Idle variation on my 1986 f150 XLT 5.0 EFI AT

[Socnarftrucks]

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]

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

[Socnarftrucks]

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

[LastDeadLast]

Any codes?

-Shannon

[Socnarftrucks]

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

[mat in tn]

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]

Ohh!!! I’ll try that then…. Thanks

[LastDeadLast]

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

[Socnarftrucks]

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

[Socnarftrucks]

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

[LastDeadLast]

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

Shannon

[85lebaront2]

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.

[mat in tn]

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]

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]

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.