FAQ
TL;DR: Cold-start misfire and white smoke on a Golf 4 1.9 TDI often trace to mechanical damage; one owner measured 24.5–27.5 bar compression and later found a scored piston. “90% of cases end this way.” [Elektroda, adam7009, post #16656200]
Why it matters: This FAQ helps Golf 4 1.9 TDI owners diagnose rough cold idle, smoke, and sudden power surges without wasting parts.
Quick Facts
- Reported cold compression on the troubled engine: 24.5–27.5 bar across cylinders. [Elektroda, sidorboss, post #16725521]
- Symptom pattern: rough idle and “unburned oil” smell for ~2 minutes when cold, then smooth. [Elektroda, sidorboss, post #16655049]
- Final root cause in this case: piston scored sideways; engine replacement solved it. [Elektroda, sidorboss, post #17869345]
- Suggested checks: VP pump quantity adjuster contamination and presence of an add-on tuning “BOX.” [Elektroda, sulof1601, post #16655095]
- Risk callout: sudden self-acceleration after oil ingestion can bend a connecting rod. [Elektroda, adam7009, post #16656200]
What are the classic signs of cold-start trouble on a Golf 4 1.9 TDI?
Common signs include uneven idle for 1–2 minutes, white smoke, and a raw oil smell on first start. After a short warm-up, idle smooths and power returns to normal. This pattern points beyond glow plugs to fueling or mechanical health. Record the duration, smoke color, and ambient temperature for diagnosis. “Uneven engine operation and the stench of unburned oil” are primary clues from owners. [Elektroda, sidorboss, post #16655049]
Could a sudden power surge mean oil ingestion and a bent connecting rod?
Yes. A diesel that suddenly “pulls like a young bull” can run on engine oil, over-rev, and bend a rod. One contributor states, “90% of cases end this way,” highlighting the risk. If cold-start smoke persists and compression drops on one cylinder, inspect for rod damage. Avoid driving hard until compression is checked. [Elektroda, adam7009, post #16656200]
How do I check compression on a 1.9 TDI with a VP pump?
Use a diesel gauge through the glow plug ports. 1) Remove glow plugs. 2) Disable fueling. 3) Crank and note pressures per cylinder when cold. This engine uses a VP rotary pump, not a common-rail rail. Comparing cylinders helps spot one weak hole that misfires cold. Document values and test conditions. [Elektroda, matimon22, post #16658146]
What numbers did the OP measure for compression, and what do they suggest?
Measured cold values were 25.5, 27, 24.5, and 27.5 bar. The spread shows one or two lower cylinders. A single notably lower cylinder increases suspicion of mechanical wear, ring or bore issues, or a bent rod. Re-test warm to compare stabilization. Keep the results with date and ambient temperature for your mechanic. [Elektroda, sidorboss, post #16725521]
What is the ‘BOX’ people mention on the injection pump?
It’s a plug-in tuning box that intercepts the VP pump harness at the oval connector. It alters fueling signals to increase torque. Aged boxes or poor connections can cause erratic fueling, surging, or rough idle. Check for any splitter and plastic module on the pump plug and remove for testing. [Elektroda, sulof1601, post #16655095]
Can the VP pump’s quantity adjuster cause rough cold running?
Yes. Swarf contamination can stick the quantity adjuster, upsetting idle fuel delivery. Symptoms include inconsistent cold fueling and surging. Inspect the adjuster circuit and wiring at the pump connector. Cleanliness and proper calibration are critical before replacing major components. Rule out add-on boxes first. [Elektroda, sulof1601, post #16655095]
Why would smoke reduce after two minutes if a rod is bent?
As the engine warms, combustion efficiency improves and cold misfire smoke can diminish, masking damage. A slightly bent rod lowers effective compression, most noticeable when cold. Warm operation can feel “normal,” yet damage persists. Do not ignore a consistent cold-only miss combined with earlier runaway-like surge. [Elektroda, adam7009, post #16655617]
I replaced glow plugs and set injection timing. Why didn’t it fix the cold miss?
Because the root cause can be mechanical. In the discussed case, new glow plugs, injector service, and timing did not help. Teardown revealed a piston scuffed sideways in the bore, and an engine swap solved it. Parts-chasing without diagnosis wastes time and money. “It’s a waste of money.” [Elektroda, sidorboss, post #17869345]
Does a healthy turbo rule out bent rods or oil ingestion?
No. A turbo can appear to operate normally later, yet a past oil-ingestion event may have already caused damage. If you experienced sudden uncontrolled pull and now have cold smoke and uneven idle, perform compression testing before assuming the turbo is innocent or guilty. [Elektroda, adam7009, post #16656200]
What does white smoke with an oil smell indicate at cold start?
White smoke with an oil smell indicates unburned fuel or oil due to poor cold combustion in one cylinder. This can stem from low compression, fueling deviations, or cylinder wear. Track duration and temperature; persistent two-minute episodes point to deeper issues than plugs alone. [Elektroda, sidorboss, post #16655049]
How do I quickly rule out a tuning box fault before deeper work?
Visually inspect the VP pump’s oval plug. If you see a splitter and small plastic module, remove the box and restore the stock connection. Road-test from cold and note if symptoms change. Any change implicates the box or its harness. Keep the module out until fully verified. [Elektroda, sulof1601, post #16655095]
What edge case should I know before spending on sensors and injectors?
An engine can pass basic checks and still have cylinder wall damage. In the case discussed, only teardown confirmed a piston pulled sideways on the bore. The owner replaced the entire engine; previously bought parts were reused on the donor. Consider compression and borescope checks early. [Elektroda, sidorboss, post #17869345]
Is temperature sensor replacement enough when cold-start issues persist?
No. A failed coolant temperature sensor can worsen cold fueling, but fixing it will not correct mechanical defects. If symptoms persist after sensor and glow plug checks, escalate to compression testing and inspection of pump controls or internal engine condition. Avoid repeated parts swaps. [Elektroda, sidorboss, post #16655049]
What’s a VP (rotary) pump in this context?
It’s the distributor-type injection pump used on this 1.9 TDI, controlled electrically with a quantity adjuster. It lacks a high-pressure common-rail. Diagnostics therefore focus on the pump harness, adjuster function, and mechanical health of the engine rather than a rail and injectors. [Elektroda, matimon22, post #16658146]
3-step how-to: baseline a cold-start misfire on a VP-pump TDI
- Remove glow plugs, disable fueling, and record cold compression per cylinder.
- Inspect the pump plug for a tuning box; bypass and retest cold.
- If one cylinder is low, add a borescope exam before buying parts. [Elektroda, matimon22, post #16658146]
