Last ditch effort…

[LongStrangeTrip]

Hey all,
New-ish member, first post. Long time lurker and consumer of knowledge on here.
I’ve got a 93 Explorer Sport, 5 speed, manual T case, manual hubs. 120k miles.

I have owned this vehicle for 3 or so years now, I’ve been fighting an issue that I just cannot seem to resolve. I drive it 2.5 miles to work every day, when I am feeling lucky. The issue has changed (sort of) over the years, but has always been present in some way. I’m at my wits end, hoping to get pointed in the correct direction here. I have crossed a lot of normal maintenance items off of the list, some 2 or 3 times out of frustration. I never get a CEL, I have a proper code reader and only ever get 556 in memory codes for fuel pump circuit.

It’s current state is as follows…

Cold start, above 50 degrees: High idle attempts to engage but is sporadic swinging between 1800 and 400 rpm, nearly dying but doesn’t. Engine is backfiring during this. After 5-7 minutes, the idle starts to improve and backfiring becomes less. After engine to operating temperature, the idle settles around 900 rpm, swinging between 800-1000 randomly. Occasional misfiring. If I unplug IAC at this point, the idle smooths out and misfire stops, idle sits at 1000rpm. Even when warm, vehicle is sluggish, seems to have some pinging, definitely have to be cautious if taking off on an incline due to lack of power.

Cold start below 50: hard pressed to keep it running with foot on pedal.
Here is what I have done over the last few years…

Spark plugs
Spark plug wires
Tested coil output
Replaced battery
Replaced alternator
Cleaned intake
Replaced all vacuum lines
Replaced EGR and solenoid
Replaced evap canister and solenoid
Replaced pcv valve
3 replacement IACs
2 Intake air temp sensor
2 coolant temp sensors
2 fuel caps (one stolen)
2 throttle position sensors
Tightened intake bolts
Replaced Throttle Body
Cleaned MAF sensor
Smoke test on engine for vacuum leaks
Eliminated items like brake booster for testing
Checked voltage at IAC
Checked voltage at fuel pump
Checked fuel pressure
Replaced fuel pump relays
Cleaned grounds
Cleaned & added dialectic to all connectors

I’m sure a few other things I am forgetting. I used Motorcraft parts when available.
I bought a used throttle body off of a runner but it was an automatic. So I thought maybe base idle trim could be different and messing with pcm. So I attempted to reset idle with IAC unplugged. Idle will not go below 1000 with IAC unplugged, and screw backed out to where no longer affecting base idle.

I keep thinking vacuum, but I can’t find it. I have put half of a can of ether in the engine compartment hoping to find a secret vacuum leak with an increase in idle, but never can.

Someone suggested replacing PCM, but sometimes it seems to run pretty good, I feel like a pcm failure would be permanent.

The colder it gets, the worse the issue is.
Can someone please help me out? I’ve always done my own wrenching, I’d hate to have to take it to someone.

 

[Rick]

With all you've already done, I'm thinking bad fuel injectors.

Have you pulled the plugs to "read" them? Reading the plugs can show if there is a dead cylinder or two, or even a wet plug which would indicate a fuel injector that is sticking open.

 

[LongStrangeTrip]

I have not done that, but will check that and report back.
Fwiw, the fuel pump doesn’t seem to run very long when first turn on the key and it doesn’t ever smell like it is running rich.

 

[Turdle]

I see coolant temp sensor mentioned in the he replacement list. Sure you replaced the correct one? Just double checking, as there are 2 coolant sensors on this engine.

 

[LongStrangeTrip]

I replaced the coolant temp sensor for the PCM, not the gauge. Twice, as I thought the first had gone bad.

I had to make a trip across town after work, so I decided to unplug the IAC. This thing runs so nice with it unplugged, just no high idle. Power is back, no backfiring. Idle around 900 which I think is still too high for a manual.

 

[410Fortune]

What is your fuel pressure when warm? Have you changed fuel filter?

 

[UtahX]

What do you have to lose, replace the fuel pump and fuel filter.

 

[Rick]

I have not done that, but will check that and report back.
Fwiw, the fuel pump doesn’t seem to run very long when first turn on the key and it doesn’t ever smell like it is running rich.

With the engine off the fuel pump should only run a few seconds before it turns off.

 

[LongStrangeTrip]

With the engine off the fuel pump should only run a few seconds before it turns off.

What is your fuel pressure when warm? Have you changed fuel filter?

I did changed fuel filter, I forgot that one. It was about a year and a half ago that I checked fuel pressure, I don’t recall what it was, but it was within whatever spec I was looking at. Sorry for the lame answer.
Fuel pump does only run for a couple of seconds. Always been a consistent amount of time, even if it sits for a few weeks

 

[Turdle]

The fuel line directly off fuel pump motor, inside tank, can be split , so, if you replace just the pump motor, check for that.

 

[410Fortune]

The pump is only supposed to run for a couple of seconds to prime the system when you turn the key to run
Once the engine starts the pcm will start the fuel pump running again, the pump will run as long as the engine is

 

[Running an Explorer]

I had the exact problem. The explorer would idle surge like yours when cold and warm. It would even do it when driving down the road. I'd have plenty of power then it would feel like I let off the gas a little and lose power then regain it a sec or two later. Replaced lots of parts. Eventually like you I just would drive it with the iac unplugged, it would run good like that. I even replaced the pcm with one from O'REILLYs and it ran good for about 50 miles then it started stalling everytime I hit the gas when going down the road or taking off from a stop. So I put the old pcm back on and it never stalled out. Idk if the surging was related to it stalling with the new pcm or just a bad rebuilt pcm.
So finally I just kept the iac unplugged and set the idle screw up so it wouldn't idle so low when cold. I never figured out what the problem was and it still runs fine this way with the guy I sold it to.

I'll second the motion for cleaning injectors. I sometimes would have a miss for a couple secs after a warm start then would go away. I always thought that it might be a leaking injector causing that but never thought that the surge and miss could be related like that.

 

[LongStrangeTrip]

I appreciate everyone’s replies. I had intended on checking plugs and reporting back, but had some things come up.
I have run a bottle of injector cleaner through this about every 6 months, for what it’s worth.
I will be back on to this on this upcoming weekend and will report back.

 

[LongStrangeTrip]

Sorry for the long delay for an update.

I started after it again, the boss (wife) insisted that I get it to a mechanic instead of spend more time on it. I’ve never been to a mechanic before, but so far they seem reasonable.

They’ve had it for a few days, and believe it to be the computer. They said they cannot get their reader to communicate properly with it, so they suspect it to be bad. FWIW, I pulled the computer a couple of weeks ago and looked it over inside, physically it was immaculate.

So, I should hear back either right before or after thanksgiving with an update from them with what results of new part. I’ll report back after that. Thanks again!