🛻 Buying a Used Duramax? Ask These 10 Questions First

(Avoid a $10,000 Mistake With This Simple Checklist)

So you’re ready to pull the trigger on a used Duramax. Nice. Whether it’s an LB7, LLY, LBZ, or even an LML — these trucks can deliver serious power and reliability...

But if you don’t ask the right questions before buying?
You might end up with blown injectors, overheating problems, or a grenade’d CP4 pump — and a repair bill bigger than the truck’s price tag.

Here are the 10 questions you must ask before buying any used Duramax.

❓1. Have the Injectors Ever Been Replaced?

Especially important on LB7s — factory injectors are known to fail early and catastrophically.

✅ Ask for:

Receipts or service history
Which brand of injectors were used
If all 8 were replaced (not just 1 or 2)

⚠️ If they say “not sure” or “just one,” assume you’ll be doing them.

❓2. Is There a Lift Pump Installed?

Duramax trucks (except LMLs & L5Ps) came without a factory lift pump. Without one, the injection pump can starve under load — especially with tuning.

✅ Ideal answer: Yes, it has a FASS or AirDog installed
⚠️ No lift pump + tuning = P0087 codes, low rail pressure, and injector issues

❓3. What Tuning (If Any) Has Been Done?

Not all tuning is equal. Ask:

Was it tuned with EFI Live, HP Tuners, EzLynk, etc.?
Who built the tunes?
Is there a DSP5 switch installed?
Were ECM and TCM tuned?

⚠️ If the seller can’t answer, assume it was slapped on by a buddy — and you’ll need to re-tune it properly.

❓4. Are There Any Stored or Active Trouble Codes (DTCs)?

Bring a scan tool or ask to plug one in.

Look out for:

P0087, P1093 = low rail pressure
P0671–P0683 = glow plug faults
P2146–P2149 = injector circuit issues
U0101 = TCM communication failure
P0101/P0106 = sensor clogging (LMM/LML)

✅ If the CEL was “just cleared,” ask why.

❓5. Has the Water Pump Ever Been Replaced?

Duramax water pumps typically fail between 150k–200k miles — especially on LBZ/LMM trucks.

✅ Ask if the new one has a cast impeller (better than plastic)

❓6. Are the Head Gaskets Original?

If you're buying an LLY or LML, head gaskets are a known weak point, especially on tuned or towed trucks.

✅ Look for signs of failure:

Coolant overflow
Bubbling in reservoir
White smoke at idle
Overheating under load

⚠️ If the gaskets haven’t been done and it’s over 200k — budget for ARP studs & gaskets.

❓7. Does the Transmission Slip or Limp?

Anything over 450HP on a stock Allison is pushing it.

✅ Look for:

Delayed shifts
Stuck in 3rd/4th gear
Shuddering under load
Codes like P0700, P0776

⚠️ Ask if the trans has ever been rebuilt or tuned

❓8. Is There Evidence of CP4 Failure? (LML ONLY)

If you’re looking at a 2011–2016 LML, this is critical.

✅ Ask:

Is there a lift pump installed?
Does it have a disaster prevention bypass kit?
Was the CP4 ever replaced or upgraded to a CP3?

⚠️ CP4 failure = $8K–$12K+ repair bill if unprotected

❓9. What’s the Balance Rate on the Injectors?

This is how you verify injector health.

✅ Use a scanner (EFI Live, Edge CTS, etc.)
✅ Acceptable Range:

LB7–LMM: ±4.0 mm³/sec
LML: ±2.5 mm³/sec

⚠️ Any cylinder consistently outside those = worn, leaking, or weak injector

❓10. Rust, Mods, and Ownership History?

Ask:

Any frame or rocker panel rust?
Is it a southern or northern truck?
Was it lifted, tuned, or raced?
How many owners?

✅ Clean frame + tasteful mods = green flag
❌ Sketchy repairs + mystery tuning = red flag

🧠 Final Thoughts

Duramax trucks are incredible machines — but buying the wrong one without asking the right questions can be a nightmare.

Ask smart. Scan for codes. Budget for upgrades.
And when you do pull the trigger?

We’re here to help you build it the right way.
👉 Custom EFI Live tuning, fuel system upgrades, injectors, and more — at PistonTwistin.org

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