Cold starting problems: 2.5tdi AFB


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Dec 15, 2012
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Hello everyone!

I have bought an Audi A6 2 weeks ago,which I'm very proud of, but now I'm seeking help with my cold starting problems.

The engine is 2.5TDI engine code AFB, it has 286000 km on board (with the camshafts replaced at 217000 km, for those who are wondering). A few days ago I personally changed the timing belts and the thermostat. This is how the timing looks like now. I think it's pretty good :) I also verified all the glow plugs.

The problem with starting first time in the morning is that it takes 5-8 seconds of turning the engine until it starts up. After the engine is warmish, it starts pretty damn well.

Today I did a little experiment, at -2 degrees celsius: started the engine(took around 6 seconds, after the glow plug light went off). After 30 seconds of running, I switched off the engine and started it again. I started in like 1 second! Switched off the engine and tried again in half an hour: at least 5 seconds of turning the engine...

For a side note, I use 15W-40 Mobil Oil and no fault codes appear using vag-com.

What else can I verify to solve this annoying problem? You guys are the last hope before going to a damn service!
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I'm way farther than that. About 1300 miles away :) I live in Romania.
If all else is well at that mileage I would chuck a can or two of diesel flush additive in a tank of fuel.
If it starts and runs live with it?

I'm guessing the injection pump cold advance could be shot (worn springs) or a bit of corrosion from watery diesel.
Check Measuring Block 08 Channel 4 to see if timing is within spec. I understand the Pump duty advance should be less than 10%.
The VP44 pump is a Big Ticket Price item! (new or recon)
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Numbers look OK to my eye but an expert may be along later.
Here is the plastic valve that can cause issues.
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Just checked mine again and on Basic Settings 004

835 RPM Late 2.8 ATDC 100% Cold Start

Not sure if that helps...
I'm not sure how to "read" that cold start valve percentage...

I replaced that plastic tee valve but the starting didn't improved.

I recorded the starting of the engine this morning at -8degrees Celsius,because a video is worth a thousand pictures, here 2.5tdi AFB cold start - YouTube
I put money on fuel delivery.... doesn't help I know.
Did you try a diesel additive/cleaner?
No, I didn't tried any additive. I'm skeptical about them. Do you have any recommendation? Maybe I should check the injectors...
I use Millers Diesel Ecomax but basically anything with cetane on it/in it. Raises the combustion 'quality' and being solvent based should clean some muck out if there is any.

As it seems to be fuel then...

Pump in Tank (doesn't do much)
Filter (easy swap but would affect all running conditions IMHO)
Tube (air or fuel leak)
Return valve (you did that)
Diesel Injection Pump (does everything and has a poor record)
Injectors (conditioner should highlight that)

Anyone else?
So, I used Metabond Megasel and the starting hasn't improved. Now, I guess, I should take down the injectors and put them on a test bench.
i know its a diesel and mines a petrol, but i had the exact same problem, turned out to be the temp sensor. before you start for the first time when cold, just unplug the temp sensor and try and start, if fires up right away then thats your problem....
Hi there

Usually, when the temperature sensor is faulty, it gives an fault code when scanning but mine didn't show any fault codes.
I tried unplugging the temperature sensor and gave it a try. It started even worse.
If it was injectors it would be all the time would it not?
Cold start cycle seems the problem only leaves fuel feed, pump cold advance, gplugs and EGR does it not.....?
Sorry :(

Try this

Ignition on wait for glow light to go off.

Turn off ignition and repeat 2-3 times

Then try a start .....

If it works then Glow Time control module needs a tweak (no not sure how)

From a BMW owner that one... :)

The glow plugs have a maximum time of running of 10-13 seconds (it says in the "manual" when you purchase one) so letting them run 20 seconds or more chances are that you will burn them.

This morning the car sat in the garrage where the ambiental temperature is around 7 degrees, while the outside was -5 to 0 degrees for the last few weeks and the car started waaaaay better, but not as I was expected.

What buffles be is that when I start the engine for 10 seconds, shut if off and then start it again, is starting like the engine is warm! My theory is that at least one of the injectors is leaking(example:holding only 90 bar instead of 300 over the night) and once they are "filled" again with diesel, the injection pump doesn't have to build up the pressure again.

My next goal is to get the injectors out and replace the diuses, but the current problem is money. It will cost me around 140 pounds.Excluding labor, cause that is done myself.
It is possible to start difficult if I'm not using winter diesel? I really never thought of that.
So, I tried winter diesel and no result. Its still starting hard when cold.
if car started worse when you unplugged the temp sensor, surley thats a sign as if it was fine it should of started the same
Have you double checked there is no air in the feed from filter to VP44 Pump?

Only ask as i changed my filter and T-valve last year and stil had some air bubling about, after a starting problem earlier in the week i changed the filter for a better quality one (o-ring was better fit to T-valve) and now have zero air bubbles and better starting.

Your prob could be a combo of weak glow plugs, cold weather and air bubbles = hard starting.

just a thought:think:
I finally found the cause!

After a routine check, I found that the oil level was higher than normal. Since antifreeze level hasn't dropped, it could only be diesel. So I took off the injectors.

