Weird weak spark problem????????

[Ramblin MAn]

Been helping a buddy with his new 91 318is. The car had a motor swap done before he picked it up and I've been working on it all week trying to run down the problem.

Finally today we have it narrowed down to a weak spark issue. When you first crank over the engine, you will get 1 good strong spark, then a weaker one and on the third and after a very weak spark, but it does spark. However, it is so weak it won't trigger a timing light for more than the first two pulses.

If I jumper the fuel pump so it is running before we crank, the car will start and immediatly die.

I have removed the intake and verified that we are getting good fuel delivery from the injectors and I even tried starting fluid with the same result.

This is happening with all 4 coils so it's stumping me what could be pulling the coils down since according to the guy that did the swap, the car was driven to the garage.

We've tried swapping computers, swapping cam and crank sensors, new plugs.


Any clues?

[colin86325]

Coils and spark plug wires? Check the engine ground and the ground near the ECU.

Where are you located?

[Ramblin MAn]

Cincinnati.

[DesktopDave]

Odd that the pump will stall the car...I'd suspect a bad pump or a low battery.

I'd do a stomp test first, see what the DME thinks is wrong.  If that code is a 1444, then I'd try to get the battery load tested at an AutoZone or Pep Boys.  You never know, might be that simple.  The coils draw a lot of current.  Could be the fuel pump relay or the main relay.  The contacts erode over the years and don't pass enough juice to keep the car running.

I'd also take a look at how many amps the pump is pulling.  Might be a good pump and a clogged filter screen or filter is making it work too hard.

[Ramblin MAn]

No the pump is fine. What I was saying is as long as I have it pumping before I turn the key, it fires for just a second. But if I use the relay, then by the time the fuel pump kicks on, the spark is too weak to fire the mixture. We drained the tank and put in fresh gas and even with starting fluid its the same thing.

So fuel delivery is just fine.

Where is G103? I was looking in the electrical troubleshooting manual and it only has the non "is" G103 located at the strut tower.

[nicknikolovski]

Check the main engine earth cable on near the passenger side (LHD) engine mount. Also have you tested your coils and leads. The coils provide the spark so check them and trace back from there. Check for battery voltage or approx 12V at each coil with the ignition on between terminal/pin 15+ and ground. If there is no battery voltage check the wire between terminal 15+ and the ignition switch. Then with ignition off, check resistance on each coil between pin 1 and 15+ (basically the pins on the coils are like this    [ 1 ] [4a] [15+]. You should get a primary resistance of 0.4-0.8 ohms. Secondary resistance cannot be checked.