Hey guys. Sorry to make my first post a needy one but my free truck isnt acting free anymore if you know what I mean.

Last spring someone I knew needed to get a 1994 f250 351w E4OD No MAF off his property asap. Hadn't been driven in 2 years due to front end issues. Told me if I take it I can have it. I showed up with a battery and it fired right up and drove away under its own power. Fixed the front end and did fluids, tuneup, and a few tires and u-joints / other misc stuff like replaced the oxygen sensor. Been driving it for the last year and put nearly 10k miles on it without any major issues. Even pulled a double car trailer from South dakota to texas and back. Was supposed to be a spare beater but my daily died on me and I replaced it with a fun car so now this guy is my daily beater. Great goddamn truck and definitely gave me an appreciation for fords.

So here is the issue. It had a evap solenoid code from day one... never turned the light on though so I ignored it. Week ago I got a CEL and it was loading up really bad and then would suddenly unload and have tons of power. Was a system too rich code (173?) if I remember right. Unplugged the o2 and she drove great. Tested the o2 sensor and it was not switching so I warrantied it out and reset the computer. Also replaced the evap solenoid while I was in there because it was leaking a tiny bit of vacuum. Fixed the code no problem. Drove great a few days until suddenly I got a CEL again! This time its running pretty dang great (Not quite 100% though) but its a code 172!

The light always sets after the truck is allowed to fully cool overnight. Then when I start it and immediately hop on the interstate going 75 the light sets after 4 minutes or so. It rarely will turn back on with successive restarts and driving for the rest of the day. Obviously I can pull the stored code though after the fact.

Things I have verified,

1. O2 sensor signal wires ohm out perfectly between eec connector and o2 sensor connector and infinite resistance to ground and to voltage so no shorts. When I measure accross the o2 sensor wires with it plugged in and the key on or running I get 1.3v no matter what though? I think ford uses a bias voltage so I chalked it up to me not knowing the system well enough.
2. 12.4v to the o2 heater with the sensor plugged in. 12.6 unplugged. This was measured pin to pin not pin to chassis. Engine off.
3. No obscene exhaust leaks before the o2.... there are a couple but they were never an issue before.
4. No MAF sensor so no worries there
5. Fuel pressure tests well within the range at idle with vacuum and during throttle blips with vacuum.
6. Cap rotor plugs and wires are all less then a year old and pass the garden hose test.
7. Base timing is set to 12*
8. Had tested the TPS a few months back and it was nice and smooth / linear. Haven't tested since then.
9. Sprayed brake cleaner over everything and there was no reaction at all with one exception. I could get the idle to jump up when spraying the "inside" of the egr valve. Obvious that there is a small vac leak there but its very small. When I feed vacuum to the valve it opens smooth and easily kills the engine at idle. I cant imagine that could be it on a non maf engine since a tiny bit of extra air that far up the intake tract should just raise the idle and not even be a factor at 75mphbut like I said I dont know these older ford systems that well so maybe it is a big issue. Primarily been a obd2 chevy guy my whole life.

Things I have not checked

1. Map sensor. Ill drag out the hand pump and multi meter to test this bad boy at some point.
2. Have heard people mention a really bad pcv valve can cause it. Haven't checked yet.
3. Have not ohmed out any injectors. From what I hear this doesnt tell you much other then the coil is alive anyways

Any help greatly appreciated. I just want it running 100% for when I load the trailer up again in a couple weeks. If this is the wrong place to post please tell me.

 

Here's a link that might help.

http://www.fullsizebronco.com/forum/7-1980-96-bronco-tech/185521-need-help-quick-172-code.html?nocache=1462983427835

 

My bet is still the 02 sensor. I have the exact same truck.


Make sure it is plugged in all the way. The connector has to "click". Don't ask me how I know


Mike

 

Changed the pcv valve last night since it was the factory unit and it was 2$ so why not. Other then being a bit loose it seemed fine though. CEL greeted me again on the way to work unsurprisingly. I took a look through that thread again but unfortunately most of it pertains to a MAF sensor that I do not have. Mine is MAP only. I am going to take the new o2 sensor out tonight and do the torch + multi-meter test. Maybe ill get lucky and it will fail. One way or another it is getting a new egr valve since it most definitely has a small vacuum leak.

