Trying to chase down a rough idle on SOHC after replacing most related sensors. Idle fine, then PCM sets code and idle goes to ****.

[Justin_]

Hey guys. I have a rough idle issue on a 98 SOHC that I cant seem to chase down. Hoping I might be able to get some help.

My Explorer is having issues with idling. It will idle rough with revving and has bad exhaust burn (you can smell it). The rough idle /mostly/ goes away when it warms up, but still has bad burn. It also has developed an issue starting up when it's below freezing (new this year). It has a bit of an issue accelerating from a stop, and the throttle response is a touch slow. It also tripps codes P0172 and P0175 (bank 1 rich, bank 2 rich) when driving in town, but not when on the highway. It will sometimes trip P0171 and P0174 (bank 1 lean, bank 2 lean) when on the highway.

Here's the thing though. If I disconnect the battery, wait a bit, then plug it back in, resetting the codes, it will work fine. You can't smell any unburnt fuel, the idle is PERFECT, the acceleration is great, throttle response is improved, and it just feels great. However, once the code is set again it reverts back to the pour performance. This happens in 10-20 miles in the city, but can take much more on the highway.

For one reason or another, in the last 2 years the MAF, IAT, front O2's, intake seals, PCV valve, injectors, plugs, wires, coils, filter (AIR), filter (FUEL), fuel pump, FPR, and fuel return line have all been replaced with new parts.

I checked the TPS out and it was fine

I removed the driver side catalytic and bypassed it with a hose (no downstream O2 on that side).

All vacuum lines have been gone over or replaced.

When I got the truck just over 2 years ago it was fine, it has been slowly getting worse ever since.

Any idea what else I should look into? Kinda at a dead end. Been chasing this for over a year and don't seem to be any closer.

This truck did roll about 18 months ago, but this was an issue prior to that too.

 

[EB4X]

Maybe a faulty o2 sensor sending bad data to PCM thus responding with incorrect compensation / adjustment ? >> If I disconnect the battery, wait a bit, then plug it back in, resetting the codes, it will work fine. You can't smell any unburnt fuel, the idle is PERFECT, the acceleration is great, throttle response is improved, and it just feels great. However, once the code is set again it reverts back to the pour performance.
Try unplugging all your o2 sensors and see how it runs in open loop (ignoring o2's and going off presets) to rule in/out. (And of course reset PCM again after doing this)

 

[Justin_]

Maybe a faulty o2 sensor sending bad data to PCM thus responding with incorrect compensation / adjustment ? >> If I disconnect the battery, wait a bit, then plug it back in, resetting the codes, it will work fine. You can't smell any unburnt fuel, the idle is PERFECT, the acceleration is great, throttle response is improved, and it just feels great. However, once the code is set again it reverts back to the pour performance.
Try unplugging all your o2 sensors and see how it runs in open loop (ignoring o2's and going off presets) to rule in/out. (And of course reset PCM again after doing this)

I will try that next time im out. Replaced them not long ago with Bosch ones, but ill try anything at this point.

 

[J_C]

I'd hook up a scan tool capable of live data and look at the long term fuel trims, as well as measuring fuel pressure.

Where did you find a FPR for a '98 SOHC? They (97-98) are rare as hen's teeth, last time I looked.

 

[Justin_]

I'd hook up a scan tool capable of live data and look at the long term fuel trims, as well as measuring fuel pressure.

Where did you find a FPR for a '98 SOHC? They (97-98) are rare as hen's teeth, last time I looked.

Need to get a new tool. Mine wont show fuel trims unless I snapshot it, and it's a pain to do that way. Any recommendations on a model that has worked for you?

Fuel pressure is fine.

I thought it might be a leaking diagram on the FPR. Ended up replacing it with a PR 409 after seeing that work for others. Ran a new 6AN nylon braided fuel line while I was at it as part of the flex line of the existing return line was kinked and a local shop thought that might be part of the problem. Technically the PR409 is ~4PSI too high, but it's worked fine for others. No real difference after replacing it though.

