5.0 running rough with rich and lean O2 codes

Good morning gents,

Apologies in advance as my questions are not for a Bullnose, they are for my 1994 F150 with 5.0/MAF. I'm not really looking for specific help, more like generic help. My truck has MAF sensor, but the 1994 EFI 5.0 is not much different from a 1986 5.0 EFI.

Anyway, when I bought the truck a few months ago and drove it 200+ miles home, I was having driveability issues where the check engine light would come on and the truck would start to lose power and stumble. The check engine light would go away and after a few minutes the truck would go back to normal and work fine again for 10 minutes, then repeat. Trouble codes in KOEO memory were 172 and 173.

#172 [HEGO (HO2S) Sensor fault / Lean.]
#173 [HEGO (HO2S) Sensor fault / Rich.]

About 2/3 of the way home, the problem disappeared and the truck has been running fine for the last couple months, and a 1000+ miles.

This past Tuesday night, the problem returned with a vengeance lol. Exact same issue, cruising on the highway, truck starting losing power and stumbling. It would continue to lose power/speed, and then boom it would come back to life and work fine again for a few minutes, then repeat.

KOEO codes in memory have returned, exactly the same as before:

#172 [HEGO (HO2S) Sensor fault / Lean.]
#173 [HEGO (HO2S) Sensor fault / Rich.]

I did a lot of reading and searching online, and this problem seems to be fairly common. I more than a dozen threads on the topic, but I only found two that found a fix for it. One guy replaced the O2 sensor, and the other guy replaced the EGR valve and EGR control solenoid, and the problem also went away. Of all the others, they either couldn't fix it or never did reply when/if they did fix it.

I removed the EGR valve and it appears to be nice and clean and it operates smoothly using a hand vacuum pump. It opens and closes nicely. I also ordered a new O2 sensor and it arrives today.

Anyway...

I'm just curious on your technical thoughts as to what would cause lean/rich signals at the same time, or back to back as it were. Faulty O2 sensor is one option, but if it isn't the O2 sensor, what else would one look for?

Fuel issues? I did try running both tanks, which each have fuel from different stations, and that didn't matter, so I assume it is not fuel pump related.

Ignition issues? Full disclosure...condition of plugs/wires/cap is unknown. I haven't replaced them yet (was leaving them for winter maintenance).

The O2 sensor wiring and connector was completely saturated with engine oil (oil pan gasket leaking). I know oil is not a good conductor...but I did find a report or two online (non Ford) where people said oil saturated wiring and connectors was causing a problem with O2 sensor signal. In that case, they cleaned with brake cleaner and the problem went away.

Anyway, I'm just hunting for thoughts and opinions on what to investigate next if the O2 sensor does not fix the issue. OBD1 is helpful, but it really only gives you a "condition", it doesn't really tell you what the specific problem is.

Cheers, and TGIF.

Since it's so intermittent, I would "lean" towards the oil soaked O2 sensor for sure.

1994 is capable of delivering live data to a good scan tool (not a parts store one).

85lebaront2 wrote

1994 is capable of delivering live data to a good scan tool (not a parts store one).

Interesting...like what kind of scan tool? Something that is still available today, or do you mean a good scan tool from back in the day?

A quick Google search and I found this...very interesting...

https://www.amazon.ca/MOTOKU-Adapter-Scanner-Diagnostic-Connector/dp/B07H7FWP24/ref=asc_df_B07H7FWP24/?tag=googleshopc0c-20&linkCode=df0&hvadid=459447164082&hvpos=&hvnetw=g&hvrand=8436391196946357287&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=1002108&hvtargid=pla-650287132553&psc=1

I'm installing the new O2 sensor today, so we'll see how that goes. Fingers crossed.

What I was curious about when I created the post was what what could potentially cause back to back lean/rich conditions?

Worse case scenario, I will install three gauges in the truck for fuel pressure, vacuum, and O2 and that when when it acts up I'll be able to narrow it down a little more...maybe.

I have a Snap-On Solus Pro that worked quite well on my 1994 Taurus. I am sure there are others out there.

I've not seen a discussion of whether or not this is an EEC-IV or V system, but suspect it is a IV.  If so disregard the following.

I'm running a Vgate vLinker MC+ Bluetooth OBD2 Car Diagnostic Scan Tool for iOS, Android & Windows coupled with DashCommand software on my iPhone.  Works great with my 1996 EEC-V ECU in Big Blue.

The beauty of that scan tool is that it is Bluetooth as opposed to Wifi, so you don't need to disconnect from your Wifi to connect to it.  And the device will automagically turn off several minutes after it doesn't detect a BT connection, so you can leave it plugged in w/o draining the vehicle's battery.

It is an EEC-IV system. The late EEC-IV units do have the capability for live data with a proper scan tool.

grumpin wrote

Since it's so intermittent, I would "lean" towards the oil soaked O2 sensor for sure.

