ok, at the risk of sounding totally stupid - what is "closed loop" and WHEN does this condition occur?
And when you say I need a "reader", do you mean some sort of specialized equipment or just an obd1 reader or volt meter? Both of which I have. (My obd1 is specifically for Ford and gives me an actual number versus having to count blinks.)

WARLOCKK, I would really like to SPEAK with you. Would it be ok if I sent you a PM with my phone number?

 

I really do appreciate all the input and ANY and all help all you guys have provided.

This thing is driving me crazy, I've been fighting with it for MONTHS now and have spent countless hours searching this site as well as a couple other Ford and F150 sites.

 

Sure you can message me. I'm not sure when I will be free to call because my schedule is crazy. It 5:30 am right now. Closed loop is when the engine is in its warm running condition. The O2 sensors controls the fuel trim and it switches rich/ lean. When you start the truck or go to wide open throttle or a few other special times it is in open loop where the fuel goes to a preset amount to warm up the engine or give full power etc. As for the reader to be honest I have never done it on OBDI. I know it can be done but I don't know if the regular code reader does it or if it is a special tool. On OBDII you use a real time scanner. You can read the voltage output directly from the 0² sensor but that doesn't help much without the fuel trim. Ill try to find out more info at the shop or in class.

 

A special "reader" is required to see the fuel trim for OBD1 systems. I've already done the research on that one. Even with the equipment to read the fuel trim it's still not going to pinpoint the source of the problem.

I'm still scratching my head over my Code 172 as well, so I feel your pain.

Every solution I've seen says it's unmetered air coming from somewhere or a wiring problem.

Have you run a KOER test?? I fail that with a Code 411, can't control rpm during low rpm check. IAC valve checks good and all other solutions point again to unmetered air coming from somewhere.

I can't make the correlation between the two but one has something to do with the other.

 

Yes, the gasket was replaced with no change noted.

I was of the understanding that the fuel canister purge valve should NEVER be open at idle and that it should only open when the engine is warm and "off idle" as determined by the voltage from the TPS.

My understanding is that on the '95 4.9L the PCM takes a snapshot of the TPS voltage when you start the engine and then uses that to determine "idle". If the TPS returns to the same voltage +/- a couple tenths the PCM should say "I'm at Idle again" and the purge valve should not open. I've checked the TPS like 100 times and it's OK.

Question is: Why is the purge valve still occasionally opening at idle, resulting in a high idle???

This is tied to the Code 172 somehow.

 

I'll have to double check on that. What you are saying sounds familiar to the point that I was about to affirm your suspicion, then I suddenly remembered that late model fords will purge any time they want and will only stop if a demand is made for power. So now I have to see if I can verify when it works on the older systems. I don't want to get confused with EGR. EGR will only operate in cruise no load conditions.

 

Let me know what your thoughts are on that. I've got a '95 (OBD1) 4.9L with a manual tranny.

The only thing I can think of is to check the TPS voltage back at the connector to the PCM since I know the TPS is good. Even swapped it with another one and the purge valve still occasionally opens at idle.

 

I realize this is an old thread at this point, but for anyone that comes across it, I still have the truck and have not found the cause of the lean code 172. It still idles high, even higher when the purge valve opens, and occasionally the check engine light still comes on. I've unfortunately learned to live with it.


The last couple of suggestions that I received said perhaps it was a cracked head or intake manifold? That could be but I've pretty much given up at this point. It's an unsolved mystery.

 

The purge valve is not large enough to have any effect on idle speed.

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Use a mechanic's stethoscope without the hard probe (open tube) to listen around the idling engine for the obvious hiss of a vacuum leak:

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The throttle body gasket is a likely suspect:

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Thanks for the reply Steve. I changed the throttle body gasket a while back. Looked like the one in your photos. I also changed the intake manifold gasket and the fuel injector O rings. I blocked the EGR valve and plugged the purge valve hose. That didn't help.

I've been all over the engine with a hose stuck in my ear and haven't' found anything. Have sprayed carb cleaner all over the place too. Nothing. I've spent a considerable amount of time on this thing and I'm baffled. Engine idles around 1200 rpm last time I had a tach on it.

With the price of gasoline it would be nice to get this thing to idle down but I'm stumped at this point.