LS1 Swap Porsche 944 odd ball issue

[Webs]

Not my car, but I'm working with an LS1 swapped Porsche 944. Vehicle runs well and has been dyno tuned, and for the most part has zero issues. However, the vehicle has a battery switch on it. It's not a kill switch, it's more of a kill the battery so it won't die switch, as in you can turn the switch while the car is running and it stays running but key off and there will be no restart.

Now here's the odd part, if kill the battery (engine is off), upon turning the battery back on, the car has a hard time starting, and runs like complete garbage, for about 60 seconds. If you wait for 60 sec before starting that car runs and starts normal. If you never kill the battery the car runs and starts normal. If you kill the battery, engine off, and turn it back on and restart immediately car runs bad. Bad means doesn't want to idle, throttle response is terrible, can't even move the car, wants to stall. Kill the engine, leave battery switch on, restart car, all back to normal.

Also of note, HP Tuners scanner will NOT connect to the vehicle for the 60 sec after turning battery back on. Disconnect/reconnect, start scanning and it will just freeze. If it doesn't freeze and appears to be connected it isn't, with key on engine off TPS won't move with pedal sweep (drive by cable). After 60 sec everything is normal again.

Is there something internal in the ECU that could cause this? Personally if it were my car I would NOT have the battery "kill" switch and buy a battery tender if I didn't drive it often and never think about this again, but I don't know why people insist on doing some of the crap they do so here I am, asking all of you

Throw whatever crap you can think of at me, lets see what sticks.

[edcmat-l1]

It's not "in the tune". It's not in the ECU. Not everything can be fixed by tapping a couple keys on a keyboard. Especially in a swap like this. I build and tune plenty of road race and drag race cars that all have cutoff switches in them. Turn the switch on, hit the key or press the button and away you go.

[SiriusC1024]

That whole running like garbage after battery disconnect almost always means that the airflow models are incorrect. The fuel trims have to repopulate to make it right. Look at the MAF and VE tables. They're off a bit.

[edcmat-l1]

That whole running like garbage after battery disconnect almost always means that the airflow models are incorrect. The fuel trims have to repopulate to make it right. Look at the MAF and VE tables. They're off a bit.

If it won't comm with the scanner for the same length of time it's not a tuning issue.

Also, in the circumstance which you describe, it should take the ECU longer to come around to running correctly. Longer than 60 seconds. It should take longer than that for the trims to start working and get it back to stoich.

[SiriusC1024]

Ah I see what you're saying.

[gametech]

My Dad's 2002 silverado has a battery disconnect. Upon reconnecting it, you have to turn the key to the run position for a few seconds, then key back off. After that it will start and idle just as it should. If you just try to start it without doing one key cycle first, it will not properly hold idle until trims kick in. I always assumed the computer just loses track of the IAC valve position, but I never knew for sure.

[Webs]

It's not "in the tune". It's not in the ECU. Not everything can be fixed by tapping a couple keys on a keyboard. Especially in a swap like this. I build and tune plenty of road race and drag race cars that all have cutoff switches in them. Turn the switch on, hit the key or press the button and away you go.

I never said it was in the tune, nor did I think it was. As I said if it were my own personal car I wouldn't even care about this. I was looking for answers as to why it would exhibit this oddity obviously unrelated to the tune file... Kinda weird you made the assumptions you did.

[SiriusC1024]

Is the disconnect on the postive or negative terminal?

[blindsquirrel]

Modern systems are really not built to deal with regular battery disconnects. If you do it, you will possibly just have to live with it doing things you don't like.

[SiriusC1024]

Has anyone ever heard of a type of safe mode boot on these ECU's?

[horsepowerguru427]

Is the disconnect on the postive or negative terminal?

I was wondering this myself. Ive had PCMs do act crazy if grond is connected before B+ or in this case, disconnect on B+ side. Which i would assume is from small power surges.

[Alvin]

I'll bet this is some sort of weird wiring issue.

This is a MAF tune. It should run decently OK even with a battery reset. Not being able to connect the scanner is a dead give away something isn't powering up like it should.

[edcmat-l1]

I never said it was in the tune, nor did I think it was. As I said if it were my own personal car I wouldn't even care about this. I was looking for answers as to why it would exhibit this oddity obviously unrelated to the tune file... Kinda weird you made the assumptions you did.

You posted the tune file, not a data log. I would assume you did that thinking someone would see something in the tune file that's causing it. I don't think what I posted was a stretch.