2001 Mercruiser 5.7 TBI Won't Keep Running

[300sflyer]

Can't keep the engine running, and I'm stumped. New cap, rotor, wires and plugs. Engine fires up right away, sounds normal for 30 seconds to a minute, and then hestitates, chugs and stalls. Fires right back up again to do the same thing over and over again. Engine scan shows a code for the MAP sensor. Have it replaced the sensor with a new one, cleared the code, and the code comes right back again right away. Funny thing is, if I unplug the MAP sensor connector, it idles just fine... Fuel seems fine, and no water in the separators.

Now to make this situation even stranger, both the port and starboard engines are doing the same thing... Really scratching my head here!

Thanks for your help!

 

[alldodge]

Which MAP code 33 or 34?

Have you checked manifold vacuum?

 

[Fun Times]

Do you happen to know if the 2 engines are connected together via a data cable running between them for communicating purposes.. Most aren’t connected but some engines are… If connected, one could be bringing the other down with it… Rare though but just in case.

Aftermarket MAP sensors have been known to continue to create a similar issue as a failed OEM sensor… Is the MAP sensor “seal” in new, good, non cracked condition.

Might be time to start checking the power, ground and signal wires to the MAP.

Are both engines creating a fault code now?

Make sure all 12 volt power and ground cables and wires are clean and tight and not losing inner wire working connection do to age, movement, etc..

Any other service work done recently that might have an affect to this situation.

 

[300sflyer]

Code 34. I have not been able to find any vacuum leaks. Yes, both engines show the same code.

 

[300sflyer]

So after running both engines again, only one has the continuous fault code 34. The second engine will still not run more than 10 seconds or so though.. Acts like it is starving for fuel shortly after startup, and dies. There are no common wires between the two engines, except for a ground wire.

 

[alldodge]

Are you using a scanner or LED?

 

[300sflyer]

I am using Diacom scanner software.

 

[dubs283]

If you unplug the IAC or remove the line from the PCV valve does the idle increase? Can you keep the engine running if you advance the throttle?

 

[achris]

I am using Diacom scanner software.

Got any printouts? (we can look at)

 

[nola mike]

So after running both engines again, only one has the continuous fault code 34. The second engine will still not run more than 10 seconds or so though.. Acts like it is starving for fuel shortly after startup, and dies. There are no common wires between the two engines, except for a ground wire.

Does second engine idle ok when map sensor is disconnected?

 

[300sflyer]

Working on the engines again today, and have a bit of an update:

Once the engines are warmed up while running on muffs, both engines eventually end up stalling. Unplugging the MAP sensor, IAC sensor, or the PVC line, will not allow the engine to keep running indefinitely. If I pump the throttles I can keep them running, but then as soon as I return to idle, both engines die. Just like before, they will start right back up again.

I am working on getting a scan printout.

 

[alldodge]

Is the 5V reading as exactly 5V as it should?

Unplugging sensors should cause more errors

 

[300sflyer]

Here are two scan pictures, both from the port engine. First is with the engine off, the second with it running. Second one shows a MAP code, despite having a brand new map sensor installed. The engine stalled just after the second picture was taken.

 

[alldodge]

You have a sensor which is pulling the 5V down to 2.55V

Turn key ON and watch the 5V reading. One by one disconnect the sensors to see which one is pulling the voltage down. If no change reconnect and go to the next one

 

[300sflyer]

With the key turned to on, [engine not running] unplugging the other sensors, did not allow the voltage to increase at all. It stayed at 4.76V. Unplugging the map sensor dropped the voltage to zero, and threw the code again. Still puzzled.

Just to clarify, I only saw the 2.55 V when the engine is running.

 

[alldodge]

The 5V should be exactly 5V running or not running. The motor will not run with the 5V below spec and it will cause other unknown things to show up as a problem when there isn't one at the location. In short, with bad 5V it can cause a MAP error when there is not one

With it not coming up it could very well be the ECM.

5V sensors are; MAP, ECT, don't think you have water pressure sensor or crank sensor. As Fun Times mentioned new MAP's might be bad if aftermarket

Use a voltmeter and measure the voltage. That said need a good digital meter. I have 4 meters and one of them reads 0.4V lower then others reading 12V

 

[300sflyer]

Here is some more info. With the TPS unplugged, the engine will continue to run, however the RPM is at about 1250 and will not drop lower. While this is happening, the MAP sensor voltage is at 1.5V. Of course this throws the TPS code, but there is no code for the MAP sensor. Does any of this make sense?

Also, the starboard engine is behaving exactly the same way, doing the same tests. There is a slight difference in the voltage, at 4.8V.

 

[alldodge]

Does any of this make sense?

Yes, But
Think of it like this. The ECM makes the 5V reference (Ref) voltage to supply the exact number as a Ref. The sensors have 3 wires, Ref, sense and Ref/Gnd.

Take the TPS and the voltage going back to the ECM at idle should be lets say 0.6V. With the Ref at 2.55V the ECM may see 0.4V, so it wants to increase fuel by opening the IAC more (note IAC at 149 and should be around 40). But wait, the MAP reading says Barametric pressure is 29Hg (Edit: should be ok) and manifold pressure is 17Hg (that's high also) and that needs to be compensated for. Its a loop where there is no fix, so the ECM shuts the motor OFF

 

[300sflyer]

Here are the scans from the starboard engine. It is behaving exactly the same way, only it throws code 22 TPS when the connector is plugged in and the engine is running.

Is it possible the TPS is defective on both engines? Could the ECMs on both engines be bad?

Here is a video of the engine running, while plugging the TPS back in.

 

[alldodge]

Apologizes
I'm seeing MAP voltage and thinking its 5V Ref.

One motor idles close 2 600 and dies, and the other idles around 1200 and doesn't die. The posting of troubleshooting tips above list bad connection and to check CKT 416 and 432 which is the 5V Ref