Overboosting / Wastegate Cycling

[garciapiano]

I'm having an issue where the engine hesitates under full throttle (full boost). The turbo will "cycle" and the boost will pull back, almost as if it's bouncing off a limiter. This causes some minor herky-jerkiness in the torque response as the turbo seems to backcycle.

This wasn't an issue before I swapped my turbo for another used OEM turbo. I have a Malone stage 2 tune, but aside from that all the rest of the engine is stock, with the stock turbo.

My guess is that the wastegate actuator may be to blame, or perhaps it may be something else like the N75 or the MAP sensor. I have no codes. The wastegate actuator adjustment has not been touched. It still has the factory paint markings on the actuator shaft.

The car actually went into an overboost limp for the first time ever on a long road trip the other week. I was at a constant 75MPH for about 2-3 hours and I went full throttle to overtake a car, after which the car decided it didn't want to boost anymore. Perhaps due to boost creep.

Limp mode was alarming given I had never experienced it before. I figured that there would be some kind of indication or CEL but apparently there isn't. The car started up and drove normally the next day.

Anyone know what I need to check to fix this?

 

[Steve Addy]

Check your turbo WG actuator to make sure it's free and moving as it should. These get dry and can end up sticking, which causes the erratic boost conditions...and limp mode.

Steve

 

[ToddA1]

Lubing and exercising the wastegate lever and replacing vacuum lines solved my limp mode issues. It only occurred at 70-80mph.

I‘d occasionally get the intake manifold pressure code (1550?), but not always. If I really beat on it, I could get it at lower speeds, but I don’t typically never did that… only when testing the fix. It’s in the what did you do thread, but good luck finding it, lol.… probably 2 years ago.

-Todd

 

[burpod]

could easily be a MAF issue. can also be cause by clogged egr. the tune you have will likely exaggerate the issue. the smoke limiter is at play here helping cause the oscillations...

taking an 011+001+003 log would point you in the right direction

limp mode will happen if the boost deviations happen for long enough with enough deviation.

 

[garciapiano]

This is when I facepalm and kick myself for not checking the wastegate duty cycle with the turbo off the car… like it was for months while I waited for parts for my engine rebuild….

very difficult to get to that actuator rod/lever with the turbo installed… but not impossible I suppose.

 

[Steve Addy]

This is when I facepalm and kick myself for not checking the wastegate duty cycle with the turbo off the car… like it was for months while I waited for parts for my engine rebuild….

very difficult to get to that actuator rod/lever with the turbo installed… but not impossible I suppose.

It's not a fun task if the engine is hot....but I managed to fix the actuator problem on a friends car in a parking lot by driving the right wheel up on a curb and crawling underneath and then using a 12" adjustable crescent wrench to work the actuator back and forth.

I burned myself more than once but got it done so I was happy...

Steve

 

[garciapiano]

I got under the car today and it’s not the wastegate… it moves freely. I’m starting to think it’s a vacuum leak preventing the wastegate actuator from functioning properly. The booster plastic vacuum line cracked a while back, and though I have repaired it with a piece of hose, a spring and a couple hose clamps, I think I still have a small leak somewhere.

I’d love to fabricate something a bit more robust than the hard plastic stock piece but I couldn’t find any suitable materials at Home Depot Motorsports after checking multiple places.

 

[ToddA1]

Have you verified all the hoses to/from the N75 are clear? Disconnect both ends and blow through the tubing, one at a time. Is the N75 working properly?

-Todd

 

[garciapiano]

Have you verified all the hoses to/from the N75 are clear? Disconnect both ends and blow through the tubing, one at a time. Is the N75 working properly?

-Todd

Vacuum lines are good. The only time I’m getting the issue is at full boost. I’m going to take some data logs and report back.

 

[Steve Addy]

Vacuum lines are good. The only time I’m getting the issue is at full boost. I’m going to take some data logs and report back.

turbo wastegate operates on pressure, check for N75 clicking, pull some numbers from vcds.

Out of curiosity what ECU are you running on your 97?

Steve

 

[garciapiano]

This turned out to be a bad N75. Threw a new one in and it now boosts normally. Parts cannon works sometimes.

Symptoms:
1. Down on power
2. Lopey, rough feeling under hard acceleration (I attribute this to overboosting/turbo not running efficiently)
3. Issues described in post #1.

In the months before the N75 finally gave out, it would buzz sometimes for up to a minute after shutdown. I have discovered that this is due to residual vacuum being let out through the diaphragm. How much of that buzzing is normal is anyone’s guess, but the buzzing had actually ceased after shutdown for several months before I finally replaced the valve. The new one does not buzz.

My guess is that the diaphragm finally gave out and couldn’t hold pressure to actuate the wastegate fully, leading to overboost.

Hopefully the new valve lasts 25 years like the old one did!

