98 SOHC idle and error codes | Ford Explorer Forums - Serious Explorations

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98 SOHC idle and error codes


Active Member
June 24, 2006
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City, State
Norfolk, United Kingdom
Year, Model & Trim Level
Hi there, I need help! Had a problem with idle and trouble codes for around a year now. I have:

P0171-too lean (B1)
P0172-too rich (B1)
P0174-too lean (B2)
P0175-too rich (B2)
P1131-O2 sensor indicates lean (B1)
P1151-O2 sensor indicates lean (B2).
The idle is crap and jumps about, once above 1000-1500 rpm all seems OK. The truck is converted to run on LPG (due to price advantages in the UK), it is a basic suction system connected to the air pipe with an emulator to loop good signals back to the PCM. It does use O2 sensor and TPS input to vary the LPG flow a little. The faults occur on both petrol and LPG.

I have done the following:

MAF sensor-cleaned and then replaced (new, genuine part)
IAC valve-cleaned and tested
TPS-replaced (new, genuine)
Spark plugs-replaced
Upper and lower intake manifold gaskets-replaced
Air filter-replaced (K&N panel)
Both upstream O2 sensors-replaced (new, genuine)
Cylinder compression test-OK
Check exhaust-OK
My local Ford dealer are telling me the PCM is next but before I waste more money I thought I would ask advice. Whatever I try next will probably be the last try before putting up with it, selling the truck or crashing the truck into a large tree!:mad: According to an LPG specialist in my area (these conversions are quite common in the UK) the PCM may have become corrupt due to the different burn characteristics/sensor inputs with LPG. They claim that when the PCM does a 60 day test it can modify some of its settings to suite what it finds (presumably originally to allow for engine aging) and that if running on LPG when it does this test the PCM has been known to change its base parameters to suite the LPG. As the LPG system is set up to mimic the petrol system the truck must work properly on petrol for this to happen, thus the problem. Does this make sense and does anyone have any advice? I have now spent over $2000 on repairs and mods and I am very rapidly coming to the end of my patience. I love this truck and don't really want to get rid of it.

Thanks for reading my rambling life story.;)

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Does anyone know if the PCM can do what has been suggested? Is it impossible, unlikely, likely or almost certain? If not how common is it for PCMs to fail? I can get the PCM cheap enough but it looks like I will have to get it fitted and synchronised with PATS at a Ford dealer which will cost money I don't really want to waste. I am prepared to risk it if it is reasonably likely to work, I have had enough of my V6 truck sounding like a diesel!

Since the last post I have replaced the PCM (second hand but guaranteed working) and no change. I am going to try a new IAC valve and check hoses/pipes but then I am out of ideas. :(

sounds like a vacuum leak, have you replaced the intake seals with the upgraded ones from the 00m12 kit? there was a TSB that explained this idle problem with the SOHC engines. The kit includes new orings for the intake as well as a new front timing chain tensioner and a pastic rod that goes into the oil passage for the front tensioner.

but with the symptoms you describe I would go with that.

You should run a vacuum test at idle to see if you are running in normal ranges, that will also be a good indicator of a leak.

I have already done the OOm12 kit with the new o-rings. I have also had a cylinder compression test done and all fine. It seems to be something associated with idle, if I sit at a junction and let it idle it is crap but if I press the accelerator to a point that I can "feel" the throttle open (around 1000-1100 rpm) it gets drastically better and appears perfect. I am not sure whether I have already mentionned it but I had a backfire immediately before the crap idle started so I am thinking this may have damaged something in the air pipe or a hose/pipe. I have tried idling the engine with the IAC valve electrical connector disconnected but it is the same as when it is connected, would this suggest the IAC valve is not responding? What pipes/hoses would be best to check as I intend to do this tomorrow?

It's all a hard call. Many items can cause similar issues. I had a P0171 and later the P0174, and it ended up being my fuel pump. I went back in and did the blue o-rings again, with no change. My idle wasn't bad though, the pump was only pushing 50-55psi, plenty for idling.

I suspect something unrelated to the LPG conversion, but you are hunting that. Have you cleaned the MAF lately, with the proper cleaner?

as Manaen asked...what is the engine vacuum?? is it steady or bouncing??

The MAF sensor was new only 3 months ago. I have no way of checking the engine vaccuum, what is the cheapest way of doing this?

The codes point to either a vacuum leak or low fuel pressure. Any cheap vacuum gauge can show you if the vacuum is steady or low. The tiny leak which could be in the o-ring gaskets etc. should not affect basic vacuum readings though. How old are the upper intake gaskets? That isn't a big job at all, I'd check the fuel pressure first, then do the o-rings(<$10).

The o-rings were done (whole 00M12 kit) around a year ago but this same problem has been present since before this was done.

you can also check for vacuum leaks with propane. Use a torch head on a propane bottle (don't light it of course!!) and run it around possible leak areas and hoses, if there is a leak the engine rpm will rise when the propane starts to get sucked into the engine.

but as CDW said check your fuel pressure as well.

How are your plugs burning? (black, white, tan, oily)

does it seem to use more fuel than it should?

Are you running gasoline at all, or can you to check the fuel pressure? As mentioned before, my fuel pump was functioning, but it was down below proper pressure. I had 50-55psi playing with the throttle, in park. Now it is at least 62psi under any load. That lower fuel pressure was causing a lean condition code P0171 and P0174.

Many things can be the cause, you have eliminated most of them. Hopefully you will find the answer before you have to start double checking things. Regards,

Plugs are fine, I have to change them every 15000 because of the LPG and they were new in August. I am pretty certain the fuel pump is not an issue as the problems I have are identical on Petrol and LPG (the vehicle starts on petrol but I manually switch it to LPG after around a mile when the engine has warmed a little-the LPG has to be warmed slightly by the coolant before entering the engine). Is there anything else I can use, other then propane, to check for leaks? I have WD40 and alcohol based cleaner, if not I can get some propane. I still think the problem is something only associated with idle as it runs beautifully above 1100rpm. I'll try the IAC and check the hoses and pipes. Thanks to everyone who is trying to help me. :thumbsup:

propane is the cleanest, but you can also use carb cleaner, although you need to be a little carefull where you spray it since it is quite caustic and flamable

I know its a little basic, but after all that work did you reset ecu and then run for a couple of days and then read the codes again?