Two days in a row of this. A couple minutes into my first drive of the day the CEL pops. Its a 93 ranger 2wd 5spd. I notice no symptoms when it pops. After 15-30 seconds it turns back off. It could just be the fact i take the same drive every day but both times it was going up a steep hill. I wasnt using a ton of throttle but using speed to "coast" up the hill to the stop sign at the top.

Yesterday i checked, it has code 172 in memory. something about a lean condition. Im not sure where i should be looking. 100% of my ignition components are nearly new. Iacv new, maf and tb cleaned in the last 5k miles. I sprayed for vacuum leaks a few months back and found nothing meaningful using propane, brake cleaner fluid, and water.

Help?

 

What engine?
2.3l
3.0l
4.0l

Lean code means computer's calculations were "lean", at no time was the engine ever running lean

The computer runs pre-set air-fuel mix until O2 sensors heat up above 650degF, which takes about 3-4min after cold start, but depends on outside temp of course
Once the computer has Feedback from O2 sensor, then it will start to calculate air/fuel mix "on the fly"

Its uses MAF sensor, throttle sensor, air temp sensor, coolant temp sensor, RPMs, and fuel pressure, to calculate air/fuel mix
After its burned, O2 shows if it was rich or lean based on Oxygen level in the exhaust
Computer then adjusts the open time for the injectors to get correct oxygen levels, this takes about 2 seconds
If computers calculations are off by more than 15% Rich or Lean then computer will set a Rich or Lean code, to let the driver know there is a problem

Sensors rarely fail, well O2 sensors do run out of chemicals and need to be replaced every 125k miles
A lean or rich code usually means a mechanical issue, air leaks or in the case of lean maybe a fuel pressure issue

If the lean code is not full time then I would start by changing fuel filter, should be done every 5 years or so
There is no fuel pressure sensor, so computer must "assume" 30psi, if it drops too much under load then "lean code", and then it goes away

If you have an EGR system then it can cause a Lean code intermittently when its used

An older dirty PCV Valve can also cause intermittent Lean, because its sticking open

 

thanks. Its a 3 liter.
New pcv valve
new fuel filter
New coolant temp sensor as well

This ONLY does it on a cold start. I went to the gym, lifted and came out an hour and a half later and drove home no issues, then to work 30 min later no issues.

 

Since its a V6 that makes it a little easier

172 Oxygen sensor not switching - system is or was lean

So its a LEAN code and only on the Passenger side of the V6 engine
1993 3.0l should have two O2 sensors, one on passenger side and one on drivers side

So could be a lower intake manifold air leak, passenger side
Or an exhaust manifold leak, passenger side
Check that the passenger side O2 sensor is tight, my 1994 4.0l loosened up and through an intermittent lean code on driver side

How old are the O2s?

If it was a MAF sensor or upper intake air leak you would get lean on both sides of the engine
So it could be the passenger side O2 is running out of chemicals, so time to change both

 

There's no way of knowing how old those o2 sensors are. I can assume as old as the truck, so 27 years. Any idea why the code isn't thrown after sitting for a length of time? The truck sat at work all day, from 8 to 430 and didn't throw a code going home today or yesterday. I would think 8 hours sitting would constitute a "cold start". So far the only time it does it is first thing in the morning at 530 when I head out to the gym.

I really don't want to have to remove those o2 sensors because its gonna require an oxy acetaline torch for sure, or a sawzall.

 

LOL

Maybe, but it would be silly to pursue any other reasons for a lean, or rich code, with 27 year old O2 sensors, especially since it only does it on one side

It may not be the passenger side O2, but its just the most likely thing it could be with the code

The computer should not even look at the O2 sensors until ECT(engine temp) sensor gets above 130-140deg, so it is an odd code to get, but................that may not be why the CEL comes on, computer doesn't always set a code when CEL comes on

 

should i try to do some type of voltage test on the sensor or should i just replace it and see if it fixes anything

 

Replace both, if you don't know the age
O2s show lean when running out of chemicals, which causes computer to run engine Richer than needed, its slow process so you don't notice, but its costing you $$

Your MPG will get better regardless and they will pay for themselves on fuel savings over the next 125k miles

The voltages for the sensor part of an O2 is .1 to .9volt so under 1 volt, you can try just unplugging the O2 from the truck harness and have a look at the slots/pins for corrosion, then plug it back in, this cleans the slots/pins
Lower voltage like this can cause resistance to build up at connectors, and the lower voltage means lean, so every .x volt matters, if dirty connection is knocking down O2 by .1volt that's enough to cause lean
Anyway free to try, lol

The voltage changes at lest 5 times a second so hard to read on a meter, and computer runs a rich pulse every few seconds to "feed Cats" extra fuel to keep them hot
The OBD2 allows an O2 sensor "average" to be viewed on a reader, the 1994 and earlier don't have that option

 

125k miles.... lol. Yeah this is a rust belt truck. If you saw the underside of this thing you'd be shocked it hasn't split in half yet. I've seen some rusty cars and trucks. Chevy's and hondas, fords. but damn let me tell you, this thing is BAD.

