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      05-13-2019, 01:51 PM   #1
pokeybritches
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Drives: ESS/G-Power Z4M, VF Z4, 996tt
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2006 BMW Z4M  [10.00]
2006 BMW Z4M  [8.50]
2003 BMW Z4 3.0i  [9.00]
Severe hesitation under acceleration - SOLVED!

I had my Z's stored while I spent a couple of years overseas. Unfortunately, my 3.0i had severe hesitation and fuel trim issues once I pulled it out of storage. Despite there being NO CODES, the O2 sensors were to blame. I hope this post helps others who may be experiencing the same issues.

The hesitation wasn't present before I put the car into storage, so I determined it was unrelated to the TTFS dyno tune. At exactly 4k rpm, the car would start bucking and porpoising, ultimately leading to a single cylinder deactivating completely (I thought I had lost the motor, because it suddenly sounded like a 5 cyl rally car). Fortunately the car was only generating 2-3 psi of boost at that rpm, and I had methanol injection to help with octane. The only codes present were generic lean fuel trim codes.

I started with replacing the DISA, since it activates at 4k rpm. This was a pain with the supercharger in place, maybe a 3-4 hour job start to finish including rebuilding the old DISA with a G.A.S. kit. The G.A.S. kit is very well made, and I recommend it to everyone who is still on their original DISA, regardless of mileage. The freshly rebuilt DISA improved low-end response, but didn't affect my hesitation issues.

My car runs 50/50 WMI, which kicks in at 4 psi via progressive control. I confirmed the meth was functional and spraying, then dropped the initial spray from 4 psi to 2 psi. No change.

Next I pulled the injectors and had them sent out to be professionally cleaned and tested through DeatschWerks. The process was fairly simple, and turnaround was 2-3 weeks counting no-rush shipping. My only (minor) issue was minimal communication... I sent the injectors off and didn't hear anything until I got an invoice a week or two later. Then again, I could have asked for updates had I been concerned. So, heads up that if you ever go this route- you'll mail your injectors off into a black hole and they will magically show up clean a few weeks later with a data sheet. Overall I highly recommend DeatschWerks, and I'm happy with the price and quality of testing. FWIW, the standard VF injectors are 30 lb/hr at 43.5 psi, and my set had a variance of 2.8%. The M54B30 uses slightly higher fuel pressure of 3.5 bar / 51 psi. Fuel injector removal is about 30 minutes if you know what you're doing, and reinstall takes about the same amount of time. Both are easy.

The hesitation issue remained. I decided to take off the front bumper and check the intake piping for leaks post-MAF and build my own over-the-radiator (OTR) intake while I was in there, since I didn't like how I had originally positioned the MAF and also plan to upgrade to the HPX MAF in the future. I ordered various piping and silicone couplers, and ended up building the contraption you see in the pictures. I found no leaks in the piping, and unsurprisingly the new intake didn't solve my issues. At least it looks nice and likely adds a good amount of power. LS-powered Holdens gain 20+ whp with OTR intakes.

At this point I decided to go all out and replace the fuel pump, filter, and fuel pressure regulator. After all, I'm running about 380 hp on a 150k mile fuel system that was meant for 225 hp. I dropped the car off at a shop for the work, as my 996tt was taking up both garage spaces and all my time. The expansion tank had also failed, and my growing frustration with the car made it best for someone else to work on it. The total shop bill was $2k, including radiator brackets and misc pieces. I was disappointed the shop told me they would upgrade the pump to an E85-capable unit, but instead they installed an OEM pump after realizing the 330i pump they ordered wouldn't fit. I'll likely have to replace it once I start running E85. FYI, shops will always blame an aftermarket supercharger/tune any time there's an issue. I had one shop blame alternator failure on a tune, and in this case they blamed hardware-related fuel trim issues on the tune.

When this didn't solve the hesitation issues, the car came very close to meeting a fiery death . There was only one thing left in the intake or fuel system that I hadn't changed - the MAF. While it was 8 years old, I'd only put about 20k miles on it. I pulled off the front bumper and swapped it out, finding only incremental improvement in the hesitation.

To troubleshoot whether the car could physically flow the required amount of fuel, I retuned the car to dump 20% more fuel in the midrange and cut the timing target to 10 deg. I'd gotten my BMW Logger data logging software to function at this point (it was VIN-locked, and VF had overlaid a 2.5i's software and VIN on their initial tune, which caused a mismatch... see the pic, as this is a risk any time you mail in your ECU to be tuned). The Z pulled buttery smooth to redline, and ran extremely rich (as expected). The hesitation issue was gone . At least I knew the fuel system could physically support the requested flow... it's just that the requested flow was somehow wrong.

In a last ditch effort I replaced the primary O2 sensors. It was the only part I could think of that controlled fuel trims which I hadn't touched. I consulted with forum member vbelsky , and he agreed... especially since the car ran pig rich with the original VF tune, which likely led to fouling. As with everything on this car's journey, replacement wasn't simple. I'd put mini-cats on the secondary O2 sensors to avoid a CEL, so space to maneuver tools was at a premium, and the primary O2's were seized in place. The shop that replaced the fuel system had wayyy overtorqued the eight 17mm panel bolts, likely via impact gun. To remove them, I wrenched hard enough to make Arnold Schwarzenegger shed a tear of satisfaction. A generous use of PB Blaster and a breaker bar later, the O2 sensors were off. I disconnected the electrical connections individually, and I used my logging software to confirm the sensors were in the appropriate banks.

FINALLY, the car ran as it should (sort of). AFR is safe but not correct, and it's pulling a scary amount of timing under boost. The MAF maxes out right around 6000 rpm. I'll fix these issues myself via road tuning. Logs attached.

rpm = white
AFR = green
MAF = red
Ignition timing = yellow

Everything that I replaced likely needed preventative replacement anyway, but if you don't want to spend $3k and 20+ hours of your own labor, start with the $130 O2 sensors.
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Last edited by pokeybritches; 05-13-2019 at 02:03 PM..
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      05-14-2019, 11:14 AM   #2
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Great news Josh.

Onto the logging my friend.
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