 

[ValveCoverGasket]

good to hear you fixed it with a valve!

Lubing and exercising the wastegate lever and replacing vacuum lines solved my limp mode issues. It only occurred at 70-80mph.

i was surprised how much of a difference this made on my swapped van (though to be fair the turbo sits a lot closer to the road and to a wheel in that setup) previously some really funky and irregular boost response, along with boost plausibility codes, after some lubing alls well again.

i imagine maybe that could be an issue on a mk3/b4 that was run in a lot of road grime or didnt have a belly pan?

 

[garciapiano]

good to hear you fixed it with a valve!

i imagine maybe that could be an issue on a mk3/b4 that was run in a lot of road grime or didnt have a belly pan?

The wastegate seizing is either corrosion buildup or carbon buildup. So it could be a combination of many factors.

Mostly, if it’s not getting exercised enough, it can seize up. So the best preventative measure is driving it hard on a regular basis - not letting it sit.

 

[little red rabbit]

Replying to an old thread i know.

I have an 05 Jetta with the BEW. Was getting the occasional overboost(followed by limp mode, no CEL), usually after long drive after encountering a long hill. The car sat for a while while I fixed a multitude of other issues. Bad MAF, boost leaks(seals were bad and intercooler actually had holes in it), rotted out actuator, mouse nest in the intake.

Problem was overboost got worse after the car was put back on the road after six months of sitting in the garage getting the bodywork done. Car worked fairly good when cold, but after it got up to temp, overboost on any long hill.

There was no vacuum issues, replaced all the lines anyway. Replaced the N18 and the N75. I don't have VCDS yet but going to make the investment as I am keeping the car until it dies.

When i replaced the actuator, tried moving the vane arm, loose as a goose. Thought about setting the thing on fire, but came to my senses.

Took the turbo off thinking it was probably toast and because a new one was close to being ordered, i took a rosebud to the turbo to heat it up and see if heat was an issue. YUP. actuator arm got sticky when the housing was heated up.

Took the turbo apart after lots of advice to just replace it and found that the housing had tons of corrosion where the vane ring sits in the housing. It would seem that when cold, no problem, but when hot, everything expanded just enough for the vane ring to stick in place. Pounded out the rust, did some general cleaning as there was no carbon to speak of and put er all back together. After the reassembly, spent three hours trying to get the little bugger to overboost. NOPE. as a matter of fact the car has never run better.

Hope this helps some other folks.

When all is said and done, this was probably caused by running the car with the belly pan missing. I live in an area with constant damp winter weather and tons of road salt and that crappy bit of plastic keeps all manner of garbage out of the engine bay. The outside of the turbo is so rusty that I'm curious when it will rot through. Through some investigation I found out that the actuator was replaced twice before I got the car. Probably a case of going after symptoms rather than getting to the root issue.

Cheers.

 

[Abacus]

Thanks for the help, but our turbos don’t have a vane ring since they’re wastegated turbos that operate from pressure and not vacuum. They’re about as far apart as turbos can get.

 

[little red rabbit]

Thanks for the help, but our turbos don’t have a vane ring since they’re wastegated turbos that operate from pressure and not vacuum. They’re about as far apart as turbos can get.

Thanks Abacus, I just looked for an overboost thread without checking to see which gen of car was being discussed.

 

[Abacus]

No worries, when there’s a problem any advice can be helpful.

 

[thechoochlyman]

I think my N75 may be on the fritz... I posted in my "rebuild" thread that I was having boost spikes. Lowering the IQ seemed to help a little, but maybe it's just coincidence or I'm playing tricks on myself. Depending on how hard I go on the throttle it'll spike up to like 20 or 25 then rock up and down by about 7 PSI until I let out of it. It's not nearly as bad if I don't stomp it, but still noticeable.

There's been discussion about eliminating the N75 in other threads, I just got done reading @Abacus' discussion from about 10 years ago. I've got a malone stage 3 with .216 nozzles and the stock turbo. I'm not sure what the programmed max boost is with the Malone, but I know stock is only supposed to be about 10. It seems mine will hold 15 pretty steady when working properly, but that's still not enough for the fuel I'm getting. What's the worry with keeping the boost so low with a K03, just the risk of blowing it up? I could definitely use 20 PSI if not a hair more.

I'm not sure what my over boost limit is before it throws a code, or how long I could hold it. but I'm going to try experimenting with a needle valve directly between the boost side and the wastegate side to see if I can smooth it out a bit. I'll probably do the same in front of the ECU so it thinks it's getting less boost than it really is. I have an Autometer gauge on my pillar and I check the ECU boost reading on my Scangauge.

EDIT: I just got a reply back from Malone. Apparently it should be set to about 17.6 PSI, and it would trigger an overboost around 22.

