Hi guys,

For the sake of the next guy searching: I have a 2001.5 Audi B5 8D A4 1.8 TQM with the AWM Engine.

I was driving the other night and I heard a "boom", almost the way a large, high pressure hose would sound if it popped off at 22 psi

The car died. I had it towed home. I tore the front end apart, to check all of the timing components (which are all new Continental Elite parts). Nothing wrong with anything. Crank and Cam timing are all perfect.

When you try to turn over the engine, it sounds like the starter isn't engaging, but I watched the entire timing set turn (Crank and Cam Gear). Everything in time and no noticeable noises.

If you try turning it over and you pop the clutch, the car will lurch forward, so it's obviously engaging the flywheel and tranny.

I get the rare "pop" from the exhaust, but aside from that.....nothing.

I did a pressure test on the entire intake system and it wouldn't hold ANY pressure at all. I'm also not getting any vacuum....and all intake & vacuum lines are good. It normally holds 30 psi no problem (during testing).

The head gasket is fine. It's holding oil and coolant at their proper levels and they're not mixing.

The car threw a cam speed sensor, engine speed sensor (mismatched values), a knock sensor code (value too high) and a MAF code (signal too low). I replaced all of the sensors, cleaned the MAF and cleared the codes. still nothing.

Can anyone help? I'm worried I may have bent a valve, but I don't see how.

Thanks!

 

If you have checked the mechanical timing and it's good then there is nothing that would have bent the valve...your more likely to wear out the valve guide/stem then bend a valve if timing hasn't skipped a tooth. You also have the timing chain on the two cams that syncs them...if that skipped a tooth your engine would log timing issues about one cam being advanced or retarded. As far as the boom sound, was it while you were accelerating? Could your inter-cooler have been punctured by a rock or piece of metal or something? How are you doing a pressure test on the intake? Could there be a ground issue that is affecting the sensors and your ability to start the engine? I once had the ground lug under the coolant expansion tank come loose and it would make the engine cut out.

 

If your engine electrical ground was disrupted while you were driving, I wonder if it's possible some fuel loaded up and back fired causing the boom you heard? Intermittent that is...

 

No. It's not holding pressure, or vacuum (even though I've checked every component). A fracture in the FMIC would explain everything! I wouldn't even notice a tube fracture (even if the fins were fine).

Thank you so much for that, because I also replaced the knock sensor and still nothing, but your idea makes A LOT of sense, Thank you again.
.

 

It wasn't the FMIC. That was such a good guess too!.


Grounds are good. Thanks.
.

 

Have you tried doing a compression test to ensure the valves are sealing? I can't imagine a single broken valve spring causing the engine not to start or try to start at all. I don't think it's common for more then one spring to break at a time.

With a broken valve spring on these types of heads I would expect a valve not to seal although your timing will be good. In other words, with a broken spring you could maybe have a valve hanging low a bit, never completely sealing on the up stroke of the cam.

If it's a broken valve spring you are looking at a head job. I wouldn't just replace the broken spring, I would replace them all...and not with OEM springs...cause Audi makes shitty valve springs (an Audi mechanic was telling me the other day how frequent new Audi's break valve springs).

You say you get a pop in the exhaust when you are cranking right? Is it skipping/flying over a cylinder? If you don't have compression on a cylinder you can hear the start crank faster on the cylinder thats not holding compression.

 

Even if you had a vacuum leak, I don't think it would be enough to prevent the car from starting...eventually enough fuel would end up in the intake to start the engine.

 

I think you're right. I think a compression test is the next logical step.....and possibly the nail in the coffin.

 

If you have a broken valve spring you could maybe change them yourself with some special tools. Or just have the head redone. However, I had a mechanic tell me that once you take a head off a 1.8 you basically need to get the block reconditioned as well or the engine will burn oil. I don't know how true or consistent that bit of information is but it basically means Audi builds **** cars. Basically you need to get the block resurfaced and cylinders honed straight.

 

Take the oil filler cap off and crank the engine over. If it doesn't sound like it's flying over a cylinder then I don't suspect you have a mechanical issue with the valves or cylinder. If your compression check is nominal then I suspect you have an electrical issue of some kind, like power supply to ignition. Have you taken your plugs out yet?