEILDS MUST BE USED. Aftermarket float bowl kits foul the stock FoMoCo parts for some years, and some OEM replacements are for Trucks and you need to be careful to make sure they are OEM 5.0 HO compatible.

    5. the Idle Sole Pot must be set correctly

    6. The Throttle Kicker must be set correctly

    7. The Float levels must be in spec, and aftermarket fuel pumps can have a wrong delivery pressure which can aggravate the static fuel level Ford calibrated the car for in 1985.

    8. The secondary float bowl supply O rings must be damage free and the front and rear float bowl casings must be free of cracks. The aftermarket rebuild gaskets and some carb kits cause major problems here.

    9. The Bowl Vent system is tied into the Altitude Compensator system under the passenger side fender. Its standard settings will be fine, but the OEM rubber lines must be verified as still being operative. Here is how to do it Link 5 See JACooks post #13

    10. Ignition advance is tied into the Air cleaner and its many vacuum and heat operated blade valves. The two stage power valve is linked to the Blue, Red and Green TVS valves on the intake manifold. Do not replace any two stage power valve with a single stage power valve unless you have to.

    11. The Load Control Valve for the Secondary light off catalyst is controlled by a large exhaust operated iron gate valve, and this must be operative.

    12. The AIR pump has a TAD/TAB system which is tied into the Heat Stove and many parts of the whole VECI diagram.

    A two stage accessory drive computer in the passenger side kick panel controls the fuel economy indicator/up shift lights, and the clutch sensor components are locked out of functioning on the 4 BBL carb Mustangs for 1985. It is not a Computer Controlled engine, and the ignition is Duraspark, not TFI, so make darned sure the parts are not mixed between the Automatic and Manual M codes

    The 13th thing...the gasoline will always cause precipitates, and the emissions crutches will have you gazumped. You can give it a 4BBL Carb frontal lobotomy, but its better to work through these 13 things.

    On the ignition side, check that these things are present. The advance curve is artificially controlled, not by computer, but Barometric pressure from two sources, and idle speed is kicked by those two sources. The action of two thermostatic valves define the advance, and the engine is not seamless as these inputs change. The red, blue or green sensors can stop working, and hang up the ignition advance, causing a lean miss.

    Not all components are well documented on 5 speed 5.0 HO's for 85

    The Duraspark BMAP sensor for the Altitude compensation is not

    Nor is the actual under passenger fender stuff.

    Nor are the parts of the Two Stage Accessory Drive

    Link 6

    For 85's, the 4V HO is still a "non feedback, non computer control engine", but its accessories are computer monitored in part.

    By these duffers.

  3. #3


    Wow! Thanks, xctasy. Talk about drinking from a fire hydrant! I may not have the technical expertise to evaluate all 12 points but I may have run afoul on #10 with installing the single stage PV from the Holley rebuild kit. I had a new 2-stage PV in hand but the threads didn't seem to match when I removed the original PV. I didn't want to force it, for obvious reasons. The single PV from the kit installed seamlessly. I'll keep looking for the correct 2-stage, if I can find one.

    I'll also investigate the Bowl Vent System identified in #9.

    One final question re #1 - I've noticed this valve as it's in front of the carb and behind the distributor cap. The valve "arm" moves back-n-forth. I assume this shouldn't be moved but, let's say, it was inadvertently, can it be reset? I read through the link you provided but I don't see anything that would describe the position of the brass arm. Maybe I missed it?

    Again, thanks for the many insights.

  4. #4
    FEP Super Member xctasy's Avatar
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    Jun 2012
    Dunedin 9011, New Zealand, South Pacific


    Just remember, the 1985 carb 5.0 HO had 103 emissions devices: no Ford passenger car ever again had as many emissions parts as the 85 5 speed 4 barrel.

    When EFi came out, each year, Ford reduced the total emission devices by electronic control sysyems eventually to about 45 devices in 1995, similar to 1975.

    So the part I dont tell you about...that's the part you needed to know about!

    Yeah, the brass arm is the Hot Idle Compensator valve. It's a bi metal strip, so if you know it's been accidentally bumped, then all you do its wait till the engine is warm with the hood closed and after 15 mInutes of running, the invar strip will bend and normally it will open the air bleed valve. If its now in the wrong position, then adjustment is to just turn the nut so the rubber seal is "just" starting to bleed air into the fuel line. That leans the mixture and settles the idle when hot. Try it if you know everything else seams good.

    The link to how it operates is Link 2 above. It's present on some Air Con 5.0s before 1985. And probably what would have stopped Nick in Nicks Garage having to do the tamper proof four corner idle readjustment when the white 83 convertible wouldn't idle properly.

    That HIC valve was a 1959 -1978 Carter and Autolite/ Motorcraft solution so the idle screws don't need to be turned when the engine was warmed up.


    Nicks advice on the emissions equipment and his knowledge....priceless!

  5. #5
    FEP Super Member xctasy's Avatar
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    Jun 2012
    Dunedin 9011, New Zealand, South Pacific



    The power valve wont hurt your tune much, but all the basic Emission spec "fuel" and "ignition" control parts will. There is more of it in a non feedback 85 THAN ANY CAR...EVER MADE

  6. #6


    Have you replaced the fuel filter?

  7. #7


    No, not in the time I've owned it. My guess is that it's probably the original that is now 36 years old and covered 36k miles. Probably time.

  8. #8
    FEP Senior Member
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    Mar 2009
    Gaithersburg MD


    Are all of the vacuum hoses hooked up? Have you checked your timing? Are you wires on the cap correctly?

  9. #9


    noac: 1) as best as I can tell, I continue to examine the vacuum connections of those I can see and will continue to do so as long as the problem persists; as xctasy noted, I've got 103 devices to check :-(; 2) yes, I've timed it; and, 3) yes, wires are correct.

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