99 7.4L MPI will not rev up, runs smooth.

A

Adamo

Member
Jun 13, 2009
37
Pacific Northwest
Boat Info
2002 410 Sundancer
Engines
350hp 3126 Cats
Hi all, 7.4L MPI in 270DA Sundancer just rebuilt is continuing to behave as it did prior to the rebuild and will not come up to full RPM. Engine runs smooth and does not miss or backfire but will not come up over 3600RPM or so. Boat behaves like it is pulling a log barge and barely planes. This was an intermittent problem in the past but since recent rebuild will not ever come up to WOT throttle spec RPM. Mercruiser diagnostic tool shows all is well with the ECM etc and suggests this is a mechanical problem. It seems to not be getting enough fuel but does not miss suggesting to me its not injectors. Any help much appreciated. My mechanic is pulling out his hair.
 
Try a different propeller. That's assuming that the hull and running gear are clean and free of any growth.
 
Just thinking "out loud" here:

I assume your guy checked fuel pressure?

Off the wall shot here, but does the prop spin easily by hand when the engine is off and in neutral?

Is the throttle linkage opening fully?
 
Prop is stock and the boat is freshly bottom painted. Also prior to rebuild the boat reached full RPM and then from time to time would behave as it does now. Very intermittent problem. Fuel water filter changed, fresh gas, new plugs, new wires, new cap and rotor even new TPS(Throttle Position Sensor).
 
Fuel pressure on the fuel rail is approx 35psi and does not drop off when at WOT but engine will still not reach any higher RPM than 3600 and sometimes barely 3200. Prop appears to hold and harmonic balancer is tight and holding. Timing has been set using Mercruiser diagnostic tool in service mode and appears to be fine. Throttle is fully opening according to TPS.
 
Final bit of detail would be that rebuild was due to water in the cylinders leading to engine seize. Very bad fuel problem due to boat sitting for 2 seasons. Tank has been flushed and fuel lines blown out up to the filter.
 
do compression test. then run boat at 2000rpm. then pull spark plug wires to make sure it is running on all cylinders.
 
Thanks, we have done a compression test and the number 3 cylinder is a little lower than the rest but we have yet to even break in this engine as it has less than 1 hour on it. It has been confirmed all cylinders are firing. What I have since found out is that there is corrosion in the Cool Fuel system and the fuel pump so even though we are getting good fuel pressure it may not be adequate. Anyone having any experience with water in the fuel that was left to sit inside the fuel system would likely know what my issues are. We are changing this all as we speak but I am still wondering if we can test the injectors one at a time rather than just replace as they are over $200 a piece. I have learned from this you can never change your fuel filter too often, especially if you let the boat sit for a long period of time.
 
Yes, you can check the fuel pressure while the boat is running. You just need the right pressure gauge.

Yes, you can test the injectors individually. It is quite easy.

  1. You need to remove the upper half of the intake manifold to expose the injectors.
  2. Connect a fuel pressure gauge.
  3. Unplug the wires from the injectors.
  4. Connect the Rinda EFI tester to diagnostic connector.
  5. Connect the fuel injector test wire from the jack on the Rinda to an injector.
  6. Crank the engine until the gauge shows nominal fuel pressure.
  7. Fire the injector test button.
  8. Record the pressure drop on the gauge.
  9. Unplug the injector test wire from the just tested injector and move to the next one.
  10. Repeat from step 6 until complete. Must start from the same pressure setting each time!
  11. Equal pressure drops indicate equal injectors and probably good.
  12. A smaller pressure drop shows a partially clogged injector.

Best regards,
Frank
 
Something else that just came to mind. Last year we had a customer who was having a similar problem - but his "top" RPM was 4,200. Compression test was fine. It wasn't until a leak-down test was performed that the problem was solved. As it was still under warranty, I don't recall any further investigation being done, so I don't know if it was intake or exhaust or rings.

