vacuum readings

[25two.stroke]

What should the vacuum read on a horn motor idling with the butterfly shut?
Have a 2.4 BP with an ECU that I can read with a DDT. I suspect the map sensor is out in the ECU cause its running super rich at idle.

The DDT reads the map and the throttle position in volts. I don't know how it can read the throttle position in volts because it doesn't have a tps. The only way it knows throttle position is through the MAP sensor/vacuum line.
Anyway, it reads the throttle position at 1.02 to 0.97 volts and the MAP at around 1 in-Hg

Based on the old A7 ecu for a BP it shows that the throttle should read 0.99vdc at idle on one of those vacuum TPS modules, but does anyone know if that is the same for the modern digital boxes that have the MAP sensor built into the box?

[ShadowFlier]

What does the DDT say for temp sensor? If that goes bad the motor will run rich.

[25two.stroke]

What does the DDT say for temp sensor? If that goes bad the motor will run rich.

Temp sensor is right on the money.

[David - WI]

I don't know the answer but can't you apply vacuum to the MAP sensor with a MityVac and see if the ECU responds on the DDT?

[25two.stroke]

I don't know the answer but can't you apply vacuum to the MAP sensor with a MityVac and see if the ECU responds on the DDT?

Yes, and it does, I just don't know how much its supposed to be. I guess I can pull a vacuum and compare the analogue reading from the needle to what the DDT says and then if they are close assume its good?

Someone out there has to know what the vacuum is supposed to be...anyone who writes fuel maps for these things.

[Stoker boy]

Did you make sure the vacuum line is not plugged or the horn fitting is not plugged?

[25two.stroke]

Did you make sure the vacuum line is not plugged or the horn fitting is not plugged?

Yes

[Stoker boy]

Is there a way to change vacuum on the engine that’s running? Other than when the butterfly opens.

It’s been many years since I put a vacuum gauge on a 2.5 but it seems it was like 3 to 5 but I can’t remember for sure. I also haven’t run a mercury box in about 15 years if I start my 2.5 up in the next few days I’ll put a gauge on and let you know.

[25two.stroke]

Nah its a bone stock horn motor. No vacuum pump or any of that stuff. I ran another one of my horn motors this weekend but it doesn't have a readable ECU. Kim at Brucato says 3-4 in so we are all in the same ballpark. That reinforces my idea that the motor is running a bit rich cause it thinks the throttle is open. I will back check everything I've done and make sure that vacuum line and fittings are clear. Maybe that explains why the motor is super hard to re-start when it is warm too.

[Stoker boy]

Sounds Good.

[25two.stroke]

So I did some swapping all around now that I have a different motor running on the o2 sensors...I put this bp ecu I was questioning on the o2 sensors and I expected it to be super rich at idle. It read damn near perfect at idle: 15.5 and 13 revved up.

Warm re-starts was no longer an issue. all around the ECU seemed perfect on the 2.4 ovalport motor which has 02 sensors, while its a pain in the a** on the bridgeport. Digital ecu with no external tps. Both motors have brand new sets of injectors. I have no idea why this ecu is not happy on the BP.

Anyway, I am still experiencing a voltage drop which I have talked to a few people about. I read consistently 11.2 volts at the ECU. Engine charging system is putting out and the actual engine voltage and battery voltage are at 12.1v with the fuel pump on. I have tested the wiring harness, tried a different wiring harness, and cleaned all the grounds and I still see a voltage drop inside the ECU. Maybe I shouldn't worry about it cause its running okay, but I just can't get over it. You will see the numbers in the pics of the DDT.

Another thing, the map psi reads 12 with the butterfly shut. This contradicts what Kim at Brucato said because 12 psi ~= 24 in-Hg and she said it should be 3-4 in-Hg. I was thinking she could have gotten in-Hg and psi mixed up, but even so I am way over 3-4 psi.
I want to know. I want the truth. What should I be reading. In my head I want to not care cause it runs good and has good o2 readings...but I also want to know what it should be for future reference.

Engine I was running it on is a carb 2.4 mod vp with block bleeders plugged off, chrome cylinders, D ported pisons, rod slots opened, 125 psi compression, exhaust chest cut, 1" reed spacer and angle cut intake.

[flabum1017]

Well, atmospheric pressure is 14.7psi then subtract your 12.1psi..... 2.6psi.

[25two.stroke]

Well, atmospheric pressure is 14.7 then subtract your 12.1..... 2.6.

Duh. I musta had a brain fart when thinking about this one. MAP = Manifold Absolute Pressure. It doesn't control for atmospheric pressure; of course.
Thank you.

Anyone know what the 2 psi readings, one above and one below the MAP number? What those even mean? Are they for engines that have exterior atmospheric pressure sensors?

And how in the world does the ECU know the coolant temp? My tan wires go to the dash, not into the ECU harness. I didn't pay attention to if that number was moving, but none of the 2.4 efi or 260 wiring harnesses I have ever seen have a way for the ECU to get engine temp info.

[flabum1017]

From what I remember ( i haven't worked on one in a while), the temp sender wire goes to the ecu, then comes back out to go to the dash,,,,, I could be wrong, but pretty sure that's how it's wired.

[25two.stroke]

On a lazer injection motor it does, but not on a horn motor.