Problem When Low On Gas

[jfk1232]

When my 1990 Laser gets low on gas (around a quarter tank), it will die when idling or being started and will have thick black smoke come out of the tail pipe. It will also have problems accelerating. If I add a little gas, the problem seems to go away.

I was thinking maybe there was some sort of sediment or bad gas at the bottom of the fuel tank that gets pulled into the engine when the gas gets low. I don't know if that would explain the black smoke. I've heard black smoke means the fuel is too rich.

[beef_bourito]

if it only happens when you're low on gas, just keep it above 1/4 tank. the black smoke out the tailpipe could be because you're running to rich. that's when the engine injects more fuel into the cylender then it needs, so some of it doesn't get burnt and goes out the tailpipe. this gives you bad fuel economy, bad power, and it's not good for your catalytic converter because it can get clogged. if you get the sensors replaced it should fix the problem, if not just keep your fuel above that magic level.

[curtis73]

Sounds like a pinhole or leak in the pickup tube in the tank. I would drop the tank, pull the sender, and check for leaks in the pickup tube. If that's the problem, what happens is it starts sucking air (or bleeding fuel pressure) when its below the level of the leak... just like a soda straw with a hole.

[mazdatech177]

sounds like you got water in the gas tank... drop the tank and check it, itll look like mercury rolling around the bottom of the tank in little blobs. make sure you put a gas cap on that thing

[jfk1232]

Originally Posted by mazdatech177

sounds like you got water in the gas tank... drop the tank and check it, itll look like mercury rolling around the bottom of the tank in little blobs. make sure you put a gas cap on that thing

Do you know if water can cause black smoke coming out the tailpipe?

Also, how does water get into the fuel tank?

[curtis73]

Water gets in the fuel tank in many ways. Primarily its a small amount from condensation. You can get bad gas from a station, or you can get some from a leaking gas cap when it rains.

Water is denser than gasoline, so it sinks to the bottom making it the first to be picked up. For that reason I don't think your problem is water in the gasoline. You can see if it runs better by putting in some gas line antifreeze. Its basically alcohol and it mixes with the water and helps get it through.

Water in the fuel will not cause black smoke. Black smoke comes from running intensely rich mixtures. Check all of your sensors for proper operation. If its an O2 sensor they typically will cause the check engine light to come on, but not always. If its a MAF sensor it sometimes just gives erroneous readings to the computer without thinking there is anything wrong. In that case it should trip the O2 sensor computer code.

It could be a leaky injector, a stuck fuel pressure regulator, several things.

[jfk1232]

Originally Posted by curtis73

Water gets in the fuel tank in many ways. Primarily its a small amount from condensation. You can get bad gas from a station, or you can get some from a leaking gas cap when it rains.

Water is denser than gasoline, so it sinks to the bottom making it the first to be picked up. For that reason I don't think your problem is water in the gasoline. You can see if it runs better by putting in some gas line antifreeze. Its basically alcohol and it mixes with the water and helps get it through.

It could be a leaky injector, a stuck fuel pressure regulator, several things.

Here are the error codes from a computer diagnostic:
13 intake air temp
15 motor position sensor
24 speed sensor
25 baro sensor

Could any of these be causing the problem?

[curtis73]

Originally Posted by jfk1232

Here are the error codes from a computer diagnostic:
13 intake air temp
15 motor position sensor
24 speed sensor
25 baro sensor

Could any of these be causing the problem?

They could all cause the problem. The IAT and Barometric sensor are two of the several sensors that directly tell the computer how much fuel to inject. That is not to say however that they are absolutely failing. It could be something else causing a cascade effect, but usually those two are pretty unaffected. Find someone with a shop manual and test them for electronic operation.

Other than that, I hope MagicRat or someone else chimes in. I can do EFI, but my strengths are with carbs. I'd hate to give you false info.

[jfk1232]

Originally Posted by curtis73

They could all cause the problem. The IAT and Barometric sensor are two of the several sensors that directly tell the computer how much fuel to inject.

When you say IAT, does that refer to the "15 motor position sensor" fault code? I have also seen the part for this fault code refered to as ISC or even IAC/ISC. Does it have different names? I'm not sure what part to ask for at the parts store.

[curtis73]

IAT usually refers to intake air temp sensor. I was referring to the 13 code you were getting.

[jfk1232]

Originally Posted by curtis73

IAT usually refers to intake air temp sensor. I was referring to the 13 code you were getting.

Where is the IAT usually located? Is it easy to install?

Do you know of any places on the internet that has diagrams of car engines so that I can look up where sensors are located?

[curtis73]

IAT is almost always super simple. Its located between the air filter and the throttle body, and usually just pushes into the rubber boot somewhere. Cheap, easy to do.

I suggest buying a shop manual for your car from Haynes, Chiltons, or Bentley. They're usually between $13 and $30 and will teach you volumes about your specific car.