It indeed looks like the O2 sensor was the problem, at least for now. I just took the truck for a drive and put about 30 miles on it, and it's working better than ever. I cleared the codes and none have returned. As for the oil soaked plug and wiring, I removed the plug and 4-wire harness and cleaned it all up with brake clean so that it was sparkling clean and dry. The wiring close to the plug was a bit sketchy so I straightened it all out and put head shrink on it, and then put some brush-on electric tape around where the wires enter the plug.

So, unless the problem returns...and it could, I'm going to assume that the O2 sensor fixed it, along with cleaning and fixing up the wiring as well.

Dumb question though...while I'm trying to wrap my brain around how this whole system ties together, since this truck has a MAF sensor on the intake, does that mean that it does not have a MAP sensor?

In all my reading, I found loads of threads where guys had replaced the ICM, or ignition control module, and it was funny because I went and looked at the distributor and there was a mounting space for an ICM, but no ICM lol. I found it mounted on an aluminum heat sink on the insider of the driver's fender, back close to the fire wall. Interesting...it looks like the same ICM that would be mounted on the distributor on previous years, but mounted on the fender. There must have been a reason for that...too much heat or too much vibration on the engine? With its own heat sink I guess that helps it dissipate some heat, rather than trying to shed heat on a hot engine.

Anyway, I plan to do all the typical maintenance stuff over the winter...plugs and wires, filters, sensors, etc.

Thanks guys!!

I hope that you've resolved the problem(s) and it/they don't return.

But I can't answer your question about the MAP sensor as I only have the 1996 EVTM, not the 1994 version.  Perhaps Bill can answer it from AllData?

But speaking of the '96 EVTM, I do have bits of it online:

Engine Control - 5.0L: This is an EEC-V system as the connector has 104 pins.  And it has a MAF sensor, sequential firing of the injectors, and doesn't have a MAP sensor.

ENGINE CONTROL - 5.8L OVER 8500 & 7.5L EEC-IV: This is an EEC-IV system with a 60-pin connector to the ECU, no MAF, but it does have a MAP sensor.  Plus bank-fire injectors

So at first blush I'd say if it has MAF it won't have MAP.  But I really don't know that because you have the "upgraded" EEC-IV system with MAF and I don't have any documentation on that.  Sorry.

As for the ICM, Ford was involved in a class-action lawsuit regarding distributor-mounted TFI modules.  They never admitted that they had a problem, even though they did, but they did quit installing them on the distributor around 1991 on the F-Series trucks, which explains why your distributor has a place for it but it is actually mounted remotely.

The 94 F250 I used to own had the ICM mounted on the fender. Moved there because of heat as you mentioned.

My 92 Bronco has MAP, and as Gary pointed out the old 92 460 had MAP.

I’m pretty sure it’s either MAP or MAF, not both.

Edit: I think my 92 Bronco still has it on the distributor, I’ll have to look again.

Gary Lewis wrote

So at first blush I'd say if it has MAF it won't have MAP.  But I really don't know that because you have the "upgraded" EEC-IV system with MAF and I don't have any documentation on that.  Sorry.

As for the ICM, Ford was involved in a class-action lawsuit regarding distributor-mounted TFI modules.  They never admitted that they had a problem, even though they did, but they did quit installing them on the distributor around 1991 on the F-Series trucks, which explains why your distributor has a place for it but it is actually mounted remotely.

grumpin wrote

I’m pretty sure it’s either MAP or MAF, not both.

Thank you gentlemen!

I had it my head that MAP was with speed density, and MAF with SEFI.

It was kind of funny, because I went to check it on the distributor (I was actually looking to see if the module was grey or black...I found several threads where people were saying to ONLY buy the black ones...whatever that means. Is it something like the Duraspark II modules with the different color grommets for the wire harness??). Anyway...when it wasn't there, I started looking around because I had to have an ignition module somewhere! Kinda neat learning these things...thanks again guys.

Rough running issues aside, the truck seems to be running even better than it was previously. I can't imagine an O2 sensor making that big a difference, but it could be in my head too;).

When the engine is first started, are these EFI systems running on closed loop or open during warm up/high idle. The high idle seems different...its not going quite as high and not staying high as long as it was previously...but again, maybe it's all in my head;).

They are open loop until the O2 sensor is hot enough to provide a signal, hence the heater in it.

FWIW, Ford did build some MAP sensor SEFI systems, I had one, a 1990 Lincoln Town Car, last year for the 5.0L engine, in 1991 the Lincolns received the new 4.6L engines, the Crown Victoria and Grand Marquis had to wait until 1992.

The change in warmup is probably due to the O2 sensor coming on-line earlier allowing the system to enter closed loop sooner.

The ICM was moved to the left inner fender starting in 1990 on trucks, big ugly (parts donor) had it there. As for the color of the ICM, the color denotes which version it is, gray is for push start, where the ICM "pushes" the dwell for a hotter cranking spark. The black ones are CCD which stands for Computer Controlled Dwell. There is one wire different, on a push start there is a red with blue stripe wire from the start circuit (branches from the wire to the relay) going to pin #4 on a CCD module it goes to the EEC.

85lebaront2 wrote

The change in warmup is probably due to the O2 sensor coming on-line earlier allowing the system to enter closed loop sooner.

