Hesitation Bucking Issue

[A4natomical]

I tried searching numerous threads but came up empty handed.

I've had this issue ever since I put in my new fuel filter in. At a constant throttle input in just about every gear gear my car bucks and hesitates sporaticly at about the 2-3 mile mark (same spot EVERYDAY) from cold startup. It loses power and regains it quickly, it cannot keep a smooth crusing speed and it lasts for about 15 seconds or so and it won't do it again until the next 'cold' start. I was thinking about a failing fuel pump that sporatically stops delivering fuel and causing this loss in power, but has perfect A/F in logs. Maybe Maf? I'm dumbfounded right now.

some other symptoms and helpful info:
-only happens once after a cold start at about 5 minutes into the initial drive
-at constant throttle, the car will feel like its accelerating on its own just enough to cause the vacuum to fluctuate from 10mmhg to about 6mmhg and then decel
-the only engine code I am getting is an implausible signal and lower limit exceeded for the MAP sensor. (replaced)
-took it to eurocode and that was no help....

Any thoughts?

[rbj325]

I tried searching numerous threads but came up empty handed.

I've had this issue ever since I put in my new fuel filter in. At a constant crusing 3200rpm in fourth gear(which is very, very little throttle input [10mmhg]) my car bucks and hesitates sporaticly. It loses power and regains it quickly, it cannot keep a smooth crusing speed. I was thinking it could be a bad N75 valve, because at that rpm I'm right at the threshold of boost/no boost. I was also thinking about a bad throttle body, closing and opening as it wishes. Or possibly a failing fuel pump that sporatically stops delivering fuel and causing this loss in power. Maybe Maf? I'm dumbfounded right now.

some other symptoms and helpful info:

-the car stuggles to keep a steady 0 mmhg when I try to climb a small incline w/o boost.
-full throttle is normal
-after the new fuel filter, at start up the car makes a groaning noise when
cranking over.

Any thoughts?

It kinda sounds like a fuel cut. You might be right with a failing fuel pump.

[A4natomical]

Bump from phone

I also notice that it starts hesitating at the same spot during the first drive in the morning. It starts at 5 minutes into the trip according to the instrument panel. Then it goes away for the rest of the drive

[F16HTON]

Probably a vacuum issue...trace all of the lines and see what you come up with. Whan was the last time you changed plugs?

[BNZ TOY]

What DV are you running? I know you said this happens even outside of boost but I would clean it just to be safe.

[A4natomical]

Probably a vacuum issue...trace all of the lines and see what you come up with. Whan was the last time you changed plugs?

about 700 miles ago.

i'll hook up my boost leak tester and see what I get.

[A4natomical]

What DV are you running? I know you said this happens even outside of boost but I would clean it just to be safe.

034 MS. I'll take a look at it, it should be clean and lubed though because I checked it out only about 4k miles ago.

[A4natomical]

not the slightest vacuum leak

[rbj325]

Do you ever have any issues starting the car or does it fire right up everytime?

[A4natomical]

Do you ever have any issues starting the car or does it fire right up everytime?

It usually starts right up when its cold, but if its hot it struggles to start sometimes or it takes two cranks to start.

now that its cold, starts up perfect.

[tlcaudi]

Had a similar issue, mine actually ended up being spark plugs. I changed the DV, coils, etc. Come to find out it was the SP's all the time. My hesitation would hit me at about 2500, to 3200 RPM......

[A4natomical]

bump.

I went to eurocode last week and we did some logging, but i couldn't leave the car overnight for them to feel the cold start hesitation.
The only thing we were able to do was adjust timing. We increased timing from T2 to T3 on the NGK BKR7E's and continued logging, and on the first run the car couldn't go past 5k rpm at WOT without misfires. Went back to the shop, I changed out the plugs to Bosch FR5DTC's and the misfiring went away and we were able to stay at B6T3 with good CF's and much lower EGTs than I had previously.

One thing I haven't ruled out is my DV. Since I have never seen the internals of a Forge 007 or something comparable, does anyone have a picture of the inside of one. Because one thing I noticed with the 034 DV is that on the plastic part inside, there is a little hole in between the boost and vacuum sides of the diaphram. And I'm thinking this little leak is what is causing all the bucking issues because it is essentially a vacuum leak. When I squeeze the vacuum line to the top of the DV a hissing sound STOPS.

Its hard to see, but that dark spot in the middle of the white is a tiny hole. Is that normally there in a normal DV?

I highly doubt this is the issue because I had the DV for about a year before the 2.0 build and it was just fine.

[A4natomical]

BUUUUUMP!!!!

