Roger Mabry

At idle, it just increases the rpm all of a sudden to 750...then after a few minutes return back to idle rpm of 600. This is with it in Drive...

It will also do this on the highway at 55-70...increase and then drop back down to selected speed by pedal.

Before it has warmed to normal temp, this increase can cause it to die but it starts right back up?? It is worse with the AC on.... normally dies.

Any ideas? Am going to check for vacuum leaks today but think it is CA feedback solenoid that is very hard to find/purchase. Passed CA smog last year
fine. Had to change the cats to pass due to years of prior poor maintenance when truck was out of the state.

Roger Mabry

Can you imagine after only 30 years Motorcraft has made them "obsolete"?

No leaks or other problems found... could not find one the last time I tackled this problem. Hoping for the best.... at least one major thing will be eliminated...

Roger Mabry

With both new parts it idled about 50 rpm higher all the time.

But, after getting up to normal temp... the surging idle was there... better
but still increasing at times.

I sprayed carb cleaner around the hoses and connections and found another
hose that was soft that provided manifold vacuum to the three port connector on the top of the firewall. Replaced the hose from the manifold port to the top connector.

That made the idle rougher than before? But, at least another problem had
been resolved.

Will continue the "fight" tomorrow.

Roger Mabry

After changing all the bad or just old hoses, plus a new PVC valve because I broke one
plastic nipple removing a hose.... it idles smoother and I actually had to increase
the idle speed in gear....

Added a old vacuum gauge to the dash so I can tell any changes and use it to improve
driving style and get better gas mileage...

Works with the AC on now and in gear without stalling.... cross your fingers.

Ford 4x4

Nice diagnosis and fix!!

Roger Mabry

I have narrowed the problem down even more... it only does this idle speed change when up to normal temp. Cold it idles smoothly and does not start this problem until off the fast idle and does it more when in gear. It does it about every minute or 90 seconds.... still increases the idle 150 rpm and then lowers back down.

There is minor moisture on intake manifold below the carb (leak?).

Got a 41 code this AM... on each test. Lean condition... Passed smog fine in CA and has near new EGR valve that is working properly. No vacuum leaks that I can find with carb cleaner sprayed around on each connection.

Have no more ideas except take off carb and fix minor leak... after sitting for several days you have to pump the gas pedal once to get it to start = lack of
fuel in bowl?

Other than this irritating condition, truck runs fine...

Roger Mabry

Key off test, after running up to normal temp and clearing the old codes first. Keep getting the same codes... adjusted air to reduce air and increase mixture.

Bought a new ECT sensor (Code 21) and it made no difference... R&R near new 02 sensor and cleaned it and cleaned up connector.. no difference.

While idling, after warmed up, the rpm just drops in Neutral at 750 to about 450 rpm and then climbs immediately back to normal of 750?? It does not die now after fixing all the vacuum problems.

Cannot find a new EGR vacuum solenoid anywhere (CA emission style only for 83-84) it tests OK but would change it just for "kicks".. EGR tests fine and is new about a year ago. Used new silicone tubing to replace many of the old connections.

Out of ideas but want to and will fix this remaining problem...

Key on test is the same codes... any idea? This is crazy and should be fixable

Roger Mabry

Got rid of the Code 41 by just enriching the mixture screws on the carb... it must have been borderline lean.. 21 inches of vacuum at idle now and even from side to side on the screws.

Still have the Code 21 = ECT voltage???

Idle drop of 350 rpm still there when running at normal temp.

Roger Mabry

Think I found the last problem... the fuel filter into the carb was "broken" inside
and something is moving around when shaken.. plus when you blow in the
direction of the fuel there is a partial blockage.

Getting new fuel filter this AM and will report results... MOST PROBABLE
PART... was replaced less than one year ago!

The old one will join my "Wall of Shame" = that are parts that failed in odd ways.

Roger Mabry

The horrible rpm dropping is gone... new fuel filter that was not partially
blocking the fuel flow made it run correctly.

Fuel filters are not supposed to "rattle"...

Will now finish up adjusting the mixture and idle rpm to the correct numbers...