Hi,
Trying to troubleshoot a recently purchased 540.

Problem is a backfire through intake while applying throttle under load from idle to roughly 1800 RPM. Seems like the motor is lean at this point. And was thinking Vacuum leak. I have an O2 sensor but also have some reversion so do not trust the reading at or around idle, but it does indicate LEAN condition during this RPM range.

MEFI-3 ECM. Cam profile 240/248 .621/.632 112 LSA. 4500 FAST throttle body. Stock GM heads

MAP at idle reads about 21 inches and 3.2 volts, however a mechanical gauge reads 8-10 inches.
Should the vacuum readings match or has the MAP value been programmed into the ECM?

Thanks for any advice.

Yes. Have tried base timing from 8-10 degrees.

It’s a new motor to me. Ran fine on the hose (no load) before purchase.

I set up the fuel system. A1000 at 40 psi at idle and holds while accelerating. Fuel system set up exactly to Aeromotive specs. Have tried moving up on regulated pressure. 45, 50, 55, etc....

Compression readings and leakdown results all good.

Yes I have tried another known good MAP sensor. Same readings on different MAP sensor.

The ECM was programmed by Innovation Marine. I have also tried another ECM with an ASM tune. (540 with 525 EFI cam) It seems to work better with this computer on it but ultimately still have the backfire through intake.

Does anyone know if the MAP reading/voltage is programmed into the ECM or should this be an organic reading created by vacuum conditions of the motor.

Thanks

Looking at the MAP sensor vacuum reading on a Rinda, should MAP vacuum and the mechanical gauge vacuum match pretty closely?

Trash,
Yes. 1 Bar MAP sensor. Tried a known good sensor from one of my other spare motors and have virtually identical MAP vacuum readings at idle. Also 2 different ECM’s and both display roughly the same MAP vacuum readings at idle. I don’t have the ability to tune either ECM as my understanding is that they are password protected.

Yes and it is tee’d to the MAP sensor. Have tried capping the side of the tee that goes to the regulator so that only the MAP sensor sees the vacuum. No change.

Have not looked to see. I do not have the MEFI burn software. That is good information. Thanks.

Pull the vacuum line off the regulator. Then adjust the fuel pressure. It’s an AEROMOTIVE bypass style. Return fuel to tank.

First thing I did after buying the motor was to pull the injectors and had them cleaned and flowed. He said they were definitely gummed up but that they ultimately cleaned up and flowed fine. The motor had sat a couple of years so figured it was due. Thanks for the suggestions. I really don’t know if this is computer related or not so very well could be something simple that I am missing. I don’t have a lot of history on the motor, like what the base timing should be, etc. Previous owner ran fuel pressure at 40 psi and said that was what was recommended by builder.