WallyP

From the 1985 tech info by year booklet...

"The idle speed contact switches in coasting fuel shut-off . No fuel is injected when car coasts with an engine speed above 1670 rpm. This continues until the engine speed drops below approx 1300 rpm . From this speed range on, the LH control unit sets in injection again (valid for an engine with operating temperature) . These switching on and off speed ranges depend on the engine temperature and vary gradually according to the instantaneous temperatures (for example at -15 ° C : coasting fuel shut-off "ON" from approx 2300 rpm "OFF" from approx . 1850 rpm) .
When the idle speed contact is closed the ignition is also switched to idle performance."

auzivision

My car is eerily quiet now on decal with throttle off. I used to have a cool sounding burble before the engine refresh and SC install. I can only guess my old TPS was either not working or not adjusted correctly. It took a couple gyrations to get it set just right by me. It clicks off at idle and clicks back on with the slightest touch of the throttle.

Supposedly my 87 ST2 harness and PEMS are in route now and soon I’ll get to play with all this stuff. I’m obviously several notches behind you guys and looking to catch up. I have so many questions, but I’ll save them till after I get some data logs.

I’m still curious if anyone has any good links, rules of thumb, or other resource related to the very basics of engine tuning. Things like under ‘this’ condition you want to do ‘that’ type stuff. Mainly related to fueling and timing… since in the simplest forms those are the only two ***** we are really turning. As I understand it, the rest are inputs (well except for one output fed back in as in input)… and there is a heck of a lot of different combinations on the ‘this’ end.

mark kibort

in a nut shell, the switch cuts off fuel and you get a quite decel and more decel braking. it will hunt for normal idle when coming off high rpm with no throttle. without it, there is no hunting. the response time is not fast enough to catch it at below 1000rpm, but it does a pretty good job for street driving. on the track, it can kill the motor (stall). the feed back loop is not fast enough.
the burble is just fuel in the system on decel. the amout of fuel needed for idle only. its enough to give the burble sounds and provide less decel braking as well. If I flick the disable switch i use, on decel, the car will coast down much further.

ptuomov

I managed to replicate what you describe but with the deceleration fuel cut off disabled. How do I fix it, i.e., prevent it from happening?

Here are some of my thoughts:

The rpm dives under 775 rpm, where the idle control algorithm and valve should presumably pick it up. Thats visible from MAF [%], which goes from 2 to 3 at about 775 rpm and then from 3 to 4 at about 600 rpm. But this doesn't work, the engine goes thru 425 rpm and stalls.

Since the MAF signal is proportional to MAF%/rpm, the rpms dropping spike the MAF signal. MAF [%] of 3 at 775 rpm => MAF signal of about 100 and MAF [%] of 4 at 425 rpm => MAF signal of about 260. In other words, the idle adjustment controller maxes out the fuel MAP. So the fueling comes from the lower left corner of fuel map. The load change may also trigger some "acceleration enrichments" but I don't know for sure since I don't understand enough about how the LH works.

Whether it's just the base fuel map or some acceleration enrichments also, the injector pulse width jumps from 1.7ms at 775 rpm to 6.1 ms at 425 rpm. Now, I don't know which one is wrong, whether 1.7 is too little or 6.1 is too much. One of them has to be wrong, there's four times as much fuel going into the cylinder per rev at 425 rpm as there is at 775 rpm. I don't know which one is wrong, but I suspect it's the 6.1 ms pulse at 425 rpm.

I am going to try leaning the high-load cells of the 500 rpm fuel map column tomorrow.

Anybody else with better ideas?

dprantl

I have not fully cured this on my car, but lowering the bottom two cells on the leftmost column to something very low like -100 made the problem much smaller.

Dan
'91 928GT S/C 475hp/460lb.ft

ptuomov

What else have you tried?

ptuomov

No more stall coming off high rpm.

I have become convinced that SuperMAF'd LH only gets to the max load cells at low rpm because of a data error. I am guessing that the MAF signal (constant*MAF%/rpm) is supposed to be close to linear in the mass of air in the cylinder at each cycle. Numbers like MAF signal 250 would in reality correspond to VE of mid range full throttle at 20 psi boost. That's not what the ISV supplies! ;-) Under 500 rpm, I am guessing that the error is simply discretization error. Those bottom left cells are only visited by error and should be tuned to respond well in the specific error conditions.

Armed with this reasoning, I took pretty much as much fuel out of the bottom left fuel map cells. I tried to get it to stall more than five times, but couldn't. No other effects on how the car drives, adverse or otherwise.

dprantl

So it sounds like it helped you too. I haven't really done much else about this for now. I too think there is something going on, but don't think it is an "error". I think there is some other LH parameter(s) that are is/not exposed. When switching to bigger injectors, this parameter is unchanged and causes some slight issues. No big deal for me though, my car is running very well.

Dan
'91 928GT S/C 475hp/460lb.ft

John Speake

Is the MAF reponding to some kind of back surge, caused by the throttle closing with some boost remaining in the intake system ?

ptuomov

No, I don't think so. John's system has bypass valves, and it takes a while for the revs to drop from the throttle shutoff. In my opinion, the only things that matter about the path that it gets to idle range are (1) rate at which the rpm drops because this likely influences the ISV algorithm and (2) how much fuel is on the port walls.

