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1984 F250, what is this under hood plug? Rough Idle issue.

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Starliner

1984 F250, what is this under hood plug? Rough Idle issue.

I'm not sure if this should be connected or not. These plugs are right near the ICM. Any guesses out there?

I'm banging my head trying to sniff out rough idle issue so I wanted to cover all bases.

After sitting in dry climate for 10 years, I replaced:
ICM,
Coil, plugs, wires, cap, rotor, most vac lines, I've blocked off the brake booster, rebuilt the carb, replaced the ignition switch,

Still rough. Carb clean isnt helping find a vac leak.
Besides idle, the truck runs great.

Any ideas would be appreciated.

JOE
1984 F250 HD 2wd auto w/air,
351W, 2bbl, Dana 60 3.73, no EEC

Gary Lewis

Re: 1984 F250, what is this under hood plug? Rough Idle issue.

Administrator

That looks like one of the connectors to the ignition module. Show me the wire colors?

Gary, AKA "Gary fellow": Profile

Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI

Starliner

Re: 1984 F250, what is this under hood plug? Rough Idle issue.

In reply to this post by Starliner

Hi Gary
They are green and black. Thanks.

JOE
1984 F250 HD 2wd auto w/air,
351W, 2bbl, Dana 60 3.73, no EEC

Starliner

Re: 1984 F250, what is this under hood plug? Rough Idle issue.

In reply to this post by Starliner

Better picture

JOE
1984 F250 HD 2wd auto w/air,
351W, 2bbl, Dana 60 3.73, no EEC

Gary Lewis

Re: 1984 F250, what is this under hood plug? Rough Idle issue.

Administrator

The better picture did it. I believe you are looking at C169, and those are the two wires going to the tach. See the right side of the first page below, from the 1985 EVTM, that shows the wire colors, and the second page that shows where the connector is, its color, and the # of pins.

Gary, AKA "Gary fellow": Profile

Starliner

Re: 1984 F250, what is this under hood plug? Rough Idle issue.

Ok, I see it there.
Since I dont have a tach, I guess it is normal for that to be disconnected. Do you thing there is any downside to plugging it in?
I have to get a factory service manual on CD for sure.
Thank you.

JOE
1984 F250 HD 2wd auto w/air,
351W, 2bbl, Dana 60 3.73, no EEC

Gary Lewis

Re: 1984 F250, what is this under hood plug? Rough Idle issue.

Administrator

There's no downside to plugging it in.

As for getting the factory service manual, it is a good thing to have. However, those schematics are from the EVTM. But in reality much of the FSM is on-line on the site, and all of the 1981, '85, and '86 EVTM's are.

Gary, AKA "Gary fellow": Profile

swampedout

Re: 1984 F250, what is this under hood plug? Rough Idle issue.

Look up DIY smoke machines. Cheap and easy to make and much more effective than spraying carb cleaner.
I cant tell you how many trucks Ive sprayed and figured didnt have leaks even with continued rough idle, only to later smoke them and find the problem in seconds.

Sam
1984 F250. 460. C6. 4x4.
MSD Ignition. Airbag rear suspension
Whole buncha problems

LARIAT 85

Re: 1984 F250, what is this under hood plug? Rough Idle issue.

In reply to this post by Starliner

Yes, that connector is for a factory tachometer.

Plug it in and get yourself a replacement cluster that has a tachometer in it.

Lucille: 1985 Ford F150 XLT Lariat

*Colors: Dark Canyon Red exterior, Canyon Red interior
*Engine: 5.0, CompCams 31-230-3, "Thumper" E7 heads, Edelbrock Performer intake, Autolite 4100 carburetor, DuraSpark II ignition, Thorley Tri-Y headers, Flowmaster dual exhaust, H-pipe.
*Drivetrain: AOD transmission, 3.55 gears, 2wd.

Starliner

Re: 1984 F250, what is this under hood plug? Rough Idle issue.

You are all so very helpful, thank you.
Of course I will be DYI-ing a smoker right away and will post an update.

JOE
1984 F250 HD 2wd auto w/air,
351W, 2bbl, Dana 60 3.73, no EEC

Gary Lewis

Re: 1984 F250, what is this under hood plug? Rough Idle issue.

Administrator

Do you have a cigar? http://forum.garysgaragemahal.com/Vacuum-leak-detection-effective-and-inexpensive-td11511.html

Gary, AKA "Gary fellow": Profile

Starliner

Re: 1984 F250, what is this under hood plug? Rough Idle issue.

Well, I went the DYI route. Burned my hand on the can, (of course), but I had smoke coming out of this valve before the PCV. This becomes another What's This? When the lever faces one way, outside air can enter, the other way, SOME air enters. What the heck is it?

I am sniffing down a rough idle but really, it looks like it is flooding at idle. I am 2-footing it now and tonight even had run it with the air cleaner off to get her home.
I bet I need to adjust those hard to get idle mix screws.

