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Thread: Low speed surge

  1. #1

    Default Low speed surge

    Looking for the size of the inlet opening for the EGR valve on a 1985 Mustang GT 5.0 with the High Altitude system. Also does anyone know what the P/N of the stock EGR valve. The decal is long gone on mine and I'm chasing down a slight low engine surge at light throttle in all gears. When surging if I give it just a bit more throttle the surge stops for just a second and then returns. Thanks.

  2. #2

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    Not sure the size of the inlet opening. Is it dirty, partially sticking open, or is the diaphragm not holding vacuum? There are a multitude of possible Ford part numbers listed with the Rock Auto replacements. Surge (lean) with light throttle in all gears (varying amounts of load) might point to a malfunctioning EGR... but it's function, when it's on or off, relies on where the throttle position is, and if all is well with the valve itself, shouldn't be posing a problem with varying amounts of vehicle load and throttle positions... if your '85 is a 5 speed and with a 4180 (or other) Holley 4-barrel, it could point to the carburetor needing to come apart and be thoroughly cleaned out and blown out and rebuilt, because some debris may be floating around and blocking some idle/transition fuel passages. The surging briefly going away with giving it just a bit more throttle may be happening because there's a wee bit (which is enough to make a difference when we are talking about the idle/transition circuit, low throttle positions) accelerator pump shot fuel to mask the issue, or the main circuit (boosters) briefly starting up. This just start happening or has it been progressively worse or going on for a while?
    Mike
    1986 Mustang convertible ---> BUILD THREAD
    Past Fox-chassis "four eyes":
    1983 Mercury Cougar LS
    1986 Ford Thunderbird ELAN
    1980 Capri RS Turbo

    Work in progress website ---> http://carb-rebuilds-plus.boards.net/

  3. #3

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    It's the carb'd "M code" engine. Went through the carb about 6 months ago and followed JA Cook and your advise for settings and it was running better than when I bought the car new in June of 85.
    The surge is something that just started a few days ago. Ford replaced the OEM valve when the car was a year old "factory update/mod" As per Ford. That's when I "lost" the OEM p/n. I replaced that one many years ago cause it wouldn't hold vacuum and after purchasing a new I discovered that by design they're supposed to "bleed" engine vacuum during operation.
    I have saved just about every part that I have changed over the years and I found the "factory update/mod valve (used only 2 yr's). You'd be hard pressed to tell it from new (looks/function). My thought is to see what the difference is between it and what Ford oridginly used and what Ford sold me and has been in use for the last 20 or so years.
    The car is also equipped with the high altitude idle compensation system and have a thought that maybe it might be part of the problem. I've noticed that over the past 6 months (why I redid the carb) the Idle would drop a couple of hundred rpm's (from 750 down to about 550) while I was setting the idle mixtures (4 screws), float levels and idle speeds. A funny thing was that when the idle speed dropped the engine ran smoother and after a second or two the idle would go back up and I would get a slight miss. The engine vacuum will vary from 19 1/2" to 20". The drop in vacuum coincides with the "miss".

  4. #4
    FEP Super Member erratic50's Avatar
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    When is the last time cap and plugs and rotor and wires were inspected? Have you verified condition of all grounds?

  5. #5

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    Good for you for doing it yourself. How did your going through the 4180 go?

    A quick check then to point at or rule out the EGR should be to temporarily remove it from the equation. Take the vacuum line off of it and plug the line with a small bolt or something, and go for another drive to see if it still does the surging. I believe the EGR should then always be closed, so if there's still surging going on, it's something else.

    A rolling idle is often associated with an idle air:fuel ratio on the rich side, with a rougher or slowed down idle on the lean side of adjustment. Might it be the solenoid lowering and raising the idle speed like that, or are you saying just when you adjust the mixture screws it does that?
    Mike
    1986 Mustang convertible ---> BUILD THREAD
    Past Fox-chassis "four eyes":
    1983 Mercury Cougar LS
    1986 Ford Thunderbird ELAN
    1980 Capri RS Turbo

    Work in progress website ---> http://carb-rebuilds-plus.boards.net/

  6. #6

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    Quote Originally Posted by erratic50 View Post
    When is the last time cap and plugs and rotor and wires were inspected? Have you verified condition of all grounds?
    The cap, rotor and wires where ether replaced or closely inspected about 3,000 miles ago.

