Measuring the ion current with a sparkplug ion sensor in a homogeneous charge compression ignition (HCCI) engine can be used to accurately detect combustion timing. HCCI engines typically operate at lean equivalence ratios. Under these conditions, the ion current becomes increasingly indistinguishable from background noise. This paper investigates various fuel additives for ion signal enhancement at low equivalence ratios and determines side effects of metal acetate addition. The fuel additives added to ethanol were sodium acetate (NaOAc), potassium acetate (KOAc), and cesium acetate (CsOAc). Concentration levels of 100, 200, and 400 mg/L of metal acetate-in-ethanol are investigated over equivalence ratios of 0.11, 0.22, 0.28, and 0.32. The engine operated under naturally aspirated conditions and maintained a constant timing of 2.5o after top-deadcenter (ATDC) at the crank angle degree (CAD) where 50% of the heat release occurs (CA50). CsOAc consistently produced the strongest ion signals, followed by KOAc and NaOAc, which NaOAc had the weakest effect on ion signal. No distinguishable ion signals were measured at an equivalence ratio of 0.11, but significant ion signal improvements occurred at an equivalence ratio of 0.22 using the fuel additives. However, the addition of the metal acetates decreased heat release rates (HRR) and peak cylinder pressures. Although CsOAc produced the largest signal improvements, it also had the largest decrease in HRR and peak cylinder pressure.