Bad gas? Bad plugs/wires?
What to check next
Diagnostic guidance. Suggested checks and possible repairs are separate from the owner's reported results.
Address the coolant loss before continuing to drive. Let the engine cool completely and arrange inspection rather than relying on repeated top-ups or the heater to control temperature. Ask for a cooling-system and cap test, including inspection of external leaks and the reservoir recovery hose and connections. Tell the technician that the radiator becomes low while the reservoir level stays unchanged, and that stop-leak was added. Repair the demonstrated leak or recovery fault and verify proper filling and bleeding. Also record the trouble codes and investigate the cold rough-running condition. If external checks do not explain the coolant loss, test for internal leakage before deciding on an intake or head-gasket repair. The two symptoms may be related, but that needs evidence. Finding where the coolant goes provides a practical starting point.
Guidance references and applicability
HELLA: Cooling-system check https://www.hella.com/techworld/ae/technical/car-cooling-system/cooling-system-check/ General reference for coolant level, leakage, temperature and component checks. OTC: Compression and cylinder-leakage testing https://www.otctools.com/sites/default/files/2007_930_otc_compression_testers_ada_0.pdf Explains possible cylinder-leakage paths. It does not diagnose a gasket failure in this vehicle.
https://www.youtube.com/watch?v=LBCpCWEFnTg
Do not continue driving it while the radiator is losing coolant or running hot. The stop-leak may temporarily slow an external leak, but it can also restrict the radiator or heater core and will not reliably repair an intake-gasket or head-gasket failure.
Please go over this guide as well.
https://2carpros.com/articles/recognizing-the-symptoms-understanding-a-blown-head-gasket-in-your-vehicle/
https://2carpros.com/articles/engine-misfires-or-runs-rough/
Discussion summary
Bad gas? Bad plugs/wires?
The owner described rough running after sitting, worse under acceleration, together with higher engine temperature and repeated radiator refilling. Stop-leak reduced some visible leakage, but the reservoir level reportedly stayed unchanged while the radiator lost coolant. The check-engine light had been on for years. Contributors requested codes and discussed possible cooling-system and gasket faults.
Reported outcome: No diagnostic codes, pressure-test results or completed repair were reported.
A head-gasket or intake-gasket failure was not established. The longstanding warning light does not identify the cause of the newer rough-running symptom.
