QuestionQUESTION: I have an 1988 gmc 1/2 ton p/u . I put a new map sensor in a year ago. In the last 6 months I have noticed That when the humity is high I get poor fuel milage. But when the weather is dry and warm itt goes up by as much as 20% can you tell me why?
ANSWER: You will get a difference in fuel mileage between dry air and humid air. Pilots are familiar with this phenomenon, you get less lift with humid or hot air, because it is less dense than cool, dry air. But, a 20% difference is way out of line. You need to check that you have the correct MAP sensor for your vehicle, and then you need to check that the air cleaner assembly is getting the cool air it requires through the intake hose (no tears or leaks in the hose, which should bring fresh air into the air cleaner from the area in front of the radiator). I would get someone to hook up a scanner and check the specs on all the other engine sensors: the air charge temperature sensor, the coolant temp sensor, and the oxygen sensor. If any of them are out of whack, they are going to force you into what is known as open-loop operation, and your mileage will go down. Something tells me that your truck is running closed-loop on dry days and open-loop on wet days. And, if by "high humidity" you mean when it is raining, then moisture could be invading one of the connections. This could get very complicated, and might cost more to solve than the cost of the extra gas you are using, but it also might lead to preventing a breakdown in the future and they can be so inconvenient.
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QUESTION: Thanks for the responce! can this also cause a mild skip in the engine. And no i'm not talking about rainy days only for the humity. when the BARO drops then my milage gose from 19 mpg to 13 mpg. but thank you for your help.
Answer I've been racking my brain to try and come up with an explanation for this. That big a difference in the mileage is crazy. I have a feeling that the MAP sensor is at fault. Was it GM or Pep Boys? A lot of the stuff that you find in the discount stores is made in China or even worse places. They're happy if it just looks right, not really worried about whether it's giving the right signal. The harness going to the MAP sensor needs a good looking over. Make sure that the plug is in one piece, making proper contact and not shorting out. You can get a replacement plug at any big parts store like a NAPA. Trace the wires all the way back to the computer, looking for any places where the harness could have been damaged. If all of this is good, and you try another MAP sensor from a reliable source, and you still have the problem, then you need to get in there with a scanner and watch the performance of the system while it is being driven, in both types of weather. You will need a really good scanner, the kind Snap-On sells for over a grand, so you will probably want to turn that chore over to a pro. It could be that the computer itself is at fault, or the PROM may be bad. Luckily both of these are available on an exchange basis for relatively low prices (compared to the ECM's in the newer OBDII cars and trucks). I would definitely try to run down what is going on, 'cause it is just going to get worse.
