Hey guys i got a 96 f250 Powerstroke that i just bought probably about a month or 2 ago. this is my first diesel so everythings kind of new to me. Im having a problem when i rev it and start driving it. its been starting to cold here in Ohio so it could be from that but I'll let you guys tell if thats correct.

when i start my truck everythings fine no issues. ill let it sit for maybe a couple minutes and then go to let things warm up a bit because i know diesels HATE cold. when i start to go everythings fine but when i get to maybe 1500 rpm it seems like it bogs down and i didnt move my foot so ill put some more pressure on the peddle and it will go to a higher rpm and will just drop to a lower rpm again without me moving my foot??

so to test if it was just from being in drive i put it in park and revved it up to 1500 rpm and then it just drops to a little over 1000 rpm and i havent moved my foot at all. i tried this two more times with the same results. it sounds like it might be a fueling issue but i read somewhere that it could maybe be the oil just cold and thick? this happened a couple days ago but i turned off my truck and 5 minutes later my uncle tried and it didnt happen. this doesnt happen everyday either or everytime i drive it just these two times so far. any help will be much appreciated

 

That kinda sounds like the exhaust back pressure valve (EBPV) closeing. It is a butterfly valve between the turbo and down pipe. It closes to help warm the engine. Try unplugging it and see if your problem goes away. It is a two prong plug on the drivers side of the turbo under the intake tube. It will not harm any thing leaveing it unplugged. Alot have it unplugged and even deleted it as it restrices exhaust flow. Welcome to FTE!

 

Alright thanks ill give that a try before I go to work tonight. Thanks for the help!

 

Hey Brendan, welcome to the site! What part of Ohio are you in?

The problem you describe could be caused by several things. First of all, these trucks don't have a throttle cable since they're all electronically controlled injection. Instead, they have a sensor on the throttle pedal that senses the pedal position -- it is called, appropriately, the Throttle Position Sensor (TPS for short). If the TPS gets a dead spot in it, you can see a symptom very much like this. The test for this is to use a multimeter to test the sensor (I generally prefer to work on the pedal out of the truck, rather than standing on my head under the dash). Generally if this is the issue, though when you push the pedal past the dead spot, the engine will continue to rev. Does yours act like this?

Another possibility, which Gabe mentioned above is the Exhaust Back Pressure Valve (EBPV). It is a flapper valve on the exhaust side of the turbo that is designed to (supposedly) help the engine warm up when it is cold outside. If that thing closes when the engine is running and is closing too farm when it isn't supposed to, it can feel just like what you've described above. When this happens it is accompanied by a very distinctive hissing (or more of a whooshing) sound from under the hood. If that's what you're seeing you probably have a plugged tube going to the EBPS (the sensor that is supposed to help operate this valve). You can unplug the valve as a test with the two wire connection under the turbo that Gabe mentioned. You can leave it unplugged too, for that matter. Lots of us have them unplugged or eliminated all together.

The last thing, and it is the most involved, is that it could be that you have some blown o-rings in the injectors. This isn't as bad as it sounds. They're able to be replaced pretty easily, but it does mean doing some wrenching to pull all 8 injectors. Pull the lid off the round fuel bowl in valley between the cylinder heads and take a look at the fuel filter and the fuel in the bowl. Is the filter all black? Does the fuel in the bowl look dark, or clean and clear? If the o-rings are leaking, the filter will be black from the oil that is bypassing the bad o-rings and making it into the fuel, and the fuel tends to look darker than normal. Again, this isn't an impossible thing to do, if you're handy with a wrench and it costs less than $100 to replace the o-rings on all 8 injectors. Just be sure to check back here before you do this, if that's what you find so we can clue you in on a couple of important details about the process.

Good luck with the project, and welcome to the site!

 

Hey DIYMechanic thanks for all the help. I'm in the North Ridgeville/Eaton Township area.

With the first thing with the pedal when you push it a little farther it picks up and goes back to sounding normal but then it just goes back to that same sound with the rpm drop.

It kinda sounds more like the Pressure valve because now thinking about it, it does kinda sound like a weird hissing coming from right around where the turbo is. I'm gonna unplug that connection and see what happens later tonight when I get out of work where she's not plugged in and cold. Would that pressure valve be the cause the sudden drop in rpm while your driving also or should the rpm drop only be during revving?

I will check into the fuel bowl and fuel filter to make sure it isn't that. Is there a better way to check it without removing the filter or just remove the filter and check it that way?

 

Sounds like you're not too far away then! It's good to have another Ohioan on here.

If it is the pedal, they typically just get a dead spot in them, and once you move the pedal past that point, they clear right up. It sounds to me more like the EBPV is closing on you when it shouldn't be. It's not uncommon for the EBPS tube to get plugged full of soot and then the EBPS doesn't read right. When the EBPV closes, you hear that whooshing and the engine more of less strains and bogs down. If it is the pedal, it would sound more like you just backed out of the pedal (lifted your foot). The drop in RPM you're describing while driving could be a lot of things. If the engine feels like it is bogging down and you're hearing that whooshing sound, then it is likely the EBPV. Unplug it and give it a go. If that fixes it, you can just tape up that wiring connector and leave it unplugged. The truck will run fine without it.

The other thing you could be seeing while driving is that the transmissions on these trucks have a very distinctive torque converter lockup. It almost feels like another gear when the TC locks. You will feel the 1-2 upshift, then the 2-3 upshift, then typically the TC locks and then you'll feel the 3-4(OD) shift with the TC unlocking briefly to make the shift and then re-locking. They feel kind of weird. Supposedly a good custom burned chip really helps them feel better, and it wouldn't be hard as the stock shift points are pretty bad. I can't attest to that from personal experience since my truck doesn't have a chip (....yet).

I'd say unplug the EBPV connector and take it for a drive. I am betting that solves (or at least helps) your problem.

 

Thanks for all the help DIYMechanic I think that's what I'm gonna do is unplug it and see how it goes. As for the tranny I don't believe its the TC or shift points but I don't want to rule anything out yet. I'm gonna test out unplugging it before I jump right to the tranny having issues and again thanks for all the help guys.

 

The EBV valve engaging should make for a distinct whooshing sound when you've opened up the throttle. It should be notably absent once the engine is warm, assuming it is disengaging properly. I read in my ford CD shop manual the valve is _supposed_ to disengage even if it is still cold, if you floor it, but mine never did that when I was living in Ottawa. Now that I'm back in SoCal, it never ever engages so I never got around to disabling it.

 

I disengaged that valve and haven't had a problem since but thank you though. And thanks to everyone that helped. Much appreciated!

 

Good deal!

 

Hopefully that took care of the problem.

 

I hope it did also,....but if it didn't,....
This issue happened to me, exactly as described above.
After replacing the "psom" it went away and cured the issue. Just a thought,
here's a link on a similar issue.

speed sensor? - Ford F150 Forum