7.3L rough idle only when cold:

I've had this truck for 20 years (300,000 miles) and know how it runs normally. This truck has never had any cold starting or running issues like this. This issue is new and there have been no changes I'm aware of that would be of concern; same driver, fuel, oil, tuner, etc. It starts fine but idles mildly rough. Rough condition continues off of idle, chugging a bit through low RPM but it never stalls. Mid to upper RPM is still a little off but increasingly smooth. Once it's warmed up this all goes away and it runs great. I would describe this condition as similar to the advanced warning that comes in the last minute before running out of fuel. Sadly, I know this condition well . A couple of the injectors were replaced 10 years ago but five or six are originals. The wiring harness under the valve covers is original and I've never had a problem with it. I do most of my own maintenance but I've been lucky to not have many fuel system problems, so I've never messed with the injectors or control system myself.

Any info is appreciated.

 

do you use any modifiers for stiction?

 

No fuel additives. Rotella T6 5w/40 in the sump.

 

https://www.archoil.com/blogs/tech/dieselpower
get some of this with a fresh oil change. Report back after a few hundred miles or if anything fails sooner

You could also pour a gallon or 2 of ATF in the fuel tank, nothing fancy, to help clean the fuel side.

 

Changed the oil a few weeks ago. I'm in south Texas so not really very cold, and I've never had this problem in much colder places. I'm inclined towards determining the cause of this issue before slapping a band-aid on it. If I've got sticking injectors I would rather replace them. I really need to determine what the problem is before I try to fix it.

 

I know where you're coming from, but archoil is not a bandaid, it actually does have a purpose in dealing with and preventing an issue that is simply inherent to heui; stiction.

You should buy a bottle and pour it in. Your truck is exhibiting textbook symptoms of stiction. Could it be something else? Certainly. But replacing anything without taking the lowest hanging fruit step of dealing with stiction would be akin to replacing your starter before checking battery cables.

To be clear, archoil is added to the crankcase...not fuel.

 

I had this same conversation today with a buddy that's had several 7.3s over the years and he advised pretty much the same as you, but with fuel additives. So I understand this, the high pressure fuel pump is driven by engine oil, right? The injectors are controlled electrically but the fuel pressure is what does the work; no oil pressure directly operating the injector? Or is high pressure oil directly operating the injector? Not really clear on how the injectors are powered. But we're talking injector problems, not pump problems, right?

I fouled a couple of injectors ten years ago by running bad fuel and it was pretty obvious that they were just dumping fuel and not atomizing it; huge clouds of white smoke. Fuel additives didn't help that situation at all. I'm extremely skeptical of additives as I've never tried a one that noticeably improved anything. I had this engine torn down at 250,000 miles for a piston replacement and it was clean as a whistle inside; looked brand new. I assume this was from 150,000 miles running on synthetic oil but that's just a guess.

 

There really is no injection pump in these HEUI engines, the injector basically basically does the work of an injection pump, but localized to each injector. It's worth reading up on "how HEUI works", its very interesting. The engine oil is what is actually pressurized for the injection event (500-2000psi) and sent to the injector, where it is used to "hammer" (lack of better term) the low pressure fuel (pressurized by a regular pump to 50-60 psi) and the injector fires based on electrical signal to their solenoid.

So, the main benefit of HEUI and the oil fired injectors, is that they last a very long time. The downside is, engine oil is nasty and leaves deposits and even cooks onto the injector components. Archoil is designed to clean and prevent these oil related deposits, and is a good idea in an HEUI engine to run throughout the life of an engine, if not every oil change, then once every few oil changes. Again, it is added directly to the engine oil and serves no/little real purpose outside of an HEUI application.

I totally understand your skepticism and agree with it - if we weren't talking about oil fired injectors. I'm not blowing smoke up your ***, and i'm not a salesman. ATF in the fuel was a secondary suggestion - atf helps clean the fuel side, but won't help you with your issue b/c it is likely on the oil side, where stiction occurs. Get yourself a bottle, add it to your crankcase, and let's talk after a few trips and heat cycles.

 

Problem seems to have gone away after an oil change; 5w40 Rotella vs the 15w40. It didn't clear up immediately but eventually went away after a couple tanks of fuel. I thought it might have been winter doing this but it wasn't very cold here. Strange stuff.

 

Use the archoil! Problem going away with fresh oil just reinforces everything we talked about above. Ambient temps have gone up for startup, and your oil is fresh. What's going to happen is, your oil will get dirty and beat up(could be as soon as a few hundred miles, and as many as 2 or 3000) and you'll start to exhibit the same symptoms. You'll probably do another oil change in the summer sometime, back to "its running good", then winter hits..........and even with fresh oil, you'll be back to the symptoms referenced above in the thread.

Since you just did an oil change, buy the archoil and pour the bottle in, FTLoG

 

Alright, I just re-read the whole thread, and man..........you're discovering for yourself what has been discovered already by so many people who run HEUI powerplants. Of course your issue "went away" with an oil change, because what you are dealing with is STICTION, and it how fresh or old the oil is directly correlates to the symptoms projected by it. If you don't listen to me now, just remember this; when you start to feel or notice the symptoms that you described above, before the oil change, let that be the trigger to buy the archoil.