05 Response has stumble after engine swap

[Bluefishcay]

Did an engine swap over the winter on our response.  Previous owner had frozen the block.  Monsoon 340, efi.  Boat fires and runs well, but has a stumble or mis when the throttle first opens.  Runs out fine in top, probably faster than it did before the swap.   Being an open loop efi, not sure where to start on adjustments or if there are any.  Cap, rotor and plugs are all new.  Fuel is fresh.  Suggestions on where to start?   I do have access to a computer if needed.  

Edited May 21, 2023 by Bluefishcay

[Bozboat]

clean the throttle body with a can of throttle body cleaner?

[PNWoke]

Sounds like having a look at the knock sensor/s might be a good idea.  A friend just found a bad knock sensor on his 2005 Supra which was causing the same symptoms you are describing.

[Bluefishcay]

Interesting on the knock sensor.  This one is new and not the one that was on the previous engine.  

[csleaver]

A vacuum leak could cause hesitation or a problem with one of the engine sensors/circuits that the ECM uses to change the fuel injector BPW multiplier (like the Engine Coolant Temperature, Manifold Absolute Pressure, or Throttle Position Sensor).

[Bluefishcay]

Update on the issue.  Boat stumbles just off idle and doesn’t seem to have quite the snap when pulling skiers.  Have now cleaned throttle blade, intake, iac.   Checked for intake leaks, replaced fuel pressure regulator, fuel filter, fuel pump, map sensor.   Verified new knock sensor is good.  Have run it with diacom and values look correct.  I believe the stumble is lean.  I have put a gauge on the fuel pressure and it’s low, 55 psi at idle and key up vs 58-62 per the manual.  May be simply the gauge reading low  The gauge swings wildly on acceleration (40-70) and then settles lower 52-53 at higher load.  Any ideas on what could be the issue?  Maybe fuel regulator circuit not correct, poor fuel flow from the tank?  
 

system is open loop, so no easy way to up fueling.  

[justgary]

9 hours ago, Bluefishcay said:

The gauge swings wildly on acceleration (40-70) and then settles lower 52-53 at higher load.  

This should not happen.  Check the regulator again.

[Bozboat]

14 hours ago, Bluefishcay said:

Update on the issue.  Boat stumbles just off idle and doesn’t seem to have quite the snap when pulling skiers.  Have now cleaned throttle blade, intake, iac.   Checked for intake leaks, replaced fuel pressure regulator, fuel filter, fuel pump, map sensor.   Verified new knock sensor is good.  Have run it with diacom and values look correct.  I believe the stumble is lean.  I have put a gauge on the fuel pressure and it’s low, 55 psi at idle and key up vs 58-62 per the manual.  May be simply the gauge reading low  The gauge swings wildly on acceleration (40-70) and then settles lower 52-53 at higher load.  Any ideas on what could be the issue?  Maybe fuel regulator circuit not correct, poor fuel flow from the tank?  
 

system is open loop, so no easy way to up fueling.  

On my 99 Indmar Monsoon, I have this issue when the cap and rotor are worn, not sure what distributor cap and rotor you have, but you might check into when it was last replaced 

[Bluefishcay]

On 7/14/2023 at 2:10 PM, Bozboat said:

On my 99 Indmar Monsoon, I have this issue when the cap and rotor are worn, not sure what distributor cap and rotor you have, but you might check into when it was last replaced 

Both new 20 hours ago at the engine swap.  

[Bluefishcay]

On 7/14/2023 at 8:57 AM, justgary said:

This should not happen.  Check the regulator again.

Going to tear into the regulator block this week.  Regulator is new from bakes.  Had the same symptoms (stumble and low power) with old regulator but did not have the fuel pressure gauge to verify pressures were the same before replacing the fpr.  Pressure was the same with both fuel pumps.  Seems like the regulator is not functioning correctly, but as it’s new and ran same before replacement  I am skeptical that it’s bad.  

Edited July 16, 2023 by Bluefishcay

[Bozboat]

1 hour ago, Bluefishcay said:

Going to tear into the regulator block this week.  Regulator is new from bakes.  Had the same symptoms (stumble and low power) with old regulator but did not have the fuel pressure gauge to verify pressures were the same before replacing the fpr.  Pressure was the same with both fuel pumps.  Seems like the regulator is not functioning correctly, but as it’s new and ran same before replacement  I am skeptical that it’s bad.  

Fuel pressure regulator? I thought high fuel pressure was a sign of a bad fuel pressure regulator?

[Bluefishcay]

7 hours ago, Bozboat said:

Fuel pressure regulator? I thought high fuel pressure was a sign of a bad fuel pressure regulator?

Fuel pressure is low.  Spec is 58-62.  On my gauge, pressure is 55 at best.  As a friend pointed out, pumps make flow, not pressure.  Pressure is a sign of restriction, so low pressure is either too much flow back to the tank or the pump not keeping up with required flow or cavitation in the pump due to line restrictions.  

[justgary]

5 hours ago, Bluefishcay said:

As a friend pointed out, pumps make flow, not pressure.

Pumps make rated flow at a rated pressure.  You can get a high-flow, low pressure pump, or a low-flow, high pressure pump.  It all depends on what you need.

What you need in your boat is a pump that consistently creates enough flow at a pressure higher than the engine needs.  The regulator reduces that pressure by returning some of the flow to the tank (or directly to the pump).

@Bozboat is correct in that the typical scenario is a bad pump if low pressure, and a bad regulator if high pressure.  Since you have changed the pump and the regulator, your issue may be the fuel lines.  They tend to decompose over time, especially with alcohol fuel.  Perhaps you can rig up a test tank and try that to see if it works OK.

[Bluefishcay]

Swapped in an adjustable regulator as there are lots available for VW’s which use the same regulator shape.  Was cheaper than the indmar one btw. Bumped the pressure up to 62 psi and had improved throttle response.  Now it doesn’t die or pop off idle.  Still a little lean as it’s not perfect off idle.  Need to pull plugs and see how they look and if I maybe have a bad injector.  

Edited August 15, 2023 by Bluefishcay