This is a '89 2.9 liter with >250k miles and it has passed all of it's smog tests in the past. It's now giving >1,200 ppm of hydrocarbons at idle, but passing/much lower figure at 2,500 rpm. Spark plugs are fairly new and look like they're supposed to. All sensors, including O2, are original. No related codes in the computer. O2 sensor was disconnected a while back to remove transmission. The truck basically runs okay. It stumbles a bit when starting out when cold but it has done that back when it was passing smog tests. This has a three wire O2 sensor and I tested resistance for the pins of the two white wires (heater?), and it gave no resistance. Have not (yet) tested if I get voltage on the other pin when the vehicle is idling or rev'd. I'm assuming the O2 sensor has reached the end of the road, and have ordered one, but I hate to throw parts at it rather than diagnose the problem. Your advice and comments specific to the 2.9 liter requested.

 

replace the O2 sensors, they are only good for 100,000 mi, they do wear out. I'm not sure your year has but newer 3.0 have 3. one on each side below the exhaust manifolds and one after the cat. the 2 upper ones control the fuel input to the motor the one behind the cat just tells if the motor running clean.

 

I'll give an update since this has a good number of views and I won't be able to get back to this project for a few days. Oxygen sensor was not the problem. Found I had a disconnected ground strap (firewall to head) and that seemed to make a big improvement, but it was a very quick check on the sniffer to see if that made a difference. After cleaning out the IAC and throttle body it is testing around 100-400 ppm HC at idle. The guy with the test shop finds the inconsistency very interesting. He checked timing for me and states it's right on the spec. My hunch now is that there's one or more fuel injector(s) that don't close completely, and that's causing unburned fuel at idle but the leaking isn't enough to make a difference at higher rpm. Will update you when I swap in new injectors. FYI, took a look at my 2021 and 2023 smog test reports and the results at idle were 32 ppm and 68 ppm. Before that it sat for about 5 years.

 

I can give this thread a closing post in case the info is someday helpful for someone. I removed and cleaned my fuel injectors and gave them new o-rings and filters from Injector Shop. I think that did fix a problem because beforehand I had little to no fuel pressure in the rail seconds after turning off the engine, but now I get a squirt out of the Shrader valve. I had to give up on getting a pressure reading using the OEM brand tester from the parts store because its fitting on the hose end pressed down on its Ford adapter so much that it would close off its Shrader valve in the down position. But while the motor seemed to run better after cleaning the injectors it didn't fix the high HC at idle. After reading a bunch of threads here and on The Ranger Station I tested for a stuck open thermostat using the 'does the upper hose get warm too soon' method, and sure enough that's why my temp gauge stayed so low in the normal range. I took RonD's advice and put in a 192 degF (he recommends 195) thermostat. But I didn't really fix the high HC at idle problem until I realized that the code 67 I was getting (Neutral Drive switch circuit open) may actually be relevant for the at idle fuel mixture. Turns out EEC-IV looks at whether you're in gear and whether you have your foot on the brakes to anticipate that an at idle engine is going to get a load put on it, and it enriches (lengthens the fuel injector squirts) the fuel mixture to handle that situation. (See the book Ford Fuel Injection & Electronic Engine Control (https://www.therangerstation.com/tec...-fuel-systems/).) Now for this to make sense I may need to tell you that the early M5R1 manuals have both a neutral switch and a reverse switch, the latter for the back-up lights. After several hours of futzing with the switches and the push rods that press on them I came to the conclusion that the push rod for the neutral switch wasn't lubricated sufficiently and it was holding the switch in the open/in gear position. After that change the HC at idle dropped and it tested today at 5 ppm at idle and 8 ppm at 2500 rpm. Idle is about 100 rpm too high right now, but the engine is dead flat at idle and it pulls at low rpm like when it was new.