I'll keep adding my concerns. The thing the bugs me the most is that this issue arises at the SAME FREAKING SPOT every morning and lasts for about 15 seconds. I can make it go away if I let off the gas or mash the gas and boost. This issue started the DAY I put in the new fuel filter. So I'm gonna put in the old filter and see what happens.

[G Men 08]

no idea if it is the same thing or not, but when i had my 3.0 a4 i had the same bucking problem if i were to just in the car while it was cold and just go i was told it had something to do with the oil not having time enough to circulate through the engine and it caused the buck somehow.. i dont remember.. but any ways the solution to the problem was to let the car run for a few minutes before i took off and it never did it again

[A4natomical]

no idea if it is the same thing or not, but when i had my 3.0 a4 i had the same bucking problem if i were to just in the car while it was cold and just go i was told it had something to do with the oil not having time enough to circulate through the engine and it caused the buck somehow.. i dont remember.. but any ways the solution to the problem was to let the car run for a few minutes before i took off and it never did it again

The oil wouldn't have anything to do with it, thanks for the help. I let it warm up for about 2-3 minutes every morning and drive it kindly until the coolant gauge gets to operating temp.

[G Men 08]

well there goes my theory lol.. good luck

[A4natomical]

well there goes my theory lol.. good luck

did the dealer tell you that?

[G Men 08]

no, i posted it on here and that was the response i got.. it didnt do it all the time, and it really only happened when i just jumped in the car and drove without letter her warm up

[AvantinUSP]

Were you hearing fuel pump moans before you replaced the filter? My guess is a failing fuel pump. Mine started the moaning which told me to replace the filter. Ran for a few months and every now and then i would lose throttle and engine would rev up and down then everything went back to normal. Eventually the pump just died and left me stranded on the road. Replaced the pump and haven't had any problems since.

[A4natomical]

Were you hearing fuel pump moans before you replaced the filter? My guess is a failing fuel pump. Mine started the moaning which told me to replace the filter. Ran for a few months and every now and then i would lose throttle and engine would rev up and down then everything went back to normal. Eventually the pump just died and left me stranded on the road. Replaced the pump and haven't had any problems since.

hmm. that is a possibility I haven't over ruled yet. The noise AND symptoms started the DAY I put the new filter in on its first crank. So this morning before work I put the old filter in (didn't throw it away, thank god) but I haven't had the chance to take it for a drive. Hopefully tomorrow when I take it for a test drive everything goes back to normal.

SIDE NOTE: the new filter was NOT OEM. It is from pure ms. So I'm thinking it is not the right filter because I believe there are 3 or 4 different types for OEM cars.

And also, today I drained the old filter of the gas that was still in it and the gas was crystal clear w/ no contaminants.

[A4natomical]

I finally had the chance to start the car with the old filter back in and the groaning noise is gone and the pump/filter are dead silent. I haven't had the chance to take it for a drive, but I'm fairly certain the problems will be gone as well because the sound and hesitation issues started at the same time when the new filter was put in.

[A4natomical]

fvckk this thing!!!! like the post above this one says, putting back the old fuel filter removed the start up sound, but the hesitation issue is still going on! and it's really getting on my nerves. I scanned the car for what seems like the 100th time and I've had a MAP intermittent code and MAP implausible signal, lower limit exceeded. So I went down to the dealer on friday hoping a new MAP sensor would clean things up, but there is no change.
Today I took the car out and in the same spot (about 2-3 miles up the road) the car starts bogging/hesitating/regaining power, then losing power, so I push the pedal down just a little to get out of the hesitation and as soon as I shift I have a backfire that sounds like a shotgun(I almost always get a backfire when this happens as well). So IMO I still think it is timing/fuel related. But the thing that bugs me the most is, why does it happen in the SAME SPOT EVERYTIME during the first startup drive of the day or a cold startup (1200rpm idle at startup)

At WOT today I noticed once I get to somewhere between 5000-6000rpm power kinda dies and boost goes down to 12psi ish... but from 3700-5000k rpm, power delivery is strong and boost holds where it should be (~22psi) then drops to 12psi at around 5200k or so...


Why can't my revo/gtrs combo be as reliable as it should.....

[old guy]

One clue to your problem is that it occurs at precisely the same time after a cold start. Consequently this sounds like programmed event. One thing that occurs shortly after a cold start is the leak detection diagnostic. Here is what happens: The ECU closes the N80 evap purge valve and then it pressurizes the tank to 30 milli-bars using the LDP (leak detection pump) and measures the rate of bleed down. The LDP is a vacuum operated pump. I suspect you have a problem with this diagnostic event. You may have a faulty LDP that is creating a significant vacuum leak when it is attempting to operate.

