Elevation effect on performance?

My XJ is a '92 with 4.0, FI & Auto. Where I live in North Texas the elevation is about 600'-700' above sea level. I'm plaining a trip to Colorado where the trails will be in the 10,000'-11,000' range. My question is will the computer compensate for this big difference in elevation? Also where is the computer on a '92?

Thanks from a flatlander :wave:

I believe that's what the MAP sensor's job is. IIRC it compensates for the air pressure somehow.

Jeep said:

My XJ is a '92 with 4.0, FI & Auto. Where I live in North Texas the elevation is about 600'-700' above sea level. I'm plaining a trip to Colorado where the trails will be in the 10,000'-11,000' range. My question is will the computer compensate for this big difference in elevation? Also where is the computer on a '92?

Thanks from a flatlander :wave:

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It will compensate somewhat, but you'll still lose a good bit of performance. I live at 7000 ft, and there's a big power difference between here and the lower elevation areas. Less oxygen means less power.
OTOH, you'll LOVE Colorado! I go back every chance I get. :woohoo:

Bill - you are correct. I do love Colorado :yelclap: . I use to live in Boulder (wish I still did).

Thanks

For every 1,000 feet increased vertically from sea level, the atmosphere loses about 3.3% of its relative oxygen. Example...here at about 6,000 feet, there is only 80.2% of sea level's oxygen. And up at 10,000 feet there will only be about 67% of sea level's oxygen. The computer on any vehicle will compensate some, but after about 4,000 feet I started to notice a slight lack in power in my XJ during my last trek out here from NY. Traveling up to 14,000 reveals a HUGE drop in power. I expect you will lose a noticeable percentage of your power, but you should be fine. That's what 4LO is for! Seriously, there are so many clubs and shops and trails out here for a reason...you'll find that out soon enough. The altitude's effect on the XJ is something you won't even be thinking about after a real short while. Enjoy your stay and have a great trip!

-Rich


EDIT-I just read your post, and I forgot the second question! But I don't have an answer, anyway. I know things changed underhood in 1992 for a lot of other things, so neither my 90 or 87 will be much help, I fear.

BillR said:

Less oxygen means less power.

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What you lose is ambient pressure. The O2 levels are the same. With less pressure, your engine has to fight to get the proper amount of air into the cylinders, therefore, you lose HP just getting more air. You essentially are choking your engine.

As an Aspen volunteer fire fighter in training, that's one of the things we discuss as it concerns fire behaviour. If you lose O2, then what's replacing it? Remember, CO2 and nitrogen are slightly heavier, so what's replacing it?

Just a thought question.

Your computer will compensate just fine. If the vehicle is from around here, they came with a high altitude CPS. It offsets the CPS and gives the engine about 3-4 degrees of spark advance, which helps.

pabloconrad said:

What you lose is ambient pressure. The O2 levels are the same. With less pressure, your engine has to fight to get the proper amount of air into the cylinders, therefore, you lose HP just getting more air. You essentially are choking your engine.

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True. To a point. If you lose ambient pressure, then you are altering the volumetric contents of each cubic foot of air taken in. With less pressure, the air expands, which is why soda bottles are so hard to open when you increase altitude, and why Twinkie wrappers almost pop on your trip up to one of our fourteeners. The ratio of the gases in the atmosphere remain constant up to around 80km, true. But for each cubic foot of air your engine digests, it is getting exponentially less O2 the farther up you drive. To be exact, 3.3% less per 1000 feet traveled vertically. You are indeed getting the same volume of air into the cylinders, but each gas in the air is taking up more space at higher altitudes. So, the O2 level is NOT the same at altitude, but the ratio is.

This is the whole reason I explained the loss of RELATIVE O2. Not Acutal. But whatever...the guy will lose performance when he drives out this way, as he made it clear he is not from a high-altitude area.

Here's a thought question or 3: Is it really possible that the loss of atmospheric pressure itself is choking the engine? Do all the cylinders really have to struggle that much harder to get CFM's moving? Or is it really just ingesting air that is quantitatively deficient in O2?

Hope I am not coming off as a jerk here. It's not my intention. I just wanted to clarify and back up some of the facts I have. Thanks, Pablo, for adding to the clarity and for volunteering to do what you do! BTW, it's mighty impressive at the ripe age of 100 that you are doing all this!

I am not sure what finding any of the computer systems in the Jeep will do for you at this point, as there are several involved, not just the ECM. I think you should just get out here and responsibly enjoy what CO has to offer!

i couldnt believe the amount of power loss i felt coming across the highest passes to and from Moab last year.. i had heard of it but never felt it before.. it was bad enough at first that i actually got out and checked my trany fluid because it just wouldnt go.... that was up around 10000' or so.. and at highway speeds... no clue how lower speed wheeling will be affected.... and for the record this was in my 99 4.0L, AW-4..
cant wait to do it all again!!!!!!!!

mike

Thanks for the replys. I thought someone would know. I just remember as a kid, my Dad's truck would strugle a little in the mountains. But back then computers were the size of small rooms and not installed in Jeeps. It will probably be alright - I just can't wait to get to the mountains (that is if I get the Jeep finished by then). Pablo, I'm a fireman in North TX - do you want to change jobs with me :laugh2: :laugh: :laugh2:

WOW! And I thought I was the only one who thought a 14-er wasn't really a 14-er unless you had a Twinkie when you got to the top!

Fourteeners - Breakfest of Champions

Or at least I thought so until we did the Maroon Bells. That was no cake walk. :looney:

I really appreciated the density of the discussion!

old-man, are you saying there is a different CPS to compensate? Do you have a part number? Why would there be a different one than those offered at sea level? Is it only for Renix systems due to the lower trim adjustments?

I just replaced my CPS with a high alt sensor. The part #04638128 if this is actually the high alt part. Originally they gave me one with the oval connection instead of the round so I just had them order the high alt one. Could be the normal one being that I wouldn't know the difference.

Around here all the dealers stock is the high altitude CPS. Advancing the timing a bit helps compensate for the altitude, but it does not do away with it. The biggest thing to make sure and change before you head up to the high country is your radiator cap. You can limp by with a cap that has lost it spring at low altitudes where the boiling point is 212 but up here, the boiling point can be as low as 190 and boilovers due to pulling a pass around here are common for flat land cars. Running premium also helps. Preignition causes significantly more heat in an engine and premium will help with that.

I also recommend running a can of BG44K fuel system cleaner on your way here. It will get rid of excess carbon buildup and clean those injectors clean as a whistle. I run a can every 10k miles in every vehicle. It's not cheap. It runs around $18 a can around here.

It is easy to tell the difference between the two CPS's. The low altitude one has the pickup centered on the metal mounting plate. The high altitude one has the sensor significantly off to one side. You should be able to see it even when mounted on the vehicle.

im interested in this high alt cps sensor.. what years will it fit?