Car shudders when hitting the throttle and slow to accelerate but smoothes out around 40mph

What is your question?

When I start the vehicle it has a rough idle and when I push the throttle down the vehicle shudders and is slow to accelerate but smoothes out around 40-45mph I changed the spark plugs and gapped them correctly I ran diagnostics and it came back with change the plugs, EVAP sensor, Ho2s sensor. Plugs are done but do I need to reset the system? And the EVAP sensor is what I'm told but they also say to check the fuel cap mine seems like it is really easy to unscrew even when it clicks when closing just doesn't fit like a functional fuel cap though it does click when tightening it just easy to unscrew

What makes this problem better or worse?

It comes and goes but mostly when acceleration up to about 40mph then smoothes out a bit but there is shaking in the steering wheel at 55mph+

How long has your 2005 Cadillac STS had this problem?

Just bought vehicle 4 days ago guy said it was the EVAP sensor though I read that it could be the fuel cap mine feels really easy to unscrew but it clicks when tightening it

Visitor

in Molalla OR

May 11, 2025

1 Reply

Popular Answer

DaveJMon May 12, 2025

Hello, Dave here, contributing to RepairPal and providing expert advice since 2010. When you say: "I ran diagnostics and it came back with change the plugs, EVAP sensor, Ho2s sensor." what do you mean? Nothing says anything. It's interpretation and proper diagnostic testing based on what is present and happening. This is my concern; it sounds like you have been led to believe that codes set are saying to take action when they are instead giving clues to perform valuable testing. I do not believe any evaporative emissions issue is going to cause the problems you are experiencing. Nor do I think you have a fuel cap problem causing any running problems. Sure, this could be contributing to emissions code issues, such as the "EVAP" code. I would be checking for intake and exhaust restrictions and proper air flow - then move to ignition - plugs and coils need to be verified as good. Hands on testing is needed. https://repairpal.com/auto-repair-near-me Click the link or call us at 877-913-1554. We'll gladly find you a certified RepairPal shop in your area.

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