Negative LTFT on idle/stop.

[edcantu9]

2003 Mountaineer. I am getting -10 to -13 LTFT on both banks on stop/idle.

Under load the LTFT stabilize to -3 to -5.

The STFTs are good.

I am thinking vacuum leak.

You guys got any ideas?

I went on ebay and ordered a smoke detection apparatus to smoke out a leak.

Or you guys think for the car being that old I should just live with the numbers?

 

[benjamintmiller]

Wouldn't a vacuum leak cause LTFT to go positive rather than negative, due to the lean condition caused by unmetered air?

Negative LTFT should be caused either by something wrong with the fuel system that's adding too much fuel, or not enough air flowing into the engine.

I'd start by changing the air filter, look at the mass air flow sensor (though they usually work or they don't), inspect the fuel system to make sure the pressure is correct and none of the injectors are sticky, and make sure the emissions controls are working -- a PCV valve that's stuck closed would result in less air flowing to the engine.

An overrich condition could also be caused by incomplete combustion, so I'd check the spark plugs if the engine has very high mileage. Modern plugs and ignitions usually last well past 100,000 miles, however.

 

[Backyard Imports]

I agree with Benjamin. You are looking at a rich condition, not lean.

That means too much fuel (or incomplete combustion leaving too much unborn fuel in the exhaust) or not enough air. In my experience, tune up issues (plugs, wires, etc) will cause a misfire code before they cause a stand-alone rich issue. Do you have misfire codes too? And a bad air filter will cause a restriction before MAF (if you have one) and will affect air getting into the engine but not the calculation of air. That is strictly an issue with the MAF. I don't know what tools that you have to monitor things but with the correct scan tool you should be able to watch things (air flow, fueling, etc) pretty easily.

I would be looking at the fuel pressure regulator. Either faulty regulator causing pressures to stay high or an issue with the vacuum lines/supply to the regulator causing it to keep pressures up. (I am not familiar with your vehicle, so I don't know off hand if it has a vacuum style regulator or if it has a returnless system. However, if it is a returnless system it will normally monitor pressures and would throw a code for that too.)

Just my .02

 

[edcantu9]

Wouldn't a vacuum leak cause LTFT to go positive rather than negative, due to the lean condition caused by unmetered air?

Negative LTFT should be caused either by something wrong with the fuel system that's adding too much fuel, or not enough air flowing into the engine.

I'd start by changing the air filter, look at the mass air flow sensor (though they usually work or they don't), inspect the fuel system to make sure the pressure is correct and none of the injectors are sticky, and make sure the emissions controls are working -- a PCV valve that's stuck closed would result in less air flowing to the engine.

An overrich condition could also be caused by incomplete combustion, so I'd check the spark plugs if the engine has very high mileage. Modern plugs and ignitions usually last well past 100,000 miles, however.

You both are right. I was wrong. It probably is not a air leak. I just checked the fuel pressure I am 60 on the on position and 66 and steady when I start it up.

No concern for the pressure correct?

By the way the truck has 98,000 miles. recently had a tune up done with spark plugs and cops.

I have plenty of tools to diagnose and fix stuff, now I am learning how to use them properly.

No codes are popping up at this time. I just got back from a 40 mile trip and still no code.

I will check the MAF sensor and PCV valve next. You know of any fast ways to test it? It not I am sure I have the proper tools to test it.

 

[Milton Shaw]

PCV valve easiest check is to remove the oil cap and see if there is enough suction to keep a piece of paper down on the tube with engine running. Also on older cars remove the PCV valve and the engine should have a big enough air leak to die. Modern cars the computer with balance out and keep it running but you should hear it change speed as it adjusts things. A lot of older cars the PCV port would stop up under the carb and be full of carbon.