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Has anyone come across erratic power and loss of lambda signal under hard acceleration? I experienced what felt like a rev limiter along with erratic lambda readings (lambda maxed out at 1.83) under hard acceleration, with rpm north of 5k and higher boost (180+ Kpa). Car drives fine otherwise, but it will not pull past 5k or so.
I tried to recalibrate the 02 sensor and ultimately just replaced it with a new Bosch unit. I just came back from a drive and am experiencing the same issue - it feels like a rev limiter is kicking in at 5k or so and the data log indicates that lambda is spiking up to 1.83 (which Peep said is equivalent to the 02 sensor no longer communicating with VEMS). FWIF, I also upgraded the injectors to 55lb/hr after replacing the o2 sensor but that does not seem to help. And I have confirmed that the rev limiter in VEMS is set at 6699, so that is not the problem.
Thanks Spencer. I gave that a try and no difference.
At Peep's suggestion, I re-gapped the speed and reference sensors (great fun) to the high end of the spec (I'm about 1.2mm now) but no difference.
Originally I thought this was a fuel issue as I was focused on the lambda, as that spikes upon this problem occurring (and recovers in 30 sec or so) but I am now thinking that the base problem may be an ignition issue. The engine revs to redline in neutral, but as I approach 5k under boost, I see the revs (per the VEMS log and the dashboard tach) suddenly drop to zero for a brief moment, with a corresponding loss of power and a few ugly exhaust pops.
Plugs are new and wires (Clewett) and cap are 3 yrs old with < 10k miles. If one of those were an issue I would expect a degradation of ignition, but not an abrupt cut.
Possibly a coil issue? Anytime I had a bad coil, the car just stopped as opposed to acting like it is hitting a wall from an ignition standpoint.
I'm corresponding with Peep who is very helpful, but also wanted to throw this out to the community as any suggestions are welcome.
Image of one of the logs is below.
Last edited by Porvair; 08-31-2018 at 08:49 PM.
Reason: add pic
What is the RPM at the 50 timestamp? It looks like you hit the rev limit at that point. It's also odd that TPS is tapering downward, maybe you are tripping the BOV to open and causing a surge?
Looking at that log, everything drops to zero at the exact same point (ignition, fuel injectors, and TPS), which to me indicates an issue with the DME power supply and/or sensor ground circuit.
I would check the typical 944/951 bugaboos, the sensor grounds at the bell housing, DME power supply circuit, etc.
maybe you are tripping the BOV to open and causing a surge?
I like the line of thought here; have seen that exact same symptom and cause at higher load (Granted an entirely different setup but I think this premise deserves some merit).
Good morning all and thanks for all of your ideas.
The rev max @ the 50 timestamp was 5,392, and the drop is the TPS was me backing off the throttle as the ignition cut is very abrupt. I have similar logs where I was at 100% TPS and it typically cuts somewhere between 5,000 and 5,200. I back off the throttle when that happens as it is very abrupt and not as smooth as a "normal" rev limiter given that this appears to be a full cut of the spark and not just holding it at the limit.
I will check all of the grounds, and I'm running a solid state DME relay so will swap in the original to see if that helps.
All that said, I hear from Peep this morning and he believes that It is a trigger error, and when VEMS sees the trigger error it cuts the ignition. He sent my info and a request for a solution to VEMS hq because he has seen that it happens totally randomly on 944 turbos, around 2-3% cars have this issue, others work just fine.
I do have a track day @ Pocono coming up in a few weeks. I'm not racing or out to set a lap record, so I can either short shift @ 5k, or may revert to stock, which I prefer not to do as its a PITA and like all of the folks on this thread I've invested time and $ and hate the idea of undoing that. Or replace the all season tires on my MK7 GTI and run that, but I really prefer the 951 as it is so visceral on the track.
Can you display the trigger error on your log? It’s an option to display that as well as the other stuff so you can easily see a correlation.
Which s and r sensors are you using?
Yes, VEMS can display trigger errors in the VEMS log (File/Open Log file).
I wonder, why in the posted VEMS log above (post #201) there is only 19.00° spark angle @ 4445RPM and no further advance @ 5000RPM. Therefore I would check the RPM/MAP Ignition table, if there is any error.
Edit: The VEMS log above (post #201) shows the view mode "File/Open Log file to separated frame" which doesn't show trigger errors by default, but you can enable this in the edit mode.
08-26-2018, 01:12 AM
