Looking for some suggestions

After a few miles on the street on country roads at normal speeds with a few occasional harder accelerations and the car running perfectly fine I then made a full throttle acceleration in second gear and just before

shifting gear at about 6500 rpm full throttle the engine completely shut off as if the ignition was turned off

For a second I tought I hitted the rev limiter so I disengaged the clutch and could slowly get to a space on the road side

Then I tried to restart the car the engine was cranking perfectly no strange mechanical noises battery was powerful but it was not starting and no signs of almost starting at all be it without throttle or with throttle

I disconnected and reconnected the battery and used a replacement DME relay but it wouldn t start so I called road assistance to get the car towed

After about one hour just before the road assistance arrived I tried again to start and the car immediately started ( as if some excessive fuel had wetted the plugs and they had the time to dry out as in a two stroke ) however it was not really runnign perfectly on throttle and I could

make it shut off by applying full throttle.

After towing it home and after one day I again disconnected the battery and ecu and reconnected all and started it then I disconnected one of the main ignition coils at the time but in both cases it continued to idle and did not shut off so I think that means that both ignititions coils are working
and that the small ignition belt is not broken

It actually runs quite well but not perfect still but I m especially wondering what can have caused the shut off under full throttle

My car is actually a 964 RS 3.8 replica but using a 993 engine gearbox and motronic ecu and ignition and the engine is a built 3.8 engine with high lift cams and individual throttle bodies with a Motronic ecu mapped in alpha-n by RS tuning in Germany
so not only everything is from a 993 but it s also modified so I would like to know if any experts of this kind of set up could give me some suggestions on what to look for

The engine and most electric parts are recent or new but I was wondering about the crank position sensor , the fuel pump itself , the fuel filter ?

 

Hi Maxi,
It is the limiter.
The non-start is most likly unrelated.
Because your intake is not stock it may take the skills of the folks who did the conversion to sort out the issue.
You checked for spark, that is good.
You may want to clean the idle control valve, a contributing factor but not a root cause to a non-start.
You will need to check fuel supply.
Also check the battery voltage at rest before turning the ignition key. If it is much below 12.6 volts the load of the starter rotating the engine may be dropping the system's voltage low enough that the Motronic DME controler is shutting down, preventing ignitin.
Andy

 

Your car is loaded with specialty parts that most people on this board will not be familiar with, myself included.

If I were you, I'd take it to a specialty shop, one that's familiar with 911 racecars. They would have the expertise to troubleshoot the ITB's, as well as be able to look at your fuel and ignition maps.

 

Does the motronic have data logging? You may be able to see what caused the car to die at low rpms. If not, I'd start with your OBD2 port and start logging some miles for anything Abnormal in the spark, fuel trims, etc....

 

I would check all the electrical connections, grounds, and wiring harnesses. If it's shutting off under full throttle, it could be because something is loose that is getting jiggled by the acceleration.

 

When you first tried to restart with no luck, did you got straight to "crank" or did you let it sit for a few secs "on" to let the fuel pump build pressure?

On my build we added a fuel pressure gauge in line. We also replaced the original fuel pump to make sure we maintained fuel press when you increase demand/displacement and go with ITBs. It is then easier to GoPro the gauge if necessary when tuning / being driven on road under hard acceleration to make sure this is not fuel starving related. We also let the fuel pump build pressure for a few secs (key "on") before asking the engine to crank / start.

As others mention, also confirm steady voltage levels (dash / cig lighter display) under all driving conditions.

Good luck chasing this down, having doubt in reliability, will distract from the driving pleasure :-(

 

Thanks for the replies

By the limiter you mean the rev limiter ?

In that case no for sure it was not the rev limiter as it was at less than 7000 rpm and in any case hitting the rev limiter do not make the engine shut off

Of course it would be easier as you suggest to deal with the tuner that did the engine setup with ITB but it was done by the precedent owner and the tuner is really far and is also in fact just a Porsche engine builder and not the one who then did the final tuning and completed the car

In this particular set up there is no idle control valve at all as when tuning with alpha-n most of the oem things as air flow meters idle control valves etc are just removed

The problem with alpha n and this type of set up is that it s somehow a really old style method of doing things and while it make things more simple as on an older race car there is also much less precision and smoothness and it s similar to how a car with individual carburators would work but still with the eventual electrical problems of older obsolete electronic parts

also keep in mind that actually after resting the car is starting perfectly each time so the battery shouldn t be the problem

Not sure if the Motronic has data logging but I can plug in the Durametric to see if I find some errors etc

Not sure if I let the fuel pressure build or not but I tried many different times without touching the throttle and then with some amount of throttle but only letting the car rest made it start again and now it will start perfectly each time

The fuel pressure gauge is something interesting to fit for sure

I will look at measuring the fuel pressure or use a new and eventually uprated fuel pump and replace the fuel filter ( I also refilled 30 minutes before the problem so maybe some not to good fuel )

Other things that I m looking to replace are the crank position sensor as I already have a new one

Not sure if there are other ignition components that are common to be replaced on thoese cars

Thanks

 

The symptoms are consistent with intermittent DME relay OR intermittent crank sensor. Not sure how confident you are that the replacement DME relay you tried is good but you can try another one or temporarily bypass it at the fuse box. If the problem persists, drop that other crank sensor in - either way, could be cheap fix.

 

Why not ask Reinhold?

 

Don't waste your time with fuel. Symptoms don't line up. Engine doesn't shut off like a light switch because your fuel pressure is off or because the pump cuts out. Your problem is almost certainly electrical.

 

Agree. Fuel delivery would be more of a sputtering/hesitation and then eventual dying (just like running out of gas).

 

Thanks for the suggestions

I m looking to first try the easiest things before bringing it to a Porsche specialist


Some little update

I wanted to eventually take out the fuel from the tank and replace both fuel filters but when I removed the tank level indicator I could see well enough inside the tank with a lamp that the fuel seem perfect without any visible humidity or water and without visible residues and even the filter iside the tank

was looking in good condition

Of course that does not mean for sure that the fuel is good or that the fuel pump is good but gives some visual indication that it should not be the problem

Also I wasn t sure if the last time it was at a Porsche specialist they replaced or not the main fuel filter but I had a look and I think it was replaced as there is a new metal collar and the filter look new


I then unplugged the battery and the ecu under the seat and started the car

The car started perfectly and after letting it at idle for a minute I shut it off then I started it again and I made some throttle blipping and everything seems about perfect

The only slight noticeable thing is that if you blip the throttle really quick there is a really short hesitation or delay as if the air is mechanically coming a bit too fast compared to how fast the TPS and injectors react in that particular situation but it basically always did that
as it s mapped in alpha-n without any air flow meter

Let s say that now the car will work well I still did not find out what made it shut off like that during an acceleration

I will probably replace the crank position sensor and fit a new fuel pump but let me know if you have other suggestions