I saw a neat idea on that Horsepower TV show. They took an old spark plug, nocked off the insulator and hollowed the rest out. Then they welded a standard air fitting onto the plug. Screwed the plug in, attached an air hose, filled the chamber with air and listened for where the air was escaping.
 
He did say the problem is intermittent. Doesn't that point to an electrical problem? If it was a compression issue the engine would always be rpm limited. A fuel flow issue could be off and on but wouldn't it still be somewhat predictable?
 
capz said:
He did say the problem is intermittent. Doesn't that point to an electrical problem? If it was a compression issue the engine would always be rpm limited. A fuel flow issue could be off and on but wouldn't it still be somewhat predictable?
Click to expand...

If I read correctly, it was very intermittent before his rebuild, but now is all the time.

What was done on the rebuild?
 
But he had the problem before the rebuild. Then it sat for two seasons. Any corroded wiring would only get worse. Did the mechanic check voltage at the TPS sensor from idle to full throttle?
 
Not discounting what you're suggesting - electrical stuff can be a real PIA!
 
Last edited:
Thanks Frank. I will share this with my mechanic. Since my last note he elected to pull off the entire fuel system and inspect piece by piece. Thus far we have found major corrosion in the Cool Fuel system Mercruiser uses and we have therefore ordered a new kit which I understand includes the Fuel Pump. As far as the injectors he has told me he is taking them to an injector shop to get them tested. I was not aware that you could do this. Lastly I have been reading here and other sites that this cool fuel system has hads some flaws and if I have it right it operates as a heat exchanger using sea water to cool the fuel and if it were to fail due to corrosion the seawater enters the fuel and therefore the engine. I just suffered a blown engine with seawater as the apparant culprit. This is all very frustrating and very expensive. Hoping to get back on the water soon and have some money left for fuel.....
 
To the question on the rebuild this is an L29 block that was completely rebuilt with new pistons etc etc. I elected to rebuild my own block and wait for it. We have checked the voltage and the throttle position sensor was just changed as it was thought to be the original problem after the rebuild but no such luck. My mechanic insists that since we get about 36PSI fuel pressure at WOT and it does not fall back at WOT that we are getting lots of gas. Others have suggested that fuel pressure on the rail does not equate to fuel volume and we may not be getting the volume we need even thought the pressure is good. Not sure of the kit that is coming might be the fix or if given its sorry state the injectors are sure to be at least partially plugged and I may have more than one problem.
 
My mechanic insists that since we get about 36PSI fuel pressure at WOT and it does not fall back at WOT that we are getting lots of gas
Click to expand...

If that was the case there would be no need for fuel volume tests. This is done on vehicles when a fuel delivery problem is suspected and pressure is good. It involves connecting a fuel flow gauge or emptying gas into a measurable container. Assuming voltage and current draw at the pump is good, I would ask him to do a volume test before he starts just throwing expensive parts at it. Especially since the lines and tank were flushed. I'm assuming the vent line and anti-syphon lines were checked?
Also where do you boat? If the outside temperature gets up around 90 could it be a vapor lock issue?
 
I'd say the next step is to check timing and the engine advance curve. On the MPI engines, base timing is set by the thick film ignition module on the distributor. Once the engine is running, ignition timing is controlled by the MEFI-3 module. I believe the switching point is when RPM exceeds 300. If there is a fault in that circuit, the engine would run with only 8 degrees of advance, which is a small amount. That would cause a lack of power.

Have the tech connect the Rinda tester to the engine and ensure that the MEFI-3 module is not in service (set timing) mode. Then he should connect his timing light and watch the timing scale. The timing should advance as the throttle is opened. If it not, that would be your problem.

Best regards,
Frank
 
Hi Capz,

I boat in the Pacific Northwest so I don't think it is vapour lock issue. I must say I wondered about a volume test instead of just a pressure test. The vent line seems to be fine but I have not checked the anti-syiphon valve. I just discovered that this is a potential problem but I am not sure how to check/repair this item and why exactly this would be a problem.IE how it would show good pressure yet provide poor volume??
 
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