 

[donalds]

Amazon product ASIN B07MQ8GHG3And forscan a free program

 

[Justin_]

Amazon product ASIN B07MQ8GHG3And forscan a free program

And ordered.

 

[donalds]

And ordered.

I suggest looking up a few videos on forscan
it is very easy to use but the more info you are armed with The better
Forscan with a elm 327 adapter is the best software for fords

You'll see

 

[J_C]

Yeah I would have suggested same, or the BAFX bluetooth for android, or BAFX wifi for android or iOS devices (both on amazon), then still Forscan app.

 

[Justin_]

So I tried running without the O2 sensors connected and it basically felt like driving after the code was set. The only differences being the idle was even worse, and the throttle response may have been just a touch better. It was still burning quite bad.



Just throwing this out there, is it possible that the system voltage being unstable could cause this? On top of running the headlights and HVAC blower motor I have 4x100W RMS aftermarket speakers on a dedicated amp, in addition to a 400W RMS subwoofer system. 800W RMS or about 1600W peak. Sometimes when I am idling (like at a stop light) I will have my voltage gauge vary from about the center, to only about 1/4 the way up. This will shift according to the music, but i'm wondering if a sporadic voltage could make it hard for the engine to read the analog. sensors.

I can get an actual voltage reading off of that in the next few days here.

The testing today was with the speakers off BTW.

 

[donalds]

If your system is off then its out of the picture
Keep looking

 

[C420sailor]

So disconnecting the battery clears all the adaptive tables and fuel trims. Something is happening that is making the truck think it needs to aggressively add/remove fuel.

As mentioned, use forscan to make sure the O2 sensors are reacting appropriately. Check all the sensors actually, make sure their readings are appropriate for the operating conditions.

You can try unplugging the IAC and EGR vac while the engine is running. See if it smooths out. See what the trims do. I had a truck that had a nasty misfire when the EGR started to open. Never would’ve found it without forscan live data.

If that all checks out, do a compression/leakdown test.

 

[Justin_]

Well I ended up waiting in a drive through line for about 30 minutes with my current OBDII tool in the truck so I wen over the sensors I could with it.

I found that the IAT sensor was reporting 66-76 degrees when its only 32-34 outside. Checked it again when I was on the road with lots of air blowing by and still the same thing. I just replaced it in June so I stopped by the parts store and they gave me a new one, however it did the same thing. Any reason why the PCM would read about double the temp on the IAT?

I will double check that when the new tool gets here, but /maybe/ that is why its worse when cold. It's probably calculating the fuel mix wrong if that is what its actually reading.

While I was there I ordered in a loner leakdown tool, they didn't have any on hand.

Should have the FORScan tomorrow and the leakdown tool by Friday.

 

[Justin_]

Once I do have the FORScan tool, what exactly should I be charting? Fuel trims, O2 data, and engine load? Then just let it sit for a bit, and drive it for a bit?

I don't want to have too much data on one graph making a mess, but I don't want to leave stuff out.

 

[C420sailor]

That’s normal for IAT. Underhood temps get wicked high while idling. You’ll see mid triple digits on a hot summer day in traffic.

Moving it’ll cool down a good bit, but with a warm motor, you’ll never see ambient.

 

[Justin_]

That’s normal for IAT. Underhood temps get wicked high while idling. You’ll see mid triple digits on a hot summer day in traffic.

Moving it’ll cool down a good bit, but with a warm motor, you’ll never see ambient.

Ok. That's what I thought at first, but thought it was strange that it still was at 64 after driving serval miles at a decent speed. Thought it would cool down at that point.

Ill check it before starting up next time to make sure.

Im sure its seen triple digits. Drove it in Las Vegas in August. It was 117 ambient.

 

[Justin_]

So, bit of an update here.

