So I've recently bought a 1999 F-250 6.8 V10 w/ 125,xxx miles on it and the truck has run completely fine for about a month now and one morning i went to work and the truck just suddenly started misfiring and running horribly. Barely any power rough idle etc. Literally over night. No check engine light came on but I decided to stop at advance and run the scan tool anyway and sure enough I had a p0300 code and a p0401 code. I know what both codes mean so with that being said I stopped at a shop and they claimed it to be my fuel pump was going out even though I knew the codes were egr i took their advise and swapped out my fuel pump and fuel filter...sure enough nothing happened. I also have had a small leak on the intake manifold on the driver side where the thermo stat meets up with the head. So I assumed maybe it was getting worse and maybe causing the misfire and figured id rip it apart and fix it while im at it. I took everything apart replaced all the plugs, new gaskets etc. while apart i sprayed the egr with brake cleaner and pb blaster along with the IAC valve. I spayed out the throttle body as well but didnt seem that bad anyway. Just finshed putting it back together last night and same problem still. Ive looked into it more now and Ive seen there is a DPFE sensor or possibly a bad vaccum line i need to check. What im asking at the end of this is there any thing else i should look for ? I want to cover all my bases and get truck up and running im tired of wasting money and time and doing nothing for the problem i originally was searching for. I fixed the intake leak which needed to be done anyway but Im runnning out of time and my full blown personal mechanic shop / Grandmas house wants the truck out lol Thanks for any advise !

 

1-Check PCV valve & its hoses and 90 degree elbow on throttle side, see if any cracks deterioration (air leak ),
2-Check MAF sensor @ air filter housing . Make sure MAF rubber base is tight no air leaks.


Good luck,

 

I would start with what wolf said and also clean the MAF if it hasn't been done recently.

 

I would fix the known vacuum leak first.

 

Hey sorry for the delayed response. I finally finished putting everything back together and got it all hookup and everything is by eye perfect. No leaks no nothing. I remember I plugged in a trailer wire adapter so that I could haul a trailer the other night and it was the incorrect one. It caused my reverse lights to stay on, brake lights didn't work on the truck etc. Is it possible I might have shorted out something in the ecu?? If so how would I check that?

 

Here's an updated more detailed post from earlier this morning. I cleaned the pcv as well as the MAF sensor. They both appeared pretty clean but sealed and properly seated. The PVC hose has been redone and looks almost new and is also re routed. so i did not find any vacuum issues there either.



So I am lost at this point any help would be appreciated. Truck was running perfectly fine except for a random high idle sometimes when put into park but if I gave it a good rev it would drop back down to normal. I assumed it was a dirty IAC or something and figured I'd get around to it later kinda deal. I also had a coolant leak from what I thought was the intake manifold to head gasket on the drivers side right in the front where the upper radiator hose meets the intake manifold so i added water here and there no issues.One night I need to pull a trailer and didn't have the wiring set up for it in the back so I stopped at advance auto and bought one of those kits that plugs into your brake wiring harness and then plugs into the trailer. I tested it in the parking lot with the trailer and it was all sorts of messed up. My reverse lights stayed on my brake lights didn't work either etc. Turns out it was for the wrong years but plugs matched up coincidentally. I returned the kit and drove home just fine. Next morning on my way to work truck started misfiring like crazy idle was bouncing all over the place lack of power etc. I took it to advance again after work even though no check engine light came on I still pulled 2 codes. P0300 which is multiple random misfire (no specific bank) and a P0401 which is an EGR Insufficient Flow code. I took it by a local shop I get my oil down at and they told me it was probably the fuel pump that they go bad in these trucks alot. So I repalced the fuel pump and the fuel filter. Nothing changed so I thought maybe that coolant leak had dripped down to the plugs or possibly the intake gasket was getting worse and leaking coolant into the air flow ports since it was thought to already be a problem. So i pulled everything off the top and firgured id go through it all why I had it apart. I replaced the intake gasket but noticed when i took it apart that it wasnt the gasket that the head it self had a small pin hole leak in the very front where the coolant fed into below that upper neck that bolts to the intake manifold. I patched it with jb weld/ quick steel/ and rtv. Yes I know it wont last but just for the time being until I figure out the main issue. Replaced the plugs with motocraft plugs checked all my coils for grime and corrosion or what not and they all seemed fine and cleaner than expected. replaced the gaskets and threw it all back together. I also cleaned out the EGR and IAC with air and brake cleaner etc. blew all my lines out with air and everything seemed fine. I also replaced my DPFE sensor. Started it up ran fine at idle for like 5 min no coolant leaks or anything so I thought I was golden then about 500 ft through the neighborhood on a test drive the exact same misfire came back doing the same thing with the idle etc. I'm going to check my fuses tonight but I'm starting to think that maybe when I plugged in that wrong wire adapter for the trailer that I may have screwed up something with the PCM. Only beacause the lights were all out of wack and literally the next morning I started having these issues. If anyone else has any advise on this to test and check etc please let me know I'm lost if its not a fuse. I would really hate to buy a PCM and not need it especially after all the parts I've bought and not needed already.

The vacuum lines all seemed fine and intact as well with no cracks or leaks.

 

UPDATE

I checked my fuses and there were 7 that did not show up on the test light i replaced them all and only 2 were fixed and no show up on the test light. the other still not working not sure if they are supposed too. I need a diagram i guess. The misfire is gone but im still idleing high and it bounces coming down as well.

 

Do you have an owners manual? That's how I figured out the fuses and relays for my trucks tow package.

Sounds like you may need an IACV. Have you thought about running some techron fuel treatment or seafoam through the tank? I also used the seafoam through the brake booster hose and it helped the idle clear up temporarily untill I bought a new IACV from Ford.

 

I use lucas injector cleaner every couple of tanks to keep everything running smoothly.

Some of your fuses will only light your test light, when that circuit is energised.
Example would be lights on , or engine running etc.

 

Ashley- that's a good point about the fuses. To add your really only supposed to test them when not plugged in with a meter to see if current is able to floww.

 

I have been hit with the misfire problem too. The engine worked fine on a trip until around 50 Kms and then started to misfire. A scan showed P0305 and P0306, which indicated misfiring at cylinders 5 and 6. I checked the plugs and they are like new. The weird thing is that, when I first start the engine cold, it runs fine with no missing until the RPM drops to normal speed and then the skipping starts. Any ideas about where the problem might be? And why is it cylinders 5 and 6? They are at opposite ends of the engine.