1994 M998, 1990 6.2L - misfiring? fuel?

Greeting all.
Start happens recently:

1. Cold start - A+, smooth idling 650-700, in a 35-40 sec, drops down to 500 with intermediate "misfiring", when warm up - back to normal. Hot start - A+.
2. Driving - 1, 2, 3th gears under 40 mph - nice and smooth, 40 and up ESPECIALLY up hill, intermediately it feels like it is "misfiring" or not getting enough fuel, kind of like if you are on the highway and 18-wheeler passing you.

At first, my idea was - maybe bad diesel fuel, so i run it until it was almost gone, filled from different gas station and add diesel injector cleaner, nothing changed.

Last year: did drop the tank, washed inside, replace gasket and pickup screen, replaced fuel pump and fuel filter.

Will greatly appreciated any advice which way i should dig.

Thank you
Sincerely
Vlad

Read through this: https://flashoffroad.com/Diesel/troubleshoot/troubleshoot.htm

When all else fails, read the manuals.

NormB said:

Read through this: https://flashoffroad.com/Diesel/troubleshoot/troubleshoot.htm
When all else fails, read the manuals.

Click to expand...

Thank you.
Lot's of info.
Saved for a future references.
Unfortunately, doesn't look like there is anything i can use in my situation.

But, Thank You again.

Based on reading the diagnostics provided by NormB, coupled with my own experience of misses and stumbling on my 998, it could be a few things.

I have a similar problem to yours, though my missing is only (I think) at idle.
There are several things we can both check first. I intend to first bleed the injectors. I did this when I first resurrected the derelict vehicle bought at auction, and I'd drained the tank and replaced all filters. It's easy to bleed injectors, and doesn't take long. Second, check your fuel filter for water or other contaminants. The drain is on the left fender underside, raise the hood you'll see a little petcock on the front of the fender rear firewall, under the control box housing area. Do this while running, and catch the fuel that comes out. Perform a visual check. Mine was fine, so that's not likely my problem.

I'm also considering having my injectors rebuilt orreplaced, since they are as old as the truck almost certainly, and I can only wonder what was done to them by the National Guard while they had the truck.

The next thing I intend to do, and am avoiding because it takes a while, is a compression test. This will inform as to cylinder or valve issues.

If you have good compression, then you almost certainly have a fuel delivery problem, either the injectors or the fuel pump. Neither repair is cheap (depending on your pain tolerance), but they both are possible improvements. I don't really want to do a compression test, because of leaning over the engine taking parts off and hooking up the tester eight times, and my back is not tolerant of leaning and twisting, but I admit it's a useful data set to have, knowing your compression. Bad compression is bad cylinder to piston fit or leaking valves, can't be anything else unless you've got bad airflow, which is unlikely.

Before I sink money into fuels system components, it makes sense to find out if my cylinders can make use of better fuel delivery of if I just need to embrace a rebuild.

I hope some of this helps.

Bulldogger

#1 - Thank you.
#2 - Here is what i did:
I start the truck.
I open up a little petcock
Truck run for a bit and stall, nothing come out from from petcock
Was not been able to start.
I loosen up OUT ( threaded ) line on the fuel pump and cranked, nothing come out
I pull out IN hose from the pump, attached vacuum gauge on the pump and cranked, no vacuum build up
I also pull out check valve from main IN line located between fuel tank and fuel pump and check it with vacuum pump, it opens up @ 6 inHg

So i guess i will order a fuel pump that i replaced about 1 year 2-3K mi ago.

Do i missing anything?

Sounds like a clogged fuel line and fuel filter. You should have gotten a pissing stream of fuel from that petcock. So, pull that all apart and blow out the fuel lines to and from the filter. The filter is located kind of behind the control box, on the firewall next to the doghouse opening by the driver side.

If you did not replace your fuel filter and water separator recently, do so and clean out its canister. Sounds like your filter is clogged and possibly dirty lines.

Your stalling and stumbling is a fuel supply issue.

Running a fuel additive is recommended for our older engines subjected to the new low sulphur fuel. Search around this site or other diesel sites and decide which one you want to use. Asking which one here will light up a firestorm of comments!

Sounds like you have an easy fix. Just be sure to clean everything out well.

Bulldogger

I did clean it and replace it like a year/3-4K mi ago

But i did order i new one and will replace it when new pump arrive.

As for additives: i am adding

I've had good results with that additive as well. Sounds like a good plan. I bet the filter will have a tale to tell.
BDGR

Quick update.
New push pump, new filter.
Runs as a champ again.

Thank you all.