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View Full Version : Intermittent bucking/surging, primarily when leaving a stop.



Mille Bornes
08-04-2013, 07:13 PM
I've spent a lot of time browsing this forum researching various things about my B5 S4 and my girlfriend's B5 A4 but now I'm kind of stumped, so this is my first post.

Recently, my girlfriend's A4(AEB 1.8T, early 99) has developed an intermittent problem. This post will be lengthy but I would like to include as much possibly relevant information as I can. So, here we go:

Not to long ago this car had a terrible misfire. It turned out to be a bad coil but when I took the coil off I found every spark plug hole was full of oil. I can't believe only one cylinder was misfiring, as the worst two actually had enough oil to completely submerge the spark plug. So, I replaced the one coil, all four plugs, and two boots. I meant to do all of them but misunderstood the way RockAuto listed their boots.
After this, the car ran fine... for one drive. Then it had a massive lack of power and an obviously audible boost leak, which turned out to be the L shaped hose on the back of the intake manifold. The previous owner had repaired this cracked hose with electrical tape, which apparently just happened to finally give at the same time the car started misfiring. What are the odds of that?
Replaced the L shaped hose and the car ran perfect for about a day and started acting up again. I do have a vag-com and at this point the only codes I was getting were Charge Limit Exceeded and System Too Rich (I've recently cleared them and can't remember the numbers at the moment). So I began checking for more boost leaks. I checked the diverter valve and it's fine, I looked all over, and eventually pressure tested. I found another hose "repaired" with electrical tape. This was the hose that goes from the turbo housing the the N75. I replaced it and the car was running perfect again... for a few hours.

At this point I'm planning to search further for more vac/boost leaks but here's what gets me: If the car is running rough, the problem goes away if you shut it off and restart it. This makes me think it's some electrical, or a failing sensor. But what are the odds that all of this problems occur at exactly the same time that the car starts misfiring? I just don't know in which direction to go.
So far I've done the throttle body adaptation and throttle body alignment a couple of times, I've checked the throttle angle readings and they seem to check out, MAF reads perfect. Around 4g/s at idle, upwards of 120g/s when doing a WOT pull through 3rd gear. I've been looking at Average Injector pulse width but I'm not sure what this should be when idling/cruising, etc..

I'm not sure if I should worry about watching lambda or not since if it's not running properly the afr may be all over the place anyways.

Here's something else that's interesting: The k03 has always made wooo noises when driving, but never idling. When the vehicle is running well, the turbo is quiet. However, when it's running rough, it woos when idling. It might be having trouble maintaining a steady idle, and if you blip the throttle a little it'll almost stall, but then it recovers, and it will idle smooth unless you mess with the throttle. But even when idling smooth, the turbo is still wooing for some reason. But then, turn the key off and restart the car, no more wooing and it runs fine.

And I just realized I never described the symptoms very well lol.
The most common symptom is lurching when leaving a stop. The car will just "chug" along, going between trying to accelerate and just completely stopping fairly quickly. If it was a tempo it might be roughly 60bpm, maybe slightly less. It does this regardless of throttle position. If I try to leave the stop gently or WOT it doesn't matter. But sometimes, if you're patient and let it buck around for a little, it'll just take off and then be fine for a little.
While cruising there is almost no issue at all, a "hiccup" every once in a great while. When tipping the throttle in when there's a hill approaching the car may stumble but it is mostly ok. It's really just leaving a stop that's the biggest issue. The car has stalled twice while attempting to leave a stop.
Idle is usually ok at first too, but it will occasionally idle real rough and, as I mentioned earlier, if you blip the throttle it has a really hard time stabilizing afterwards.


I cleared the codes after I replaced the hose I mentioned off the N75 a couple days ago and haven't checked to see what has come back, but I will tomorrow. Sine I replaced that hose there have been two instances where this problem has occured. Both times restarting the car has resolved it temporarily.


