1999 gmc 4.8l

[Redleader4]

I have a engine light issue and a low idle that that moves 200-300 rpm or surges i guess.I have just bought the truck and the previuos owner has taken out the catalytic converters is that okay for this engine or is that part of the idling issue.thanks in advance for any help.

 

[Molson02536]

I have a engine light issue and a low idle that that moves 200-300 rpm or surges i guess.I have just bought the truck and the previuos owner has taken out the catalytic converters is that okay for this engine or is that part of the idling issue.thanks in advance for any help.

Have the Sierra scanned for the DTC codes, and also re add the Catalytic Converter. The PCM was programed to have the engine run with the Catalytic Converter in place.

Without the Catalytic convert in place, it would cause less back pressure which is changing the vacuum pressure on your engine like when you replace the cam with a larger cam. It will throw off the vacuum needed for the sensors to be read properly by the PCM to make any changes to have the engine run 100%.

keep us posted and good luck.