Elevation and power loss

1990 XJ with the 4.0 and the AW4 tranny. Around 110k original miles with a steady oil consumption of around 1/2 quart every oil change (3k miles)...not bad for an old engine. No oil burning issues though, I lose that 1/2 quart out of various small leaks from the RMS and valve cover.

Anyhow, I bought the jeep a few years ago on the coast at sea level then drove it home to the mountains where I live at around 2400' elevation.

I'm curious if the change in elevation or relative humidity could affect how the jeep runs. I have driven it back down to the coast for family visits a few times and it always idles around 75-100rpm higher and seems to make 'more power' with 'better acceleration' by the seat of the pants test when I am on the coast, only to return to it's slightly low idle, sluggish acceleration and generally slightly less power when I get back home in the mountains.

Can someone school me on this, or at the very least tell me I'm crazy and there's no way that performance should suffer?

Performance suffers as altitude increases. I take my stroker down to sea level and it scares me. At 14000 feet it definately loses a ton of power.

Ive never noticed any significant losses at 14,000 ft.(in the jeep anyway,me,thats another story)!

old_man said:

Performance suffers as altitude increases. I take my stroker down to sea level and it scares me. At 14000 feet it definately loses a ton of power.

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I figured that power loss would be apparent at elevations like you mention, but I'm not really that high up. Basically at higher elevations it always feels as though it's 'bunged up', kind of like the exhaust is restricted or something weird. This has led me to NEARLY replace the cat several times, only to have it return to great throttle response the next time I drive it down to the coast, and so on and so forth.
Is there any reason to adjust the TPS any differently (RENIX era models) at high elevation VS. lower elevation?

Chrysler produced a high elevation CPS for the Renix 4.0 engine (meaning up to 1990 only). It slightly advanced the timing to make up for higher elevations. I have not tried it. It can also slightly counteract lagging timing from worn bushing in the distributor, supposedly.

The higher your elevation the less oxygen there is to burn. Since there's less oxygen, the PCM will pull out fuel too. Rule of thumb is about a 5% power drop for each 1000' of elevation for naturally aspirated engines as air density decreases by about 3%.

Here's an interesting site with more info in regards to altitude density http://www.2-stroke-porting.com/altiden.htm

At 2,400 feet the power loss should not be that bad. I would check the entire intake system ie; the air cleaner, air hoses, trottle body;.... the injectors. Sea Foam the combustion chamber. Excessive carbon buildup could cause the fuel to evaporate faster leaving less for spark plugs to burn.
I agree on the CPS change.

what would be the most aggressive decarbonization method out there? I have used 3 cans of Mopar Combustion Chamber Cleaner over the last year and the first 2 made the biggest difference, but the third didn't seem to do too much. I know for a fact that the previous owner drove the XJ with a faulty thermostat for YEARS, so it never got up to proper temps and I believe there was MASSIVE carbon buildup in there. I think that with the odd high RPM blast and several treatments with MCCC I have it under control, but I still have the feeling there is lots more left in there.

Don't know if there's one around you but the NAPA by me has Seafoam additives. I did the combustion chamber/crankcase one a couple weeks ago.... big difference.

I regularly go from my house in Riverside, Ca. (817 Ft.) to my Mom's place in Running Springs, Ca (6K + Ft.)

I really dont notice a power decrease at all. I feel that it actually runs a little better when im in Running Springs as opposed to Riverside.

Renix with 210K miles...

Just changed the fuel filter, plugs and air filter in my folks driveway this morning and it seems to have a bit more power, even though I'm still down here at sea level. The old fuel filter looked pretty nasty and when I emptied the gas out of it, it came out all brown and dirty looking...couldn't have been good for my injectors! Added a can of injector cleaner to the tank and we'll see if the improvements continue. I think I may have fixed a large part of the problem though...it doesn't feel as 'constipated' as it once did, that's for sure! Unfortunately I'll be parking the XJ at my folks place while I go travelling for the winter so I won't be able to see what impact the new fuel filter has at elevation, but it's looking promising!