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R and S sensors are brand new Bosch. I swapped those in a couple of days ago and I regapped them to approx 1.1mm per Peep's advice. That exercise had basically no effect on this performance issue as I had similar behavior before and after the replace and regap. The issue is occurring in 2nd and 3rd gear; I cant try it in 4th or 5th without fear of losing my license...
I changed the view of the logs of my post in 201, and there, in fact, no trigger error. Below are two screenshots - one right before the cutout, and another immediatly immediately the rpm dropping to 0. The spark angles do look way off vs the base tune map which is embedded in the lower right corner of each pic. I'm perplexed. Maybe overwhelming the coil and it is giving up momentarily?
I have not tried a new coil yet. I may do so, but I feel a bit that I am throwing parts at this as I swapped in a new WB02 and speed and reference sensors in the last few weeks. The existing coil is stock and likely has a lot of miles, but I was hoping to get some better direction before buying new parts.
Thanks again for the log info.
In the latest logs I see MAT at negative 100C, which is basically the same as having no sensor connected. This is an electrical issue. Check the wires, connectors and sensor. An easy test is to connect the logger, position your laptop so you can see it while you're under the hood, key on-engine off, then wiggle each wire, connector etc until you see MAT spike/ drop off.
Also, the engine rpm dropped to zero, which should be impossible if your description of events is accurate. I suspect another electrical issue between the VEMS and your SR sensors.
MAP sensor also dropped to zero kpa, which is absolute vacuum and not a real reading. Is the map and mat sensors powered by the same wire?
Thanks again for the log info.
In the latest logs I see MAT at negative 100C, which is basically the same as having no sensor connected. This is an electrical issue. Check the wires, connectors and sensor. An easy test is to connect the logger, position your laptop so you can see it while you're under the hood, key on-engine off, then wiggle each wire, connector etc until you see MAT spike/ drop off.
Also, the engine rpm dropped to zero, which should be impossible if your description of events is accurate. I suspect another electrical issue between the VEMS and your SR sensors.
MAP sensor also dropped to zero kpa, which is absolute vacuum and not a real reading. Is the map and mat sensors powered by the same wire?
I agree. The 944 is prone to sensor ground issues at the best of times (I have seen all kinds of issues with poor bellhousing grounds on stock 944s and 951s at my shop over the years), and I suspect that something with the VEMS system combined with typical 30+ year old 951 wiring issues manifests itself in unique trigger issues with the 2-3% of cars mentioned by Porvair.
My VEMS-equipped '86 951 with 170K miles and all original DME wiring harness runs great, both with older Bosch speed/ref sensors and recent cheapo FAE replacements (the Bosch sensor connectors were broken). However, I am leery of the old wiring harness; I recently placed an order with Lindsey for a few parts and had them throw in a new speed/ref sensor wiring harness to have on hand.
I would check and clean all relevant power and ground connections, starting with the bell housing grounds, but also battery, alternator, coil, fuse/electrical box, etc. It is entirely possible that the original Bosch Motronic system is more forgiving in terms of clean electrical signals than is the VEMS . . . .
Thanks all - I will check the grounds and sensors.
More urgent issue this morning; I decided to join some Porsche buddies for a 2 hr drive to Lime Rock for the vintage festival. I left the house with my 2 sons (age 9 and 23) at 4:45am with a plan to meet up in Queens, and got about 2 miles from home and the car just quit. Tach to 0, and no spark. I was on the Northern State Parkway on Long Island, which has some oppressive towing rules with exclusive towing franchises, so AAA could not tow me. We caught an Uber home (note: the Uber app does not really expect folks to be picked up on the side of a limited access highway) and got my Cayenne and a tow strap, and the older boy towed me back home (man, I'm too old for this crap...).
Got home and confirmed no spark. Good 12v at the coil, and I checked the coil resistance, and primary and secondary are in spec, When I test the spark however,( https://clarks-garage.com/shop-manual/ign-04.htm ) I get a very weak, pathetic spark. So perhaps the coil is part of the issue. Not sure if I can find one locally today so I may need to wait a couple of days before diving back into the morass.
And I swapped DMEs while on the side of the road but no effect. And the VEMS display on my phone does indicate 170 rpm when I crank the engine, which I would think means that VEMS is sending a signal to the coil?
