heater control valve leaking, P0171 code

[rfingon]

OK, strap in, because this may be a long one.... This 4.0L OHV (166K miles) will throw a P0171 code during a regular, in town drive, but will never throw this code during highway driving. Also worth mentioning, the MPG is steadily going down the crapper. She used to throw both a P0171 and P0174 code (same conditions as above) before I replaced the air intake manifold gaskets. No P0174 code since fixing this. Prior to doing this a while back, the engine started misfiring on a regular basis, and I received a P0304 (cyl. 4 misfire). Took that plug out, and the ceramic was cracked IN HALF! Replaced that plug, fixed the misfire, but the P0171 and P0174 codes persisted until I did the upper gaskets. Last Thanksgiving while I was visiting relatives, I started to drive home and noticed that the engine was missing and knocking like a bastard. This time, P0303. Took that plug out, same thing - cracked ceramic. I did notice that when I was taking this plug out, I could see coolant slowly dripping out of the vacuum-activated crank part of the heater control valve. I then replaced the heater control valve. One day, however, while under the hood and the dash switch on the "off" position, I noticed coolant spewing out of the crank part of the NEW switch. I quickly switched the dash control to the defrost position, and the leaking stopped (i.e. after the heater control valve engaged by vacuum). Upon warming up the engine and opening the thermostat, I moved the dash switch to "OFF". No leaks were detected out of the heater control valve. I did have to add some coolant to the reservoir.

At this point, I am going to go Quentin Tarantino on your ass with a flashback. This is what I have troubleshot/replaced thus far:

1) A while back, when all this code crap started, I replaced all O2 sensors along with Bosch Platinum +4 spark plugs (truck had 101K on it when I got it, so I did the replacement out of principle).
2) cleaned the MAF sensor with CRC MAF Sensor cleaner, and later replaced the MAF (wasted $130, not the problem--did this before checking vac leaks and replacing the air intake manifold gaskets).
3) checked for vacuum leaks with carb-choke cleaner, as well as replaced any dry rotted vacuum hoses large and small, including the vacuum lines to the 4WD. Pretty confident that no vacuum leak exists from hoses or the air intake.
4) replaced the fuel filter.
5) checked fuel pressure at the rail (64 psi).
6) cleaned the IAC and replaced the filter under the cap with some cotton wadding. My truck had that crazy humming sound before doing this. Thanks TRS peeps for the humming fix.
7) cleaned the throttle body (did this when I replaced upper intake gaskets).
8) replaced the DPFE sensor (wasted $53, not the problem).
9) replaced every spark plug again (Autolite platinums) after the second cracked plug was found and bought a new ignition wire set.
10) after still occasionally misfiring, replaced the ignition coil (wasted $79, not the problem).

At this point, I went back through the ignition system, one wire and plug at a time, measuring wire resistance and inspecting each plug. All wires checked out and are tip top. What I observed on the spark plugs was rather interesting. Cylinders 1,2,5 and 6 had clean insulators on the spark plugs, but cylinders 3 and 4 did not. The ceramic was rust colored. After perusing the TRS forums, I concluded that I must have a lower intake manifold leak, seemingly worse on cylinder 3 (deduced from the constant emergence of the P0171 code).

So why is the new heater control valve leaking? I don't think the heater core is plugged (no coolant smell in the cab and the heater works great). Is it possible that my new heater control valve is faulty? I have an Autotap, and I can see that all O2 sensors are working properly, and the engine temperature is steady when driving. When I do rip into this truck, I will replace the thermostat anyway. Unfortunately, I can't rip into this truck yet, because the garage has my original daily driver in it with the transmission ripped apart. I am going to finish my tranny rebuild to get my Frankencar back in action at this point. This way, I will not be without transportation and can take my time on the Ranger, so I can do this job right.

With that being said, any insight or feedback would be greatly appreciated. I am expediting on my car right now, because I want to baby this truck at this time to minimize any more damage to the engine I may be causing by driving it. Most of all, I appreciate your patience by finishing reading my long-winded post. Thanks in advance.

 

[rfingon]

Update on the truck. I replaced the lower intake manifold gasket, valve cover gaskets and fuel rail to lower intake gasket. The truck throws these codes: P0171, P0174 and P0305. You could tell where air was getting to cylinders 3, 4 and 5 by looking at the fuel rail to lower intake gasket. However, the truck is still sucking air somewhere, but damned if I can find the leak with a can of carb cleaner.

The next logical thing was to do a compression test. I wish I would have done this before I ripped off the top of my motor. I couldn't get a good read on cylinders 2 and 3. Cylinder 1 averaged 100 psi, cylinder 4 read 110 psi, cylinder 5 read 40-45 psi, and cylinder 6 averaged 110 psi. So I guess this means that cylinder 5 is definitely a problem, perhaps a bad valve or ring. The engine burns no oil (is the piston ring OK then?), and I don't see any coolant present in the oil. Please advise. Thanks in advance.