Just FYI under the same conditions as the previous two days, the CEL didn't come on at all. I suppose I could unplug them and clean the contacts on the plugs and reassemble, and check the grounds for corrosion. I added an additional ground when I was trying to smooth my idle out.

 

Just unplugging a connector and plugging it back in cleans it, don't need to disassemble

This is why new sensors "work", sensors rarely fail, just unplugging and plugging the connectors back in fixes the problem 99% of the time, so "new sensor" worked, because you had to unplug the old one and plug the new one in, lol

O2s are different, they are like batteries, they run out of chemicals over time, so have to be changed just like a battery, but not as often, lol

 

So can you maybe try explaining this code to me again? I don't understand how the computer can throw a code for lean but not actually be running lean when it happens.

I cleaned the throttle body and the MAF and reassembled everything. I pulled the EEC fuse and "dry cranked" the motor to confirm the ECU was actually unpowered (it was). Put EEC fuse back in, truck runs mostly the same. Got the CEL for 30 secs, then turned off, then it turned on again a quarter mile down the road, and turned off again shortly thereafter. That was yesterday

This morning I left my buddies house, truck sat overnight. Got the CEL a couple mins into the drive again and it stayed on for like 5 whole minutes before shutting off again and staying off.

I checked those o2 sensors. The wires are like 2 feet long and I can't reach the plugs because they sit way back behind the dizzy on top of the clutch housing

 

Alright so lets just make double sure I interpreted the CEL flashes correctly. Jumpered the connector under the hood:
one slow flash, pause, 3 slow flashes, pause, 3 slow flashes, pause, 1 slow flash, longer pause, then the 1 - 7 - 2 blinking flashes for the 172 code.

 

The computer has a pre-programmed air/fuel mix table, for the engine size in memory, to use when engine is cold
The O2 sensors are the computers only feedback for lean or rich, but they don't work cold

O2 sensor's chemical reaction to detect oxygen can't work when they are under 600degF, so they need to warm up first, which is why they are heated, to warm up faster, and of course exhaust temp is 800+ degF, so it takes a few minutes to get them hot enough to work

After O2s are warmed up the computer can then do "on-the-fly" air/fuel mix calculations for best power and economy
It uses MAF, Air Temp, engine temp, RPM, engine size, fuel pressure and throttle position to do its calculation for injector OPEN TIME
Its all about injector "open time"

Lets say the computer calculated that the open time for each injector is 100ms(milliseconds), this would be 0 STFT(short term fuel trim)
After 2 RPMs computer gets feedback from O2 sensors on the 0 STFT burn, if there is too much oxygen then lean, if too little oxygen then rich
If lean then computer changes open time to +2 STFT which is 102ms open time
2 RPMs later if its still lean, then +4 STFT, 104ms open time
Computer will continue to add to open time until O2 shows a "good burn" correct amount of oxygen in exhaust
This takes about 2 seconds
So engine was never running Lean, or Rich, well for 2 seconds or so, lol

An engine will ping/knock if running lean, or blow smoke if running rich

If computer gets to +15 STFT or higher, that means 15+% error in the calculation so computer will set a code to let the driver know about the error
+15 STFT = Lean code
-15 STFT = Rich code

That's what lean or rich codes mean, 15% or more error in computers base(0 STFT) calculation

And there is LTFT(long term fuel trim), this is an average, over time, of STFT number, this is so the computer doesn't have to "relearn" STFT every time its restarted and to help with cold start running
Say there are a few cracked vacuum hoses, small leaks after hoses get 20+ years old, lol, so average of STFT has been +6, so LTFT would be +6
On cold start computer uses the tables in memory and ADDS 6% open time
And when doing "on-the-fly" calculations 0 STFT is actually +6 STFT but shows as 0 STFT
And if there were more air leaks and "new" 0 STFT was averaging +3 STFT then LTFT would go to +9 LTFT
If LTFT gets to +15 then computer will also set a Lean code

 

well god damn so why is the computer unhappy? It's saying theres more than 15% variance in fuel trim than it wants to use, correct? I'm wondering if the computer will compensate this indefinitely, just burning more and more fuel.... I suppose my question to you is why does the CEL turn on and off? Usually in cars, even in my old OBD1 hondas, the CEL would STAY on even if it was just a momentary error. This ford turns it on and off as I drive. I'm just trying to understand why it only does it on the first drive of the day. A bad o2 sensor could cause this I am sure, but maybe there is something else I could be looking for. I remember 5 or 6 months back I was telling you about how I replaced damn near everything under the hood and it still shakes and vibrates at idle and seems a bit unsteady, and you were telling me to get the timing checked or set. Real quick lets recap the new underhood parts
fuel filter, plugs, wires, dizzy, ignition coil, coolant, ECT sensor, coolant gauge sensor, thermostat, brake booster check valve, PCV valve, radiator cap, IACV. MAF has been cleaned a couple times, air filter is clean, throttle body was cleaned heavily a couple times, and the idle was set according to FORD. I've sprayed for vacuum leaks like I said before. I just don't know where there could be an air leak causing the lean condition. I keep thinking its an air leak because the engine has always idled a tiny bit lumpy. In fact, sometimes on warm starts it sounds like an old cammed v8, its honestly really pleasing to hear it loping like that but then again I got a full custom exhaust and no CATS