JOE
1984 F250 HD 2wd auto w/air,
351W, 2bbl, Dana 60 3.73, no EEC

Gary Lewis

Re: 1984 F250, what is this under hood plug? Rough Idle issue.

Administrator

I have no idea what that valve is. I've never seen one and it isn't supposed to be there. I'd put a new PCV hose on and ditch the valve.

As for the idle, it may be the idle air screws, but I'm betting you have a blown power valve.

Gary, AKA "Gary fellow": Profile

Starliner

Re: 1984 F250, what is this under hood plug? Rough Idle issue.

Geeze, I just rebuilt it so I will go to mix screws first. Thanks Gary.

JOE
1984 F250 HD 2wd auto w/air,
351W, 2bbl, Dana 60 3.73, no EEC

grumpin

Re: 1984 F250, what is this under hood plug? Rough Idle issue.

In reply to this post by Gary Lewis

I’d ditch the valve too, but I would like to know what it is for!

Dane
1986 F250HD SC XLT Lariat 4x4 460 C6-Sold
1992 Bronco XLT 4x4 351W E4OD
1998 GMC Sierra SLE K1500 350 4L60E
Arizona

Gearhead

Re: 1984 F250, what is this under hood plug? Rough Idle issue.

In reply to this post by Gary Lewis

That valve is factory. The explanation I received, decades ago, was that it was emission related. Can't remember specifically what it did. It can go.

Gearhead
1980 F350 Ranger Lariat SuperCab 4x2
400 2bbl C6, air, cruise, attitude.
A work in progress.

myrl883

Re: 1984 F250, what is this under hood plug? Rough Idle issue.

It's called a hot idle compensator. Under high under hood heat conditions (traffic), the bi-metal spring opens the valve to create a slight vacuum leak that leans the idle mixture and raises idle speed slightly.

Pretty clever engineering for the 60's, before we had electric throttle kickers!

Personally, if it works properly, I would NOT eliminate it.

And, sorry Gary, but that's exactly where it belongs.

Ford Parts Monkey since 1985
1981 F100 Flareside - Black, 302-4V Roller/AOD
1986 F150 Flareside - Medium Fire Red 302/AOD
1989 F150 Standard Cab 4x4 - Dk Shadow Blue 302/AOD
1993 F350 4x4 Crew Cab - 7.3 IDI/ZF-5
I think it's a sickness...

ArdWrknTrk

Re: 1984 F250, what is this under hood plug? Rough Idle issue.

Administrator

So this is something that causes a bigger vacuum leak than the PCV itself?

Isn't the TVS in the distributor advance line a better solution to the issue?

Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.

Gary Lewis

Re: 1984 F250, what is this under hood plug? Rough Idle issue.

Administrator

In reply to this post by myrl883

myrl883 wrote

It's called a hot idle compensator. Under high under hood heat conditions (traffic), the bi-metal spring opens the valve to create a slight vacuum leak that leans the idle mixture and raises idle speed slightly.

Pretty clever engineering for the 60's, before we had electric throttle kickers!

Personally, if it works properly, I would NOT eliminate it.

And, sorry Gary, but that's exactly where it belongs.

I sit corrected. Good to know. Thanks, guys.

Gary, AKA "Gary fellow": Profile

1986F150Six

Re: 1984 F250, what is this under hood plug? Rough Idle issue.

Administrator

In reply to this post by myrl883

myrl883 wrote

It's called a hot idle compensator. Under high under hood heat conditions (traffic), the bi-metal spring opens the valve to create a slight vacuum leak that leans the idle mixture and raises idle speed slightly.

Pretty clever engineering for the 60's, before we had electric throttle kickers!

Personally, if it works properly, I would NOT eliminate it.

And, sorry Gary, but that's exactly where it belongs.

This was found with a search [Mike's carburetors]:

Hot Idle Compensator
by Mike | posted in: carburetor | 0
The Hot Idle Compensator is used on some carburetors to offset enriching effects caused by percolation during hot operation. Generally used on vehicles with air conditioning. When the engine is running hot and then stopped, the fuel can boil or percolate and vaporize causing a rich condition at idle.

Some Hot Idle Compensators were mounted to the carburetor:

And something else:

https://secure.cougarpartscatalog.com/valve-hot-air-compensator-used-1967-1968-mercury-cougar-/-1964-1968-ford-mustang.html

https://www.ebay.com/itm/64-65-66-67-68-Mustang-Galaxie-Fairlane-Torino-289-260-390-hot-idle-compensator/124144270652?hash=item1ce7933d3c:g:CjEAAOSw2d5ei1zc

Apparently, GM used similar technology:

https://www.ebay.com/itm/GM-Compensator-CarbAirator-Add-On-Hot-Idle-Vintage-Carburetor-NOS-RP-555/184249404361?hash=item2ae61ecbc9:g:AnUAAOSwov5ekheh