No power at altitude

[sdnative]

I was up in the mountains (7000' +) last weekend and my truck had very little power. It was fine at the lower elevations.

We ran a couple of the trails up there, and it did ok for the most part, except on any type of steep incline, ledge, rock, etc I had to climb. The first time I noticed a problem was a little way into the first trail there was a small "waterfall" climb of about 2' or so. I was able to get my front wheels up, but after that I just wouldn't climb. I was in low range first gear and the pedal was floored. It reved up to about 2500rpm and didn't move. After a couple of seconds, the wheels started to move really slow and after some back and forth movement I was able to get up it with a little momentum.

Another time we were driving a forest service road to a campsite, and there was a fairly steep part (maybe 30 degrees). I couldn't get up it in high range at all. I had to shift into low range to continue.

Neither of these situations would be a problem under normal circumstances. I have done much steeper climbs at lower altitudes without taxing the engine at all.

Towards the end of the day, someone in the group smelled gas fumes. Turns out my gas tank was super pressurized. I opened the gas cap and let it vent, which took a good minute or two. After that I could clearly hear the gas boiling in the tank. This was at ~9500'. I never did any climbs after venting the tank, so I don't know if that was the problem.

Once I got back down to the bottom of the mountain, it ran like a brand new truck. Plenty of power.

Is it normal for the gas tank on these to pressurize at altitude? Shouldn't the charcoal canister relieve this pressure? Would a plugged canister cause the pressure?

What would cause this extreme lack of power? I would expect some, but nowhere near this much. Any ideas?

This is in my 94 FZJ80, 226k miles. Plugs, wires, air filter, etc were replaced a few thousand miles ago. Fuel filter (by engine) was replaced last year. I was not towing and had maybe a couple hundred pounds of cargo.

Thanks for any help with this.

 

[Hefty]

Engine isn't getting the correct o2 to fuel ratio re calibrated. Air is thinner up higher. I deal with it here in colorado all the time. For every 1000 feet in elevation your engine will lose about 3% of its power. Multiply that by 7k and your losing 21% of your engines HP and torque. That is a very noticeable difference. If you stay at that altitude for a while your ECM should re calibrate to a point.

Edit: No the gas tank shouldn't pressurize to that extent but yes they will pressurize some

 

[fourtrax]

you really need a supercharger if your going to spend any time at altitude, or you could opt for some lower gears.

 

[sdnative]

I wish I had gears, but I don't think it would have helped much. Gearing doesn't magically create power, just moves the RPM into the power band. I clearly had no power.

None of the other vehicles I was with (several FJs, a jeep and a rover) had problems like this. Something was definately wrong, I am just trying to figure out what.

I had two issues, and I'm not sure if they are related: loss of power and excessively pressurized fuel tank.

What would cause the excessive pressure? Plugged up charcoal canister? Plugged vent line? I'll have to check, but isn't the pressure releived through the canister to atmosphere?

Would this cause a fuel flow problem leading to loss of power? I wish I would have tried to climb something after venting the tank...

Is the ECM not working properly?

Could it possibly be something else other than the engine? Torque converter? Tranny?

 

[fourtrax]

i had to replace my gas cap resently cause the o ring was hard and not making a good seal. I only noticed (smelt gas) it going uphill with drivers side lower, and before i replaced the cap, it would still make a little noise (gas venting or air getting sucked in) when i opened it. i'm defintly not a mechanic but it sounds like you might be on to something with some blockage in the system. I found this pdf that kind of explains how the canister works. http://www.endwrench.com/current/spring04pdfs/EvapEmission.pdf

here is the 94 info

 

[tirewater]

Could be a plugged line in your EVAP system. Check to see if someone has blocked off (i.e. w/ BBs) the vacuum lines from the charcoal canister to the engine. Also check for any kinks or crimps in the lines.

Generally there is a vacuum in the gas tank.

Obviously something is blocked. AFAIK, nothing is pressurizing your gas tank. The pressure build up was most likely the change in altitude and reduction in atmospheric pressure.

