FRporscheman

Isn't the mixture supposed to be rich in open loop? I.e. cold-start enrichment? How does the engine run, is it smooth or rough?

I have a similar (but probably not the same) issue on a 968 so I'm reading threads trying to get ideas...

mahoney944

Cold start only:
On first startup right after it tries to start it stumbles, the rpms have a hard time building and it wants to stall. About half the time it does. When I go for the second attempt it fires up very rich and works it's way to 14.7.. when I jump the isv the ratio drops to the 12s again and the logger shows that closed loop is off. I'd say the engine is drive able but not smooth. I also had a bad fuel check valve and my fuel feed hoses we're getting soft and collapsing. I have new everything on order from the tank to the hard line that runs up front to the rail. So I'll post if that changes things when I get everything.

mahoney944

Installed new fuel pump, lines, check valve. The car holds pressure now over night. However the car runs rich at warmed up idle with the isv jumped. 11s to low 12s. The car idles at 14.7 without the isv bypassed. I can adjust the fqs to -11% to bring it up to mid 13s, It seems to help the car be more smooth. Still trying to figure out why it's so rich before the isv does its corrections.

Tom M'Guinn

Are you bypassing the ISV via the diagnostic port? There are circumstances where that also disables the O2 feedback adjustments. What happens if you just pull the connector off the ISV? Or disconnect the O2 sensor?

Droops83

"Open loop" simply means that the DME is ignoring the O2 sensor signal and running solely off the fuel map in the DME. This of course happens to be rich when the engine is cold, and gradually leans out as the engine warms up. Even if you unplug the O2 sensor, a perfectly-running 944 engine should be near stoichiometric when warm at idle and during light load/cruise conditions. The fuel table is rich during heavy load/high RPM conditions, so when the engine enters open loop as determined by the throttle switch/sensor, it simply reads off this table.

At my shop, I often unplug the O2 sensor on '80s 944s and 911s to get an idea of how the base mixture is tuned (and if there are vacuum leaks or other issues), and if the O2 sensor feedback is masking something. A really well-tuned engine with a good O2 sensor will not run very different with the O2 sensor connected or disconnected.

Of course, in the 968 the engine management system is more sophisticated and will set a "check engine" light immediately if you unplug the O2 sensor. However, it is not sophisticated enough to have an entirely different "open loop" map. The only sort of default/failsafe mode that I know of is if the camshaft position sensor is disconnected or the sensor/wiring is faulty, the timing will be retarded by 6 degrees and the DME will revert to batch-fired injectors like in the 8V 944.

mahoney944

I didn't try unplugging the isv but unplugging the O2 sensor makes it stay in open loop and it idles very rich.

Another interesting find is my car is using a new solid state dme relay, for some reason it keeps the pump on constantly when the key is on instead of just for the 3 second timer. I contacted them about it and they sent me another one to try. Oddly, it does the same thing. Looks like there's something on my car doing it.

Tom M'Guinn

The "open loop" thing on your logger is just a way of telling you when your DME is making fuel adjustments based on the O2 sensor, so if you disconnect the O2 sensor, it's "open loop" by definition and that indicator doesn't give you an useful info (other than that the logger is working I guess). At the risk of over-simplifying and belaboring the point: the DME collects info from a bunch of sensors like the MAF, TPS, Temp, RPM, O2, etc. Based on that information, it goes to its "map" and looks up how much fuel to supply to get to the desired air-fuel ratio. At part throttle, it usually aims for something in the neighborhood of 14.7:1. So, after reading all the sensors and squirting that much gas, it then looks at the reading from the O2 sensor. If it sees that the motor is really running richer than its target, the DME will cut back a little on the fuel to get to 14.7:1. If it does that and ends up at leaner than its target, it then adjusts back the other way a little -- always trying to hone in on 14.7. It's a never ending process of adjustments. That's what "closed loop" is. If your car is in "open loop" it just means the DME is not making those adjustments. Sorry if that's all well understood already, but the point is this: if your car runs overly rich without the O2 sensor, then the O2 is just masking something since, as Chris has been saying, the air-fuel ratio should be in the right ball park with or without the O2 sensor. So why is your car running rich? Could be lots of things, but if everything else seems to check out, it could be as simple as the M-tune maps just being programmed that way and needing a tweak.

mahoney944

Yeah I believe the maps need adjusted. Everything else seems to be working. Cutting over 11% and it's still rich at idle (isv bypassed) seems like a lot to adjust though? I'm out of ideas for what else might cause it to be rich. The LR m-tune version seems a bit glitchy..... Not sure what else to check.

Tom M'Guinn

I was going to suggest putting the stock parts on to see if the issue goes away, but people tend to hate when I say that. Maybe ping Joshua to see if he can help during his coffee breaks or something?

mahoney944

I don't have all the stock parts anymore lol. But my running / idle issues started or at least became noticable once I installed the LR m-tune kit. It definitely runs very rich before the isv compensates. I believe this extreme correction to be the reason why things aren't as smooth as they should be especially when it's switching tables based on throttling. but I can't find anything wrong and I've replaced everything.

Tom M'Guinn

Not sure when your set up was sourced, but see this thread and post 76 in particular. No guarantee that's your issue, but certainly worth considering as a cause.

https://rennlist.com/forums/944-turb...ig-rich-6.html

mahoney944

I received the update chip. Didn't fix it unfortunately.

Droops83

I notice that you keep saying something about the ISV compensating----are you saying that the idle mixture goes rich when you bypass the ISV OR when you unplug the O2 sensor?

How does the engine run in midrange/higher RPM with the O2 sensor unplugged? If it is rich everywhere, it is possible that your overall fuel map is too rich. Don't be so fixated on the idle mixture (which can easily be compensated for) until you figure out how the engine runs overall.

mahoney944

Its rich over all. If you disconnect the O2 sensor the DME will not allow the car to run in closed loop and it idles around 11.5 to low 12s. I used the fqs to position 6 on my m-tune (-11% fuel) to bring it up into the 13s....with everything connected properly it will go into closed loop and idle near 14.7...so what's happening is the base mixture is too rich so the isv/ oxygen sensor compensates to bring it to the correct mixture. I believe that when you rev the motor it seems choppy due to switching from closed loop to open loop to closed loop again with this rich condition in effect.

mahoney944

As mentioned before I'm using lr m tune and I have a new solid state dme relay with pump prime . I noticed my pump prime stays on constantly with key on engine off instead of just for 3 seconds. I was sent another with the same results. So I read the DME computer can cause it to keep the pump on in certain instances (I have a rogue dme computer). So I unplugged my DME computer and keyed forward. The pump ran for 3 seconds and turned off as it should. I then plugged in my stock dme computer without chip and it ran 3 seconds and turned off, still working right. Finally I removed the m tune chip and daughter board from my rogue DME computer to imitate my stock dme without chip and the pump stayed on. Apparently my rogue DME keeps the pump on ? Might be part of the problem.

DME sent out for review....