Fouling Plugs at Altitude

I bought my van this april and did some basic maintenance( replace air intake, new plugs, new air filter, new fuel filter, oil change, etc.) then drove it across the country 2000 miles. I noticed after about 500 miles that my check engine light would come on when climbing hills and as I drove more Ifound it to be slowly becoming underpowered little by little. As I suspected it was fouling plugs. After about 1800 miles of driving I ended up at the ocean and the check engine light stopped coming on no matter how steep the hill was when at altitude even slight hills would cause it to come on. It also started running stonger. After a few weeks I drove back to altitude, and sure enough at about 2000 ft. the check engine light started coming on again and it started running rougher when climbing hills. I suspect that it's not adjusting the fuel mixture properly to altitude, but I have no idea how I would go about remedying that and would love any advice people have to offer.

Thanks,
Dusty

First thing to do is to check the codes (which is what the "check engine" light was trying to tell you — new trouble codes).

Here is how to "pull" the codes and here is how to find out what the codes mean.

Gwen

So I finally got around to checking my diagnostic codes and I'm getting a 2-1 code and a 5-1 code. The 2-1 code is a O2 sensor malfunction which surprised me because from what I've read that usually comes with significantly decreased gas mileage and I've been getting 22 to 27 depending on the type of driving I'm doing. The 5-1 code says it's a switch code which I'm not entirely sure what that means other than that potentially there's and issue with the ECU itself. I guess I should start calling salvage yards for a O2 sensor, but if people have any other thoughts or advice I would greatly appreciate it. Thanks

Don't waste your time with a used O2 sensor. Code 51 is almost always an throttle position sensor.

Go here: http://www.toyotavantech.com/forum/s...=oxygen+sensor for code 51 or search for "throttle position sensor".

Go here: http://www.toyotavantech.com/forum/s...=oxygen+sensor for the O2 sensor.

Make sure you read through all the posts. Some part number info has changed.

So I've been slowly plugging away at fixing all the little odds and ends wrong with my van. I found a bad vacuum hose and after replacing that my O2 sensor goes off less often. IT still goes off though when I'm driving up steep hills and the van is working hard. I looked for exhaust leaks and did find that I'm missing the front exhaust manifold bolt, but even when plugging the exhaust to listen for leaks could not detect any coming from there or anywhere else in the system. This leads me to think that most likely I either need a new O2 sensor or my catalytic converter is partially clogged. If I remember right the 88 was the first year that there was two O2 sensors, but i have only been able to find one upstream of the cat. Am I mistaken that there are two O2 sensors on 88's and if there are two where would the second one be? Could my O2 sensor be getting tripped by anything other than an exhaust leak or a bad cat as well that I should be aware of?

'88's have 2x o2 sensors, the 2nd one is behind the muffler, ahead of the rear axle.

You have your van listed as an '87, if it IS an 87 it will only have one sensor.
If you are unsure of your actual year, best to decode the VIN

If you suspect an issue with the cat, split the exhaust on the input side and do a visual inspection.

Given how cheap o2 sensors are, just replace yours and take it from there.
The fact you keep getting a failure code, indicates its compromised, driving with a failed o2 WILL cause cat damage, as will MANY other things.

So I realized why I had it listed differently than I thought. My title and registration all says that it's and 88 but the vin says its a Dec of 87 so I'm not sure if there was a mix up at some point on the title or since it's and end of 87 if it was sold as an 88. That might explain why I only have one O2 sensor. I also crawled underneath to double check and it definitely only has one sensor but it is the 88 four wire style instead of the 87 one wire style so I suspect that it may be from when they were phasing the new sensors in.

Production date is not the same thing as the VIN# which is 17 digits long, please don't confuse the two.

That is why its important to go by VIN# rather than production date.
The general rule of thumb is that anything from August onwards will actually be the next model year as most new models are launched in September.
That said, many models are now being launched even earlier than September so the VIN is even more important.
If your's was built in Dec 87, it is indeed an 88 model.

That means there SHOULD be a 2nd o2 sensor aft the muffler/ahead of the axle.
Many shops feel the 2nd o2 is redundant and think nothing of deleting it, problem is the ECU is looking from input from that sensor in order to accurately adjust fuel trim.
My van was "modified" in this way, correcting that idiotic mod solved my o2 sensor code logging that I was experiencing.
I suspect (hopefully) you'll find a 2-wire connector flapping in the breeze in the general area of the rear muffler.