5.0 running rough with rich and lean O2 codes

[Rembrant]

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.

[grumpin]

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

[85lebaront2]

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

[Rembrant]

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.

[85lebaront2]

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.

[Gary Lewis]

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.

[85lebaront2]

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.

[Rembrant]

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!!

[Gary Lewis]

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.

[grumpin]

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!!

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.