FAQ
TL;DR: Normal temp is 100–103°C; "Normal temperature in this engine is 100-103 degrees." Boiling points to lost pressure, trapped air, or a fouled radiator. Close a good cap, use proper coolant, bleed the LPG reducer, and confirm thermostat/radiator flow. [Elektroda, lopr_pol, post #17135422]
Why it matters: This FAQ helps Astra H 2009 Z16XER owners fix post-repair boilover without damaging the engine.
Quick Facts
- Operating temperature is 100–103°C under light load; the system must stay pressurized by a good cap. [Elektroda, lopr_pol, post #17135422]
- Proper coolant raises the boiling point to about 130°C; plain water boils near 100°C. [Elektroda, rs07, post #17136511]
- The Z16XER uses a map-controlled thermostat with an electric heater engaged by the ECU under load. [Elektroda, rs07, post #17136511]
- With LPG installed, bleed the reducer at its highest coolant hose connection; self-bleeding is unreliable. [Elektroda, enhanced, post #17136471]
- Small leaks aerate the system; feel the radiator’s left lower corner (fan sensor area) to verify heat. [Elektroda, grzegorz1478, post #21395217]
Why is my Astra H Z16XER boiling after head gasket/thermostat work?
If you used water or ran with the cap off, it will boil before the thermostat opens. The thermostat opens near 105°C, while coolant raises boiling to ~130°C under pressure. Keep the cap tight and use proper coolant. [Elektroda, rs07, post #17136511]
Will leaving the expansion cap off cause overheating?
Yes. The cooling system needs pressure to raise the boiling point and purge air. “Never, under any circumstances, unscrew the cap when the engine is hot, or start the engine without the cap tightly closed.” Running unpressurized invites boilover and aeration. [Elektroda, ociz, post #17138190]
Should I use plain water for testing after head work?
No. “Never, under any circumstances, pour water into the place of the appropriate coolant.” Water boils at 100°C at 1013 hPa; at high altitude it can boil near 80°C. That can mislead diagnosis and trigger boilover. Use the correct coolant and keep the cap sealed. [Elektroda, ociz, post #17138190]
How do I bleed the LPG reducer/cooling loop properly?
Bleed the reducer at its highest coolant connection. 1. Locate the reducer’s highest hose fitting. 2. With the engine cool, loosen it to vent air until a steady coolant stream appears. 3. Retighten, top up the reservoir, and confirm large-circuit flow. This prevents trapped air from stalling circulation. [Elektroda, enhanced, post #17136471]
How can I quickly test the thermostat on this engine?
Remove the thermostat and place it in a pot of hot water to watch for opening. If oil contaminated the system earlier, check for a clogged radiator as well. Oil sludge can restrict flow even if the thermostat works. Clean or replace the radiator if needed. [Elektroda, tomek4, post #17135672]
What does the thermostat’s electric heater do on the Z16XER?
It is a map‑controlled heater the ECU energizes under load to improve cooling. “This heater is turned on by the engine controller when the engine is heavily loaded to ensure better cooling.” That allows higher base temps with extra cooling on demand. [Elektroda, rs07, post #17136511]
The hose after the thermostat stays cold—what should I check?
Check for air and circulation. Feel the radiator’s left lower corner near the fan sensor; it should warm before the fan starts. If the bottom stays cold while hoses are hot, the system is aerated or flow is blocked. Inspect for leaks, then bleed thoroughly. [Elektroda, grzegorz1478, post #21395217]
Can a tiny coolant leak really cause boiling and poor circulation?
Yes. Even a small hole can aerate the system, reduce pressure, and trigger boiling. Eliminate all air and fix leaks first. Verify the cap, bleed the system, and confirm the radiator heats top and bottom. Air locks will keep the fan from engaging correctly. [Elektroda, grzegorz1478, post #21395217]
How do I verify the expansion cap (plug) is working?
Ensure the cap seals and holds pressure; the system must remain closed. Replace a suspect cap and recheck behavior. A failed cap prevents pressure buildup and lowers the boiling point, mimicking overheating. Do not run or test with the cap off. [Elektroda, lopr_pol, post #17135422]
Is 100–103°C a normal operating temperature for the Z16XER?
Yes. Light‑duty operation sits around 100–103°C. The closed system is designed for this temperature range. Do not open the system during operation. Keep a good cap installed and verify pressure integrity to prevent boiling at these temperatures. [Elektroda, lopr_pol, post #17135422]
Could the radiator be clogged after oil-in-coolant from a blown gasket?
Yes. A “muddy” radiator restricts flow and overheats the engine even with a new thermostat. Clean or replace the radiator and flush the system if contamination occurred. The short loop should still warm, but poor flow will spike temperatures quickly. [Elektroda, mod22, post #17136635]
Could swapped hoses cause my no‑flow or overheating symptoms?
Yes. Swapping two rear-engine hoses can reproduce a no‑heat or no‑flow effect. Verify hose routing against a diagram and correct any mix‑ups. This edge case has occurred and caused similar symptoms to thermostat failure. [Elektroda, andrzej20001, post #17135833]
I replaced the cap and thermostat, but it still boils—what next?
Hunt for leaks and trapped air first. Check if the radiator’s fan‑sensor corner heats up, top and bottom tanks warm, and the fan cycles. If leaks are absent and hoses are hot, suspect a fouled radiator or flow restriction. Bleed thoroughly and re‑test. [Elektroda, grzegorz1478, post #21395217]
My LPG reducer is plumbed in parallel—does air there still matter?
Parallel plumbing reduces impact on the main circuit, but air can still block gas operation. Vent the reducer anyway at its highest point. If it remains air‑bound, you may also experience issues switching or running on LPG. Bleeding restores proper flow. [Elektroda, enhanced, post #17136471]
, I only poured the water on trial to check if it mixes with the oil again, but the liquid is already flooded and the effect is unfortunately the same 
yes has gas.