M1008, Rock Crawl stall. Full Rich. Help me Obi-Wan

[azex]

Alright.

We were out on Camp Navajo this past week, participating in the "disabled veteran elk hunt" out in the rough roads of the 221,000 acre installation.

Out on the rough rocky roads, I had several instances where the truck went "full rich" which stalled the truck, causing hard restarts and in one instance we had to leave it and come back later with some WD40 to act as starting assist and got it started again. During these episodes the truck would "miss" and smoke terribly from excess fuel being "half-burnt" in the engine. After a couple minutes of this sputtering and smoking, the engine would smooth out and run fine. Once we got back onto smoother roads it wouldn't do it anymore. It seemed the bouncing would initiate the issue.

I thought at first it was a fuel pump intermittent failure or electrical problem, but it's not fuel starvation, it's fuel dumping into the engine, which just bogs it down til it stalls.

From my town to Camp Navajo is over 200 miles each way and the truck ran perfectly (although the mileage sucks) the entire trip up there and back down the mountain. No worries. I have replaced the 4:56s with 4:10s and it's a "respectable citizen" out on the Interstate now.

I'm really interested in what the hive mind may think is causing this failure, and of course, how to fix it.

Believe me, it's no fun hiking through the woods, in the dark, when it's 25 degrees, for 3 miles to get help.

D.

 

[pd2fly]

ratio change?

I would love to hear how you converted your 4.56s to 4.11s? I've been told the easiest way would be to just replace the entire rear/front assemblies, which sounds difficult and more work/expense than necessary.

I think I would actually prefer the 3.73 ratio, but even the 4.11 would provide benefit in fuel saving to someone who is not not a hard core off-roader.

As for your engine problems, I've always considered the 3 most important things about diesel engines to be 1) clean fuel, 2) clean fuel and 3) [wait for it] clean fuel. Beyond that, it's time for me to call the mechanic. Sorry I couldn't be more helpful.

Bruce

 

[Keith_J]

If the engine is at operating temperature and the air intake is not clogged, there shouldn't be any black smoke. The only other issue would be timing, either in the injection pump or in the drive from crank to cam.

Diesel engines have variable mixture, running from very lean at low power to just less than 14:1 air/fuel at wide open throttle. In other words, power is modulated by injected quantity, not manifold pressure.

I would say the most likely situation would be in the injection pump, specifically the timing adjust.

 

[kassim503]

here is what I think- a line on the suction side is partially cracked or leaking which lets air into the system when its bumping around that masks itself when its not being jarred around. I really believe timing jumps around when air is being pumped through

 

[Keith_J]

That makes sense. Given the distance and mileage, it is possible the sock on the fuel line in the tank is aspirating air into the supply.

 

[azex]

But if it was sipping air through the tank, why would I get all the fuel smoke and "missing" until it smoothed out?

D.

 

[azex]

Regarding the gear change. Complete axle swap in the back to a 3/4 ton mid-80s floating halfshaft axle assy.

Slightly smaller drums but not by much. I don't overload the truck so it's not been a problem. The shock towers had to be welded in the proper place for the correct lineup from the truck body. Springs were left stock.

Front: Complete gear and bearing set from an online retailer. Less than 300 bucks delivered with ring, pinion, shims, new bearings and seals, gasket and gasket sealer.

I cruise at 65-70 without the engine sounding like it's gonna come apart. I no longer have old ladies in 30 y/o Buicks giving me the finger as they pass me on the Interstate and choke on diesel fumes. I wouldn't go to 3:73 as I already lost some of the "jump" off the line at stoplights, it's noticeable but not a deal breaker. I think the thing would be real dog in stop and go traffic with the 3:73s. But this is just speculation on my part having driven the 4:11s for the last 6 mos.

D.

 

[Keith_J]

But if it was sipping air through the tank, why would I get all the fuel smoke and "missing" until it smoothed out?

D.

Late injection timing will make it smoke as if there were "excess" fuel. If you had excess fuel, you would most likely to have a run-away diesel engine.

Power and consequently speed in a diesel engine is controlled solely by injected quantity. In a gasoline engine, power and speed are controlled by quantity of fuel-air mixture. Manifold vacuum means much less fuel-air mixture.

This is an indirect injected diesel engine. When at top dead center, most of the air is in the precombustion chamber in the cylinder head. If the piston has dropped significantly when the injector squirts, there is little air in the chamber. The air in the cylinder has cooled to below the autoignition temperature so the sooty flame emanating from the precombustion chamber is extinguished.

 

[trukhead]

Late injection timing will make it smoke as if there were "excess" fuel. If you had excess fuel, you would most likely to have a run-away diesel engine.

Power and consequently speed in a diesel engine is controlled solely by injected quantity. In a gasoline engine, power and speed are controlled by quantity of fuel-air mixture. Manifold vacuum means much less fuel-air mixture.

This is an indirect injected diesel engine. When at top dead center, most of the air is in the precombustion chamber in the cylinder head. If the piston has dropped significantly when the injector squirts, there is little air in the chamber. The air in the cylinder has cooled to below the autoignition temperature so the sooty flame emanating from the precombustion chamber is extinguished.

Daang, I always enjoy your posts because they are so rich and full of fact. I am happy to have your clear, reasoned logic help me figure stuff out. Buy you a .

 

[azex]

Great reply Keith. I'll start at the IP and work my way back. I'm suspecting the "sock" like some, and I might start at both ends and work my way to elimination.

The vacuum lines which run from the vacuum pump to the IP, should I look at those too? Or just start replacing soft hose on principle? It's not like the vac tubing costs a lot and could be the whole source of the problemo.

D.

 

[Keith_J]

The vacuum lines to the IP are only for the transmission since the TH400 relies on engine RPM and manifold vacuum from a typical gasoline engine to modulate the shifting. They have nothing to do with the engine operation.

The sock in the tank normally keeps the fuel intake from unporting or sucking air. But the Nylon material can crack, causing air to be entrained in the fuel.

The injection pump varies timing based on fuel pressure inside the pump case which varies with engine speed. Air bubbles allow compression which retards timing.