Very weird rough shaky idle/driving situation

[Datjosh]

So I have 1994 7.3 idi factory turbo that starting running rough after a drive on the highway. It starts perfect cold and hot and never stalls. The weird part is it seems to fix it’s self every time I take the supply line from off the filter but after a drive for 3-5 minutes it goes back to being rough and having grey smoke. I’ve tested all my injectors they seem to check out and all the spray from loosening them seems consistent. I’ve took out the fitting with the glass ball and it seems to function correctly and blown out my return lines out as well with having no effect on the rough idle. The only thing that seems consistent is at 1500 rpm’s it’s always a bit more shakey.
I’ve narrowed down the problem is in the engine bay by allowing it to feed from a fuel can. Any diesel mechanic I find can’t explain why it acts better when I unhook the feed line then reconnect but resumes running rough after going for a drive.

 

[BlindAmbition]

Sounds like you're getting air intrusion from the supply side of things. Most commonly return lines and caps are the issue, but air intrusion can happen basically anywhere on the fuel system. Check your rubber lines down by the tanks and make sure they are good. If it happens more when fuel is low then your showerheads might be done for.

 

[Jesus Freak]

Definitely air somewhere.

 

[IDIBRONCO]

Most commonly return lines and caps are the issue, but air intrusion can happen basically anywhere on the fuel system.

No. Those can only cause fuel drain back. There is a difference. Air intrusion only happens while the engine's running because air is being drawn into the fuel system.

 

[Old Goat]

Air....

Sounds like you need to find some more knowledgeable Diesel mechanics.


Goat

 

[IDIBRONCO]

The more I think about this, the more I think that I'd start with the lift pump, the connections to it, and the rubber hose that goes from the frame to the lift pump.

 

[Cant Write]

The weird part is it seems to fix it’s self every time I take the supply line from off the filter

I assume the supply from the lift pump to the FF, once you remove it, where do you connect it?

Do you have the mechanical lift pump?

Are you able to get the symptoms without driving it, or is driving it a requirement?

I’m with @IDIBRONCO. Are you able to connect 5-gallons to the suction side of the mechanical lift pump.

Could it be a timing issue like my van, which I thought was air intrusion...?

 

[IDIBRONCO]

Are you able to connect 5-gallons to the suction side of the mechanical lift pump.

Yes he mentioned that already. I was thinking about your timing issue, but got hung up where he says that he can disconnect the fuel filter supply and it runs right for a short time. That doesn't sound like timing to me. On the other hand, Datjosh, it certainly wouldn't hurt to have your timing checked.

 

[Cant Write]

@IDIBRONCO ah gotcha, I just couldn’t decipher it all!! Thanks for clarifying

 

[IDIBRONCO]

@IDIBRONCO ah gotcha, I just couldn’t decipher it all!! Thanks for clarifying

You're welcome. I agree with you on the timing 100%. It can cause some crazy things to happen. I never thought that slow timing could cause the issues that you were having until you had them.

 

[Datjosh]

Sounds like you're getting air intrusion from the supply side of things. Most commonly return lines and caps are the issue, but air intrusion can happen basically anywhere on the fuel system. Check your rubber lines down by the tanks and make sure they are good. If it happens more when fuel is low then your showerheads might be done for.

Shower heads are definitely done for but I’ve ruled out it being the lines cause I ran it on a gas can on the ground and it still had the issue. It’s almost like the injection pump isn’t able to take what the mechanical lift pump is giving it cause ones I loosen the feed line and relieve pressure it idles and runs good till I drive again but all my returns aren’t blocked off

 

[Datjosh]

You're welcome. I agree with you on the timing 100%. It can cause some crazy things to happen. I never thought that slow timing could cause the issues that you were having until you had them.

I was also thinking timing but I don’t have the pulse detector for my timing light I’m currently looking for someone nearby who does. I surely do not want to mess with timing unless I can get it perfect. No diesel mechanic I’ve talked to seems to be able to put together what it could be. So my two ideas are the filter on the cone of the fuel inlet on the pump and timing. One’s hard to check with it on the vehicle and the others hard to find…. Also @Cant Write it seems driving is the requirement to make the problem start back, when I get some free time I’ll make it act right and let it sit and idle and see if it does it on it own.

 

[BeastMaster]

Could this be a plugged fuel filter causing the IP to pull vacuum at it's inlet?

 

[IDIBRONCO]

And yet another good possibility. removing the feed line from the fuel filter could relieve the vacuum if this is happening.

 

[Datjosh]

And yet another good possibility. removing the feed line from the fuel filter could relieve the vacuum if this is happening.

Filter is new I changed the filter after the problem started thinking it was it and when I remove the line it doesn’t seem like is sucking air more like it has plenty of pressure so the feed line definitely isn’t draining back either