A couple of questions for the seasoned mechanics among us

"Desu-San-Desu" (Desu-San-Desu)
08/12/2014 at 18:45 • Filed to: None

So, I've got a few niggling issues remaining with the Audi. They're the sort that are not immediately indicative of disaster, but they are annoying and frustrating in both their persistence and vague nature. I have yet to get a definite diagnosis, so I wanted to pick Oppo's brains and see what opinions here are.

1) Rattlesnake in mah boots

Sometimes, generally at low revs when I haven't revved the engine up enough before letting the clutch out, there is a brief 'rattle/grinding' sound that comes from the bell housing. Now, I don't think it's a synchro because all of the gears shift fine and the noise isn't specific to any particular gear(s). It also doesn't occur in neutral. It's only at very low revs, at minimal torque, with the clutch released and the throttle engaged. It usually last about half a second and then goes away. My initial thought was the clutch, but my father and his mechanic buddy Jeff both drove it said the clutch is fine and doesn't need to be replaced. Their diagnosis?

Old pilot bearing. Now, that sounds fairly serious to me, but they keep assuring me it's not. They say it's just noisy when you don't match the revs well enough and the engine bogs down due to the flywheel not having enough rotational inertia built up yet. According to them, it's just noisy, but not bad enough to worry about for a good while. They said it'll easily wait another year or so without any risk to the transmission, clutch, or fly wheel. They say that when it starts making the noise even at normal revs, then consider replacing it and the clutch.

What does Oppo say of this?

2) "Gimme a second, I'm...I need to catch my breath."

This one's a real doozie. It comes and it goes, but today it was particularly noticeable and I want to get it fixed before the big Mountain Run in October. It boils down to hesitation and noticeable bogging at higher revs.

Up to about 4,500rpm, the car pulls fine. I mean, it's still slow as molasses in a straight line, especially up hill, but not so slow as to be unsafe pulling out onto the highway. What do you expect with 130hp?

But right around 4,500-5,000 revs, it's like the powertrain...sucks. It's not quite a wall, but there's a definite shift. Suddenly it doesn't pull nearly as hard, but the engine still makes plenty of noise. The revs build slower, but they still build, however it's not linear. It'll sometimes hiccup and then 'grab' again, but these hiccups are fairly subtle and don't seem to go as far as the transmission. The engine just gets bogged down- like something is holding it back or it's not getting enough of something.

Now, I've had a dozen and more 'like causes' thrown at me from local shadetree mechanics. Clogged catalytic convert, bad fuel pump, bad injectors, timing belt getting worn, bad MAF, bad oil pump, etc. etc.

Now, in the past year, I've installed a new fuel filter, plugs, wires, distributor cap, air filter, cleaned the MAF sensor and throttle sensor plate, and I'm replacing the timing belt and lifters next week. The car has been doing this on and off for months and I personally am leaning towards fuel pump, but I want to get opinions from here as well before I lay down money.

What say ye, Oppo?

Keep in mind, I'm on a very tight budget. After replacing the tie rods, getting an alignment, installing new pads and rotors, and ordering lifters, the timing belt kit, springs, dampers, and tires, I'm going to be very lean, financially, for a while. But I'm trying to get as much done on the Audi while I can so that it's reliable moving forward. It's a touchy balance and it's stressing me out a good bit.

DISCUSSION (14)

Nibbles > Desu-San-Desu
08/12/2014 at 18:53

Throwout bearing is a BIG DEAL, and it sounds like they're talking about the throwout bearing, as that has input on clutch usage where the pilot bearing keeps the pilot shaft aligned with the crank. Had one grenade on me once, on a road trip. Had do complete the outing and return trip by dry shifting.

505 - morphine not found > Desu-San-Desu
08/12/2014 at 19:04

I'm no mechanic, so take this with a grain of salt but 2. does sound like it runs out of gas. That can be fuel filter, or, the ECU deciding it needs less because of something. You drive a 90 2.3, right? My 100 2.3 had the nightmarish KE III Jetronic, and the things i would check before replacing anything is

- are all vacuum lines in place and airtight? I had a major headache of non-starting when at operating temp bugging me for weeks with mine before i found the disconnected air hose causing it......

- does the fuel pump deliver enough? I think you can measure this by getting the fuel lines off from the engine, and directing their flow into measurement glasses, then turning ignition on for a measured time. Or something along those lines.

- if both are right, then it's probably the ECU cutting suply, the question is, why? Something clogged? An electrical wire broken or disconnected causing it to mismanage the situation?

- or maybe it could be spark advance - if it's not correct you'll notice that at exactly the situation you mentioned. Yeah this seems something worth checking out. What controls your advance? Is that unit working correctly? Is the ECU getting correct reading of that? If you find the advance unit, try disconnecting it and then driving like that, to see if the issue comes up.

Just ideas i know, still hoping some of that helps move you onto the right track....

RamblinRover Luxury-Yacht > Nibbles
08/12/2014 at 19:10

That's what I thought at first, but it's hard to be sure. Either could be making noise briefly with the clutch out, the only difference might be the throwout bearing chirping or rattling for a moment after he finishes letting out (i.e. still spinning). In that instance, the pilot's forced to come to a dead stop. The symptom as presented, hard to say, and might not be worth gambling on.

RamblinRover Luxury-Yacht > Desu-San-Desu
08/12/2014 at 19:12

I'm seconding Zack/formerly known as Nibbles on it possibly being a throwout bearing, but it still could very well be a pilot bearing. Either can make noise with the clutch depressed, but only one can make noise with a foot on the pedal but the clutch not in slip. That's your check. - if that gets you noise, panic.

As to #2, very possibly fuel pump or vacuum.

