Starting problem. cranks but no ignition. (CPS) ??

#1

Hello to the se30 crew!
So, I just replaced the cylinder head on my car and reconnected all hoses and electrical items per usual. It cranks over fine but does not start. No sign or sound of combustion. :frowning:
I have spark at all 6 plugs and fuel in the rail. The voltage at the positive coil terminal is ok. The distributor rotor is present and looks fine.
My research says maybe the CPS has failed. The Bentleys manual says to check the CPS resistance; I did this and it’s OK. However, I read elsewhere that it could still be faulty.

Any suggestions for checking the CPS? Or do I just buy a new one?
Thanks.
Jon

#2

Make sure the timing isn’t 180 degrees out?

#3

you’ll get this symptom if the small +ve wire from the battery is not connected as well, so maybe double check your electrical connections.

#4

If you have spark on each of the cylinders, the crank sensor is probably not your problem. You don’t say what year, but if an 88 or later that has the C191 connector under the intake manifold, check that connector. It connects the injectors to the main harness.

#5

What is the +ve wire? How do I identify and find it? Thanks.

#6

The car is March 1987, 325i. I checked under the intake manifold, and that connector is OK.

#7

Unless the engine harness has been swapped out, an 03/87 doesn’t have a C191 connector. That change in the harness occurred late in 88.

The +ve wire isn’t your problem either. It supplies power from the battery to the DME (and a few other things). Spark at each of the cylinders can only occur if the DME has power and also has timing reference data from the CPS.

At this point I’d start by verifying cam timing (since you had the head off). Then I’d check the pressure at the inlet to the fuel rail while cranking over the engine and check to see if firing pulses are present on the injectors.

#8

alfageorgia wrote:

start at the battery …

in my 89, there are two wires attached to the positive battery post - small and large. The large one leads to a junction block on the passenger side of the firewall in the engine compartment. The small one directly feeds some key components. Do you have an ETM? (check under Links, E30 Resources)
cheers,
bruce

#9

If you have that connector under the intake for the fuel injectors I have had problems with it before. Take it out of the bracket and make sure it is connected. Use a meter to make sure continuity.

Michael

#10

Thanks for all the tips.
Let me clarify, the six wires from the injectors go directly into the harness.
After more consideration, I realize I am not hearing the fuel pump noise when I turn on the ignition. Hmmmm. maybe a relay or pump problem; that means insufficient pressure. So tomorrow I will check the pump and relays.

#11

I just wanted to close out this thread with the solution to aid us all in the future.
So the engine would crank over but not start. New CPS. I checked for spark. fuel pump. the relays ok. fuel pressure ok. timing belt was on correctly. grounds were good. Noid light test was ok. Tried a new coil. Got into the Motronic connector for testing - mostly OK, but very confusing.
So, I had spark and fuel but not ignition. after many hours of checking and re-checking, I gave up and took it to a local BMW shop.
They started fresh and found the problem: I had installed the distributor rotor adaptor incorrectly when I reassembled and installed the cylinder head. It was about 30 degrees off (there is a little guide pin and hole for alignment that I missed… duhhh). So the plugs were firing nicely, just not in sync with the fuel and compression. The distributor rotor adaptor is the item with three female thread holes that holds the rotor. Now it starts like a champ and I am ready to rolllllll. BTW, the shop said I was not the first dummy to make this error. :blush: :blush: