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Idle variation on my 1986 f150 XLT 5.0 EFI AT

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Socnarftrucks

Idle variation on my 1986 f150 XLT 5.0 EFI AT

When the truck is idling at a stop light it feels a little rough and shaky, when in park the shaking is less and there’s a cyclic variation between 750 rpm’s and 1000 rpm’s. New spark plugs, wires, TPS, and IACV. Any help is appreciated.

Gary Lewis

Re: Idle variation on my 1986 f150 XLT 5.0 EFI AT

Administrator

Sounds like it is lean. Have you checked all the hoses to see if something might be off or cracked?

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

Socnarftrucks

Re: Idle variation on my 1986 f150 XLT 5.0 EFI AT

I have. Tried finding vacuum leaks with the non flammable break cleaner approach. Also tried a power balance disconnecting the driver side injectors.

LastDeadLast

Re: Idle variation on my 1986 f150 XLT 5.0 EFI AT

Any codes?

-Shannon

1985 F150 XLT
1965 Mustang Convertible
2013 F150 Platinum

Socnarftrucks

Re: Idle variation on my 1986 f150 XLT 5.0 EFI AT

No, sir. Occasionally the Emissions light will flicker for a few seconds especially during a cold start.

mat in tn

Re: Idle variation on my 1986 f150 XLT 5.0 EFI AT

nonflammable will not burn! in or out of the cylinders. therefore, it will not find a vacuum leak. you need a fuel in order to get a combustion response.

Socnarftrucks

Re: Idle variation on my 1986 f150 XLT 5.0 EFI AT

Ohh!!! I’ll try that then…. Thanks

LastDeadLast

Re: Idle variation on my 1986 f150 XLT 5.0 EFI AT

One thing I did with my 85: I got a vacuum gauge and made a baseline reading. In my case right at 21psi.

After that, I started unplugging and plugging individual lines from the intake manifold; brakes, EGR, cruise control, etc. and see if there are any changes in vacuum. This won't tell if you if you have a vacuum leak in the intake, but it might tell you if you have a significant leak in one of the various vacuum components. And of course it only works at idle.

Also, If you already don't have one, I would also grab a code reader from AutoZone or Amazon. I have this one: Innova 3145 with this extension: Innova 3149 Extension.

-Shannon

1985 F150 XLT
1965 Mustang Convertible
2013 F150 Platinum

Socnarftrucks

Re: Idle variation on my 1986 f150 XLT 5.0 EFI AT

Ooh wow!! Didn’t know there was OBD1 code reader!! Would it register anything even if there’s no “Check Engine Light” on?

Socnarftrucks

Re: Idle variation on my 1986 f150 XLT 5.0 EFI AT

Many thanks! I will use that approach and also order the reader!!!

LastDeadLast

Re: Idle variation on my 1986 f150 XLT 5.0 EFI AT

My 85 EFI doesn't have a check engine light. YMMV.

Shannon

1985 F150 XLT
1965 Mustang Convertible
2013 F150 Platinum

85lebaront2

Re: Idle variation on my 1986 f150 XLT 5.0 EFI AT

Administrator

In reply to this post by Socnarftrucks

The erratic idle may be the same thing my son had on his 1986 F150. His problem was an aftermarket TPS. The idle stop screw on these trucks is adjusted based on a voltage reading. If the voltage is a little high the EGR vacuum and vent solenoids will chatter as the system tries to adjust things.

Bill AKA "LOBO" Profile

"Getting old is inevitable, growing up is optional" Darth Vader 1986 F350 460 converted to MAF/SEFI, E4OD 12X3 1/2 rear brakes, traction loc 3:55 gear, 160 amp 3G alternator Wife's 2011 Flex Limited Daily Driver 2009 Flex Limited with factory tow package Project car 1986 Chrysler LeBaron convertible 2.2L Turbo II, modified A413

mat in tn

Re: Idle variation on my 1986 f150 XLT 5.0 EFI AT

please use motorcraft tps (and other electronics ) only. i only say this because of the problems we have had in the year in the shop with "replacement parts". it seems that there is a specific response/range that the ecu works the best with . just a theory for me but we have ended up fixing many situations when we backed up to motorcraft parts. and tps is one of the biggest. have had a few iac's also and one now that i believe will be solved with a coolant temp sensor but it has not been back to me yet.

Socnarftrucks

Re: Idle variation on my 1986 f150 XLT 5.0 EFI AT

In reply to this post by 85lebaront2

Interesting. I do have an authentic Chinese imitation one from Autozone. I wonder, what if I delete the EGR?…. One way to find out…..

85lebaront2

Re: Idle variation on my 1986 f150 XLT 5.0 EFI AT

Administrator

In reply to this post by mat in tn

Mat, my point exactly! "close enough" will work sometimes, but not always. 1987 up systems are not as sensitive as they take the voltage at startup and use that for closed throttle, the EGR controls also changed from two solenoid valves to a single pulse width valve.

The other item I have found is the EVP sensor (EGR Valve Position), I have had a number of them where the plunger was too long, not by much, maybe 1/16-3/32" too long so the computer thought it was open when it wasn't. A little work with a grinder and problem solved.

Bill AKA "LOBO" Profile