The good news is that your gas cap is working correctly.

I wish I had gears, but I don't think it would have helped much. Gearing doesn't magically create power, just moves the RPM into the power band. I clearly had no power.

None of the other vehicles I was with (several FJs, a jeep and a rover) had problems like this. Something was definately wrong, I am just trying to figure out what.

I had two issues, and I'm not sure if they are related: loss of power and excessively pressurized fuel tank.

What would cause the excessive pressure? Plugged up charcoal canister? Plugged vent line? I'll have to check, but isn't the pressure releived through the canister to atmosphere?

Would this cause a fuel flow problem leading to loss of power? I wish I would have tried to climb something after venting the tank...

Is the ECM not working properly?

Could it possibly be something else other than the engine? Torque converter? Tranny?

 

[rgsiii]

My '94 ran poorly at altitude. It is a heavy truck 6,200 pounds and when I went to high elevations, like the Eisenhower tunnel, it was frighteningly slow.

I just moved to NM and CDan suggested advancing the timing, this helped. It created an overheating problem which was solved by changing the fluid in the fan clutch. The overall result is a vehicle that runs better and does not overheat. It is still not fast, but I can accelerate up most hills. The altitude where I live is 6,500 feet and the highest highway elevation that I has been used is about 8,000 feet. I feel like trying the Eisenhower tunnel now (not really, I'll use my SC'd 80).

 

[Nay]

I wish I had gears, but I don't think it would have helped much. Gearing doesn't magically create power, just moves the RPM into the power band. I clearly had no power.

You've never seen a rig with a crawler box, have you? Gearing is critical.

You may have a problem. However, your description doesn't sound terribly far off from the simple problem of 3 tons under-geared and under-powered at altitude.

One of your examples was trying to climb a 30 degree slope in high range at high altitude. If you can ever make that work on a naturally aspirated undergeared 80 I'd like to see it - I'm 13% overgeared and can't do squat in high range at altitude offroad. I am in low range and stay there until we are done - one of the things I love about the 80 is the AWD low range feature. I use it all the time.

Your other example of needing 2,500 RPM for a ledge climb is more interesting but not out of any normal range. I ran for 18 months with stock gears and 35's, wheeling at 7,500 - 12K feet. What finally got me to do gears was high elevation ledge climbs as I was starting to incur some damage due to throttle compensation. Better to spend that money on proper equipment.

My 80 wasn't as bad as you described, but you are only a little bit of narrative color away from what anybody who lives and wheels at high altitude experiences full time.

If you don't find anything, remember that gears help this big pig tremendously for rock crawling at altitude (I still don't have enough gearing offroad while being pretty low onroad).

The good news is 35's and gears can be next on your mods list

(and there is a reason I went 5.29's the first time)

 

[sdnative]

I understand your argument on the gears. Meybe they would have helped. I am just not convinced this is perfectly normal behavior.

What about the fuel pump? Maybe it's not delivering proper fuel pressure? What about the sock filter in the tank?

To clarify: It's not that I needed 2500prm to climb the ledge. The engine reved up to 2500rpm, wouldn't go any higher and wouldn't spin the tires. It just ran out of power. It felt like my foot was on the brake at the same time.

There were several other vehicles with me and AFAIK no one experienced this. I know the newer vehicles have better computer controls, but..

 

[sdnative]

Since everyone loves pictures, here's the ledge in question. Not my vehicle, viewer discression advised. This is on Gold Mountain in the Big Bear area.

 

[rgsiii]

I understand your argument on the gears. Meybe they would have helped. I am just not convinced this is perfectly normal behavior.

What about the fuel pump? Maybe it's not delivering proper fuel pressure? What about the sock filter in the tank?

To clarify: It's not that I needed 2500prm to climb the ledge. The engine reved up to 2500rpm, wouldn't go any higher and wouldn't spin the tires. It just ran out of power. It felt like my foot was on the brake at the same time.

There were several other vehicles with me and AFAIK no one experienced this. I know the newer vehicles have better computer controls, but..

