My truck gets 12-13 mpg highway, misses intermittently(more under load), stalls sometimes coming up to a stop(restarts immediately), sometimes I can hear it backfire thru intake. NO POWER! I need HELP! The only code I have is from EGR, which is caused by having EGR vacuum line plugged (when plugged in it would run worse,way worse). I have changed cap, rotor, wires, wire routing, plugs, coil, ignition module, PCM, rear fuel tank & fuel pump, fuel pressure regulator, compression, cleaned throttle body & IAC motor. I have owned truck since new & it got around 17-18mpg on hwy. Please someone, anyone I need help!

 

Have you changed the fuel filter? It could be starving for gas.

 

Changed filter just prior to fuel tank & pump, but I'll try it again

 

check plugs.

are you positive plug wires are on correctly?

which motor?

they have different routings for different years for some of the v8s.

 

I have replaced plugs, plugs that came out looked fine. Printed proper wire routing for my yr truck, followed it to the "T". 5.0L is the engine size.

 

How about a clogged cat? It may be preventing a full flow of exaust. From your original post, it looks like you have done everything you can so you may want to double check it. As QL2 says, and as I have seen many times, it is easy to cross wire the plugs. Tripple check those.

Have you checked compression? You may have a head gasket or rings going out.

 

what brand and kind of plugs?

even new ones can fail, that is why i would pull them and read them.

 

Loss of power could be loss of compression, incorrect timing, engine running too lean to name a few. If you did hear a backfire through the intake manifold that would indicate premature detonation, which could destroy the top half of your engine. If that is happening I would be more concerned about that then loss of engine power. If air/fuel charge doesn't always detonate when the spark plug fires or should fire, then it might detonate sometime later before the next complete cycle for that cylinder. If one cylinder is not firing at all or only sometimes, it would cause an excess amount of fuel in the exhuast some of which would get sucked up via the EGR system and end up making the rest of cylinders run to rich. Just some things to think about as you continue your trouble shooting. I have a o/2 meter in the cab of my truck which can tell me if the engine is running too rich or lean and I have found it to be the most valuable tool for trouble shooting these types of problems.

 

Autolite 25 plugs. When I replaced the plugs(3000 miles ago) the ones I pulled out looked fine & it is doing the same thing, so I can rule out plugs. I have checked compression, don't remember #'s but they were good & all even. Converter is about 2 1/2 yrs old. It doesn't miss @ WOT only @ cruise speed. Power @ WOT is slighty down from when motor was new but pretty far away from original motor. Motor replaced about 60000 miles ago.

 

I would check all your vacuum lines to make sure you don't have any leaks. At WOT the vacuum is low and the computer doesn't utilize vacuum for controlling it's sensors at that time so it kind of goes into a default mode. While running at cruise, you should have a good intake vacuum and the computer will be making adjustments, many of which are controlled via the vacuum system.

 

maybe the timing chain has streched.

seems like you tried everything.

i am stumped.

 

When this problem first started I did have a vacuum leak that would trigger a check engine light but spent the better part of a day w/ a vacuum pump & checked each and every vacuum line on the truck. I even found a leak in the line for the bend door on A/C. I have since repeated this process w/ the vacuum pump & could not find any leaks. Even used carb cleaner on intake, TB, etc & didn't find anything wrong. Can you still check for leaks w/ carb cleaner oon todays vehicles? HELP!! I am just about to throw in the towel & take it somewhere & pay out the *** for them to tell me whats wrong.

 

Have you checked the timing? It should be set at 10°BTDC. I hope the timing change hasn't slipped, as that could be causing these issues. There are ways you can locate TDC for #1 cylinder to see if it appears the timing change had slipped, but I would first verify the timing is at spec. I agree with quick, we are running out of ideas.

 

there is a TSB on plug wire routing for these trucks. there should be s sticker somewhere under the hood that shows proper wire routing. there are two cylinders that fire one after the other and i believe they aare the 7 and 8 cylinders. if the 7 and 8 wires are touching anywhere you will get whats known as induction crossfire. this means that these two cylinders will fire at the same time, resulting in drivablility issues. go back and be sure you have these wires routed properly.

 

I agree. As I said in my earlier post, tripple check the wires. With everything else you have done and checked, this is the last thing I can think of.