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A valve spring tension test measures how hard each spring pushes at its installed height and at full valve lift, then compares those numbers with the engine maker's range. Route 66 Auto Repair in Upland uses it during head work and when an engine stumbles only at high rpm. Weak springs let valves bounce, which costs power and can bend parts. Our ASE-certified technicians record every reading on the repair order, so you decide on replacement with numbers in front of you.
When should valve springs be tested?
- Any time a cylinder head is apart for valve, guide or seat work
- A stumble or misfire that only appears near redline or while merging hard
- A broken spring found on one cylinder, which raises questions about the rest
- An engine that overheated badly, since heat can soften springs
- High-mileage heads being reused in a rebuild
What does the test actually measure?
Each spring gets three checks. Free length shows whether it has sagged. Squareness shows whether it leans, which side-loads the valve. Pressure at installed height and at open height shows whether it still controls the valve at speed.
A spring can look fine and still fail the pressure check. We also compare springs against each other, because a spread between cylinders points to heat damage. Results feed straight into valve spring replacement decisions.
Our valve spring testing process
Step 1: Remove and tag
Springs come off in order and are tagged by cylinder and position.
Step 2: Measure length and square
Free length and lean are checked against the factory numbers.
Step 3: Test pressure
Each spring is compressed to installed and open heights on a spring tester and the readings are logged.
Step 4: Report
Pass or fail for every spring goes on the repair order with the spec beside it.
What's included in a valve spring tension test
Included on the work order
- 01Free length check on each spring
- 02Squareness check on each spring
- 03Seat pressure and open pressure readings
- 04Side-by-side comparison across all cylinders
- 05Written results with factory specs listed
- 06Recommendation to reuse or replace, with the reason
Do weak valve springs need attention now or later?
Need now: call
- Any spring that is broken or far below minimum pressure
- Springs that fail while the head is already apart
Need later: book
- Springs slightly under spec on a mild daily driver, noted for the next head service
- A lean within tolerance that we log and watch
How pricing and timing work
When the head is already off for other work, testing adds modest labor. On an assembled engine the springs must come off first, which costs more, so we usually recommend testing only when a symptom or an engine rebuild calls for it. Your written estimate shows every step, and replacement springs are quoted separately after you see the results. AAA members get 10% off parts and labor, up to $75, with the card at time of service.
What happens after the test
If all springs pass, they go back in their original spots. If some fail, you decide between replacing only those or the full set, and we explain the tradeoff. Either way, the numbers stay on your repair order for the next time the engine is opened.
Frequently asked questions
Can a weak valve spring cause a misfire?
Yes, mostly at high rpm. The valve does not follow the cam and bounces off its seat, so the cylinder loses compression for a moment. Low-speed misfires usually have other causes, which we check first.
Is this test needed on a normal engine repair?
Not always. We recommend it during valve and head work or when symptoms fit. We do not add it to jobs where it would not change the outcome.
Should all springs be replaced if one is broken?
Often yes, since they share the same heat and miles. Testing the rest lets you see whether that is true for your engine before you pay for a full set.