Well I switched out computer from my old one to new one and it had no change. Possibly something in that wire circut

 

Hey Ron. So I am plugged the O2 sensor and it still did the same thing. I tested my fuel pressure to. Runs about 38 with the vacuum taken off the pressure regulator. Runs around 30 add about 2500 RPM and At idle. I have a question though. Do you know what sensor that is that screws into the side of the engine block right above the oil pan but in front of the starter on the driver side of the engine. It has two wires coming out of the clip that go up to the wiring harness and there’s a third one that comes out with a little black deal on it at the wiring harness. Could it be some kind a knock sensor or is it also a temperature sensor of some sort

 

Yes, that would be the knock sensor, seen here: https://www.broncoii4x4.com/forum/viewtopic.php?t=4784

And yes it can cause low power but usually not rough running

read here: https://www.therangerstation.com/for...sensor.137799/

Replacement knock sensors are not available new

 

So I tested the fuel pressure. 38 with the vacuum line taken off of the fuel pressure regulator. A steady 28 to 30 when Idling. And a steady 28 to 30 when revved up around 2500 RPM. Do these values seem good to you. One thing I noticed is I had to turn the key on and off a couple times to get the fuel pressure over 30 with the engine off. Could be the gauge but when I first turn the key on it stopped at 10 then I turn the key Off and then back on and it kicked it up another 10 and then did it again and it kick it up another 10. Maybe that’s just coincidence or just to gauge being a little sticky. But if it was a fuel pressure problem it seems like it wouldn’t run good at all and it actually does eventually. Either way I got new fuel filters to put on and I’m going to change the thermostat for the heck of it.

 

35psi is spec in 1997 and earlier Rangers, so yours is fine

But................Pressure should hold above 20psi with key off for a few MONTHS, not minutes, hours or days..............months

Yes, fuel pump only runs for 2 seconds with key on, and that 2 seconds = about 10psi, its a safety thing, the 2 seconds

But engine should starts with just 20psi, and when RPMs are above 400 fuel pump will come on full time, so it might stumble with only 10psi but then jump up in RPM when pump comes on full time

The Pressure Regulator(FPR) should hold pressure, key off
The fuel injectors should hold pressure, key off
The fuel pump has a check valve(part of the pump) that should close when pump is off to hold pressure, key off

These 3 things are what hold pressure in the fuel system when key is off.

You can pull off the Return fuel line on the FPR and then cycle the key on and off to make sure FPR is holding fuel in
Unless engine is flooding, smell like gasoline with key off then all injectors are holding
Which leaves fuel pump
All this assumes no gasoline is leaking out under the truck, lol

It isn't a big deal to cycle key on and off twice before starting, it won't cause running issues

 

Hey Ron I had a thought. I told you that I tested the coolant temperature sensor and the readings were good. Is it possible that the line that has the voltage drop as it warms up is sending this to the computer possibly be in lost somewhere like a bad connection or splice. I was looking at a wiring schematic it seems that there is some kind of connector at some point and also a splice in with the TPS and map sensor. I’m just curious if it makes sense at all. I’m just not sure if when the car is warm and I shut it off for just a minute if the coolant temperature sensor is still in play at that point To cause the symptoms it does during cold and warm start ups. What do you think

 

Hey do you also think I can Eliminate A possible intake leak from the gasket. I was reading a couple things and it seems like some people were having that problem. But it does run really good.

 

Yes, in 1986 the orange/white wire is the 5v Reference voltage sent from pin 26 on computer to several sensor, i.e. TPS, MAP, EGR

And each of those also has a Ground Reference from computer pin 46, Black/white wire, this ground is also used for IAT(air temp), ECT, Knock sensors, these use pull up resistor circuits,so they are not "sent" 5v but register like they were when tested

So the Black/white wire from pin 46 connects to TPS, MAP, EGR, IAT(ACT), ECT, and knock sensor, as the reference ground

Ford didn't use a connector just a splice with tape, in the engine bay, S127 in my diagram, splice 127

The 5v from pin 26 uses S126, splice 126

 

So is there anyway to test if that signal is getting back to the computer Or is it tied into too many other things. And what do you think about the possibility of there being an intake leak From the gaskets and as it heats up it seals it self. To me it doesn’t seem likely because of how good it runs

 

Hey Ron what do you think about the possibility of a bad fuel injector. Not long after this problem started I checked all my spark plugs and the one in the very back on the passenger side actually was missing the end of it the electrode whatever you call it. Surprisingly the card and run that badeven though I discovered this. I was reading some stuff in it kind of sounds like it could be a possibility

 

Not sure how a broken spark plug would be related to a broken fuel injector?