The injector nozzle has 5 spray jets. 5 of the injectors had only 4 jets going well, 1 jet of each nozzle was clogged.They were spraying diesel at 210-220 bar. Even that, they were not THAT bad...
One of the nozzle barely had 2 jets working, under heavy load, but on light load it was "*******" diesel instead of spraying. This one could only keep a pressure of 150 bar.

That's why the engine needed a lot of turning, so the injection pump could build up the pressure again, so the 5 other injectors could spray diesel into the cylinders.

With a price of 67 euros per nozzle in Romania, I searched the net. I finally ordered a set of 6 nozzles from Germany with a staggering 133 euros per set. They are on the way and will keep you updated when the new nozzles are in place.
I'm having the exact same problem with my 2.5 tdi a4 mine has the ake engine I fear that ur problem is not going to go away with the new nozzles on the injectors as I've replaced my injectors completely and made no difference whatsoever I have heard that the diesel lines can crack and leak the tiniest bit which allows air into the system while not being driven! I had the car up on a lift last week and noticed diesel at the bottom of the bulkhead on the right hand side I couldn't see where it was coming from tho there's a section where the diesel lines are covered has anyone else ever heard of this there appears to be a bit of stripping involved to access the lines
That's why the engine needed a lot of turning, so the injection pump could build up the pressure again, so the 5 other injectors could spray diesel into the cylinders.

Not 100% true, its not a CR engine so if 1 injector is faulty the other will still work. I would say you have found a fault, but wait and see when you have fitted new injectors.
I'm back with updates. The engine starts way better with new nozzles, but it doesn't start ... suddenly, it starts few cylinders at a time, just like some glow plugs are faulty. I rechecked them, they are all fine, but I noticed that some of the glow plugs connectors are not in perfect condition. Some of them make that click noise when you connect the glow plug, some doesn't. I believe that those connectors that doesn't click may be the cause of the rough start.

I can also note that the engine pulls way better than before :)

But now I have another problem: Out of 20 engine starts, 2 were very long and different than usual, so I checked for errors and I get one fault code:
01268 - Quantity adjuster (N146) -intermittent.

I believe that N146 is part of the injection pump and I'm worried about this fault code.
So I got the problem solved :arco:

The problem of that fault code, and the hard start(cold and hot) was caused only by the pressure valve located on top of the injection pump, as shown here Biodiesel Myths: Diagnosing TDI Performance Issues (Part 2) - YouTube
I must note that I changed the valve. In the video it shows that you can tap the tip to get back to position. Once it's out of position, I think the valve is faulty.

I also replaced the glow plugs connectors and now the engine starts like a charm in the morning.
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Only trouble is the valve on top of the VP44 pump is not like the valve on the VP37, the v6 tdi valve has a cage mounted to retain the guts of the crank relief valve. I suspect the seal was faulty hence the hard starts from cold ! I have a similar b5 audi with a tiptronic gearbox which exhibits similar cold starts but it's fine from hot ! The fuel runs back into the tank if left nose up on hill but will start instantly when parked nose down. It's my missus runabout and i will break it for spares soon. I noticed the previous owners fitted non return valves on feed and return hoses but still no wiser. It's got better once i have replaced the fuel filter and plastic tee valve to genuine Hengst/VAG parts...
I should add that I mounted a valve from a VP37 on my VP44 (without that cage). Keep us updated once you replace some parts for your problem.
I should add that I mounted a valve from a VP37 on my VP44 (without that cage). Keep us updated once you replace some parts for your problem.
And why would you do that for ? VP44 pump runs higher case pressures than the vp37 will ever do ? You better swap it for the old one, just replace the seal...
What are the difference of pressures these pumps work?

It doesn't matter seal that much. The thing that is faulty is that little rod inside it. If it is out like in this picture it's faulty. I cannot order a new valve from the shop because they only order an entire pump, so for that matter, I took one from a scrap pump.
Trust me mate, the seal is the problem in your case, the valve you have will spill the guts much quicker than the vp44 valve will. Look for keystone mod on tdi forum, might be able to save some grief later on !
Not sure if this has been said. I had a 2.5 Tdi with cold start problem. I used 2 stroke oil in the diesel. not the expensive stuff the cheap stuff is better. when i topped up. after a few miles. it got rid of the cold start problem. i think it was a fuel delivery issue air getting sucked in. but the 2 stroke fixed the cold start problem. and not only that it made my car a lot smoother.

the oil will also lube up and keep your fuel pump and full fuel system in good nick. some forums will tell you that it makes your car better on fuel , i didnt se any change in mpg but my car did run a hell of a lot smoother.
@vag_man, I checked what you said and I tried myself with a combination of 1:200. That's 300ml for 70l of diesel. I don't know if the cold start has improved or not(it starts well anyway), but the engine runs better, at least at idle. It vibrates less,as I can notice the gear lever shakes less at idle.

Before the 2T oil, I had a sqeak at idle, no matter what the condition were. If I accelerate at ~1000 rpm, the sqeak disappeared. Now, after I put the oil into diesel, that sqeaking noise disappears after the engine is warm. At 90 degrees C, I never hear that sqeaking noise.