Atom22

I have been working through the FSM to figure out why my car will not start after my coolant flush. I am getting the C41, Err (ECM), H72 code which is the Loss Serial Data Connection. The car was running fine and started up immediately prior to coolant flush. The coolant flush consisted of drain radiator and from both knock sensors. All the grounds I can find and circuits I have checked have been good. I have checked and all my spark plugs are firing and the connections all have correct voltage at the ignition coil module. I am in the process of getting to the CCM to check continuity to ECM. The ECM has continuity to the ALDL and everything else I have checked. I find it coincidental that my ECM failed right after this coolant flush.

Is there something I am missing? I have read that the knock sensors, even bad, would still allow car to start. I checked that ground near them and it had continuity. Before I rebuild or buy a new ECM, does anyone have any suggestions? So frustrating that the car was running strong near the end of my repairs and now what turn on.

Also, a little bit of coolant was bled from both valves above water pump, but since I am getting sparks I assume its not my opti-spark. I do not think its a VATS issue because I get a crank plus the codes that have been thrown are not VATS related to my knowledge.

I would appreciate any ideas. Thanks,

3D-Aircrew

It may have been an inadvertent disturbance to the wiring to the ECM that might have happened when you were messing with the knock sensors. I would go through the engine bay with a fine tooth comp and check all the connections to the wiring harness. You might find a loose connection or an open wire.

dizwiz24

Look at all the easy stuff first, like connectors / grounds.

But Im worried something got spilled into the optispark

i know you said you got spark...

note the opti seals shrink/crack and the top of my 93 opti was basically an open slot (due to shrunk/crackd seals) / corroded @ 26000 miles !

Atom22

If the optispark got wet would it throw the C41, Err codes? I also sprayed starter fluid at air filter and it started right up then died. So I know the C41 code is not allowing ECM to pulse injectors.

B757captain

Same thing happened to me after changing out my opti - I'm not close to the car or manuals and my memory is a bit fuzzy but I seem to remember something about the low coolant sensor (?). Mine wouldn't start until there was enough coolant to register. Any chance you've got an air pocket? I don't even remember where the sensor was but after filling the coolant reservior mine started right up

Atom22

That is a good idea. I just bought the car a month ago and I am trying to put it back to stock. One thing that was missing was a thermostat, so I bought one and put one in. The coolant sensor is below that area. It is very possible that the sensor is not reading coolant.

Atom22

The coolant temperature sensor checked out fine and the water pump is filled with coolant. There is coolant in reservoir, but I do not know how to tell if low coolant sensor is working.

I am am still working on getting to CCM to check serial data wires from ECM, such a pain.

Any other suggestions. Maybe it was completely coincidental this happen after coolant flush.

Atom22

Do you recall if it was the low coolant sensor, the coolant gage sensor, or the coolant temperature sensor?

jrclark32

Not trying to hijack your thread, but I have an almost identical situation with my 1992. I did a coolant flush yesterday, and refilled the system today.

It fired right up after I filled it, I got good flow through both bleeders, and was letting her get up to temperature when it just died, as if I’d turned the key off.

Now she cranks and cranks, with no attempt to fire.

I’m hopeful about this coolant sensor idea. It seems pretty unlikely that we both had a coincidental ignition failure after performing the same procedure.

Atom22

Jrclark,
Did you remove knock sensors for coolant flush too?

jrclark32

I did

Atom22

This weekend, I plan on replacing both coolant sensors (one in water pump and one in engine block). I’ll let you if that fixes the problem.

vette196

just a side note, make sure you bought the right thermostat, the LT1 uses reverse flow (something like that..), probably not related to your problem, but since this vette is new to you (not sure if you've owned other LT1 vehicles) I figured I'd point that out to you

Atom22

So after a couple months I finally got down to the problem. I tore car apart and checked every wire harness I could find and went through every FSM diagnostic i could find. At the end of all this I sent ECM into SIA Electronics and they found some bad solder joints. They repaired joints and sent ECM back. I plugged it in right away and car had no codes and started up immediately.