First of all, sorry about my poor Eng. and grammar. i hope you can understand or guess what i'm trying to say.
I have 85 GT and I need help!!
When I drive my car faster than 80-90mph or over 4000rpm for more than 5-10min Engine check light come on. I found out this happen because of bad oil. I try many kind of oil,but it doesn't matter what kind of oil i use(it run a lot better when it's new, i mean first 500km). Also, when water temp is lower than first line of gage, it never happen. However, i took out thermostat so temp will never go up when i'm driving high way and engine check light never come on.
Does anybody had a problem like this or know what is wrong with my car?

Let's get this straight. You live in Vancouver (according to your Profile) and you've taken out your thermostat so that you can drive on the highway.

Can you say "Engine sludge"?

Remember that phrase because you'll be hearing it in the future. You'll probably also be hearing some other not so nice things regarding your car if you keep that thermostat out.

It's there for a reason. The engine is designed to operate within certain temperatures. Oil is designed to lubricate best within certain temperatures. With no thermostat, your engine will never reach these temperatures. Not in Vancouver. Especially not this time of year. I know, I live there.

Was there a problem with the old thermostat? Was it stuck shut? Did you try putting in a new one?

By the way, where is it that you're able to drive around here at 90 mph for over 10 minutes? If I tried that I'd have more problems than just "bad oil".

I think I'll leave someone else the task of diagnosing this problem. The rain has finally stopped here in Vancouver, and I want to enjoy the sun while I can.

I have an 85GT as well and have that sort of problem, if i drive at a fast rate of speed for long period of time, lets say 120 or 140 kp/h then i use to get the check engine light on, i checked the codes and they were the tps and map sensor and the one about the egr, i replaced the two sensors at bout 45 buck cdn for each of them, now the car runs better, like it did before they went flaky and im getting my 2150 bucks worth out of the engine rebuild. The oxygen sensor is next but it rained and i d rather stay inside.

Patrick, as for your question about where you can drive at 90 miles an hour for 10 mins, well try the number 1, or marine drive between boundry and new west, u know where it reeks like sulfur, or you come out here to slurry, opps im meant surrey and do 200 + kph from white rock to guildford. Ive done at least 90 in all those places, but im scared of those photo radar traps.

Thank you for your concerned Patrick and Thank you for information Punkndrublic.
By the way, i moved to Seattle six month ago;however, i come back to Vancouver every weekend, so i enjoy highway drive every weekend from Seattle to Vancouver(exspecialy between exit 240-270 on I5 North).
Patrick, you can try over 150kmh drive at Punkndublic told you and olso you can try anywhere from Vancouver to horseshoe bay on #1 highway. Anyway, thank you and i wish i can see you guys on highway, as you know my car is Yellow85GT(I still have BC license on my car)!!

I'm familiar with all those places that you've mentioned. I'm probably old enough to have driven everyone of those roads long before you guys were even a gleam in your father's eye.

I decided to gas up the ol' Fiero and revisit everyone of those areas. My plan was to do 90 miles per hour for ten minutes straight on each piece of highway, just like the two of you suggested could be possible.

This is the result:

1) Ran over three cats, two raccoons, one unidentified critter, and had near misses with two deer, four dogs, eight cyclists, and a dozen hitchhikers.

2) Took out three roadside signs. (Including the one that said Welcome to Surrey.)

3) Acquired eighteen speeding tickets along with five citations for dangerous driving.

4) Have lost my driver's licence for the next decade.

5) Will spend the next two months in jail.

Thanks guys.

PS. One ‘87 Fiero for sale. Cheap. Runs good but needs front end bodywork.

And I thought I had a bad day.....

The last I saw of your dog, he was climbing a tree to get out of the way...