Ahh yes, OK, understood. That makes perfect sense and explains why the high idle is a bit different. The change was subtle, but a change regardless. The high idle doesn't go quite as high as previous and it comes down much quicker.

I found a cracked spark plug boot tonight so I just ordered a new set of plugs and wires. I was going to do it over the winter, but might as well do it now while I can still road test the truck. I also noticed that it has Motorcraft Platinum plugs in it...not sure if that's good or bad, but I ordered a set of SP450 copper core plugs to replacement with. The emissions decal under the hood calls for ASF-42C as the correct originals, but I understand that the SP450's are direct replacements for them. It has ASF-42P's in it now. I was going to get Autolite 25's, but there were only 3 available on RockAuto.



I don't know if the cracked plug boot makes much difference or not, but I do get some vibration sometimes. Somebody threw away all the plug wire separators and they're all intertwined with each other, so I ordered a couple packs of Dorman separators so I can get everything routed properly.

Thanks again for all the help folks. She'll be running perfectly before winter at this rate;).

Wanted to tell you something I learned, it’s in the owners manual, when you cold start these the idle is up for a bit.

You can kick it off by tapping the pedal, like an automatic choke.

Found that interesting. And as I mentioned in another thread, I haven’t had my tester hooked up to this Bronco forever! Just working great!

grumpin wrote

Wanted to tell you something I learned, it’s in the owners manual, when you cold start these the idle is up for a bit.

You can kick it off by tapping the pedal, like an automatic choke.

Found that interesting. And as I mentioned in another thread, I haven’t had my tester hooked up to this Bronco forever! Just working great!

I did not know that! I'll give it a try, but she's coming down pretty quickly now so not a big deal. It seemed like it was really too long though before the new O2 sensor.

I had a similar issue, it actually ended up being the O2 sensor harness, I found it after my engine started smoking on a no crank. This was on a 1986 tbird sefi 302, there was a bare wire in the O2 sensor harness that rubbed on another wire, and apperently there was no fuse or fuseable links for the heated o2's in my 86, so it would short out and kill the computer. Was driving down a mountain pass, downshifted to accelerate, and the car just shut off. Fired right back up, with intermittent no starts until I saw smoke with the hood open.

You can also run an "engine cylinder balance test", not sure how well it works with batch fire, but on sefi, it revs the engine up and shuts off one injector at anytime and identifies bad/underperforming cylinders, I found a bad head gasket this way. You just run codes (I use a paper clip and a test light or analog volt meter), and within a minute I f the codes being reported, you just go WOT. You can run the test 3 consecutive times, passing two in a row is within spec.

The maf cars and trucks still have a map sensor, but it no longer reads vacuum, but atmospheric pressure and is now called a BAP sensor with no vacuum line. The map sensor was essentially your air to fuel mixture, if you have ever had the vacuum line fall off, I would get 6mpg instead of 20+. It uses the map sensor for instant vacuum readings for air fuel mixture, and then compares it to the O2 sensors and uses a table to calculate any fuel trim. Stock computer can only modify the fuel table by about 10%, and ethanol gas (e10 in my state) takes up 7%. The engine has to be in pretty good tune to not throw a check engine light, as you essentially only have 3% fuel trim for both elevation and wear and tear before the computer freaks out and throws a code.

"85lebaront2"
They are open loop until the O2 sensor is hot enough to provide a signal, hence the heater in it. 

I didn’t realize that, assumed it was timed since it was OBD1.

Interesting stuff here!

Haystack wrote

The maf cars and trucks still have a map sensor, but it no longer reads vacuum, but atmospheric pressure and is now called a BAP sensor with no vacuum line. The map sensor was essentially your air to fuel mixture, if you have ever had the vacuum line fall off, I would get 6mpg instead of 20+. It uses the map sensor for instant vacuum readings for air fuel mixture, and then compares it to the O2 sensors and uses a table to calculate any fuel trim. Stock computer can only modify the fuel table by about 10%, and ethanol gas (e10 in my state) takes up 7%. The engine has to be in pretty good tune to not throw a check engine light, as you essentially only have 3% fuel trim for both elevation and wear and tear before the computer freaks out and throws a code.

Not all MAF trucks have MAP or BARO sensors, I challenge you to find one on a 1996 OBD-II system. GM and Volvos I have worked on have both and for two reasons (a) quicker response to sudden manifold pressure changes as you mentioned (b) better mixture control at low air flow rates (idle).

The EEC-IV systems can only be "adjusted" with a piggyback system, such as a TwEECer or Moates Quarterhorse. EEC-V systems can be completely reprogrammed with proper hardware and software (I use Binary Editor and a MongoosePro cable to do them).

I have two Flexes, a 2009 and a 2011, they have 4 O2 sensors, 2 at the manifolds and 2 at the catalyst outlets. The 2 at the manifolds are interesting in their function, they are a dual band, one narrow and one wide and provide the two separate signals to the EEC. The downstream ones are simply a catalyst monitor. These also have only an MAF sensor.