If you get Pinging/Knocking noises from Lean mix that can break spark plugs(and pistons as well), but...................your 1986 fuel injection is Batch Fire not Sequential EFI
On a V6 this means 3 fuel injectors open for each 1 rotation of crank shaft, 2 on one side 1 on the other side, then it switches to the other 3 on the next rotation

So a single injector failure won't effect its cylinder that much, the lower intake on that bank is full of air/fuel mix from the other 2 injectors and each cylinder just sucks in what it needs, if an injector stopped working then O2 sensor would show Lean and computer would open injectors longer to compensate

 

Hey Ron I have a couple things to report it may help. I went ahead and changed out my injectors and replace the fuel filters. Today I changed out the fuel pump. The high-pressure one. It has been doing that usual starting issues and then running OK although I did notice if you hold the gas pedal down at about 2000 RPM it does do a slight surge. And that it might stop and start doing it again. You don’t really feel it when you’re driving . Anyway I’ve switched out the high-pressure in line pump i started it up and I could not get it to stop doing its problem at all. So I jumped out of the truck and kept going with my hand and was shaking stuff around to see if I can get any shorts.i noiticed it was doing some white smoke and it had done that today when I was driving it alsowhich It didn’t really used to do. Anyway it was doing that while I was keeping it going so one of the things I did was pull off the vacuum line to the map sensor and I had to keep it revved up because of the vacuum leak to keep the engine going. It started to smoke really bad and the engine kind of changed I put the map sensor on it it started to idle smooth. I thought that was kind of weird so I let it idle for a bit and turn off the car. As usual when I started again a minute later it was doing it’s thing but would not stop again. So I did the same thing with the map sensor it started to smoke really bad when I pulled off the accum line I kept it revved up so it wouldn’t die And then I popped the vacuum line back on and it idled smooth again. As smooth as it will anyway it does Idle higher than it should. So it was a change of events with that new pump and then all the smoking it was doing now and this map sensor thing that changed the shaking. I had replaced the map sensor already and it made no difference from the old one so I’m not sure what this was all about. Any ideas

 

I forgot to mention I also change the gasket on the intake manifold. When I switched out the injectors I went ahead and took off the intake and replace the three part gasket just to be sure there wasn’t some kind a vacuum leak

 

The MAP sensor tells computer the vacuum pressure in the intake manifold, at idle, throttle close, the vacuum is high, when you open the throttle the vacuum drops, the MAP sensor reports the vacuum drop, computer then adds more fuel because driver wants to go faster

When you unplug the MAP then thats 0 vacuum, computer thinks you "floored it", wide open throttle, so computer DUMPS alot of fuel into the engine, hence the smoke, but grey or black smoke

White smoke is water or coolant being burned in the cylinders or exhaust system, its not unusual to get a bit of white smoke on cold start, thats just the condensed water in the exhaust pipes, Cat and muffler being vaporized as they heat up, but doesn't last very long

You can usually tell if cylinder has a leak from bubbles in the radiator
Cold engine, remove rad cap
Start engine
Let it idle or as least keep it running
No coolant/water should come out the rad cap
No bubbling up
If that happens then you do have a cylinder leak, i.e. blown head gasket or cracked head

Do a search here for "Glove Test" free and simple way to confirm leak and find which cylinder has it

 

Hello and good afternoon sir so I have the exact same issue I have replaced the majority of the things you replaced and I cannot figure it out it’s driving me nuts! Please let me know if you got it figured out!

Greatly appreciated ,

Regards

Jef

 

Actually I did figure it out it was a loose valve seat on the passenger side rear cylinder head. I could hear the banging around more than I could hear it when it warmed up. Finally pulled ahead and found two loose valve seats. My motor was under warranty so they sent me a new head and problem solve

 

OMG! I printed this whole thread and studied it for a while before trying the suggestion to disconnect the battery, turn the headlights on for 3 seconds, reconnect the battery and start the engine. That is the first and ONLY thing I tried.
It worked!!! My truck runs great now. I have been trying to figure out this same problem for months.

Thank you for the very thorough explanation of all the troubleshooting ideas by RonD. I have been using this forum since I purchased my truck in January 2022. I couldn't have fixed up my truck without your help.