It’s a 91 stock for ranger with original 139xxx miles on it and it won’t run right. I have put a new pcm (computer) in it and nothing and a mass air flow sensor and still nothing, I have also done everything you can think of to do with the fuel system like the injectors are new, fuel pump is new, gas line is new, fuel filter is new. I even put some new vacuum lines on the darn thing. What els could it be???? What it’s doing is when I his the gas it will shut off or when I plug in the mass air flow sensor it won’t run at all and I even replaced the mass air flow sensor and the plug as well. I can get it down the road if I hold the gas pedal to the floor but it spits and sputters and back fires like crazy. Oh and no there is no water in the gas I have checked that as well and put fresh gas in the truck. Thanks for looking if you know of anything please let me know thanks!!

 

Welcome to the forum

Reads like you have done alot of stuff

The computers and electronic stuff on engines make them much more reliable than before, but I am sure you are not thinking that at this point, lol.

The mechanics of gasoline engines haven't changed, they still need 3 things to run right.
Spark, at the right time
Fuel, in the right mix with air
Compression, above 120psi

Spark and fuel can be intermittent or variable, so outside of a quick test they should be the 2nd and 3rd thing tested.

Compression is either good or bad, not intermittent or variable
So it is the best place to start
You either find out it IS the problem, so don't waste time and money on non-fixes
Or you find out it is NOT the problem and can move on to spark and fuel

One thing that often gets overlooked with engine problems is exhaust flow, people focus on the air getting IN but ignore the fact it has to get OUT as well, lol.
A clogged exhaust system cause all sorts of issues, Cats and mufflers can break apart inside, usually with this the engine will idle, but run poorly as RPMs increase and exhaust can't get out.
But if its a bad clog, potato up the tail pipe, lol, then even idle is hard to get

Cold engine compression test
Remove all 6 spark plugs(V6) for the test, keep them in order in case there is a problem found.
Test 1 cylinder at a time
crank engine to get 5 or 6 "hits"
WRITE DOWN the results

1991 3.0l runs 9.3:1 ratio, so should be 155psi-165psi, but compression gauges are often not calibrated
Point of compression test is to make sure all 6 cylinders are close to each other, within 10%.
But all should be above 150psi

If compression test comes out OK then you can move on to Fuel and Spark


For Spark on the 1991 3.0l you will need a Timing Light
First check distributor
Pull off the cap and see if you can wiggle rotor shaft, if possible have someone crank the engine while you watch rotor turn, make sure it is not wobbling
A bad distributor bushing can have you chasing your tail for weeks, been there done that.
Ford cylinder numbering:
3 6
2 5
1 4
front

Cap should be marked with #1, this is important for electronic ignitions
Distributor rotor turns clockwise
1-4-2-5-3-6 is the firing order

Unplug and remove all the spark plug wires and put them on the "bench"(do this during compression test)
Then start from scratch with #1 and put them back on, 1 and 4 will be the longest wires

I have miswired spark plug firing order for 45 years, and still do, and it took me 20 years before I accepted the fact that I do make mistakes, lol, so I KNEW it was wired RIGHT, every time it wasn't

The 3.0l used a TFI Remote ignition module, usually mounted on the radiator support
This is what it looks like but not its location: http://www.therangerstation.com/tech...emote_tfi1.jpg

You need to find this because you need to find the SPOUT connector that will be near it
SPOUT connector looks like this: https://www.ford-trucks.com/forums/1...r-460-efi.html
Scroll down to post #7

SPOUT connector must be removed to set based spark timing, 10deg BTDC, with timing light

Fuel injection is what the computer is for in 1991, that's all it runs, and some transmission control if automatic, but just basic on/off for that

Spark is done by TFI Module, not computer, computer sends TFI module "suggestions" on the SPOUT(SPark OUT) wire but thats all
Engine will start and run without SPOUT wire connected, as seen during base spark timing adjustment.

Computer has 2 operating modes
Open Loop and Closed Loop
Open Loop is used on cold start and if there are sensor problem, computer has Factory Tables in memory that tells it what air/fuel mix to use for a 3 LITER engine at any given RPM and throttle position.
Open Loop also has Choke mode, gasoline engines still need to be "choked" when cold.
Choke means more fuel than normal, higher idle, advanced spark timing.

Closed Loop is when the O2 sensor(s) are hot enough, an O2 sensor needs to be above 600degF to work, O2s used a chemical reaction to detect Oxygen in the exhaust, that chemical reaction only takes place above 600degF.
This is why O2s are heated and why computer has to wait, Open Loop, until they warm up.
O2s also wear out, chemicals are used up, just like a battery wears out.

Computers has a 5volt power supply that runs a few main sensors, MAF, TPS, and ECT
All 3 get 5volts on the same wire from computer, so good to test if all 3 are getting this 5volts with volt meter, key on engine off.
MAF also gets 12volt with key on, MAF has a heater as well.

On the fuel rail is the FPR(fuel pressure regulator) it has the Fuel Return line attached, also a Vacuum line
Remove this vacuum line and check it for gasoline, it shouldn't be there, if so replace FPR, it will flood out the engine

1991 fuel pressure should be 30psi, no lower than 15psi with engine off, no higher than 40psi with engine running

 

 

Hey I was going through the exact same problem doing the exact same things but what I had to do to fix it was to put the engine back into time at 10