Northstar Engines and System Technical Discussion Discussion, 1999 Deville PCM codes - can you tune it? in Cadillac Engine Discussion; ok
so i have a PCM code(s)
my 98 deville is sucking premium gas at 12mpg
does it need retuning ...
This was moved from "PCM tuning" because your question is not about the possibility of "tuning" your PCM for high performance.
You have codes - what are they and what are the code definitions ?? Your gas mileage is horrible, so you probably have multiple problems - none of them with the PCM itself. The "P" in the codes refers to the basic system in which the code is, the code number itself is the fault locator. Use the sticky post ^^^ "How to pull codes" and study the section for your car, year group and dash button layout. Get any codes stored, write them down with the suffix - Current or History - and return to the sticky to open the link with "obd2" in the address - that takes you to a Master Index of all P,B,C, and U codes. decipher all your codes and post it all here.
A "new" PCM for your car must be programmed by a GM dealer - that box is flashed for YOUR car and your car only.
FPR - Fuel Pressure Regulator. Under the engine beauty cover, on the right (rear) run of the fuel rail - the silver domed thingy with the vacuum line sticking out the side. Carefully remove the vacuum line - any raw gas drip out of the nipple or line ? That would indicate a ruptured inner diaphragm . If so, replace the unit. Hairpin clip, pull it out - 2 O-rings on the body. Make sure the lower one comes out. Oil the new unit O-rings, press it in, clip. Hook up vacuum.
You didn't offend anyone - we all recommend checking the FPR during checks for poor gas mileage. Your car should get about 12 - 16 mpg city, 17 - 21 suburban, 25 - 27.5 long open highway runs.
Your instrument panel (DIC) fuel mileage readouts - instant, average, range, fuel used - change as conditions change. The average and gallons used can be reset - read your owner's manual. Understand that instantaneous mileage is for fun: it is worthless in calculating true gas mileage. The average - long term - mileage is the one to use.......... But understand that it's a long term average: You reset it and start the engine and sit at idle - your gas mileage is 0. Get out on the Interstate at 65 for a half hour and it should slowly click up to the high 20's. The longer you drive, under all conditions, the more accurate the average overall gas mileage will be.
EDIT: Go up a thread or two from this one - read the sticky posts there - Acronyms, for one. You'll see a LOT of abbreviations used here and in the (real) GM/Helm service manual hard copy.