 

First,......."new" parts now a days means untested parts, so new doesn't mean "it works" any more

So putting in "new" sensors or controls is actually a bad thing to do wholesale, it just adds layers of possible issues, so if there was one problem and you swap in 2 new electrical parts you then have 2 possible problems, lol

DIY mechanics do this all the time, gotta use your brain first and wrench second or you can just go into a downward spiral of "parts"

Computer is not burning more and more fuel unless your recent MPG is in the toilet, i.e. 8-10MPG, which you would notice
Computer is burning just the right amount of fuel unless engine is pinging or blowing smoke

There is no fuel pressure sensor on any year Ranger, computer is programmed to expect 30psi in 1997 and earlier, and 55psi in 1998 and later, Rangers

So lower fuel pressure can set a lean code

As far as the CEL coming on and going off that may not set a code, could be a misfire or a sensor dropping out for a second or two from a loose connection
Computer can only set a code if a problem is seen long enough for it to decide which code to use, there is a long list

I assume you are clearing the codes?
Does the lean code keep coming back?

 

yes i cleared the codes when i pulled the ecm fuse. 172 came back quick

Any idea how i can test my fuel pressure? Never done it before on any car or truck

 

There is a test port on the engine, on the fuel rail
It looks like a tire's Air Valve, because that's what it is, a schrader valve, works the same way

IF you have a screw on pressure valve gauge that goes to at least 50psi it will work on 1997 and older Rangers, later model need 75psi at least gauges
You can rent these or buy them at most auto parts stores

 

does it need to be a screw on? I have a dial gauge for tire pressure but you just have to press and hold it on. and I am assuming it is 30psi with the engine off, and 30psi with engine on?

 

It would spray fuel every where so no, not a good idea to do that

Running pressure should be 30psi +/- 4psi, but you also need to REV engine to above 2,000rpm and hold it there for 20seconds or to make sure pressure is not slowly dropping, that would be a sign of weak pump or filter issues, inline or in tank "sock" filter

Engine off pressure should hold above 15psi but not a big deal if it drops to 0psi just means it may not start first crank

A CEL within the first few minutes of cold start can't be a lean or rich CEL, because O2 sensors have to be working before computer will start to use them, that takes at least 2 or 3 minutes depending on outside temp

 

thanks. And addressing your last point, the light comes on after 3 or 4 mins. The only CEL code ive gotten is that 172. Nothing else

 

@RonD so do you have any idea how I am supposed to reach the connectors that sit back behind the dizzy up on top of the clutch bellhousing that the o2 sensors plug into? If you say changing the o2 sensor will fix this then I need to be able to solve this issue. The only thing I can think of is climbing a step stool and kneeling on top of the engine, but I'm not sure my forearm will fit in that gap back there.

 

I don't have your model here so not sure I can be much help in telling you how to get to or remove a part or connector

On my 1994 4.0l the two O2 connectors are hard to get to at the location described but I was able to get to them
There is a left and right O2 and the connectors are the same for both so don't mix them up, there is only 1 wire color that is different

Diagnostics is not specific, best guess is all you can do

l.e. You come out and have a flat tire
So must be a hole in the tire, right
sure could be
but what about a leaking valve stem
or leaking wheel/bead

Things are usually not as cut and dried as we would like, lol, so always just a best guess

 

I have a 93 3.0 2wd 5spd
But I get what youre saying. The passenger side o2 seems easy enough to get to, I will take off the passenger front wheel and have enough room to use an o2 socket in there. I was just concerned about the plug. Since I need front brakes I could do these at the same time.

 

If it was a bad fuel pump or faulty fuel pressure regulator, wouldnt there be more lean codes in the ecm, and on both banks (not just 1)

 

Yes, if there was a general issue, like upper intake air leak or MAF issue, or low fuel pressure, you would expect both banks to be effective IF both O2 sensors were less than 150k miles old
Once O2s are past the 125k mark its hard to say how accurate they would be
Lean and rich codes are directly effected by O2 sensors ability to detect oxygen accurately, because that's the only feedback the computer has for its calculations, computer has no redundancy for this

General "rule of thumb" on V engines with 2 upstream O2s, is................
If both banks show lean or rich look for a general issue, i.e. upper intake vacuum leak, MAF issue, low fuel pressure
If just one bank is effected then look at lower intake air leak, exhaust blockage(if DUAL exhaust) or O2 sensor issue

If both O2s are older or unknown age then change both