And I swapped DMEs while on the side of the road but no effect. And the VEMS display on my phone does indicate 170 rpm when I crank the engine, which I would think means that VEMS is sending a signal to the coil?
It shows that the VEMS is getting a good signal from the SR sensors at that rpm. I'd focus on diagnosing the erratic sensor inputs into the VEMS. garbage in = garbage out. Your weak spark could be due to a weak battery from cranking it repeatedly on the side of the road.
BTW I tested the cranking speed on my car with a new battery and new starter and it reads 210 to 220 rpm, so you're not far off.
Thanks SamGrant951, but I think I am good for now. I ended up picking up an MSD blaster from the local speed shop.
Got home, installed the latest $part, and a big nothing. No spark. Nada. Nil. So I ordered a speed and reference $ensor harness (as that was very brittle) and new cap and rotor.
In frustration, I decided to pop in the stock (Lindsey chip)Motronic boxes and ... it started. Ran like crap as no AFM and missized injectors, but it ran.
With that, I just grabbed more wrenches, screwdrivers, and implements of destruction and reinstalled the AFM and stock injectors. Took it out for a spin and it pulls like it should up to 6500+.
Not sure of next steps. I hate to give up on the VEMS, but after leaving me and my kids on the side of the highway I'm gonna have a hard time feeling comfortable with that system in my car, especially as the factory parts work reliably.
Thanks SamGrant951, but I think I am good for now. I ended up picking up an MSD blaster from the local speed shop.
Got home, installed the latest $part, and a big nothing. No spark. Nada. Nil. So I ordered a speed and reference $ensor harness (as that was very brittle) and new cap and rotor.
In frustration, I decided to pop in the stock (Lindsey chip)Motronic boxes and ... it started. Ran like crap as no AFM and missized injectors, but it ran.
With that, I just grabbed more wrenches, screwdrivers, and implements of destruction and reinstalled the AFM and stock injectors. Took it out for a spin and it pulls like it should up to 6500+.
Not sure of next steps. I hate to give up on the VEMS, but after leaving me and my kids on the side of the highway I'm gonna have a hard time feeling comfortable with that system in my car, especially as the factory parts work reliably.
Thanks for the update. It sucks that the VEMS hasn't worked out for you, but I really feel like the issue is not something inherent to the VEMS itself, but a combination of its requirements and the marginal 30+ year-old 951 wiring. The reason that VEMS has become prominent in this application is that it is one of the few standalone engine management systems that plays nice with the unique speed/reference sensor setup of the early Bosch Motronic systems, which require an inordinate amount of processing power to accurately interpret. The VEMS seems to work great for most of us, but a small yet significant percentage of cars seem to have the exact same issues. Hopefully we can all collaborate and figure out what the issue is.
In the meantime, I went back to the basic VEMS installation manual, as I remembered it had some specific instructions about separating the ECU sensor grounds from the "power grounds" (electrically noisy components such as fuel injectors, idle control valve, etc), as outlined on pages 5-7 of this PDF manual:
My interpretation of the factory 944/951 wiring diagram shows that the sensor and power grounds terminate at the same point on the bell housing. To anyone who is having these issues, I strongly recommend cleaning these grounds to start, and looking into replacing questionable sections of the DME wiring harness. Perhaps running a separate ground for the sensitive speed/ref sensor circuit would cure some of these issues . . . . . I also wonder if "noisy" ignition wires or other EMI issues are at play.
Thanks Chris. I'm hoping that a refresh of the speed and reference sensor harness helps, and your points on noise and possible grounding issues make sense.
Before running across the inability to rev over 5200 or so I had a really annoying issue with a bouncy tach, both at idle and at speed. It was not due to varying rpm but rather a noisy signal, which I thought was perhaps a grounding or other interference issue somewhere along the line. Peep did not have any advice other than to swap the tach feed pin from EC36 pin 4 and to EC18 pin 4. That cured it, but I still notice a slight pulse in the other gauges (factory boost and fuel) which very well could be a grounding or noise issue.
I will probably leave the VEMS harness in place including the MAT and WBO2, so swapping between factory and VEMS would then be a 30-minute process. I will attack the bellhousing ground when I install the new R and S sensor harness. And then, when I feel the need to punish myself, I can try to get this working like it should.
09-01-2018, 10:44 PM
