Puzzling Z4 35i jerkiness, rough/hunting idle at low RPM

J

jingai

Member
Hello,

I am just about at my wits' end on this one. Have for a while had as the subject describes, but recently got 29e0/29e1 codes which led me to finally try to diagnose the issue. LTFTs were pegged at 30% and obviously fuel economy was... bad. Initially suspected vacuum leak, so I smoke tested the system and found a tiny leak at the charge pipe, which I then replaced with VRSF charge pipe. Subsequent smoke test revealed no more leaks.

Then replaced the (already fairly new, less than 2K miles) coils and plugs. Symptoms persisted. Bit the bullet and did a new set of index-12 Vitesco injectors and had it walnut blasted at the same time. Fuel economy mostly back to normal now, but the jerkiness/bucking and rough/hunting idle at low RPM persists..

Everything done so far in a list:

1. Smoke test, fixed small leak, smoke test again,
2. New coils and plugs,
3. New injectors,
4. Walnut blast,
5. Reset all adaptations,
6. Checked O2 sensors,
7. Checked HPFP pressure under load,
8. AFRs match now and LTFTs around 5% after injectors.

Tried turning A/C off completely in case it was the compressor drawing a bunch of current, but no change.

Am at a loss now.

Anyone have an idea where I could look next? Have thrown a fair bit of money at the problem, so trying to be as methodical about this as I can. At this moment, there are no codes thrown.

Appreciate any help anyone can provide.
 
Several rounds with AI and I got these suggestions, which sound reasonable:

1. Valvetronic limit‑stop learn
2. Remove oil filler cap to test PCV
3. Swap VANOS solenoids, monitor cam deviation

Do I need ISTA for #1 or is it possible to do with mobile app like Bimmergeeks ProTool or MHD?

For #2 what would be the fix?

For #3, does anyone know where I can get new solenoids that aren't stupid expensive? lol
 
Just in case it helps anyone else, the following solved all of my problems:

1. Pulled and cleaned the VANOS solenoids. No more rough/hunting idle. I think the carbon blast might have contributed to this.

2. Shift bog fix was accidentally checked in MHD.. unchecked it and did valvetronic diag cycle (ignition 2, throttle to the floor 10 times within 15 seconds, start car). Shifts much more smoothly again and no more bucking at low throttle.

3. One or more injectors were definitely bad. Fuel economy good again.

Not the cheapest fix ever, but not too bad relatively since I was able to sell the old injectors for a decent amount. $3K all said and done, including upgraded charge pipe.
 
Used to be car fixer unit near work, guy specialised in Land Rovers.
He bought off old boy Rover 75.
Only had it a day ran poor.
Asked a friend in the trade and was told it was worn or stuck injector.
So all the pressure dumps through it and starves other injectors.
So found replaced injector, all good.
Apparently common on Rover cars.
Must be same all injector cars.
 
So when you said injectors were bad..those were the originals or the index 12 ?
 
B21 said:
So when you said injectors were bad..those were the originals or the index 12 ?
Click to expand...
Originals were index 12 as well. Was a little surprised to have issues with them at only 50K miles, but 29e0/29e1 codes along with fuel economy at 13.8 MPG.. New injectors are Vitesco index 12. No more codes and fuel economy back to 20 MPG.

Probably only one injector was bad, but I was over trying to find the problem and just bought a whole set from FCP Euro. So at least have lifetime warranty now. Plus managed to sell the old injectors for $800, so not too upset about it.
 
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jingai said:
Just in case it helps anyone else, the following solved all of my problems:

1. Pulled and cleaned the VANOS solenoids. No more rough/hunting idle. I think the carbon blast might have contributed to this.

2. Shift bog fix was accidentally checked in MHD.. unchecked it and did valvetronic diag cycle (ignition 2, throttle to the floor 10 times within 15 seconds, start car). Shifts much more smoothly again and no more bucking at low throttle.

3. One or more injectors were definitely bad. Fuel economy good again.

Not the cheapest fix ever, but not too bad relatively since I was able to sell the old injectors for a decent amount. $3K all said and done, including upgraded charge pipe.
Click to expand...

At last some real world live actual experience for a change…!! 😃

Thanks for posting this jingai, good info, glad you’re sorted.

How much sound difference have the VRSF pipes made…? Can you feel the turbos spooling up any quicker..?
 
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john-e89 said:
How much sound difference have the VRSF pipes made…? Can you feel the turbos spooling up any quicker..?
Click to expand...