You can isolate the LDP to see if it cures the 5-minute stumble. There are two hard plastic vacuum lines that run from underneath the manifold to the left front fender. One of them goes to the vacuum canister and the other one goes to the LDP. Both should have a quick disconnect on them. The line going to the vacuum canister is tied in to the N249 and the N112 valves underneath the manifold. IIRC the one going to the LDP only has a check valve and no other connections coming from it. Disconnect the one going to the LDP and be sure to block off the open line coming from the vacuum source (manifold) and see if this cures the problem with the next cold start cycle. Be aware that this will probably throw some evap codes. If it doesn’t cure the problem let me know since the problem could still be with the leak detection diagnostic and there are other things you can try.

Good luck

[Sales@RAI]

Old guy has a good hypothesis.

Usually when a misfire is occuring or a missed spark you can trace it.

- fuel pump
- fuel filter
- plugs
- coilpacks
- ground for cps
- MAF

I would check all of those. Obviously it could get expensive, I would see if you could trade with a local guy one thing at a time. The plugs might be good, but I would do te 7Es again and back the gap down.

How much have u logged? Is anything noticable from the logs? MAF readings, misfires, etc?

[A4natomical]

One clue to your problem is that it occurs at precisely the same time after a cold start. Consequently this sounds like programmed event. One thing that occurs shortly after a cold start is the leak detection diagnostic. Here is what happens: The ECU closes the N80 evap purge valve and then it pressurizes the tank to 30 milli-bars using the LDP (leak detection pump) and measures the rate of bleed down. The LDP is a vacuum operated pump. I suspect you have a problem with this diagnostic event. You may have a faulty LDP that is creating a significant vacuum leak when it is attempting to operate.

You can isolate the LDP to see if it cures the 5-minute stumble. There are two hard plastic vacuum lines that run from underneath the manifold to the left front fender. One of them goes to the vacuum canister and the other one goes to the LDP. Both should have a quick disconnect on them. The line going to the vacuum canister is tied in to the N249 and the N112 valves underneath the manifold. IIRC the one going to the LDP only has a check valve and no other connections coming from it. Disconnect the one going to the LDP and be sure to block off the open line coming from the vacuum source (manifold) and see if this cures the problem with the next cold start cycle. Be aware that this will probably throw some evap codes. If it doesn’t cure the problem let me know since the problem could still be with the leak detection diagnostic and there are other things you can try.

Good luck

thank you so much for this post because I was fully unaware that this took place and it totally makes sense. During the rebuild of my motor I followed the steps posted by 'absolutegtr,' on the evap/vacuum line removal. I pm'd him about those two plastic lines and he said they could just be removed and left AT the quick disconnect (thus leaving them both open to the atmosphere/vacuum leak). So if I reroute those two lines back into the intake tract I (in theory) should get rid of the problems?

[A4natomical]

Old guy has a good hypothesis.

Usually when a misfire is occuring or a missed spark you can trace it.

- fuel pump
- fuel filter
- plugs
- coilpacks
- ground for cps
- MAF

I would check all of those. Obviously it could get expensive, I would see if you could trade with a local guy one thing at a time. The plugs might be good, but I would do te 7Es again and back the gap down.

How much have u logged? Is anything noticable from the logs? MAF readings, misfires, etc?

truth, it has shown to add up pretty quick $$. Plugs made the car run better actually. I'll try the old guy hypoth and see where that gets me.

and about how all those little things add up. I stopped by the dealer today to grab all four exhaust gaskets (manifold/head, manifold/turbo, turbo/cat, cat/downpipe) to replace them this weekend, along w/ an oil change to get rid of my obnoxious exhaust leak that I can't get rid of by tightening any of the bolts. I guess I have a fun weekend ahead of me.

[old guy]

For starters you should definitely block off those two hard plastic vacuum lines. Leaving them open is just creating a vacuum leak. I am assuming that you do not want your SAI system or your N249 to function. Personally I recommend that you reattach the one running to the LDP. Then make sure you have a proper connection from the N80 evap purge valve to a vacuum source. It is critical that you have a check valve between the N80 and the vacuum source. There is really no benefit to disabling the evap/tank purge systems and some definite safety issues by disabling it. You may find that this cures your daily stumble.

[A4natomical]

These must be the two plastic lines you are referring to. During my build I asked what I should do with these two lines and I didn't get an answer, so they both were left not hooked to anything and are still in that state. Should I hook them up to a vacuum source or plug them?

[old guy]

Plug them. Otherwise you will have a vacuum leak.

[[email protected]]

Results from plugging those lines?

[A4natomical]

Results from plugging those lines?

Well we'll find out this weekend (if halloween doesn't get in the way) while I change all my exhaust gaskets and change my oil, I'll tackle these two lines while I'm in there.

