I've got a stock 1994 Toyota truck with the 3VZE, and approx. 240,000 miles. I've been experiencing a loss of power in two circumstances...when the gas cap is on and when I drive at elevation. The first is easily taken care of...leave the gas cap off/replace it, but the second one I have not found a solution for yet even after reading all of the associated posts.

The change in elevation can be as little as 1000 feet and gets worse as I go higher. At 6000 feet going up hill I'm struggling in 3rd gear when I should be able to simply down shift to 4th. The check engine light does not ever come on (I've checked and it works properly) and it is showing no codes (I've checked for flashing). I have a new catalytic converter, new VAF meter, new plug/wires, and a new distributor cap/rotor (all OEM parts). I've not replaced the fuel filter yet but I have one in the shop, just need time to do it. It passed smog with no issues or high readings. It starts fine in the morning/in the cold, and does not ever run rough or have a rough idle. The tach does jump around on occasion but not constantly.

While it happens mostly at elevation, I have had it act the same with no change in elevation...I simply notice it more at higher elevations. As for the gas cap, when I leave it on it builds up a lot of pressure (vacuum) and releases it when I pull it off. I figure the seal has gone bad and it is not venting properly.

I'm about to trace vacuum lines looking for leaks and dig further into the EVAP and EGR system but before I do I'd love some advice on where to start and make this right. Thanks in advance for your help.

 

I think the general rule of thumb is a 3-4% power loss for each 1000 feet of altitude, figuring from sea level.
So a 300 Horsepower car at 10k feet would basically lose 30-40 HP at that altitude. So your motor got some age on it ..it goes to figure you have 24% loss at 6000.

 

Agreed. Any car would struggle at that elevation as the air is thinner. Same goes for us if we were to hike up that high.

 

I always thought it was that 35 years of Lucky Strikes I smoked.

 

The gas cap should let air into the tank, but not out. Excess vapors from the tank should go through the charcoal canister for scrubbing, and then the clean air be exhausted to the atmosphere.

As has been said, some power loss is inevitable at altitude. However, if your engine is stumbling or otherwise running poorly, something else is wrong.

Also, the VAFM used on the 2nd generation Toyotas does not do a particularly accurate job of compensating for altitude. The error increases with the square root of altitude, and is such that a properly tuned engine at sea level will run rich at high altitudes. This is inherent in the physical principles on which the VAFM works, using ram air pressure to infer mass air flow, rather than measuring mass air flow directly as later model fuel systems do.