Any help is greatly appreciated. I'm a bit stumped and I'm not sure what direction to go and it's hard to diag anything when it's so intermittent. I'm having a hard time getting my mind off of ignition since it all started with a misfire but the ecu doesn't recognize any misfires. I haven't checked fuel yet but I'm not really sure what would be so intermittent, yet cured by restarting. I did check vacuum before I replaced the N75 hose and it was at -10inHg when idling smooth, I haven't rechecked it but I'll have to do that. This is just a DD, not modified at all, so there's no boost gauge.

Again, I appreciate any insight.



-Joe

Seerlah
08-04-2013, 07:16 PM
Cliff notes?

RENOxDECEPTION
08-04-2013, 07:26 PM
Overboosting could be the wastegate not opening. maybe due to a torn hose between N75 and the wastegate, or a faulty n75. my $0.02

boostfix
03-14-2016, 09:40 PM
Joe,
Did you ever solve your problem as described by your quote "The car will just "chug" along, going between trying to accelerate and just completely stopping fairly quickly. If it was a tempo it might be roughly 60bpm, maybe slightly less. It does this regardless of throttle position. If I try to leave the stop gently or WOT it doesn't matter. But sometimes, if you're patient and let it buck around for a little, it'll just take off and then be fine for a little.".
If so, what was the problem as this describes exactly what my B5 1.8T is doing at times.
Thanks...Dan

zevond
03-15-2016, 01:25 PM
It sounds like the same problem I was having, then I replaced my MAF and the stumbling when trying to leave from a stop went away and also it fixed my rough idle. Mine was intermitted, but started to come more often until I replaced it.

walky_talky20
03-15-2016, 02:13 PM
repaired this cracked hose with electrical tape, which apparently just happened to finally give at the same time the car started misfiring. What are the odds of that?

Misfire = engine kicking about = stress on L hose. Mystery Solved. Next.


This makes me think it's some electrical, or a failing sensor.
Good thought on the failing sensor. We shall circle back to this.


I just don't know in which direction to go.
I would say up. Or forward. Both are solid choices. Onward and upward, you see.


Around 4g/s at idle, upwards of 120g/s when doing a WOT pull through 3rd gear.
Dude, just...just...Nice. This info is usually like pulling teeth. This is good stuffs right here.


I'm not sure if I should worry about watching lambda.
Good thought on the lambda situation. We shall circle back to this.


Woo noises.
Noted. "Woo".


I'm not really sure what would be so intermittent, yet cured by restarting.
I think this is key. Again, circling back.


This is just a DD, not modified at all.
Good for you. No, GREAT for you. Diagnosing somebody else's modified mess is always 100x harder (or impossible). When it's stock we actually have some baseline expectations and can be more helpful.

Oh, almost forgot to circle back!
So we have to ask ourselves, what "resets" when you shut it off -- such that it is running for crap, then fixes itself when restarted? My guess is the oxygen sensor. When you start it, the car will run off the map for a bit until it is certain the sensor is warm, then it switches to closed loop. Shutting the car off and restarting will probably reset this timer.

WalkyTheory #1: Oxygen sensor was fouled by raw fuel due to the all the misfiring. Now it gives false readings that cause it to run either really rich or lean (not sure which one). As such it has a lumpy idle and is absolutely gutless off the line.

WalkyTheory #2: Forthcoming once you shoot down the first one. ;)

Note: I am not a mechanic. I do not fix cars for money. I have troubles fixing my own car. I do not suggest you go out and buy a new oxygen sensor just because I suggested it (even if I had the audacity to label it "WalkyTheory #1" - I mean what kind of pretentious douchebag refers to themselves in the 3rd person?). What I do suggest is that you consider a faulty oxygen sensor and perhaps take some steps to look into that possibility. I might look at fuel trim data. I might try just unplugging the sensor and driving it for a bit to see if that fixes your problem. Not conclusive, but gives you some more data.

SN95Audi87
03-15-2016, 04:28 PM
This is why I love the forums. When you get the problem fix post up results.