Howdy all -
Quite a few updates since my last post (in my signature, but posting here)
1990 2WD f150 Custom 4.9L
New Front fuel tank + pump due to tank rot and failed pump | New EGR valve | New TAB / TAD / EVP | 90% of vaccuum lines replaced | New Coolant temp sensor | New Coolant Thermostat | New IAC | New Distributor cap | New Rotor | New plugs | New Bosch O2 Sensor (heard this might be an issue vs motorcraft) | New FPR

I managed to clear my EGR codes by replacing the whole EGR so that's great. Up until this morning i've only run up to KOEO tests not KOER. I've had code 41C for a couple months now

41C (HEGO (ho2s) sensor voltage low / system lean OR No Ho2s switching detected.

This morning I ran a KOER test to see if it would point me in a direction to clear 41 and I got 42R
42R code 42-exhaust gas oxygen(ego) sensor: voltage signal always "rich"(high value)-does not switch.

I'm at a bit of a loss as to where to go now. I read the Fuel pressure regulator is often a culprit, but with the new one in it hasn't made a difference.
My CEL light typically comes on after about 20-30 minutes of city driving (NYC)

I read through this thread https://www.ford-trucks.com/forums/7...ally-mean.html but his solve was with the FPR, unfortunately mine is Napa new and my 41 / 42 codes are still there.

any thoughts as to what to check next?

 

Those definitions are generic. The definitions on this page are specific to these trucks:

(click this text)


Codes do not indicate parts to be replaced; they indicate circuits to be diagnosed. The first diagnostic step for any code is to confirm that the indicated condition actually exists. IS the engine running rich? The tailpipe would be black with soot instead of toasty brown or rusty.

"New" is not much description for parts. Brand, PN, & source would be more useful to know. And for EACH part: what diagnostic test did it fail, and did the replacement pass the same test? Which coolant sensor did you replace - the 1-wire or the 2?

 

Got it thanks - in that case it's
41 --c No HEGO switching detected
42 -r- HEGO sensor circuit indicates system rich

I can pull the part #'s when i get a chance

Also forgot to mention - the CEL comes on after about 15 - 20 minutes of city driving, but no noticeable difference in performance. But after an hour - hour and a half, if I try and accelerate a bit, it stumbles until I floor the pedal, then it stumbles but all of a sudden picks up. Not sure if that's helpful in any sort of diagnosis.

I replaced the IAC because it had a bunch of buildup on it, cleaned it off and it just looked a little worn so I figured might as well replace while i'm in it.
Coolant temp sensor - same story, tip had some wear on it so i replaced.
Thermostat I replaced because it had a 180 thermostat in there, replaced with a Napa 195 which is the stock temp.
Distributor cap / rotor / plugs / pug wires - I replaced all of these because my plugs were absolutely toast. The wires weren't in great condition either so I went ahead and replaced them + the distributor cap and rotor since I was in there.
I changed the FPR due to my fuel pressure being out of spec. The new FPR brought my fuel pressure within acceptable range.
Coolant sensor had 2 wires. Replaced with a copy
photos of sensor + Plug + exhaust below

o2 Sensor

Exhaust - doesn't look rich to me?

coolant sensor

OLD plugs - think they were factory

new plug without about 50 miles on it

 

No, 1990 spark plugs wouldn't look that nice. Not that it's necessarily a problem, but I don't think that's the right oxygen sensor. Was there a lot of slack in the wires? And you didn't say why you changed it. I'd put the original back in. That tailpipe does NOT look like it's connected to an engine running rich. That the CEL comes on is not informative - WHY it comes on is what matters. Did you read that whole page about codes?

 

Sorry for the delay - holidays!
I'll check the o2 sensor in the am for slack. I changed it at the suggestion of my local mechanic, he said it was the factory o2 sensor and it wouldn't hurt to try swapping it as it's 31 years old. I'll check the bosch for excess slack and report back. Unfortunately I don't have the original anymore, but have no issue ordering the motorcraft replacement. I did read through the whole code page - you're referring to checking the circuits instead of just spray and pray part replacements yea? I'll hunt around for the correct way to test the o2 sensor tonight and give it a shot tomorrow

 

Rockauto.com shows the Bosch O2 sensor P/N 15716 with an 11" wire, and 15719 with a 22" wire for the 1990 4.9L. I used the 15716 in my '95 4.9L and it made a noticeable difference in the "smoothness" of the engine! No tests performed, I just did it cause I like crawling under my truck to replace parts!!

 

Thanks for the heads-up! Maybe that's part of the problem - I believe i have 15717 in there right now, but i'll double check.
And oh believe me I feel you. I noticed the lower intake manifold mount nearest the cabin was cracked over the holidays. So what started as a relatively simple job of pulling the manifold to fix the crack turned into new valve cover gasket / new exhaust manifold gasket / new Intake temp sensor / etc. ha

edit - just confirmed the only difference between 1516 / 1517 / 1518 is the harness length, but considering just getting a new motorcraft one. I keep reading that these non oem o2 sensors fail quite often out of the box.

 

Yes, blindly replacing factory parts without diagnosis or reason CAN hurt, and often does. It's an antique, with antique wiring & other parts. Disturbing them without reason risks additional damage. The oxygen sensor is NOT on the maintenance schedule for a reason: it NEVER needs to be changed unless it fails a published test. Follow the links on this page:

(click this text)

 

Thanks for that - I'll order a replacement motorcraft one just to level set it.

 

Hey Steve - Thanks for access to the manuals. I'm currently reading through the emissions manual for my 90 f150 trying to figure out how to test my HEGO.
I'm not entirely sure how to read follow these instructions - i have a 4 wire HEGO - so to connect my DVOM, do i have to backprobe the hego sensor while it's fully connected?


Assuming that the HEGO sensor passes this first test - moving on to the next step requires a breakout box which don't seem to be an easy purchase - is there any way to follow this step without a breakout box?