I don't have a pressure gauge at home, I'll have to grab it at work tomorrow.

I'm flashing the codes. I don't remember if the scanner I have at work shows live data on these older EEC-IV systems or not...been a long time since I used it on something this old. Its a Snap-on MODIS.

I pulled the oxygen sensor back out so I could back probe both sides of the connector. It zeroed out on all three pins so the connection is not the problem. The sensor tip is black, so I pulled all of the plugs and checked them. They all look normal.

 

I keep forgetting to ask how your scanning the computer & coming by the code numbers?

 

I'm connecting a jumper wire between the signal return and self test input pins. Turn the key on and it flashes the key on, engine off codes first followed by the continuous memory codes.

When the light first came on over a month ago I did use an old, obsolete MT2500 scanner at work and it showed code 41 at that time too.

 

I've been doing more research online. The oxygen sensor has three wires and as I understand one of them runs to a fuse link to supply power to the heater? I don't have any detailed wiring diagrams at home. Should this pin on the connector be hot all the time? Would an inoperative heater circuit cause this problem or would that just delay or prevent it from going into closed loop?


I should also add whenever the light comes on its almost always when I'm coasting or under light throttle. Usually happens right after I slow down to make a turn.

 

BBB Industries.com has a wiring help deal,,,just make a user name and password and your in,,,i looked up your truck and the hot wire to sensor is GY/Y,,,i assume this is grey with yellow strip,,,,should be hot with koeo and it is a low voltage,,,,i think 0.10 volts or less but will not swear to that voltage,,,,if you go to site just make sure to click it to wiring not TPS to see wiring diagrams,,,,i see pawpaw is now helping you out,,,,,he is very good with these old rangers

 

i still wont swear to the voltage but i think that voltage changes from 0.01 to 0.09 volts depending on rich or lean needs,,,,,but cant seem to find exact voltage anywhere yet

 

I'm going to follow along, have the same issue on the same truck just an 89.

 

The verdict is in: it was the fuel pressure regulator. I took it in to work today and got there early so I connected my scan tool first but on these older EEC-IV systems I cannot view live data with it. With the Snap-on scanners you can't until about 1990 or 91 on Fords. So, I connected the fuel pressure gauge, turned the key on and the needle spiked at 105 psi!!! I started the engine and it held steady at 105. That regulator must have been completely blocked and wasn't letting any return fuel flow back. I called around but nobody had a new one in stock and the cheapest quote I found was $75, so I figured its worth trying a used one. During my lunch break I ran home and pulled one off one of my parts trucks and put it on tonight after work. Now I'm getting 43 psi KOEO and about 35 psi with the engine running. Big difference. I cleared the keep alive memory again and so far no check engine light. I won't know if my fuel economy improves for a few days yet but that had to be the problem. I'm surprised it would even run with the pressure that high, let alone run normally. I no doubt shortened the life of my in frame fuel pump.

I still don't understand why such high fuel pressure was causing a lean exhaust code. I would expect the exact opposite. I never suspected the regulator was restricted because the engine was running as smooth as ever and had absolutely no driveability issues.

 

Ok, good find & feedback on the fuel pressure check. Way to hang in there & not give up on your trouble shoot & to hear of the positive results.
Will be interesting to hear how fuel consumption checks, now that pressure is within proper range. Don't know that I've heard of pressure that high before. Bet the fuel injectors appreciate it being back in range.

 

hhhhmmmm wonder if running that rich could have hurt the cat,,,assuming a 88 has a cat,,,a can of cataclean would not hurt but it is kind of pricey at $29.95 or is it $19.95

 

Yes, it has a cat. Time will tell I guess. Both of my parts trucks still have good cats so I have backups in case this one does go south. I never saw any black smoke while it was running so I don't know how much unburned fuel was ever reaching the cat. I didn't notice any difference in how it ran after switching regulators either. Very odd that it ran as well as it did. I'm still completely stumped as to why it kept throwing a lean exhaust code.

 

Seeing as how you were reading the codes via a blinking CEL & the difference in a lean, or rich code is only one number, all you had to do was miss one blink!!!!

We don't know How rich it was likely running, but if you come to suspect a damaged cat converter you can perform a test with your vacuum gauge, by scrolling down & following test scenario 14 here Vacuum Gauge is an Invaluable Tool when Diagnosing Engine Problems

 

I know I counted flashes correctly and I watched it until the sequence was over. I checked it several times and on several different occasions. Every single time it came up with 41. That's what it showed when I had the scanner connected when this problem initially started and again yesterday too.

I do have a test plug for checking exhaust back pressure. Remove the oxygen sensor, screw the plug in and connect a pressure gauge to the port on it. I'm not too concerned with it though...for as long as it ran like this its not showing any signs of a restricted exhaust. They don't do emissions testing where I live either.

 

Ok, will be interesting to hear how fuel consumption goes now that fuel pressure is back in range.

 

Did you ever measure the fuel injector pulse width Before & After the fuel pressure regulator was replaced, if so post up the Numbers.