diff --git a/gtsam/navigation/PreintegrationBase.cpp b/gtsam/navigation/PreintegrationBase.cpp
index dc5fd8647..f2a8d41fd 100644
--- a/gtsam/navigation/PreintegrationBase.cpp
+++ b/gtsam/navigation/PreintegrationBase.cpp
@@ -368,11 +368,11 @@ void PreintegrationBase::mergeWith(const PreintegrationBase& pim12, Matrix9* H1,
                                    Matrix9* H2) {
   if (!matchesParamsWith(pim12))
     throw std::domain_error(
-        "Cannot merge preintegrated measurements with different params");
+        "Cannot merge pre-integrated measurements with different params");
 
   if (params()->body_P_sensor)
     throw std::domain_error(
-        "Cannot merge preintegrated measurements with sensor pose yet");
+        "Cannot merge pre-integrated measurements with sensor pose yet");
 
   const double& t01 = deltaTij();
   const double& t12 = pim12.deltaTij();
@@ -382,9 +382,9 @@ void PreintegrationBase::mergeWith(const PreintegrationBase& pim12, Matrix9* H1,
   Vector9 zeta12 = pim12.preintegrated();
 
   // TODO(frank): adjust zeta12 due to bias difference
-//  const imuBias::ConstantBias bias_incr_for_12 = biasHat() - pim12.biasHat();
-//  zeta12 += pim12.delPdelBiasAcc() * bias_incr_for_12.accelerometer() +
-//            pim12.delPdelBiasOmega() * bias_incr_for_12.gyroscope();
+  const imuBias::ConstantBias bias_incr_for_12 = biasHat() - pim12.biasHat();
+  zeta12 += pim12.preintegrated_H_biasOmega_ * bias_incr_for_12.gyroscope()
+      + pim12.preintegrated_H_biasAcc_ * bias_incr_for_12.accelerometer();
 
   preintegrated_ << PreintegrationBase::Compose(zeta01, zeta12, t12, H1, H2);
 
diff --git a/gtsam/navigation/tests/testImuFactor.cpp b/gtsam/navigation/tests/testImuFactor.cpp
index b83c91e55..f1d761cb0 100644
--- a/gtsam/navigation/tests/testImuFactor.cpp
+++ b/gtsam/navigation/tests/testImuFactor.cpp
@@ -790,8 +790,8 @@ struct ImuFactorMergeTest {
 
   int TestScenario(TestResult& result_, const std::string& name_,
                            const Scenario& scenario,
-                           const imuBias::ConstantBias& bias01,
-                           const imuBias::ConstantBias& bias12, double tol) {
+                           const Bias& bias01,
+                           const Bias& bias12, double tol) {
     // Test merge by creating a 01, 12, and 02 PreintegratedRotation,
     // then checking the merge of 01-12 matches 02.
     PreintegratedImuMeasurements pim01(p_, bias01);
@@ -831,40 +831,41 @@ struct ImuFactorMergeTest {
   }
 
   void TestScenarios(TestResult& result_, const std::string& name_,
-                     const imuBias::ConstantBias& bias01,
-                     const imuBias::ConstantBias& bias12, double tol) {
+                     const Bias& bias01,
+                     const Bias& bias12, double tol) {
     for (auto scenario : {forward_, loop_})
       TestScenario(result_, name_, scenario, bias01, bias12, tol);
   }
 };
 
 /* ************************************************************************* */
-// Test case with identical biases where there is no approximation so we expect
-// an exact answer.
+// Test case with zero biases
 TEST(ImuFactor, MergeZeroBias) {
   ImuFactorMergeTest mergeTest;
-  // TODO(frank): not too happy with large tolerance (needed for loop case)
-  mergeTest.TestScenarios(result_, name_, kZeroBias, kZeroBias, 1e-3);
+  mergeTest.TestScenarios(result_, name_, kZeroBias, kZeroBias, 1e-4);
 }
 
-//// Test case with different biases where we expect there to be some variation.
-//TEST(ImuFactor, MergeChangingBias) {
-//  ImuFactorMergeTest mergeTest;
-//  mergeTest.TestScenarios(
-//      result_, name_, imuBias::ConstantBias(Vector3(0.03, -0.02, 0.01),
-//                                            Vector3(-0.01, 0.02, -0.03)),
-//      imuBias::ConstantBias(Vector3(0.01, 0.02, 0.03),
-//                            Vector3(0.03, -0.02, 0.01)),
-//      0.4);
-//}
-//
-//// Test case with non-zero coriolis
-//TEST(ImuFactor, MergeWithCoriolis) {
-//  ImuFactorMergeTest mergeTest;
-//  mergeTest.p_->omegaCoriolis.reset(Vector3(0.1, 0.2, -0.1));
-//  mergeTest.TestScenarios(result_, name_, kZeroBias, kZeroBias,
-//                                      1e-6);
-//}
+// Test case with identical biases: we expect an exact answer.
+TEST(ImuFactor, MergeConstantBias) {
+  ImuFactorMergeTest mergeTest;
+  Bias bias(Vector3(0.03, -0.02, 0.01), Vector3(-0.01, 0.02, -0.03));
+  mergeTest.TestScenarios(result_, name_, bias, bias, 1e-4);
+}
+
+// Test case with different biases where we expect there to be some variation.
+TEST(ImuFactor, MergeChangingBias) {
+  ImuFactorMergeTest mergeTest;
+  mergeTest.TestScenarios(result_, name_,
+      Bias(Vector3(0.03, -0.02, 0.01), Vector3(-0.01, 0.02, -0.03)),
+      Bias(Vector3(0.01, 0.02, 0.03), Vector3(0.03, -0.02, 0.01)), 1e-1);
+}
+
+// Test case with non-zero coriolis
+TEST(ImuFactor, MergeWithCoriolis) {
+  ImuFactorMergeTest mergeTest;
+  mergeTest.p_->omegaCoriolis = Vector3(0.1, 0.2, -0.1);
+  mergeTest.TestScenarios(result_, name_, kZeroBias, kZeroBias, 1e-4);
+}
 
 /* ************************************************************************* */
 int main() {