I only did the charge pipe and really mostly because it was cheaper than the plastic OEM part. It's nicely constructed for sure, but it didn't really make a difference in sound. I'm sure it could handle more boost as well (it's pretty beefy), but I haven't yet done catless downpipes.

Would love to do downpipes so I could move to stage 2+, but a) I've spent enough money for now, and b) tbh, the car is fast enough for my needs already. I'm at speeds that could land me in jail within seconds as it is lol.
 
jingai said:
I only did the charge pipe and really mostly because it was cheaper than the plastic OEM part. It's nicely constructed for sure, but it didn't really make a difference in sound. I'm sure it could handle more boost as well (it's pretty beefy), but I haven't yet done catless downpipes.

Would love to do downpipes so I could move to stage 2+, but a) I've spent enough money for now, and b) tbh, the car is fast enough for my needs already. I'm at speeds that could land me in jail within seconds as it is lol.
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Catless down pipes only add 10 BHP ish so not very cost effective ..unless you’re going big turbos
 
B21 said:
Doesn’t make much difference between stages..it’s still 10 BHP ish
Click to expand...
Kind of curious then: why does MHD recommend downpipes for stage 2+ if it equates to just 10 BHP? And, in your opinion, is that recommendation legitimate? That is, in your opinion, would stage 2+ be OK without them?

Obviously I wouldn't hold you to anything if my car bursts into flames, but kind of curious now :)
 
jingai said:
Kind of curious then: why does MHD recommend downpipes for stage 2+ if it equates to just 10 BHP? And, in your opinion, is that recommendation legitimate? That is, in your opinion, would stage 2+ be OK without them?
Click to expand...
B21 will know more than me but i'm pretty sure downpipes are required because the stage 2+ tune (boost, ignition timing, fueling etc etc) is mapped around improved exhaust flow. If you throw on a stage 2+ tune without downpipes or high-flow cats, then the tunes assumptions are off from the get-go, and then you are looking at exciting things like hotter temps followed by the turbos going. :geek:
 
jingai said:
Kind of curious then: why does MHD recommend downpipes for stage 2+ if it equates to just 10 BHP? And, in your opinion, is that recommendation legitimate? That is, in your opinion, would stage 2+ be OK without them?

Obviously I wouldn't hold you to anything if my car bursts into flames, but kind of curious now :)
Click to expand...
The de catted pipes only really have a significant effect on torque / BHP when you really start to push things…using E85 is another issue..

The increased back pressure from catted pipes does cause higher exhaust temps and like FMIC it depends on where and how you are using the extra power…I’ve done dozens (over 50 maybe closer to 100) on both my 20i and 35is over 10 years with all sorts of air boxes/downpipes/fuels/turbos/tuns/etc etc

In the UK with limited hot temps the cost / reward equation is out on decat pipes / fmics / airbox mods etc..

IMHO far far better to make sure your filters are premium and fresh…decent PAO/ester engine oils and all your coils/plugs/injectors are spot on..

And of course a walnut blast first ..

Remember the engine is just an air pump..
 
B21 said:
In the UK with limited hot temps the cost / reward equation is out on decat pipes / fmics / airbox mods etc..
Click to expand...
I am in a humid subtropical area where temperatures are regularly above 30C.

B21 said:
IMHO far far better to make sure your filters are premium and fresh…decent PAO/ester engine oils and all your coils/plugs/injectors are spot on..

And of course a walnut blast first ..
Click to expand...
Yep, did all of that first. Also have upgraded FMIC due to the high ambient temps we get around here.

But are you suggesting stage 2+ would be potentially OK without decatted pipes? As I said, I'm pretty happy with the performance as it is, but just kind of curious.
 
jingai said:
I am in a humid subtropical area where temperatures are regularly above 30C.


Yep, did all of that first. Also have upgraded FMIC due to the high ambient temps we get around here.

But are you suggesting stage 2+ would be potentially OK without decatted pipes? As I said, I'm pretty happy with the performance as it is, but just kind of curious.
Click to expand...
Yes should be fine..if you're 'hot' a bigger 5" stepped or 7.5 " stepped cooler more important..much..
 
jingai said:
Yeah, given the climate a CSF stepped intercooler was one of the first things I did after I bought the car.
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I saw some minor improvements moving from stock to Wagner evo 1 to evo 3..but then again ..Scotland!

If you live in a place where ambient is 25c+ then E89 N54 temps start to climb really badly when driven hard without prodigious heat management!
 
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