2TR-FE low idle after steep descent in 4L

This morning I was going down a long and steep descent, about 50% grade for half a mile or more. Terrain was very rough. I was in 4-low in first gear and also having to apply some brakes as well to keep my speed at a minimum. When I got to the bottom my truck was idling very low (around 400RPM) and rough due to the low RPM (similar to when I bog the engine using too high a gear). It has continued intermittently throughout the day. No codes. No other symptoms. Drives fine. Doesn't stall. Anybody experience this?

 

Have you checked for any vacuum leaks that may have shaken loose or a maybe a line cracked. Typically a sensor like tps would throw a code. Might be worth checking fuel pressure if you can replicate it to see if the pump is going out. Can't think of much that would be related to the descent

 

^^Agreed, sounds like something came loose or cracked. Usually you'll see a code, but sometimes it takes awhile for the ECU to deem it a problem and throw the code. I wouldn't be surprised if you see one soon if it keeps acting up, check around the engine bay for loose, disconnected or cracked hoses.

 

I've gotten that about 5-6 times. All after long steep 4lo descents.

 

I'll keep an eye on mine next time, but I haven't noticed that before...then again I'm not really looking for it.

 

Mine would idle so low that the whole truck would shake. A couple of times it wouldn't even stay running without constant throttle input

 

Yeah that's not right....and it goes away after a bit? If it was a vacuum line lose or something like that I don't imagine it would just go away. Does it stay like that until you shift out of 4lo and then it gets better?

 

Might have nothing at all to do with being in 4lo, maybe it's just the steep incline/decline and the truck is struggling to get fuel...bad fuel pump or dirty/clogged injectors maybe?

 

Usually goes away after shutting the truck off for a little while.

 

Maybe the pump is sucking in some air when the gas all shifts to one side and causes the engine to run lean until it's all worked out?

 

Most times it'll stay with the low idle till I shut it off or when I'm back on the highway in 2wd and going through the gears

 

Hasn't done it in a while though. I wheel at least once a week and go up and down the same steep hill. Really hit or miss of it'll do it or not.

 

Have you noticed how much gas is in your tank when it does happen? If it's fuel delivery related I bet it's when you're low(er) on gas.

 

Yes! It's actually happened once before and went away after driving around a bit/shutting it off for awhile.

And low and behold after daily driving the truck today it's gone.

Regarding fuel...yes, I was a tad low, had 1/4 tank when I started down the slope.

 

My best guess (I'm no mechanic so take it for what it's worth) is that it's fuel related then, and not related to 4wd or 4lo for that matter.

I wouldn't go so far as to say it's normal behavior, sounds like the truck is struggling to find the right amount of fuel it needs when you're low and on a decline. At 1/4 tank you should still have enough in it to keep from sputtering, even on a slope, IMO.

 

It's happened with anywhere from full to quarter tank.

 

I have no idea then lol. But I'd still lean more towards it being a fuel issue than anything else. Hopefully someone else chimes in with some experience on the matter!

 

I was thinking maybe it had something to do with vacuum, because descending in low gear is a very high vacuum condition.

I'll try the trail again with a full tank.

 

The 4cyl will do that sometimes.. mine goes all fucky once in a while it'll just idle low for a bit but then is back to normal after a bit.. what it really hates is when you stall trying to crawl over a rock takes like 4 tries sometimes to get it started again

 

Yeah I hate that too. Found that if I stall, I turn the key off for about 15 - 20 seconds and it starts easier.