One of the few things I hadn't worked with yet was the IAC valve. I was on the train of thought that 'Its running rich, it must be a fuel or fuel sensing system'. However I was reading a bunch of threads and a lot of them mentioned the IAC. At first I didn't think much as I was looking at it from the point of view that if it was stuck /open/ it would just be adding more air, making things lean out. I wasn't thinking about how if it was stuck /closed/ that would prevent there from being enugh air at idle as it would effectively be making things rich. Since it is quick and easy to do anyway I pulled it off, cleaned the crap off the filter, sprayed the inside down with carb cleaner, and wiped it all down in there.

I put it back on and cleared the PCM by unplugging the battery. It took idling for 10/15 minutes and a full 35 miles to set the code this time (15 of that was highway though). This was over the cores of turning the car off and on 5 times. After the code was set there was no rough idle/surging. The engine would stay within about 30/40 rmp at all times centered around 645 (in park, AC off). It was still burning rich, but the throttle response was acting normal and I think its acceleration was normal as well.

Will see what it does in the morning, but the surging may be fixed now. Ill. add that to my list of things to do when I get a new explorer. Its something ive never touched before.

I had the rich code the entire time I have had the vehicle, but the surging is what has been new and developing. It seems it may have just been 2 issues, some fuel trim issue (yet to be determined), and a carbon buildup on the IAC.



On the rich code end, ive been noticing that once the codes are set I am hearing what I think is a misfire when idling. It comes out as just a quite poof from the exhaust, so you only hear it if you are outside or have the windows down, but it is definitely poofing. Most of the time its once every 3-5 seconds, but it can be as frequent as 2-3 times a second. Make of that what you will.

The reason I pulled the driver side cat was it was making pinging sounds when I was idling. I figured it just hit a rock a few too many times and had fallen apart (still possible), but its possible that this is the same sound.

For what it is worth, bank 1 always sets the code first, but bank 2 is never far behind.

 

[Justin_]

Getting closer

After digging over dozens of threads on Explorer Forum, Ranger Forum, Ford Trucks, and more I decided to stop searching about the Ford 4.0 SOHC and just search for error codes p0702 and p0705 in general.

After digging over several more threads I ended up on Hyundai Forums on a thread about somebody having issues after replacing their evap purge solenoid on a Accent.

The thread talked about how the EVAP system normally only runs when the vehicle is under load and not at idle, but if the solenoid is broken it can effectively just constantly pull extra fuel out of the tank and into the intake.

I remembered how quite often when I fill up after driving for some time there will be a hissing sound, almost like cracking a vacuum when I remove my fuel cap.

So, I was wondering if that is what was happening to me. It would make since, especially since I am running out of other systems to test.

So I removed the EVAP lines from the upper intake and taped over the holes to test it out. After that I cleared the codes, but did not reset the computer. At idle when 1/2 warm I was still getting the puffs from the exhaust, but it seemed to be burning much better.

Ran it 40 miles up and down the highway, then when I got back to town I saw that the P0702 and P0705 codes were pending, so I turned the truck off and on again, and shortly after I got the CEL back for those codes

HOWEVER!

The truck feels GREAT now! Better than I think I have ever felt it. The power and response are super! If there wasn't codes I would have guessed it was running prefect (aside from the exhaust puffs and ever so slight gas smell still coming from the exhaust)

I will need to actually fix the EVAP system, but since the entire fuel tank is somehow able to hold a vacuum I would imagine I just need a new valve.


This is turning out to be a bunch of little problems rather than one big one. Cant really complain, but it is interesting to see the improvements after every modification.

 

[boominXplorer]

Get a factory maf and swap out the aftermarket one. I've had so many weird rich bank codes caused by an aftermarket maf.

 

[Justin_]

Get a factory maf and swap out the aftermarket one. I've had so many weird rich bank codes caused by an aftermarket maf.

I would definitely believe that. I do still have the original MAF and I can try that back in again now that I have some of these other issues chased down.

After a bit of googling I do not see anywhere to buy an original motorcraft MAF from online new. It seems there's a couple sources left for the V8 MAF, but they are all around $350 which seems kind of crazy.