1990 Jetta losing power

I'm trying to get my '90 jetta gl running. She runs fine for about 10-15 min. @ idle (5-10 driving) then the car starts to have a very rough idle, smokes and loses lots of power with max speed topping only about 35mph:banghead: . To me it seems as if the engine is not getting enough fuel. I changed spark plugs, wires, dist. cap, and ran that seafoam stuff though it. Any ideas of what is happening. By the way I did have a new fuel pump (externel) and filter replaced already.

sounds like you are getting too much fuel. Black smoke indicates a rich fuel mixture. I had the same problem with my old gl. It turned out to be a loose connection from my O2 sensor. I would start with your sensor connections. Especially your O2 and water temp sensors (on an 8v, it should be mounted on the block inline with the inlet hose from the radiator). When you start your car, the fuel mixture is rich and as it heats up, it leans out. If your ECU is getting a bad read from your O2 or water temp sensor, It won't lean out and your plugs will start to foul. Then, you start to get studdering, powerloss, and black smoke.

Good luck

ok thanks, didn't know the temp. sensor off the block would do that. Is there a way to bypass or check it first to see if its the problem or just change it. I think you are right though because it does throw some mean black smoke.

Properly testing your sensor can be time consuming. You need an ohmmeter to check your temp sensor. You also need to know the temp of the sensor. Here is how I do it. get a pot and fill it with a quart of water. Then insert a candy thermometer (preferably digital). Then you need to suspend the business end of the sensor in the water. Connect your meter to the sensor and measure your resistance values. Then begin heating the water and checking the values as the water temp changes.

I can't find my spec list for 90 and up cars, but i'll bet they haven't changed much form the 89 model. here are the values you should read at different temps.

55deg. - 3000-3800 ohms
65deg. - 2200-3000 ohms
75deg. - 1800-2500 ohms
85deg. - 1500-2100 ohms
150deg. - 400-550 ohms
200deg. - 200-270 ohms

good luck

I have an 87 jetta doing the same thing. but if i turn it off then back on it will run for 10 more min.

Look at your spark plugs. If you have a lot of carbon built up on them, it's a good indicator of a rich mixture.