These symptoms are not too dissimilar to mine. I was concerned about the fuel, but the mechanic asked if it had a CEL come on and the answer was no. He felt that it shouldn't be the problem. This was borne out to be correct.

 

[sdnative]

Thanks for the help guys. Keep the ideas coming. Call me stuborn, but I dont think this is normal.

 

[Nay]

Thanks for the help guys. Keep the ideas coming. Call me stuborn, but I dont think this is normal.

It should rev higher than 2,500 RPM. That climb was not huge.

Did you notice if you were in overdrive during the climb, relying on the transmission to downshift? If you were in OD it may have stayed there as enough undergearing that isn't at least speedo corrected does adversely affect the shift points, and you might not have had the gearing leverage to get the RPM to spin up and transmission to downshift.

Now if you were locked in 1st and it flat ran out at 2,500 that's a different story. You should be able to redline it in first if you want to.

I leave my power button on all the time and generally lock out OD for technical trails, and for steeper climbs where I need control it is always locked in first. You try stuff like that in a higher transmission gear with high altitude and undergearing and you could absolutely hit a wall at relatively low RPM.

If the feeling was pretty much like your high range hill climb attempt, I bet you weren't in first gear...

 

[SUMMIT CRUISERS]

Last year I drove the Durango, CO to Ouray (550) with my 92 and a M-416 trailer loaded (probably about 850 lbs total). Power was so bad I had to use low, 2nd gear on most of the climbs up hill (pavement). This year I changed my fuel sock and pump because of lack of power and stuttering after driving about an hour this summer. That problem is fixed. Doing the same trip next month, I hope mine is better. Fuel sock was definitely sucked in, probably original (300K miles) and bad.
I'll definitely post my results on this thread at the end of July.

 

[Nay]

Last year I drove the Durango, CO to Ouray (550) with my 92 and a M-416 trailer loaded (probably about 850 lbs total). Power was so bad I had to use low, 2nd gear on most of the climbs up hill (pavement). This year I changed my fuel sock and pump because of lack of power and stuttering after driving about an hour this summer. That problem is fixed. Doing the same trip next month, I hope mine is better. Fuel sock was definitely sucked in, probably original (300K miles) and bad.
I'll definitely post my results on this thread at the end of July.

You can look forward to 2nd gear high range this year

 

[SUMMIT CRUISERS]

You can look forward to 2nd gear high range this year

That will be sweet. Maybe I'll even be able to pass a few on bicycles.

 

[jlg32281]

You could always move. I doubt my truck has ever been to 1000 ft above sea level, or even 500 for that matter.

 

[sdnative]

Did you notice if you were in overdrive during the climb, relying on the transmission to downshift? If you were in OD it may have stayed there as enough undergearing that isn't at least speedo corrected does adversely affect the shift points, and you might not have had the gearing leverage to get the RPM to spin up and transmission to downshift.

Now if you were locked in 1st and it flat ran out at 2,500 that's a different story. You should be able to redline it in first if you want to.

I leave my power button on all the time and generally lock out OD for technical trails, and for steeper climbs where I need control it is always locked in first. You try stuff like that in a higher transmission gear with high altitude and undergearing and you could absolutely hit a wall at relatively low RPM.

If the feeling was pretty much like your high range hill climb attempt, I bet you weren't in first gear...

For that ledge I was in transfer case low range, and transmission in low range. I manually shifted to low on both shift levers. The CDL and rear were locked, but not the front. OD button was pressed in.

Does OD even affect anything in first gear? I thought it was only active in third?

 

[sdnative]

You could always move. I doubt my truck has ever been to 1000 ft above sea level, or even 500 for that matter.

No thanks, I love San Diego

 

[Nay]

For that ledge I was in transfer case low range, and transmission in low range. I manually shifted to low on both shift levers. The CDL and rear were locked, but not the front. OD button was pressed in.

Does OD even affect anything in first gear? I thought it was only active in third?

You should have been fine, the stock gearing is not bad enough on 295's to do what it did at any wheeling altitude in the contiguous 48.