altitude problems

ROGLSTUBB · #1 (**permalink**) 12-30-2004, 12:51 AM

Here is the situation. My 94 ZJ 4.0 runs fine normally. I took it on a short trip to Reno Nv. It got more and more gutless as I climbed in altitude. I understand the air is thinner as you climb in altitude which changes air/fuel ratio. I remember back in shop class "some 10 years ago" that the MAP sensor on some vehicles take a atmospheric measurement as you turn the key on before the engine starts. The jeep did seem better after a few days up there and now seems to be little too responsive on the trottle now since I am back but I not sure if it just me over compensating for the lack of power when I was up there. I have drove it up there before and I dont remember it doing it last time and my wifes has never acted like that.

My question is what tells the computer atmosheric air pressure? obvious something does so it can adjust air/fuel to altitude. Is it possible to be delayed? If it is the MAP would an adjustible one be a solution?

graewulf · #2 (**permalink**) 12-30-2004, 06:53 AM

It takes time for the computer to adjust to new conditions. Even doing something like installing a K&N filter instead of stock will take some driving for it to readjust. I just did a tune up on my 95 ZJ and the computer says my fuel economy is worse, but I know it will get better as the computer readjusts its settings to the new hardware.

As for reading atmospheric pressure, that is not what it does, it reads the ppm of oxygen entering the system (as well as fuel and other factors) - you probably have a Mass Air Flow sensor, not a MAP sensor, they serve a similar function. The computer constantly readjusts its mixture as you drive. An air filter that allows better air flow (like a K&N) will help some during the transition from one altitude to another. In the carbureted days you used to have to re-jet the carb to get the performance back, now the computer adjusts for you. I hope this helps....

ROGLSTUBB · #3 (**permalink**) 12-30-2004, 11:57 AM

It felt like air flow problem. In fact I checked the air filter thinking it might be dirty when I was up there. I have been considering doing one of those performance filters but I know if I get one the wife is going to want one on hers and I got to have the funds for two.

I was reading my haynes a little closer last night after I posted and according to it in the emission section the MAP does give the computer atmospheric pressure but I did not say if it just took this reading before start up or if it took it constantly.

How long does it take for the computer to readjust?

graewulf · #4 (**permalink**) 12-30-2004, 01:23 PM

I'm not sure how long a Jeep will take, it can be anywhere from 1/2 hour run time to several hours (or more) run time to adjust. The MAP reads air pressure, the MAF reads air flow, 2 different animals, not sure which is on the Jeep (I seem to remember it being a MAF but could be confused). Also, higher octane fuel will help when going to higher altitude until it adjusts itself.