Aaron James > Desu-San-Desu
08/12/2014 at 19:19

1st one sounds like the throwout bearing which is different than the pilot bearing. The pilot bearing goes in the flywheel and is what the small end of the transmission input shaft rides in.

2nd one could be due to low octane gas. The cars computer will retard the timing to prevent knocking (pre-detonation) which reduces power. Try running a tank of premium fuel through it and see if that is what is going on.

can't add a picture because trolls, here's a diagram of the pilot and release (throw-out) bearings. http://repairpal.com/images/managed…

Desu-San-Desu > RamblinRover Luxury-Yacht
08/12/2014 at 19:31

I just went out and did a lap around the neighborhood to try and recreate it with my camera mounted to the outside of the car, but there was too much wind noise to really hear anything.

But here's when it happens:

Low speed, usually accelerating from a stop or near-stop, such as in first or second gear when the revs are below the torque curve. Clutch pedal pressed in? No noise. Releasing the clutch and the clutch released? The noise is there sometimes , but I have to really be giving it gas while the revs are too low for the gear for it to be heard consistently. The clutch pedal can be mostly released or entirely released, but if I press the clutch pedal in, the sound goes away. The sound also is not present if I rev-match the shift or raise the revs before releasing the clutch. It seems to be very particular circumstances, but it's annoying because I know it can be heard outside the car and, as stupid as it sounds, it makes me self-conscious as well as worried about what's going on in there.

I plan on replacing the clutch assembly anyway, but replacing it in 6 months would be a lot easier than replacing it in 2 weeks. I've been living with it for over a year with no discernible increase, so I'm hoping that whatever it is, it's just minor and embarrassing.

Desu-San-Desu > Nibbles
08/12/2014 at 19:31

Copied from my reply to RamblinRover:

I just went out and did a lap around the neighborhood to try and recreate it with my camera mounted to the outside of the car, but there was too much wind noise to really hear anything.

But here's when it happens:

Low speed, usually accelerating from a stop or near-stop, such as in first or second gear when the revs are below the torque curve. Clutch pedal pressed in? No noise. Releasing the clutch and the clutch released? The noise is there sometimes , but I have to really be giving it gas while the revs are too low for the gear for it to be heard consistently. The clutch pedal can be mostly released or entirely released, but if I press the clutch pedal in, the sound goes away. The sound also is not present if I rev-match the shift or raise the revs before releasing the clutch. It seems to be very particular circumstances, but it's annoying because I know it can be heard outside the car and, as stupid as it sounds, it makes me self-conscious as well as worried about what's going on in there.

I plan on replacing the clutch assembly anyway, but replacing it in 6 months would be a lot easier than replacing it in 2 weeks. I've been living with it for over a year with no discernible increase, so I'm hoping that whatever it is, it's just minor and embarrassing.

Desu-San-Desu > Aaron James
08/12/2014 at 19:34

Is the throwout bearing something that degrades slowly over time or is it something that just goes out one day? On my dad's old Chevy S-10, his just gave out one day with no warning or symptoms. Also, I'll copy over my reply to RamblinRover if that helps at all:

I just went out and did a lap around the neighborhood to try and recreate it with my camera mounted to the outside of the car, but there was too much wind noise to really hear anything.

But here's when it happens:

As for the fuel issue, I generally run 89 Octane, with a tank of 93 about once a month. I'm due for a fill-up in the morning, so I'll try that and pay more attention to when it happens in regards to what octane fuel I'm running.

grahamrh808 > Desu-San-Desu
08/12/2014 at 19:38

European indy tech here. Lots of people have already mentioned the throw-out and pilot bearings, and both are good guesses from your description, though like you mentioned a video would really help. Throw out bearing noise is more likely when you're stepping on the clutch, so you mechanic buddy Jeff might be accurate with the pilot bearing idea. But honestly it doesn't really matter which bearing it is, if you going to do all the work to take the transmission off, you should really change both, and probably the clutch as well while you're in there.

The driveability issue is a bit trickier...maybe I missed it, but what year and model Audi do you drive? Assuming it's new enough, the first thing to do is scan the car and see if there are any fault codes. That'll point us all in the right direction. If there are no fault codes, check the fuel pressure: On board diagnostics are good at picking up a lot of faults, but faulty fuel pressure often isn't one of them.

Aaron James > Desu-San-Desu
08/12/2014 at 19:43

A throw out can grenade itself but usually they just wear out and start making noise. If it's been doing it for a while without getting worse than it could very well be just fine to leave it until you replace the entire clutch and bearings. I have a Honda Accord with a noisy throw-out and I don't worry about it. It's been making noise just like yours for 3 years now. I'll replace it when the clutch goes completely out.

RamblinRover Luxury-Yacht > Desu-San-Desu
08/12/2014 at 20:33

That sounds more like the throwout bearing, then. Not the pilot bearing, if it's a bearing and not your clutch. The total picture sounds like you need to get into your bellhousing regardless.

Desu-San-Desu > grahamrh808
08/12/2014 at 21:08

1991 Audi 80 quattro. Pre-OBDII. No Check Engine Light but I know that can't be trusted.

Desu-San-Desu > Aaron James
08/12/2014 at 21:10

Yeah, the clutch seems to have plenty of bite and spring left so I'm hoping I can hold off on that for a few months. My buddy Mike at KoniMarket can get me a Stage 1 SPEC clutch assembly for the same price as an OEM kit, so I'll be going with that when it comes time.

Nibbles > RamblinRover Luxury-Yacht
08/13/2014 at 08:14

Agreed. That sounds a lot like the noise my old Mustang was making as the throwout took a shit. Fun part is, it could hold entirely fine for GOD KNOWS HOW LONG or crap out immediately