[[email protected]]

Well we'll find out this weekend (if halloween doesn't get in the way) while I change all my exhaust gaskets and change my oil, I'll tackle these two lines while I'm in there.

Cool...def post up the results

[old guy]

Forgive me while I rant a little. this is not directed at anyone in particular, but rather just a general comment. If you are going to start ripping out vacuum lines it is in your best interest to take the time and effort to figure out what they do and the effects of removing them before you do so to avoid a lot of unnecessary problems.

Now on to your problem. You have disconnected two different hard plastic vacuum lines. The straight one connects to the vacuum canister. If you are not using the N249 or the N112 to operate the combi valve you can just disconnect and plug that line. If you don't plug it you will have a vacuum leak. How do you think the vacuum canister gets charged? By having a vacuum pulled on it!!

Now about the one with the angle after the disconnect: That line serves two functions. It provides a vacuum source for the LDP and it also provides a vacuum source for the evap system. You need this to operate the N80 as well as the LDP. I recommend that you reattach this line. The N80 also has a secondary connection. The line described above provides the vacuum when you are off boost. Once you start boosting the check valve stops the boost signal from reaching the N80. At that point the second connection to the N80 provides the required vacuum. That line splits off downstream of the N80 and connects to the TIP. I strongly recommend you reattach that complete circuit. This should be done to provide the proper ventilation required for your fuel tank. Running an open tank vent isn't a very smart move. I am pretty sure if you do this you will cure your problem. Just plugging the two lines will not accomplish what you need to do. You have already disconnected the LDP but the other end of the system (N80) is still trying to perform the leak down test. Do it right.

[NightRoller USP]

Does it feel almost like a drivetrain bump????? If it does then I get it too.....I drives me NUTS!!!

[A4natomical]

Forgive me while I rant a little. this is not directed at anyone in particular, but rather just a general comment. If you are going to start ripping out vacuum lines it is in your best interest to take the time and effort to figure out what they do and the effects of removing them before you do so to avoid a lot of unnecessary problems.

Now on to your problem. You have disconnected two different hard plastic vacuum lines. The straight one connects to the vacuum canister. If you are not using the N249 or the N112 to operate the combi valve you can just disconnect and plug that line. If you don't plug it you will have a vacuum leak. How do you think the vacuum canister gets charged? By having a vacuum pulled on it!!

Now about the one with the angle after the disconnect: That line serves two functions. It provides a vacuum source for the LDP and it also provides a vacuum source for the evap system. You need this to operate the N80 as well as the LDP. I recommend that you reattach this line. The N80 also has a secondary connection. The line described above provides the vacuum when you are off boost. Once you start boosting the check valve stops the boost signal from reaching the N80. At that point the second connection to the N80 provides the required vacuum. That line splits off downstream of the N80 and connects to the TIP. I strongly recommend you reattach that complete circuit. This should be done to provide the proper ventilation required for your fuel tank. Running an open tank vent isn't a very smart move. I am pretty sure if you do this you will cure your problem. Just plugging the two lines will not accomplish what you need to do. You have already disconnected the LDP but the other end of the system (N80) is still trying to perform the leak down test. Do it right.

I 100% fully agree to your opening statements. You probably know this if you have read through my build thread, but my dad being an engineer, he always asks if I have any schematics he could study before we/I start removing anything that had a purpose in the car to understand what is going on and what is wanted to be done to see if it makes sense............because his famous words to me are, "and you believe everything you read on the internet?".....

I have the N80 on a vacuum source btw...I didn't follow that part of the DIY....didn't make sense.

I'll have to plug the quick disconnect 90* to a vacuum source with a check valve and plug the other 'longer' plastic line.

[F16HTON]

FWIW I have zero vaccum lines disconnected on my car

[A4natomical]

Shown in this picture. The N80 valve (hard line) is hooked into the intake manifold back where the brake booster is. The line off the N80 that went to the intake box has been plugged.

[A4natomical]

FWIW I have zero vaccum lines disconnected on my car

I know. I'm starting to regret taking them all out when I did. I should have just removed the ones that needed to be removed (aeb head)

[old guy]

I have the N80 on a vacuum source btw...I didn't follow that part of the DIY....didn't make sense.

I'll have to plug the quick disconnect 90* to a vacuum source with a check valve and plug the other 'longer' plastic line.

Your N80 should have two vacuum sources, both protected with a check valve. One runs down to the TIP. This provides a vacuum source when you are on boost. The other one connects to the manifold to provide vacuum when off boost. In the original configuration there was a vacuum line coming off the manifold just behind the throttle body that connect to the N80 as well as the LDP. The quick disconnect was the leg running to the LDP. If you have all the original components I would recommend hooking that part back up and see what happens.

Good luck!

[Sales@RAI]

I love that mine are all gone