  7. #7

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    Every time I figured out a cruise surge issue it was because it was running too rich.
    Black 1985 GT: 408w, in the 6's in the 1/8 mile
    Bimini Blue 1988 LX 5.0 Coupe 5-speed, Hellion turbo, zero options
    Grabber Yellow 1973 Mustang Mach 1: 351c, toploader
    Black 2012 5.0 GT, 6-speed, Brembo brakes, 3.73's
    Wimbledon White 1966 F-100 Shortbed Styleside, 390, Tremec 3550, FiTech EFI

  8. #8

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    Quote Originally Posted by Walking-Tall View Post
    Good for you for doing it yourself. How did your going through the 4180 go?

    A quick check then to point at or rule out the EGR should be to temporarily remove it from the equation. Take the vacuum line off of it and plug the line with a small bolt or something, and go for another drive to see if it still does the surging. I believe the EGR should then always be closed, so if there's still surging going on, it's something else.

    Love working mechanical things/troubleshooting. Spent most of my life in that type of work.
    Working the 4180C wasn't much different from other Holley carbs. Also have a couple of "spare parts" 4180C's on the shelf.
    Plugging off the vacuum to the EGR was my 1st thought but, before I start playing with It I wanted to get the OEM P/N and inlet orifice ID size. Part of the fun is getting all of the info on something before I tear into it. It's a habit that I picked up from working in quality control and being a QC auditor for more years than I care to remember (ha). As they say "The proof is in the details".
    Once I have the details of the EGR worked out and if the problem still exist I'll then focus on the distributor vacuum advance.


    A rolling idle is often associated with an idle air:fuel ratio on the rich side, with a rougher or slowed down idle on the lean side of adjustment. Might it be the solenoid lowering and raising the idle speed like that, or are you saying just when you adjust the mixture screws it does that?
    As far as the Idle varying during the carb adjustments go and after I eliminate the ERG and distributor vacuum advance I'll look into the "High Altitude" compensation system that's mounted up inside of the RH fender under the wheel well liner. From my preliminary research that system use's several barometric pressure sensor's and vacuum solenoids to bleed in raw air to compensate for altitude changes. If it malfunctions that would definitely affect the idle speed and fuel mixture at idle. Out here in Phoenix, Az we're around 1,100/1,200 feet, so that's a possibility if the other things check out.

    I enjoy going into such detail cause I have two excellent reasons, I don't need the car on a daily basis and I'm retired, Yea ! Thanks for your interest.

  9. #9

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    pinch off the brake booster hose, too. I've had intermittently leaky boosters cause similar concerns.
    Jim DeAngelis
    Cornucopia of Useless Knowledge
    Connoisseur of Dearborn Ferrous Oxide
    '83 GT hatch, currently under the knife
    '79 Capri 2.3L n/a, Medium Copper metallic, survivor
    (bought from MRausch82)

  10. #10

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    Quote Originally Posted by FB71 View Post
    pinch off the brake booster hose, too. I've had intermittently leaky boosters cause similar concerns.
    That's a good thought but, not to high on my list of probabilities because I've replaced that last year and about 2,500 miles ago.

  11. #11
    FEP Super Member erratic50's Avatar
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    I've had the same one on my 86 since new and it has nearly 1/2 million miles but I've seen them fail right out of box. I'd pinch it off and drive it on a back road.

  12. #12

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    Just wanted to give you guys an update. I've been out of town and unable to work on it for the past couple of weeks and I was finally able to do some work on it today. I did a vacuum check on both sides of the high altitude compensation system and it passed no leaks.
    Next I pulled the EGR valve and found some leakage where the pindel seats and by cleaning it did seal. Also did some checking on port size for the vacuum and the exhaust gas flow restrictor. The vacuum nipple ID is .032 and the flow restrictor ID averaged .510. Did some checking on a Motorcraft replacement EGR valves and found that the replacements come with several sized flow restrictors and you install the size of restrictor as per a chart that come with the valve. The closest I found was .495 and I installed that one. My existing valve matched the replacement valve in all other aspects.
    I started it up and brought it up to temperature and checked for leaks and I decided to check the float levels I found both the primary and secondary to be a bit low. Not high enough to run out of the sight plug but, if you rocked the car slightly it would. I reset the float levels to where the fuel was just high enough to reach the plug hole threads.
    Took the car out for a test drive and it ran much better but, there's still a slight surge still. About the only thing I can think of now is to find someone with an exhaust gas analyzer to see what the carb is doing.

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