P0305 on Cadillac: How to Diagnose & Fix
Cylinder 5 Misfire Detected
2 Cadillac models · 2 documented issues · $400-$1,200 repair
P0305 on Cadillac by model: symptoms, diagnosis and fix
The direct-injected 3.0L LF1 fitted to U.S.-market 2010-2011 SRX vehicles can develop valve deposits that contribute to rough running or misfires. The U.S. SRX changed to a 3.6L engine for model year 2012. GM preliminary information PIP5029 lists the 2010-2014 SRX with the 3.0L LF1 among direct-injected engines where major carbon deposits on the intake and/or exhaust valves can cause misfires, often more apparent on a cold start. Misfires have several possible causes; valve deposits should be confirmed rather than inferred from codes alone. GM bulletin 16-NA-383 superseded PIP5029 and addresses carbon-related valve sticking across a broader gasoline-engine population.
Common Symptoms
- Misfire, often more apparent on a cold start
- Check engine light with misfire codes (P0300-P0306)
- Rough idle
- Extended crank or crank-no-start
How to Fix
Follow current GM diagnostic and cleaning instructions after confirming deposits or sticking valves as the cause. Bulletin 16-NA-383 includes an approved chemical cleaning process; walnut blasting is one option, but it is not the only accepted remedy. More extensive mechanical work depends on inspection and the response to the specified procedure. Confirm injector, ignition and mechanical condition as part of misfire diagnosis, and check VIN-specific coverage separately.
Sources for this issue (2)
The XTS 3.6L V6 (LFX, and the LF3 twin-turbo) is a direct-injection engine: fuel is sprayed straight into the cylinder, so it never washes over the backs of the intake valves. Small amounts of oil that get past the valve seals bake onto the valves, and with no fuel to clean them the deposits build up over time. GM's own bulletins also point to fuel: contaminated fuel and incomplete burning add to carbon on the intake and exhaust valves. GM bulletin PIP5029C lists the 2013 XTS with the 3.6L LFX, and bulletin 16-NA-383 (May 2017) covers GM gasoline engines through 2018. GM notes the problem should not show up before about 5,000 miles. In bad cases valves can stick, and compression on several cylinders can read zero on a cold engine. Some engines also suffer valve damage or cam followers that are out of place.
Common Symptoms
- Rough or unstable idle
- Misfire or stumble that is worse on a cold start
- Check engine light with misfire codes (P0300-P0306)
- Extended cranking or crank-no-start in severe cases
How to Fix
Check first: if the check engine light is on with misfire codes (P0300-P0306), especially on a cold start, have the shop do normal misfire diagnosis (plugs, coils, injectors) and a compression test. GM's bulletins say routine diagnosis may not find the cause if the valves aren't sticking at the time of the test. If everything outside the engine checks out, ask the shop to look at the intake valves for carbon. GM's fix (bulletin 16-NA-383, which replaced the PIP5029 series): with the engine warm and held at about 2,000 rpm, a technician slowly feeds ACDelco Top Engine Cleaner (P/N 19355198) through the throttle body with a pressurized tool. The engine then idles for 2 more minutes, and the oil and filter are changed. A second treatment may be needed. GM lists 0.5 hr for the cleaning plus 0.3 hr for the oil change. GM warns that feeding the cleaner in too fast can hydro-lock the engine and bend a connecting rod, so this is a shop job. GM also says not to use aftermarket top-engine cleaners, because some may harm components or seals and may void warranty coverage. If the valves or cam followers are already damaged, the engine has to be repaired first, and the cleaning is done afterward. For heavy deposits, some shops instead remove the intake manifold and walnut-blast the valves, which means blowing crushed walnut shells over the valves to scrub off the carbon. Removing the manifold is why walnut blasting costs more than the chemical cleaning. Prevention: GM tells owners to use Top Tier gasoline, change the oil on schedule, and add a bottle of GM Fuel System Treatment Plus (P/N 88865595) at each oil change. Consumer Reports notes there is no permanent fix, so older direct-injection engines will likely need cleaning from time to time. Coverage: this is a technical service bulletin. It is not a recall or a warranty extension. The bulletin includes a warranty labor operation for cars still under GM warranty. Otherwise the repair is paid maintenance.
Sources for this issue (5)
- GM PIP5029C - Engine Misfires Due To Major Carbon Deposits On The Intake And Or Exhaust Valves (Apr 2013)manufacturer-bulletin (NHTSA-hosted)
- GM 16-NA-383 - Rough Idle, Crank No Start, Extended Crank or Misfire Due to Excessive Carbon on Top of the Valves or Sticking Valves (May 2017)manufacturer-bulletin (NHTSA-hosted)
- 2017 Cadillac XTS TSB 16-NA-383 listingtsb-index
- Consumer Reports: What Is Walnut Blasting for Cars?article
- How Much Does Walnut Blasting Cost?article
P0305 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0305 mean on a Cadillac?
P0305 stands for "Cylinder 5 Misfire Detected." This code means the engine control module detected a misfire on cylinder number 5, meaning that cylinder is not burning its air-fuel mixture properly on some combustion cycles. The computer senses misfires by watching for tiny fluctuations in crankshaft speed each time a cylinder fails to fire correctly. A misfire wastes fuel, can make the engine run rough or shake, and dumps unburned fuel into the exhaust, which can overheat and damage the catalytic converter. It generally points to a problem with spark, fuel delivery, or compression specifically affecting cylinder 5. On Cadillac it is documented across 2 models: SRX, XTS.
What are the most common causes of P0305?
The most common causes of P0305 are: Worn or fouled spark plug on cylinder 5, Failed ignition coil or coil pack for cylinder 5, Faulty or clogged fuel injector on cylinder 5, Vacuum or intake air leak affecting that cylinder, Bad spark plug wire or boot (on applicable engines), Low compression from worn rings, valves, or head gasket, Carbon buildup or sticking valve on cylinder 5, Lean or rich fuel condition affecting the cylinder. The specific cause varies by vehicle.
How much does it cost to fix P0305 on a Cadillac?
Published repair estimates on Cadillac range from $400 to $1,200 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.