Have you talked to the mechanic about it?
I would pull the T-stat and make sure it was installed in the right direction (spring-side facing the engine).
Something else to keep in mind:
ARTICLE NO. 99-23-8
• GAUGE—TEMPERATURE GAUGE FLUCTUATION AND/OR “KNOCKING/THUMPING” NOISE FROM COOLING SYSTEM—4.0L OHV ENGINE
• COOLING SYSTEM—PULSATING COOLANT FLOW —NOISE—“ KNOCKING/THUMPING”— AUDIBLE IN PASSENGER COMPARTMENT—4.0LOHV ENGINE
FORD: 1995-2000 EXPLORER, RANGER
This TSB article is being republished in its entirety to include 2000 model year vehicles and to provide Service Part(s) and Labor Time information.
ISSUE
A temperature gauge fluctuation and/or a“knocking/thumping” noise may be audible in the passenger compartment on some vehicles. This noise is most pronounced with the engine at normal operating temperature. The noise is caused by steam formation in the cooling system. The noise is then transmitted through the heater hoses and heater core.
ACTION
Install a Coolant By-Pass Kit. This increases coolant flow which reduces steam formation. Refer to the following Diagnostic and Service Procedure for details.
DIAGNOSTIC AND SERVICE PROCEDURE
1. Check for excessive exhaust system restrictions. A restricted exhaust could elevate cooling system temperatures.
2. If dash-mounted cooling temperature gauge fluctuates, check that gauge and sending unit are functioning properly.
3. Check for restrictions or kinks in the radiator or heater hoses.
4. Pressure check cooling system for leaks.
5. Confirm proper Engine Coolant Temperature (ECT) sensor signal to EEC-V processor.
6. Check for correct 50/50 coolant mix. Verify that freezing point is -37*C (±-15*C) or -35*F (±5*F).
7. If concern still exists, install service Coolant by-Pass Kit and verify that concern has been resolved. Refer to the Application Chart in this article.
PART NUMBER / PART NAME
F8PZ-8522-AA / Coolant By-Pass Kit (‘95-‘96 Ranger)
F8PZ-8522-CA / Coolant By-Pass Kit (‘97+ Ranger)