Advice with a starting condition

[Chestnut]

Have a Motronic 1.3 2.5i swapped e21 323i with a very odd starting condition that I will describe to the best of my abilities.
As the starter motor turns over the engine, it sounds like the injectors will fire at intermittent intervals, usually for about 1-3 seconds. During this time, the engine will usually stumble a few times and then catch.
I need to know what may be causing this so I can fix it because it is very frustrating. I know somebody on this forum knows what the cause of this may be.

[Adrian_Visser]

Uhhhhh that's what happens when you start a car...

What exactly is the problem? the stumbling? How does it run at idle and throughout the rpm's?

Could be lotsa things really.

Worn out plug(s)
Dirty injector(s)
Worn out dist. cap/rotor
...

[Chestnut]

Sorry, when the engine cranks, I can hear the injectors come on and then the injector ticking stops for a few moments and I mean all injectors stop then all you hear is silence, which means that the injectors are not delivering fuel. When the engine starts, it runs very rough at idle, not the famed sewing machine idle that BMW six cylinder engines are so famous for, runs like crap as a matter of fact, and it stumbles real nice when the accelerator is depressed.

There is also a charming knock that is intermittent that I can't diagnose by ear, I'm guessing it's the faulty Slovakian timing belt tensioner but probably my faulty job putting it in, who knows, it may all be an interrelated clusterf*** and that is why I came to R3V.

[jlevie]

Since an intake leak can produce the same hard starting and stumbles, that should be investigated first by having a smoke test run on the intake. If the problem persists after eliminating intake leaks you'll need to start troubleshooting the engine management system. That includes verifying DME power, operation of the CPS, coil power and firing signal, and injector power and firing signal for starters.

[Chestnut]

I plan on testing for vacuum leaks by spraying starting fluid around everything that could possibly have a vacuum leak and hopefully hit a point where the idle will raise a bit.

Jlevie, could you explain how the function of everything that you mentioned is checked, because I have a feeling that I will not find a vacuum leak, that sounds like too straight forward of an issue for what this car has thrown at me. Also, do you have any suggestion as to what my intermittent knock/rattle could be? Thanks:bow:

[Bmil951]

Well, the injectors stopping is odd, but the hard starting, rough idle and the "stumble" when revved sounds like bad piston rings. Check your spark plugs, if there's oil on them your piston rings are probably bad. Ask me how I know....

[jlevie]

I plan on testing for vacuum leaks by spraying starting fluid around everything that could possibly have a vacuum leak and hopefully hit a point where the idle will raise a bit.

Jlevie, could you explain how the function of everything that you mentioned is checked, because I have a feeling that I will not find a vacuum leak, that sounds like too straight forward of an issue for what this car has thrown at me. Also, do you have any suggestion as to what my intermittent knock/rattle could be? Thanks:bow:

For the engine to run the following conditions must be met:

Power on DME pins:
27 Start Input
18 Un-switched Power input
37 Power Input from Main Relay

Ground on DME pins 2, 14, 19, 24

Timing data from the CPS on DME pins 47 & 48 from a rotating engine

To have spark power must be present at the coil positive and ground pulses
from the DME's pin 1 must reach the coil negative. Power to the coil is
controlled by the ignition switch via C101. When checking for spark, use the
output lead from the coil to eliminate the distributor, rotor and plug wires.

To have injector firing power must be present at each injector and ground
pulses from the DME's pin 16 (Bank1) and pin 17 (Bank2) must reach the
respective injector bank. Note that the injectors are wired as two banks of
three. With cylinder 1,3,5 being bank 1 and 2,4,6 being bank 2. Power to the
injectors is controlled by the main relay.

The fuel pump relay must have power on pin 86 (relay coil) from the main relay
output (pin 87) and power on pin 30. The DME will ground pin 85 to turn on the
relay and power the pump(s) via pin 87. Of the above, only the fuel pump power
is fused. So if the there's power at pin 87, but not at the pump, check fuse
11.

The main relay and DME pin 18 receive power from the smaller of the to wires
that connect to the battery's positive terminal. That wire incorporates an
in-line fuse. When the DME is presented with a start signal, it grounds the
main relay pin 85 and furnishes power to the fuel pump relay, injectors, and
DME.

Troubleshooting:

Disconnect the battery and the DME cable. Then:

1) Disconnect the coil negative and check continuity from that connector to
DME pin 1. Also verify that from DME pin 1 to ground is an open circuit.

2) Check the resistance across DME 47 & 48, which should be 500-560
ohms. If the CPS is dismounted, the resistance can be seen to change
from about 500 to 540-540 when a ferrous object is brought to the face of the
sensor. Neither pin should be grounded.

3) Check for continuity from DME 36 to main relay 85 and from DME 3 to fuel
pump relay 85.

Reconnect the coil, remount the CPS (air gap should be 0.040"), plug the
relays back in, reconnect the DME, and connect the battery. Then do the
following checks:

1) With the key off, verify that power is present at DME pin 18 and main relay
86 & 30.

2) With the key on, verify that power is present at DME pin 27 and pin
18. Power to pin 18 is from the main relay and there should be power to the
injectors and fuel pump relay.

3) With the key on, verify that no voltage is present at the DME grounds (2,
14, 19, 24).

4) Verify that power is present at the coil positive and at fuel pump relay
pin 30. Those get switched power from the ignition switch via C101.

The engine will start and run (if poorly) with only those connections to the
DME in place. The other signals from Cylinder ID, AFM, temp sensor, etc., are
necessary for proper operation. But they won't prevent the engine from firing.

IMPORTANT:

A power check means seeing a voltage within about a tenth of a volt of what
you measure across the batter terminals, which should be at least 12.6v on a
charged battery.

A continuity check means seeing less that 1 ohm of resistance.

An open circuit means seeing a resistance of at least 100k ohms.

A good quality auto-ranging Digital Multimeter will make these tests much
easier.