I have a got a mystery afoot

[dangerranger406]

Okay, where to begin,
I have 90 ranger with 2.9,
Issues I know of,
EVERY injector pig tail has clips broken off (have new ones to wire in on the way) cause some issues
The mystery however, is when I first start it, runs great, drive for approximately 12 miles and it starts running like dog turds, missing, no power
Timing was a little off (retarded) fixed that, issues remains, has high oil pressure, when I adjusted the timing oil pressure went to normal levels for about two days and it's back up like my blood pressure,
I'd just swap the motor for a 4.0L but I don't have the funds for it,
Any ideas on what or why?

 

[RonD]

Pre-1995 Rangers have separate spark and fuel systems
TFI module on the distributor runs the spark system
The PCM(ECM), engine computer, runs the fuel system(pump and injectors)

Engine computer has 2 operating modes, open loop and closed loop
Open loop is used when the engine is cold, also called Choke mode
Computer uses air/fuel mix tables in memory(factory tables) for a 2.9l engine
So the "compute" part of the computer is not being used
Basically just needs RPMs, engine temp and throttle sensors to look up the air/fuel mix in the tables

To compute an air/fuel mix "on the fly" the computer needs all the sensors working
One of the main sensors is the O2 sensor, and it can only work once its heated above 600degF, that takes 5 to 10minutes depending on outside temp

Once the O2 sensor is working the computer changes to Closed Loop
In closed loop the computer calculates air/fuel mix then checks the O2 sensor voltage to see if that mix was lean or rich when it was burned in the cylinders, it then adjusts the injector open time to correct a lean or rich voltage
This is done in milliseconds, so very fast corrections
And there is no "backup" sensor for an O2 sensor, if its wrong computer won't know it, so engine runs poorly
Computer saying, "garbage in = garbage out", and its quite true

You describe a fault in Closed Loop, i.e. after engine warms up, 12miles?
O2 sensors only last 12 years or 100k miles which ever comes first
So if you don't know the age of the O2 sensor then change it, may not be the problem but its the ONLY sensor that has a known time or mile limit
You just have the one O2 sensor on a 1990 2.9l

Next would be the MAP sensor, its on the firewall and have a vacuum hose from the intake and a 3 wire plug in
Check the vacuum hose for leaks, unplug and plug back in the 3 wire connector to clean its contacts
You should do the unplug and plug back in on any sensors you see to clean the contacts
90% of DIY sensor replacements correct the problem just because sensor's connector was unplugged and plugged back in, old sensor was just fine
All sensors only use 5volts, so any extra resistance in a dirty contact changes the voltage the computer sees so can/will cause poor running
Again, "garbage in = garbage out"

Next would be the computer itself
The pre-1995 computers have 2 or 3 radial capacitors that can leak/fail after 20+ years, under a $5 fix to replace them and clean up the circuit board
Worth a look, and another reason is to unplug the 60 pin computer connector and to plug it back in, cleaning all its contacts

 

[dangerranger406]

yeah, i replaced the O2 sensor and the MAP sensor, still having the same issue, i found damage in the wiring harness, whoever was in the engine and wiring harness before i got it was really careless when trying to test components, alot of spots that need to be repaired because the wires are kinked and/or the coating is broken open exposing bare wire, i definitely have my work cut out for me on this one, lol, i have a new ICM to install, (was free so figured install it just to rule that out) some of the damages in the harness are for critical components like the O2 sensor, injectors, MAP, and TPS, I also noticed that someone had the valve covers as well as the intake manifold off, and did a really sloppy job putting it back on,
The deeper i dive into the rabbit hole, the worse it gets, it will eventually make a really nice truck, till then lol, there's Lots to fix