LMTV rough idle on startup?

[Dexis]

Well out of nowhere a new issue, go figure.

Parked the truck after cleaning up grounds the other day. Drove it around a bit, acted perfectly normal. Now today, after being parked a few days I fire it up and after slightly longer cranking than usual it catches but has a loping, choppy idle. Engine has always been 100% startup, idle, and running, smooth as butter until today.

By the time I got on the brakes and dropped it into gear it started surging, hard enough that the truck was lurching forwards and backwards.

A slight throttle input and the surge goes away but engine is still rough and shaky under any amount of load.

Any ideas where to start on this one?

 

[Ronmar]

What year truck?

with diesels I always start with primary fuel supply. A loose fitting or filter seal, or pinhole in a line will allow the fuel to drain back out of the system and cause hard starting, then the pinhole will feed a steady supply of air that causes baseline fuel pressure and supply issues.

unit injectors RELY on proper baseline fuel pressure in the fuel gallery to fill and inject properly…

there should be a test port near the fuel filter on the side of the engine between lift pump output and the gallery thru the head where you can connect a gauge to measure baseline fuel pressure. 10-20 psi cranking, 20 psi at idle ramping up to 60psi by peak torque rpm(1200-1500RPM). There is a spring loaded pressure regulator at the output of the head that the lift pump pushes against to create this pressure….

I did some fuel system videos on Youtube for my A0/3116 but the A1 system operates in the exact same way(lift pump, filter, gallery, regulator), only flowing the opposite direction thru the cylinder head(A0 back to front, A1 front to back)…

you can use a basic mechanical oil pressure gauge, but you must restrict flow to dampen the pulsations from the piston lift pump in the governor or HEUI pump…

Here is an overview of my 3116 fuel system and where I connect a mechanical gauge and restrict flow to it. There is also a vid there on my channel of what a small air leak(not bad enough to effect engine operation) looks like on a pressure gauge in my dash…

if this is an A1 and baseline pressure is OK, it could be a sensor issue and you will probably have to read the ECU to help diagnose it…

Good Luck!

 

[Dexis]

Its a 2001 A1 with a 3126. None of this sounds good as i dont even have a pressure gauge to plug up to the fuel system and Im guessing all my obd2 diagnostic gear is useless here as well. So how screwed am I?

 

[Ronmar]

Well find the pressure sample port near the main fuel filter alongside the head(not the water separator with the primer). It should be a T fitting before or after the filter in the hard metal piping that feeds fuel thru the filter to the front of the head to feed the fuel gallery.

determine what size it is and go get the necessary size fitting to adapt it to 1/8” National pipe thread, then get a simple mechanical oil pressure gauge kit at the auto parts store(gauge, clear poly tubing and 1/8” NP to poly fittings usually all in the kit) or online, to attach to that test port. Then you can use a small c clamp to squeeze/restrict the line to the gauge to dampen the needle pulsations and check the pressure. Without restricting fuel the needle will swing between 10 and 90 psi

What you describe sounds like fuel starvation, especially since you did not mention a check engine light, and most issues HEUI related will throw a CEL… Baseline fuel pressure is NOT monitored by the ECU and it is easy enough to rule out, and you really must rule it out before you proceed(Good houses have good foundations)…

sorry, no OBD for you… if the fuel supply tests OK, you will need a laptop with an adapter and the cat ET software to read the ECU parameters… like I said, no CEL in the dash, I suspect fuel supply, which is a very common issue…

Here is what a tiny air leak looks on a pulsation dampened gauge… without putting a gauge on it you would never know. Fuel supply issues will destroy unit injectors…

 

[Skyhawk13205]

My truck started to have a rough idle on start up, after it is warmed up it seems to even out, I suspect loosing prime. The manual has a diagnostic where you put a sightglass on the return to see if it is excessive air bubbles, and how to find a leak.

 

[hike]

On our 2003 M1078A1 with the 3126 the tee fitting @Ronmar is mentioning is aside the primary fuel filter to the rear under the rear cab support arch.



If I recall correctly it is a 3/8 ORB fitting (9/16-18 with an o-ring), which appears consistent with the diagram in CAT's SIS2GO app.



For reference: 1— tube; 4— assembly: tee, two o-rings; 15— assembly: plug, one o-ring.

 

[Ronmar]

Yep, everyone should check this. It is the foundation of diesel operation and a VERY common point of failure. Since all the system from tank to lift pump is under constant vacuum, and the return line is downhill and has no real restriction(no pressure buildup) any leak only sucks in air and never shows any outward sign of leakage…

if you fail the running pressure test, pressure is how you find the air leak. Run the engine to get the system full of fuel and shutdown. disconnect the return line and cap it, disconnect the supply line and connect an air pressure regulator and apply 15PSI to the supply line, then go look for the wet spot. The tank fittings are JIC-6.

My first leak after I got my truck home was the clear filter bowl on the primary filter. It was perfectly clean and dry until I hooked up the rig below and put 15PSI on it, then the bowl started to drip fuel(it was cracked)… also had a line problem… if you put a valve on the inlet to the air pressure regulator, you can pressurize it and close the supply valve and seal the system under pressure to do a long term leak-down test. sealed at the tank supply and return like this it should hold 15PSI of pressure indefinitely…