Another brain teaser. Some time ago I posted about difficulty starting after fitting bigger [28lb] design 2 4-hole injectors. Car runs well when hot but cold starting was problemmatical.

After some discussion on the list, in ST2 I played around with the warm up mapping increasing the settings until the motor picked up when cold [that being a relative term here].

The problem seemed to be fixed but then seems to have started again now that the cooler season has kicked in here [day temps now a nice 20 to 25 degrees C or so]. I was starting to suspect the temp 2 circuit given my wiring loom is not in great shape. This morning, a nice 20C day, I measured the resistance at the LH terminals - read 2.1k ohms both channels and cross checked on the sensor itself- same readings. Could not find any signs of discontinuity or leakage to earth when the LH and sensor connectors were disconnected so I am now tending to rule out temp 2 problems.

The only other change I made was to the breather system which currently vents to atmosphere from the filler neck via a pro vent. Also the tank breather connection and the vent from the filler neck that combine and go back to the ISV area are currently disconnected. I assume that the ISV opens a bit more to compensate- just wondering if maybe it is maxxed out and cannot open any more. Guess I can test for that by reconnecting the small line from the filler neck.

To start at the moment I have crack open the throttle and let the system run for a minute or two then it stabilises. Maybe the air flow through the breather is needed to start the car noting that the MAF does not read this component.

Thoughts appreciated.

Regards

Fred

 

Have you tried increasing the cranking fuel pulse length ? On the fuel parameters screen.

 

So here's a good question. On both my cars (each with 42# injectors) the cold start is very similar to what Fred describes, except that they start fine, but hunt like crazy for the first minute or so, then settles down.

I always thought because of the bigger injectors we would want to DECREASE the pulse width or the settings in the cranking map and the warmup map to reduce the possibility of flooding and to compensate for the bigger injectors.

 

Fred If you have not already done so I would suggest the cut off the temp 2 connector and wire in another waterproof connector\
cut back to where you find good conductors.
also check the the vacuum lines for fuel at each damper and FPR

 

I know it is counter intuitive, but I remember that Tony, had lots of starting problems with his s/c car, and in the end increased the cranking pulse duration. IIRC....

If your cars start OK and then hunt, then you need to change the warmup map.

 

John,

I reinstated the small breather connection and that made no difference so that is eliminated. I have not adjusted the cranking pulse width- to be fair I would have no clue where to target adjustments for this parameter.

To start the car I have to depress the throttle, it then seems to pick up OK after a few cranks and hold it for about a minute at 1k rpm- after that it is fine. Indeed just took it for a run and it was running really well with cooler air now on tap.

I will check the fuel pressure regulator and dampers to check they are holding vacuum. I tend to forget that the next round of fun is just about to dump on me given I have been running this 928 for some 6 years since everything was changed out.

What is the significance of cranking pulse duration by the way?

Regards

Fred

 

Well, the cranking pulse does what it says on the tin - it is the sole arbiter of the engine fuelling at those low engine speeds. The pulse is modified by temp 2 and battery volts, but the MAf plays no part until the engine fires and the rpm pickup.

 

I had a warm start issues with my boosted car and had to crank up the cranking pulse width to 3.00. Take a look at your idle maps and see how smooth they are in the 4 cells surrounding where it sits when smooth.

 

Well that makes sense -durrgh!

With bigger injectors presumably logic suggests that should be reduced? So if something is not working correctly presumably by opening the throttle the MAF kicks in with more fuel as the brain thinks the cruise map has kicked in?

Sounds like it's time for another tinker with ST2.

Regards

Fred

 

Sean,

When setting the idle I look for AFR 14 and then set the 8 cells surrounding the idle point at the same values. I ffind that gives the smoothest tickover- that and advancing the EZ to about 15 degrees.

I'll try modifying the cranking PW -at least I have a number to shoot at but my problem is cold start not warm..

Regards

Fred

 

Counter intuitive, for sure. Wow... to think I may have been doing this wrong for the past 4-5 years... I'll play with that the next time I SharkTune. The GT is about to go under the knife and get a frontal lobotomy and some open hear surgery.

The Euro is pretty much put away for the winter, and unfortunately it's a bit of a PITA to hook up the STer to that one.

 

Beofre attempting some corrections in ST2 - having thought about what is happening would I be correct to understand that when ST2 scales for bigger injectors does it cut back on things like cranking pulse width?

I can understand what might have happened if in cutting back the pulse width ST2 overshoots the mark assuming there is a component for opening and a component for the opening duration. Having said that when I fitted the current 28 lb injectors I adjusted the warm up map and that helped. Does anyone understand the relationship between AFR and engine temperature as the engine is warming up? It would be good to check this if someone knows the values to look for.

I do not like changing anything without a reasonable understanding of what the potential impact is. I have been reading through my copy of Probst to see if could find something to guide me but not spotted anything to date. My symptoms may well be connected ot cranking pulse width but I do not think that is the full story.

Does anyone know when the cranking pulse stops and the idle map takes over? Presumably at some set rpm like 500rpm or so?

Regards

Fred

 

The cranking pulse is not rescaled by the ST when changing injector size.

I think the relationship between what is recorded by the water temp sensor and the acutal A/F ratio required is going to be a variable from one engine design to another. It is going to be an iterative process optimising he warmup mapping.

IIRC the changeover point to the "live" map is at 250rpm.

 

Constant @ 3079 ('88 EPROM), hex CD.

It's coded to bypass the cranking map with a value of hex FF.

My '88 with 24# starts fine with the cranking map bypassed (@40F/4C.).

 

John,

Any thoughts as to why the cranking pulse width might need to be increased givwen a bigger injector flows more fuel per pulse? Could it be that the opening time component of the injectors I have is so much longer than the stock injectors that there is no time left for injecting fuel in effect?

At the moment I am having to hold the throttle open a tad when cranking to get it to fire and ahve to keep it there for about a minute of two gradually releasing the throttle bit by bit until it settles into a steady tickover pattern.

Regds

Fred