 

[burpod]

maybe they can figure it out but i've said this before and i'll say it one more time.... but tunes that don't use real IQ values will *always* have poor boost control, and always even worse the faster you mash the pedal. in order to work well, the boost control maps need IQ values to be correct (or at least reasonably close), since that's how it's calculating how to control the n75 wastegate or vanes to make target boost. if 30mg is actually 40mg, the ecu simply won't be calculating how to operate the wastegate/n75 right! (makes sense, how could it??? fuel drives boost) combine that with start of injection maps that aren't well matched to the fuel/boost, boost not well matched for the fuel, maybe some little hardware issues, etc. gets messy fast. will likely never be correctable by linear changes like IQ (and/or rod length for a vnt). might "fix" it in some areas... but the cycling of boost/n75 up/down is a very common issue when the fueling/timing is wrong for the boost and/or in case of vnt, n75 map not right as well. of course a bad n75/hardware issue isn't out of the question. sometimes a poor tune will run better on a n75 that isn't working right, just depends how the errors add up.

 

[thechoochlyman]

Well, my N75 is about 7 years and 133k miles old. So I wouldn't doubt it could be bad.

 

[garciapiano]

My boost spikes and then levels off. It can sort of wander around under 100% throttle.

My guess is that turbo is just not supposed to operate in a higher boost range so the computer is doing its best to correct for the boost that it's seeing.

I haven't graphed requested vs actual boost, but I probably should just to see what's going on.

 

[burpod]

My boost spikes and then levels off. It can sort of wander around under 100% throttle.

My guess is that turbo is just not supposed to operate in a higher boost range so the computer is doing its best to correct for the boost that it's seeing.

I haven't graphed requested vs actual boost, but I probably should just to see what's going on.

in other words, the tune isn't right. thats whats in the "computer". you should take a boost log. thats only the tip of the iceberg, but it generally will be a starting point to see what else isn't right..... boost spikes don't need to happen. the tune can control the turbo to make whatever boost it wants. whether its a safe amount or not is another matter

 

[volksguy]

Hello with all this involved what company out there. Is the Best that
Takes the IQ Value in there Tunes For The GQ ECU

 

[thechoochlyman]

in other words, the tune isn't right. thats whats in the "computer". you should take a boost log. thats only the tip of the iceberg, but it generally will be a starting point to see what else isn't right..... boost spikes don't need to happen. the tune can control the turbo to make whatever boost it wants. whether its a safe amount or not is another matter

Have you been able to complete tune-out boost spikes without compromising performance? Seems to me like there's going to be some lag in the system regardless of how well it's tuned, it just depends on whether you want to slowly ramp the boost up or ramp it down after it spikes. The N75 and Wastegate do have physical limitations in regard to their response time.

 

[thechoochlyman]

I'd totally be down to take some more logs and let you write me some sample tunes now that I've changed injectors and probably need something a little different. But in the meantime, I DO think my N75 isn't working right. I have a couple spares to try out later.

 

[burpod]

Have you been able to complete tune-out boost spikes without compromising performance? Seems to me like there's going to be some lag in the system regardless of how well it's tuned, it just depends on whether you want to slowly ramp the boost up or ramp it down after it spikes. The N75 and Wastegate do have physical limitations in regard to their response time.

not 100% for every setup... still working on it it IS possible, i know that. it's more complicated than just n75. its accuracy of the fueling, quantity of fuel, rpm, timing (and timing depending on the air/coolant temps), the target boost map, smoke limiter... probably some other stuff too but you really don't need to sacrifice performance to eliminate any signficant spikes or have good boost control. big spikes simply means one or more of the above isn't done right.

 

[burpod]

sure, try out different n75s can't hurt.

one thing i've learned is.. and i'm near 100% positive of this, large boost spikes are often a strong indicator that (among other problems) timing is too advanced... fwiw

 

[thechoochlyman]

On my way home yesterday I pulled the blue line off my N75 just to see how my car performs with the wastegate completely unhooked. Other than triggering limp mode several times, it accelerated a ton better. No smoke at all and no surging. If I was careful enough I could hold the boost around 20psi. At wide open throttle I could hold 31 PSI until I let out of it, but obviously it's not the best idea to do that very often. Lol

I really don't understand why VW didn't just use a purely mechanical wastegate system for these cars. It's not like a little extra boost than you need is going to hurt anything. I wish I could disable limp mode completely so I could mechanically tune it how I want without the ECU getting mad.

The replacement N75 didn't help anything. I ordered the hose barbs I need from Amazon because nobody local had them. Hopefully I can set the wastegate to dump at 20 psi and won't get limp mode. I actually triggered limp mode a couple of times with the N75 plumbed in just because of how high it surges when you stomp it.

 

[garciapiano]

You’ll get more boost with the wastegate disconnected, no surprise there.

 

[thechoochlyman]

Definitely not a surprise, but I'm a little shocked that it could hit 31.