Air in fuel when priming Injection Pump -IP is loud

My 2003 TDI Auto has 315k miles.

I have replaced two seals on the IP that were leaking. I have replaced the fuel filter and filled it with Diesel Purge. Engine starts and runs fine after I set the injection timing and adjusted fuel quantity.

I have replaced the alternator pulley, tensioner and idler. I was getting the marble rattle noise on start up before replacing those parts and now I am not. Only unusual noise is the loud IP. It reminds me of mechanical engine tappets having too large a gap. Only other symptom is constant air in the fuel when I draw it through the return port on the IP.

I have not yet primed the fuel filter. I will do that tomorrow. I have ordered a new return check valve and will replace it in a few days. If that valve is faulty, then air could flow from the return line to the filter back to the IP. I believe the check valve is present because the return to the fuel tank may contain air.

My theory is that air is getting into the IP. It is small enough that fuel still gets into the cylinder and explodes. But, the presence of the air causes a strike of metal against metal. The IP is designed for fuel only and is likely expecting a hydraulic resistance from the injector fuel line at the end of the injection stroke. Since there is a little bit of air in the fuel, the plunger compresses the air and travels unopposed for a short distance, creating a gap between the plunger and the lobe. Then, I believe a spring slaps the plunger against the lobe, causing the noise I am hearing. At least, that is my theory.

As a test, I will clamp off the return line from the check valve back to the IP. I will prime the pump again. If I get no more air in the fuel, that result would seem to support my theory. I could then leave that clamp in place and dump the return fuel to a container while I run the engine briefly. That test might tell me a lot.

I seek comments on my understanding of the IP design and operation as well suggestions for further testing. At this point, I do not belive the IP is failing.

Tim

I think you are calling the thermostatic tee on the filter a check valve. It actually returns fuel to the pump until it reaches the proper temp. Then it stops recirculating and sends it to the tank. It will recirculate some air until the fuel warms up.

The injection quantity is controlled in the pump by a moving collar on the high pressure plunger. When the correct amount of fuel is injected the collar uncovers the spill port and injection pressure drops. At idle the amount is less than at full load so this is a constant thing. The return springs prevent overtravel of the plunger and prevents the contact you're speculating about.

You could run both the supply and return from a container of fuel to eliminate air as the source of the noise.

When priming you can disconnect the return line from the pump and plug it then pull fuel from the fitting on the pump. Playing with the tee while doing this might reveal if it is letting air in.

Thanks. I will try to test the tee today and will report back.
Tim

If that pump has 315,000 miles on it it likely has some worn internals. If you've got all the air out and the car runs good, drive on.

BobnOH

I had a 2000 Jetta TDI with 400k and it NEVER sounded like this. It is way too loud. But, if I get the bubbles gone, I will drive it until symptoms get worse, like unable to hold injection timing.

Anyway, I plugged the return line between the IP and filter. I was able to draw fuel without air bubbles. I put the hose back in its proper spot on the filter and I was UNABLE to draw fuel without bubbles. That result may be normal, although it seems all IP repair procedures refer to priming the IP until the bubbles are gone.

I am awaiting a new T connector/check valve. I will advise if the bubbles clear.

Tim

I got the new T today and swapped it out. I was able to draw fuel without air if I sqeezed the Mighty Vac slowly. That is an improvement over the old T. However, if I squeeze rapidly, air is present in the fuel. The noise I am trying to eliminate is still present.

I will try the same vacuum test on the line from the fuel pump to the fuel filter. If bubbles are present, then I know there is a leak of some kind on that line. If not, then there is a leak at the fuel filter, connecting hoses or IP itself.

I will advise of the results.

Tim

Anecdotal- I had a pump on my 99.5 that had trashed internals at 60k miles, results will vary. When I changed out the injection pump, I simply sucked on a hose connected to the fuel return from the pump, got solid fuel, done. So it must be getting air from somewhere. You'll find it, you're on the right track.

All - I tested two configurations to see how air is introduced into the IP.

1. Pull fuel from the tank. Removed the input line to the filter and attached Mighty Vac. Pulled fuel without bubbles regardless of level of vacuum. I conclude that there is no air introduced between the fuel tank and the fuel filter.

2. Determine if the pump can hold a vacuum. I removed the return lines from the #4 injector and the fuel filter. I tested two sections of plugged tube to be sure they could hold vacuum. Each held 20 in/Hg. I attached those hoses to the two return ports on the IP. I then attached the Mighty Vac to the input port of the IP. It would not hold vacuum, losing vacuum slowly but steadily. I conclude that the IP is unable to hold a vacuum and is likely introducing air into the fuel.

So, here are some questions.
1. Is my result normal? Is it expected that the IP will NOT hold a vacuum?
2. If the result is abnormal, what next, short of replacing the IP. Could other components cause the result? Note that I can detect no fuel leak anywhere.

My plan.
1. Await response and ideas from the community.
2. Spend a little money to have the situation evaluated by a local TDI guru.
3. Research the purpose and behavior of the fuel return connector on the IP. When I examined it, there appear to be a small orifice and moving parts inside. I need to better understand what it does.
3. If the only fix is to replace the IP, I will put the car into service as is and perform major maintenance in about a year - IP pump, timing belt, water pump and vacuum pump, maybe cam shaft and followers if indicated. With a little luck, it will last that long under light use.

I look forward to your responses. I will post again after I get the opinion of the TDI guru.

Tim

After further review....

I saw a post that said an advanced injection timing would be louder than a retarded one. I did adjust the timing a few weeks ago and may have advanced too far. I replaced the alternator pulley as it was making noise and may have masked the noisy IP after I advanced the timing.

So, I am still looking for opinions but I will take out my VAG 401 and see if I can dial the timing back a bit.

Tim

I dialed back the startup timing a couple of degrees. No change in symptoms. I now believe the noise is normal for a well used IP. Barring a fault that affects performance, I am going to ignore the symptom and hope it goes another 50k or so. At that point, I plan to perform major maintenance - timing belt, water pump, vacuum pump, inspect cam and followers, and maybe replace the IP. If it gets no worse over 50k miles, I will just drive it until it fails.

Thanks for your feedback. I will not post to this thread again unless there is a significant change in the situation.

Well done, now you're where I was at interweb guessing. Turbos and IPs are just so damn expensive. And they're sometimes repairable, but they're not really rebuildable.

All - new info on this loud IP. It occurs ONLY under light load. When going up a steep hill or under accelleration, the noise ceases. I plan to fiddle with the IP and adjust the IQ using the mechanical bump method. I have already messed with timing with no change in sympoms. The fact that the noise disappears when more fuel is demanded gives me a little hope that it may be an adjustment issue.

And another dead end. I leaned the pump out as far it would go, pusing the top piece as far toward the timing belt as it would go. No joy. Same noise, starts and runs great. I checked my Xtool VAG 401 and at 3k, quantity requested = quantity delivered. So, if the IP is going bad, it is not giving any warning signs.

So, I am back to drive it until it breaks.

Tim

All - Continuing to define the noisy IP issue.

It is NOT loud at idle, but makes considerable noise when driving under most conditions. When it's under load (rapid accel or going up a steep hill) the noise disappears. It seems to correlate to throttle position, being noisy in a certain range of throttle position. At idle, not an issue. Under load, not an issue. Occurs only during normal driving.

Any suggestions appreciated.

Tim