Cadillac Repair: gm 3800, mass airflow sensor, intake plenum

Question
hey Bill i know this isnt a buick forum but i assume you know gm pretty good.....im kinda new to this site....but i hope you can help me, i know my way around engines pretty well, i just bought 96 lesabre ,79,000k miles, well the old man i got it from told me it sat for a year, i bought it because i saw it needed an intake plenum....so i did the upper/lower, everything is in the right places,i also put new plugs, wires,fuel filter,i broke the map sensor so i replaced it, then the iac just because it didnt look so good, and well thats about it...... the idle gets erratic sometimes after i drive it then throw it in park or neutral...like up n down but almost like it wants to stall and will sometimes stall....it even stalled in a parking lot when i let of the gas and then it shifted but then cut off.....note it starts right back up and then runs fine again..... it just seems it has trouble redeeming its normal idle.......today i noticed oil was getting past the pcv valve and so i replaced that o ring........i then had just took apart the throttle body and cleaned the hell out of it........there is still an idle problem........what else can i do? i also hear onec a day or more some thump sound coming from under the car after i start it......like someone punched the gas tank or something.....what do you think i look at ....i dont want to be married to this thing....any help will be highly appreciated!


Answer
Hi,

    If my suggestions don't pan out then I would repost in the Buick area because this is kind of a weird one that only someone with Buick or 3800 experience would know.

    That said what I would try next is to remove the screen from the mass airflow sensor and clean it will carb spray or MAF sensor spray cleaner. You might need to take the throttlebody off if you can't get to the backside. Reason being is I cleaned an otherwise good looking top me MAF on a 3800 and it made all the difference in how the engine ran.

    The computer/PCM itself might be at fault and hooking up a scan tool and reading codes or seeing what the sensors are doing might point you in the right direction. Also with the computer down and on the floor tap on all side litely with a screwdriver handle with the engine running and it might act up meaning a bad solder joint or loose connection.

    Also check both battery cables where they attach to the battery for corrosion and clean as needed. Then trace the negative cable and remove and sand the end and the place where it attaches...could have a high resistance to ground.

    As for the noise it could be a vacuum building up inside the gastank due to a vent valve being restricted. Try running the engine with the gascap off and see if the noise continues.

    Thats all I have. Hopefully I have been of some help. Bill