 

Any vacuum leak however small can trigger a lean code. This also applies to exhaust leaks upstream of the O2 sensor. Fix the EGR, fix the exhaust leak then run the truck again.

You stated "Fuel pressure tests well within the range at idle with vacuum and during throttle blips with vacuum." What were the values?

A loose PCV valve can also be a source of unmetered air.

Do not forget there are several vacuum reservoirs that can also leak. The coffee can on the passenger side fender is known to rust our on the bottom. There is also a small plastic reservoir on the HVAC blower motor case. I have found them cracked a few times.

The other common issue is leaking throttlebody gasket and/or worn out throttlebody bushings.

 

Fuel pressure was within 1psi of whatever the service manual said. Something like 35 - 40 if I remember right. Pcv valve is brand new and not leaking as of yesterday. I have the plastic vacuum canister so no rust for me. I cant say I have ever seen any other one on my hvac system though? Throttlebody gasket and bushings are both good. Zero reaction from water or brake clean. Egr definately gets a nice raise in rpm when I spray it so that is next along with testing the o2 sensor tonight.

 

The O2 sensor wires travels with the starter's wires past the oil pan & up to the battery/passenger side fender harness. Friend found a wear spot in one of his wires that was causing a repetitious blown fuse recently on his 5.8- 95. Inside the wrap i think or close to the seam down where it goes over the front cross-member.

 

Now here is where stuff gets weird... I only have one oxygen sensor because this is the 8600lb+ chassis and it comes out of the transmission harness. that runs accross the trans crossmember. Now I have absolutely no idea if this is how it is supposed to be or not. The harness truly looks factory. Truck was converted from 2wd to 4wd at some point in its life. I do know that the correct two pins referenced in the engine wiring diagram (29 and 49) go to this connector and have good connection. I have not peeled back the conduit but the eec connector and the o2 connector both look unmolested.... in fact 99% of the wiring is unmolested on this truck other then the usual trailer wiring shenanigans and a very poorly installed radio that was drawing V constant power from the lighting circuit. Last I checked I have nothing oxygen sensor related running through the starter harness and it all appears to run down the frame rail on the drivers side.

 

OBD-I trucks have only one O2 sensor. My old '92 F350 had the O2 sensor wiring in the transmission wiring like yours.

 

Idle fuel pressure should be ~30-32 PSI with the vacuum hose on the Fuel Pressure Regulator. Without vacuum the pressure should go to ~40 PSI.

A photo of the vacuum reservoir on the blower motor/evaporator case:

courtesy of Subford

 

Oh thank god. Its damn near impossible to find anyone referencing the plug being there and everything is a variable when the whole truck was in pieces at some point in its life.

Im going to change the egr valve tonight probably. Ill check the reservoirs for leaks tonight as well. Honestly just assumed that line on the hvac was just another actuator or something that makes the magic hvac blow ice cold air in the summer. Coldest A/C of any vehicle I have ever owned.... 80% of the vac lines are original with the occasional rubber splice. Probably will bite the bullet and redo all of them if the egr doesn't take care of it.

 

Those vinyl vacuum lines are way beyond their service life. Great idea to replace those. Just looking at them too long may cause it to disintegrate.

 

LOL....I just fired mine u last night, and I didn't get 1/2 block, it started running real rough, it was down 2 cylinders, pulled over counted to 30, restarted ran fine with no problems, but like the OP, my lines are all factory, except for the occasional rubber splice.

 

Ford does not use a bias voltage. Voltage should be 0.7-0.9v rich, and 0.0-0.4v lean.

 

So.... I get 1.3v across the two pins at all times with my meter. Pretty sure I get that even with the o2 sensor unplugged. Is my puter not long for this world?