1998 Ford Ranger XLT 3.0L 4x4

So, my ranger is dieseling when i turn it off.

I have ran sea foam in the tank and through the intake.

This has seemed to help but it still diesels when I shut it off.

Any suggestions??

 

You most likely have a leaking injector

Dieseling is when spark in a gasoline engine is cut off but residual heat in a cylinder or cylinders can self ignite a gasoline/air mix, in a carburetor system this occurred because the rotating engine could suck in more gasoline air mix from the Jets
Dieseling is almost unheard of with fuel injection because injectors are powered off with key off, so there is NO gasoline to keep engine spinning just air

So you have gasoline leaking in from somewhere..................and on a 1998 that could only be from an injector, but..............
1998 and up did have a Pulse Damper on the fuel rail, it has a vacuum line attached, look for it, remove vacuum line and check it for gasoline, if found then Damper is leaking, replace it, thats the source of the gasoline

Test for leaking injector
Start cold engine and warm it up for 5 minutes then shut it off

Turn key on and press gas pedal down to the floor all the way<<<this turns fuel injectors OFF
Hold pedal down and try to start engine
It should not start, it should not fire at all

If it fires then you have a leaking injector

Let engine cool off
Unplug the coil pack's 4 wire connector so no spark

Now do the same test as above but crank it a few times, do key off and key on between cranks, pedal to the FLOOR the whole time
Now start pulling out spark plugs
The leaking injector spark plug tip will be WET with fuel, others will be dry

 

Okay so, I did the leaking injector test. It does not start when you hold the pedal down.

And as for the pulse diaphragm on the fuel rail where is that located? It it right under the ignition coil round thing sitting on top of fuel rail ? And does it have to be started to see if it’s leaking?

 

Yes, thats the pulse damper, no just pull off that vacuum line and see if there is gasoline in it

But if it doesn't fire on the "Clear Flooded Engine" test then there is no leaking fuel, from injector or Damper, so engine can not "diesel"

Really don't know whats going on

Not even a diesel engine can 'diesel' without fuel, lol.

 

Well pulled that and no gasoline....

so im not sure what else to check cause it still diesels when I turn it off sometimes.

 

Not sure either

One thing you can check and it is called the "anti-diesel" screw, lol

Although thats because it looks like an idle screw and it ain't

Any way on the throttle linkage there is a screw that looks like an idle screw, as said it ain't because you can't use an idle screw with fuel injection

After engine is warmed up and idling
Unplug the IAC Valves 2 wire connector, RPMs should drop to 500 or engine may even stall, either is GOOD it means screw is OK

If RPMs do not drop down low then, turn that screw counter-clockwise and RPMs will do down, set it so engine is barely running, then plug IAC Valve back in and RPMs should go up

 

Unplugged the IAC VALVE and engine just sits about 1,000 rpm.

turned screw counter-clockwise and it did nothing still idles about 1,000 rpm when iAc is unplugged

 

You have a vacuum leak somewhere
With IAC Valve unplugged, unplug one hose at a time on the intake and cover port with finger, if RPMs drop then follow that hose to find the leak

 

Those are all good suspects, but if they all check ok, my next suspect would be the EVAP canister purge valve or as Ford calls it, Vapor Management valve. Mounted on firewall next to brake booster. Has large hoses with quick disconnects, one from canister in rear of vehicle and the other to intake manifold port, also one small vacuum line that operates the valve when computer energizes the vacuum solenoid. Pull off and plug the small hose and pull off larger hose to intake and plug it or just pinch it shut with vise grip , carefully. Hose is usually deteriorated over the years.. Drive vehicle till warm and under same conditions to see if dieseling goes away. If problem goes away, you can unplug the electrical connector, which should keep any vacuum bleeding past valve pulling fuel vapors from canister if it is working correctly. This will confirm valve is bad when unplugged electrically.

 

Ok so I don’t have anything on the firewall like that... but under the bed at the rear of the vehicle I have it.

 

Actually I think I found it it’s not on the fire wall but in front of the booster

 

That's the EVAP canister, not that.
The EVAP valve is located on the fender well near the fuse box, there will be a single large vacuum line coming from it down low, below the EGR valve.
Pull the line off that and plug the hole in the intake manifold and see if the dieseling stops.

Could be an EGR problem too, but if it's that faulty to cause dieseling, there would probably be a code for it.

 

That's it...

 

Another thing to be checked is the PCV valve and it's vacuum line.
There is also another line that comes from the oil filler cap to the air tub, make sure there are no vacuum leaks there.
Any leaks in the EGR tube will become unregistered air and may cause problems like dieseling.

Check the "O" ring on the dip stick too, make sure it's even still there.

 

Okay so I plugged the two lines coming from the purge solenoid.

I still have high high idle so I’m certain there is a vacuum leak still somewhere. And it only diesels when it’s high idled . If I let it come down by it self it shuts off perfectly

the oil filler - air filter tube line is good no leaks

tomorrow I will be looking at the pcv line cause I can hear a hiss back there.

 

Update

blocked off the two lines to the purge valve and no more dieseling if I let it idle down.

still have high idle

unplugged the IAC valve and now how normal idle