FAQ
TL;DR: “Warm TDI needs around 250 rpm to ignite.” That benchmark explains most hot-start smoke on Golf V 1.9 TDI. Check starter, battery CCA, grounds, crank sensor, and consider start‑fuel dose. This FAQ shows quick tests and fixes. [Elektroda, k124l, post #19209372]
Why it matters: For Golf V 1.9 TDI owners battling warm-start smoke and long cranks, this prioritizes proven, low-cost fixes before tuning.
Quick Facts
- Warm-cranking threshold: approx. 250 rpm; below this, expect long cranks and smoke. [Elektroda, k124l, post #19209372]
- At −20°C a healthy lead-acid battery has about 50% capacity; starting current drops sharply. [Elektroda, seba694637, post #17130780]
- Service the starter and clean high-current connections before tuning; poor cranking causes hot-start failures. [Elektroda, k124l, post #19209372]
- Typical remedy after checks: raise warm-start fuel dose via ECU calibration by a competent tuner. [Elektroda, piotrek1994, post #17132475]
- Verify battery inrush current (CCA) with a shop tester; it should align with the case label. [Elektroda, seba694637, post #17132799]
Why does my Golf V 1.9 TDI smoke heavily and crank long when warm?
Warm-starts need about 250 rpm. If cranking slower, fueling lags and unburnt diesel smokes. Common causes are a weak starter, poor battery-to-starter connections, or starter wear. Address these first, then evaluate calibration. This order saves cost and restores clean starts. [Elektroda, k124l, post #19209372]
How do I quickly rule out a bad coolant temperature sensor?
Do a warm-start unplug test. 1. Warm the engine, shut it off. 2. Unplug the coolant temperature sensor. 3. Restart and compare cranking and smoke. If behavior changes, the temperature input is influencing starts. Use this result to guide next steps. [Elektroda, Ireneo, post #17124497]
Can a crankshaft position sensor fail when hot without logging a code?
Yes. A crankshaft position sensor can degrade when hot and misread position. The engine then cranks longer or fails to start. It may not log any fault code, complicating diagnosis. Replace if suspected after other checks. [Elektroda, wojtego, post #17125122]
Does a diesel really need more current to start when hot?
Yes. A warm diesel needs more electrical power to start. Check starting current after an overnight rest and after heat soak. Weak batteries often first show as hard hot starts. Verify capacity before chasing other issues. [Elektroda, seba694637, post #17125571]
How can I check my battery’s starting current (CCA) correctly?
Use a shop CCA test. 1. Let the car sit overnight. 2. Have a workshop measure inrush current with a tester. 3. Compare the reading to the battery’s label value; they should be similar. Replace the battery if results are consistently low. [Elektroda, seba694637, post #17132799]
Is a gel battery a problem for hot starts on the Golf V?
Battery type matters less than available starting current. Focus on tested inrush current versus the label. Any battery that meets its rated starting current should crank the engine adequately. Use a professional tester to confirm. [Elektroda, seba694637, post #17132799]
Could my starter be the real culprit even with a new battery?
Yes. Starter wear, such as damaged brushes, can slow cranking and mimic a weak battery. Rebuilding or replacing brushes restored instant starts on a 1.9 TDI case. Inspect the starter if warm cranking feels sluggish. [Elektroda, Tom2204, post #19209337]
When should I consider increasing the warm-start fuel dose (map)?
Consider it after hardware checks pass. Many hot-start TDIs improve when a tuner increases the start-fuel dose in software. Show the tuner a warm-start attempt to evaluate need. Choose a specialist with this exact service. [Elektroda, piotrek1994, post #17132475]
Disconnecting the coolant sensor makes it start instantly—what does that tell me?
That behavior matches community experience. If unplugging the coolant sensor makes it start instantly, temperature input affects starting. Inspect the sensor and connector, and review warm-start calibration later. This simple check can save time. [Elektroda, Gwizdinho90, post #21301530]
What cranking RPM should I see on a warm start?
Target about 250 rpm during warm cranking. “Warm tdi needs around 250 rpm to ignite.” If the tach shows less, service the starter and connections. Then reassess starting behavior. [Elektroda, k124l, post #19209372]
I feel the starter “locks” when hot—what should I do first?
Address the starter and high-current cables first. Regenerate or replace the starter, and clean battery and ground connections. Poor current delivery can make the starter stall or bind when hot. Confirm cranking RPM after service. [Elektroda, k124l, post #19209372]
Why does it start at −15°C but struggle after a short stop?
Cold weather reduces a battery’s available capacity sharply. At −20°C, a healthy battery has about 50% capacity. A warmed engine then demands higher starting current, exposing marginal batteries. Test and replace if results fall short. [Elektroda, seba694637, post #17130780]
