Audi A3 2.0 TDI long cranking before the engine starts up

[jgjurchinoski]

Hello all,

I am new to the forum. I have an Audi A3 2.0 TDI for 4.5 years now. Since I bought the car it had a rough start, but in the recent period the problem got much worse. At one point it cranked for almost 12 seconds on a cold start. I have to point out that this cold start was done one day after a shop replaced the seals on the tandem pump.(The outside temperature whilst the cold start was around 5 degrees Celsius.)

After that, I had the brushes replaced on the starter. The second video shows a cold start after the replacement of the brushes. (Keep in mind that this video was shot on the 2nd of January 2021 but the temperature was around 10 degrees Celsius.) The shop tested the Injection Quantity Deviation of the 4 injectors and 3 of them were only by 0.4 out of the 0 mark and only one injector was at 1.30mg/str. which is also in the acceptable range from -2.8 to 2.8 mg/Str.

There is no check engine light on the dash nor some message on the diagnostic tool indicating that the glow plugs are faulty. 

One mechanic had an idea that there is a sensor that tells the plugs that the engine is always hot, so that they do not heat up on a cold start. But I don't think that is the problem.

On the last two videos I start the engine when it had been running for about 5-6 minutes after the brushes on the starter were replaced. My concern is that even when the engine is on full working temperature, the start is rough, it cranks a little before starting.

 

As I don't have any experience I am asking for help and I want to say thank you in advance for any tips or similar problems from fellow Audi drivers. 

 

[cliffcoggin]

Jovanche.

Your hot starts are not unusually prolonged to my way of thinking. There's a rattle for a few seconds after firing, but it's an old engine and the oil has drained back faster than it would in a new engine.

Your cold starts are a different matter. I have the same engine and the ambient temperature here is much the same as where you are, however my engine fires after turning over for less than three seconds. I suggest you investigate:

[1] The compression pressures. A worn engine takes time to develop enough compression to ignite the mixture.

[2] The two fuel pumps. Worn pumps can allow fuel to drain back to the tank, hence it takes time to refill the injectors.

[3] The temperature sensor and heater plugs. Your mechanic's theory about the lack of a cold signal to initiate the heaters is perfectly feasible, though it is not something I have come across.

[jgjurchinoski]

Thank you so much for the reply Clifford! As I have become sort of a DIY guy, can you please advise me if you have any experience on how can I test these three areas?

For #1 I think I need a tool that somehow connects to the engine via the hole for the glow plugs.

For #2 I did not know that my car has two fuel pumps. Maybe the second pump is the tandem pump or am I completely in the wrong here?

For #3 can you please (if you know) explain to me where the temperature sensor is located? And lastly, yes the heater plugs are the next step for me. As I have the car for almost 5 years now, I think it is time to change them even though there is no message that suggests I should do so.

[cliffcoggin]

Jovanche.

[1] Yes a compression test gauge is needed for a compression test. Whether it is worth buying one for a single use when it is something any garage will be equipped to do is up to you.

[2] Having checked the manual I see the fuel pumps are combined into one tandem pump. You said the pump seals have been changed, but have the valve seats been checked to ensure they do not pass fuel backwards? The pump is not capable of being adequately tested by the average home mechanic, so it would be better to get a garage to do the job.

[3] The heater plugs use the same temperature sensor as the panel gauge i.e. the engine sensor, which means it signals via the ECU. I suggest you do not replace the heater plugs without clear proof they are defective. The chances are that if anything is interfering with fuel delivery it will be the ECU or one of the sensors that signal it. That again is not something most home mechanics can deal with.