2001 Mercruiser 5.7 TBI Won't Keep Running

[300sflyer]

Both motors will run indefinately at about 1200 RPM, with the TPS unplugged. Both motors return to normal RPM once the TPS is plugged back in, but then stall out seconds later. Both throw the TPS code when it is unplugged. The port continues to throw the MAP code when it dies. The starboard does not.

 

[nola mike]

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

I'm thinking a bit differently on this. Is that 2.55v the reference voltage, or is it the voltage that the PCM is seeing from the sensor? Easy way to check would just be to rev the engine and see if that voltage fluctuates. To me it seems like the manifold pressure is high at idle, like you said. Also, the idle speed is low. Fuel input is going to be based on map, so a bad MAP reading will result in the wrong fuel mixture. Question is, is that map Reading correct or not? I would put a manual gauge on there and see how they match up

 

[nola mike]

Both motors will run indefinately at about 1200 RPM, with the TPS unplugged. Both motors return to normal RPM once the TPS is plugged back in, but then stall out seconds later. Both throw the TPS code when it is unplugged. The port continues to throw the MAP code when it dies. The starboard does not.

What does the std engine show for a MAP pressure at 600 rpm? With the TPS disconnected the PCM is using a fuel map that guesses the TP.

 

[300sflyer]

As I recall, it was about the same as the port engine at 17.

 

[alldodge]

First and foremost we are learning nothing by unplugging a sensor. All that is doing is causing the ECM to get mixed info, so anything that happens afterwards means nothing

As FunTimes said previous, is there a data cable between the two motors?

Do the motors have a MEFI 1 or 3?
Have a motor serial number?

Normal readings

 

[300sflyer]

There is no data cord between the two engines. MEFI 3 S/N 0M017939

 

[nola mike]

Time to start running down that troubleshooting chart that fun times posted. First step is to see whether or not the PCM is getting accurate map readings by plugging in a vacuum gauge. If not, you need to determine what is causing the discrepancy.

 

[achris]

Is the new MAP sensor genuine or a/f market? I don't see a 'sensor voltage' reading on those printouts, but it may be that MEFI3 doesn't report it. Would be nice to see the sensor voltage, even with just a DMM..

Have you checked fuel rail pressure?

Chris.

 

[alldodge]

Key ON, Voltage reading -
Pos meter probe on J2-4 and Neg probe on J2-3, should read 5V
Pos meter probe on J2-19 and Neg probe on J2-18, should read 5V

Doing the above will check supplied power and grounds to sensors

Ohm out sensor wiring

 

[nola mike]

Is the new MAP sensor genuine or a/f market? I don't see a 'sensor voltage' reading on those printouts, but it may be that MEFI3 doesn't report it. Would be nice to see the sensor voltage, even with just a DMM..

Have you checked fuel rail pressure?

Chris.

Are you looking for reference voltage to the sender or sensor output? I mentioned earlier (and I think AD confirmed) the MAP voltage reading on the lower right is the MAP output

 

[achris]

Are you looking for reference voltage to the sender or sensor output? I mentioned earlier (and I think AD confirmed) the MAP voltage reading on the lower right is the MAP output

The 5v reference supply to the sensors...

 

[300sflyer]

Chris, The new MAP sensors are aftermarket made by BWD. OEM ones are no longer available. [861249A1] I have not checked fuel pressure yet. I made the assumption it is ok, as they will run at 1200 RPM just fine. [TPS disconnected].

 

[achris]

Why were the sensors replaced? (and since you replaced them, on both engines, both engines are having problems... Coincidence, I think not)

 

[achris]

Why were the sensors replaced? (and since you replaced them, on both engines, both engines are having problems... Coincidence, I think not)

Have you tried putting an old sensor back in one of the engines?

 

[300sflyer]

I would not have replaced the sensors if there wasn't a problem in the first place... Putting them back does not solve anything.... I tried that.

 

[dubs283]

Its possible you have an issue with the wiring harness, testing circuits for a short to ground is a good starting point.

You are almost at the point of swapping components/parts between the motors. Twin engine setups are nice in that regard as you have a complete parts room in the bilge

 

[300sflyer]

Alldodge, The voltage at both TPS and MAP sensors on both engines, measures 5.05V with a digital volt meter.

 

[300sflyer]

Where does one tap into the fuel system on a TBI engine to check the fuel pressure? Unlike an MPI engine, there is no fuel rail with a schrader valve.

 

[nola mike]

I would not have replaced the sensors if there wasn't a problem in the first place... Putting them back does not solve anything.... I tried that.

Where does one tap into the fuel system on a TBI engine to check the fuel pressure? Unlike an MPI engine, there is no fuel rail with a schrader valve.

Are you planning on doing the troubleshooting steps posted by @Fun Times ? Not understanding why you aren't trying to find the actual cause of the one code being thrown

 

[alldodge]

Alldodge, The voltage at both TPS and MAP sensors on both engines, measures 5.05V with a digital volt meter.

Do the wires ring/ohm out?

Where does one tap into the fuel system on a TBI engine to check the fuel pressure? Unlike an MPI engine, there is no fuel rail with a schrader valve.

If you have a plug (item 3) it can be removed and pressure gauge attached
https://www.mercruiserparts.com/bam/subassembly/31844/2704/110

Otherwise you would need something like the Merc fitting to put inline where the fuel line attaches to TB