Rough running after warming up

[ToyoTribe]

I can't find my notes. DH wanted me to post about this behavior of our 94 Tercel DX. I will try to describe it from memory.

When we first start the Tercel in the morning, it runs fast but smoothly. Once it gets warmer, it gets rougher. DH has sped up the idle to help keep it from dying.

Even while warm, though, it runs better when it's going fast, like on the highway. But at a stoplight when fully warmed up, it idles roughly. (I tend to keep light pressure on the gas to keep it running smoother.)

There are about 3 exhaust leaks. We wondered if the exhaust leaks might be preventing the O2 sensor from getting a true picture of how the engine is running.

IMHO, since it runs better when first started (and then runs/starts more roughly after it warms up), I thought that the O2 sensor might be the key. If it keeps the mixture rich when it doesn't need to, because it doesn't get a correct sampling of the exhaust, might we see this behavior?

One more thing--the other night the alternator came disconnected on the highway (DH figured this out by the way the headlights dimmed), and the car ran with more power and smoothness than it has in recent memory.

What's up with that???

I have been driving it for the past 3-4 days, and I find that when I drive at around 40MPH, it runs pretty darn smoothly. But it doesn't have the same perky pickup from a stop that it did when we first got it.

It still has the first dose of Auto-RX in it. I think it's possible DH said that the engine ran a bit rougher once he put the Auto-RX in.

Ideas?

[92tercel]

I have a 92 tercel and it does the same thing that yours does i also have exhaust leaks mabey your on to something that that may pose a problem on how the car idles i don't know just trying to figure out why my car does it to

[NickR]

Originally Posted by ToyoTribe

One more thing--the other night the alternator came disconnected on the highway (DH figured this out by the way the headlights dimmed), and the car ran with more power and smoothness than it has in recent memory.

Now this is very strange. And somewhere in this fact must lie the clue to the cause of the problem.

Just because the headlights dimmed while driving doesn't necessarily mean that the alternator came disconnected. Did DH actually open the hood and observe that the alternator had become disconnected? If so, I would repeat the experiment by disconnecting the alternator again, and seeing if the car runs better. (The battery should be able to keep it running for about 5 minutes, or more). If you get the same results, I would start checking the voltage across the battery terminals with the alternator connected, and the engine running. Could it be overcharging? Could this be creating misleading signals to the ecu?

[ToyoTribe]

Good idea. I will pass it on to DH. And, yes, he did lift the hood and see that the alternator had become disconnected. The dimmed lights were his first clue about the situation.

FWIW, DH has been able to drive to and from the farm (probably 80-100 miles) on batteries. Unless he's using the headlight(s) , I think he can make the round trip on one battery, though he always carried a spare. We've been charging the batteries at home. But DH recently fixed the oil leak(s) that were allowing oil to drip on the alternator...that's why we depended on several batteries. Anyway, now we don't have to keep the battery charger out by the car all the time.