Hi,
Long time Reader, first time I post here.
I just bought a GenV 502 EFI Mefi1 egnine. Tried to start and it ran, but only in idle. If you try to accelerate it will stall.
I read the codes, Code 33, Code 42.
On Code 33 MAP sensor, I did just replace MAP sensor but same Code. I started to measure all the references regarding to the Manual 23, Sensor works great, harness is ok, even the ECU shows the right voltage readings on the Rinda tester. Don't know why it always sets the code, maybe toast ECU?

Code 42 is more difficult. The Ignition doesn't change from Distributor Mode to ECU mode. I replaced the Ignition module in the distributor and the pickup coil. Same code. I started measuring all the voltages and Ohms referring to the Manual.
The manual says i shouls unplug J1 and J2 connectors from the ECU, ignition on. Measure Ohms on 423. should be over 3000. trigger 424 with positive, 423 should change from over 3000 ohms to unter 1000 ohms.
BUT: when i measure Ohms, I have unter 1000, and when i trigger 424, I get over 3000. So it does exact the opposite thing what it should do. replaced the ignition module a second time, no changes.
Later i found that when I unplug the ground wire connection from the mefi harness, it is as it should. same when i unplug the 2-pin connector between ignition module ion dist. and the ignition Coil.
Thought maybe bad Ign. coil,and replaced. Same. Checked harness completely, all grounds OK, no shorts, no opens in harness.
Problem is, I got the engine as is, I never seen it running properly and noone knows what happend to the engine and who meddes around with it.

Hopefully someone can help!

Thanks
Phillip

Is the ground a "good" ground? Have you checked for resistance between the location where that ground was connected, to the negative battery terminal?

Yes,
The ground is good, nearly 0 ohms. The ecm has also a reference ground to that sensor, wich is also 0 ohms from sensor to the ecm terminal.

the engine is on a crate, no battery is hooked up, it gets power from a power source.

Explain this power source. Thanks.

this is a 230V to 13.5V converter (230v because I'm in europe) wich is hooked up to the 10pin plug and to the engine battery + and - ports.

Being a MEFI 1 it might have a VST and should be using manual 16. Tell us more about the motor, VST or cool fuel, TB or EST ignition. With code 42 it should have EST

Yes it has the VST system. The distributor, I don't know what is what, so how can i identify? it is the distributor with the pick up coil and the ignition control module inside it. 2 connectors, the 4-pin is all for the ECU, Ref LOW, REF HI, BYPASS and IC. the 2-pin is + and "C" to the ignition coil.

Please don’t continue doing this.

Batteryless efi systems (motorcycle, snowmobile, etc) use a capacitor (my sled uses a 22,000uhf) in place of a battery.

Please install a battery. Also make sure alternator is charging. Your ECU and electrical system will thank you.

Okay thanks for the input!