In any case, it seems to me that with SuperMAF there's some critical rpm level under which the MAF signal computations are such that MAF signal is essentially unrelated to the volumetric efficiency. I get readings such as MAF signal 250 with the idle switch on and ISV operating. I wouldn't be surprised if this is an unavoidable error, the ECU we're working with is a quarter century old. The MAF voltage and MAF [%] are not off, they are about right, but the computed MAF signal makes no sense if it is to be interpreted as proportional to the volumetric efficiency. Discretization error in the computation MAF signal = constant*MAF [%]/rpm would be my first guess.

In any case, I don't think there are any actual situations in which the bottom-left cells of the SuperMAF map are ever visited except because of various data errors (where a data error is defined broadly as a situation where the MAF signal is high despite the volumetric efficiency being low). Therefore, I see no downsides with tuning the bottom left corner of the SuperMAF map to the lowest possible fueling level to avoid the rich condition that seems to cause the stall under 425 or so rpm.

ptuomov

Now that the car is no longer stalling because of me flooding it with fuel, maybe it's time to start thinking about the idle ignition map again.

The stock S4 idle ignition map is something like this:
RPM 340 390 460 560 620 660 800 930 1100 1200 1400 1700 2200 2800 3500 5200
Deg. 11.8 11.8 11.8 19.6 11.7 9.8 7.4 5.5 5.8 5.7 6.7 8.1 10.8 19.9 34.1 42.5

The hump at 560 degrees is there, I believe, to help the engine make a lot of torque when the rpm falls thru the idle speed. I have already moved the hump to the right to work with the higher idle speed. As a next move, I was considering extending the spark-advance hump to the left, lower rpms. Say making all those cells on the left about 20 degrees.

Does anyone see any problems with that?

To get some guidance, I went back to the book, "Four-Stroke Performance Tuning," quoting of which I have gotten a many bucket loads of **** here. Here's what the book says:

So A. Graham Bell seems to approve of extending the hump to the left.

Here's what Autospeed says:



Thumbs up or thumbs down idea?

jcorenman

I think you are right about the "hump" at 560, being there to goose the rev's back up (quicker than any throttle fiddling can do that). But why a "hump" and not a "ramp"?

So yes, I agree with 10-ish degrees over the idle range, and then ramping in more advance at lower RPM's. I would do it as a ramp however, and not a step -- 11 to 19 deg from 620 to 560 is pretty steep. But it all depends-- too much advance at very low RPM isn't good either.

Remember that you can lock the ISV with the latest Sharktuner software, to a fixed setting. Then fiddle with the idle advance map so that it is stable-- you can perturb it with the aircon, or by letting out the clutch slightly (in gear, brake on). Then re-enable the ISV and see if it is still stable

ptuomov

The reason why I don't want to do, say, a linear ramp for the whole range from idle to 340 rpm is that, for a given mixture density and turbulence, the time it takes to burn the mixture is constant. Although the turbulence increases with the revs the ISV lets more air in at lower revs. My guess is that the burn time doesn't vary that much in the range from 340 rpm to idle.

If the burn time is about constant, simply keeping the spark advance constant from idle to 340 rpm about halves the number of degrees from spark to the peak pressure point. Suppose that the peak pressure point is at 20 degrees ATDC with 20 degrees of total advance at 680 rpm. Holding the burn time constant, 20 degrees of total advance at 340 rpm will move the peak pressure point to TDC. The engine will make more torque with peak pressure point at 20 degrees ATDC than at 0 degrees ATDC. So the ramp in torque now slopes the wrong way.

The burn accelerates with more turbulence, so the effect is not that extreme in reality. However, I believe that a flat timing curve already causes a ramp in the peak pressure point. There's a risk of overshooting 15 degrees ATDC with a ramp in total advance.

More generally, when thinking about the stability of the system, I think it's more useful to think about the torque that the engine generates rather than just the total advance.

UPDATE:

I used the new ST2 features to fix the ISV and then tune the idle ignition maps. I set the 340, 390, and 460 idle ignition map cells to 20. With my maps and the ISV fixed to 50%, the engine now idles about 125 rpm above the set point without the AC on and 75 rpm above the set point with the AC on. When I reenable the ISV, it pulls the idle down to the 775 set point as it should. I then experimented with revving the engine up and letting it drop from high up. The engine catches the idle now very quickly from under the set point.

What remains to be tested is letting the RPM drop quickly from a high-load, high-rpm region. Dropping from the high-load, high-rpm region leads to a rich spike because of the wetted port walls. There's still a chance that the engine stalls in those conditions, but I am optimizstic that it doesn't. Tests to resume this afternoon.

John Speake

Glad to hear the new ISV feature is proving useful :-)

janesmith

Hey thanks a lot for this all information..its awesome....very helpful and much easier to understand..i had a confusion about this but now i am sure your advices will help a lot..

Thanks