Did not read all the posts but rough idle, bad gas mileage, hard starting etc sounds like your intake manifold gasket. Had that happen on a 94 V10 and a 98. Did not throw any codes. Engine is sucking way too much air but the computer is so good it just pumps enough gas in to keep it running relatively smooth. If the gas mileage hadn't dropped so bad not sure I would have really noticed. Pretty easy fix and even after 400K miles on each of them neither had it happen again.

 

I recently had to change the valve cover gaskets. Removing the intake. The book called for new manifold gaskets so I changed the gaskets before I put it back together. Would water be leaking from a bad gasket?

 

Nope, no water leak, it isn't Technically part of the cooling though it does seal off some of the cooling passages. Never saw one that leaked water but it could happen I guess. If you replaced the gasket it is very possible you did not get it on completely right and it has sucked in a corner or two. They aren't real hard to put on but you want to follow the service manual procedure exactly and you also want to use a factory gasket. This is one place OEM is no where near as good.

Easy test, spray same carb cleaner with a thin nozzle close to the gasket manifold joint, with the truck idling. if it is sucking air you should hear a difference as it sucks in the Carb cleaner.

 

We already did that test when we looked for vacuum leaks and the engine never changed. It's been almost a year since I replaced the valve cover gaskets.

 

All I can say is both of the ones that did it to me had almost 20,000 on them before they went. Warranty covered both. If the leak is at the back it may not react to the carb cleaner. My thoughts are, you have gone through pretty much everything else. The vehicle is clearly burning a lot of gas to keep running and you say it is firing on all cylinders so the only other cause has to be that somewhere it is getting a bunch of surplus air, thus leaning out the mix and causing the computer to dump more gas to keep the mix right. That or there is something wrong with the computer and it is actually running way too rich. IF it is rich, the plugs should be black, if it is lean, white and if the computer is making up enough gas they will be normal brown. If the plugs aren't black then you have extra air getting in somewhere.

 

Roger that! Plugs weren't black. Mechanic said they looked brand new. Because they are. I'm gonna head to the shop and discuss what you guys have discussed with me! Thank you very much! Nothing bugs me more then to have wasted horsepower! Cheers gentlemen!

 

So, Ive been fighting this badly run n truck for months now! Gas mileage is almost nonexistent. It was 4.9 the last time I fueled up. I've replaced a ton of parts and did a bunch of tests. Atleast it used to run like crap! But it ran. I started the truck the other morning to let it warm up and it started rough, backfiring and running terrible. My mechanics said your injectors are plugged. So I removed all the injectors sprayed them off and cleaned them with break cleaner. Replaced the O-Rings and oiled them before I reinstalled them. It's still dead! Won't even try to start? I charged the battery for a day. So it was crank n good. Dead in the water! The tank is full. I replaced the fuel pump twice in the last year. 50# of fuel pressure at the rail. Checked the coil packs, checked for spark, changed a few sensors and it won't start! Happy holidays everyone! Nothing like not being able to go to grandmas for Xmas!

 

Are you getting good spark TO the cylinders? Are the injectors actually firing? Are the plugs fouled? What kind of plugs are in it?

 

We passed the spark test. The plugs I replaced with the ones the sticker under the hood asked for! Mechanic said spark plugs look new. Because they are. Replaced the fuel pump twice in the last year and a half. New fuel pressure regulator. I've changed a bunch of sensors. But I haven't changed any sensors that run thru the PCM? Like the crankshaft sensor, the camshaft sensor and the speed sensor? I saw a video that shows the 8 volt feed from the PCM dropping to 3-5 volts which wasn't enough voltage to start the truck?! So they hooked up a jumper wire from the battery at 12 volts. The truck started! When they built a resistor. It took him a few tries to get the right ohms resistor to bring enough voltage during cranking to start the truck. It's Xmas. So I don't know if I get any time to fix it today. Just a new angle I suppose? PCM's are not cheap!

 

So I pulled out the Haynes Bible and found Pin 7 (orange wire). It is the 8 volt feed wire that goes to the VSS, CPS, CAMSHAFT POSITION SENSOR, ICM and also runs the injectors, plus the coil packs. Gonna test pin 7 for voltage. If that fails I will then run a 12volt jumper wire to the orange wire, pin 7 and see if that starts it? Stand by!!!