7.4 Horizon inboards surging at Fast idle..Basic questions

[rothfm]

Hi,

Trying to help a family member with a new to him boat. 1998 Sea Ray 33 express with twin 7.4 Horizon MC inboards. We have done a lot of research on the topic and understand the interrelationships of AID, Throttle plates, TPS etc....We replaced AIC's for a Sirge/Stall symptom because when throttling back to dead idle (in Neutral), sometimes the motors would stall. The New AIC's seemed to fix the stall Sympton, and that area was fairly Grungy.

However, what we are left with is the same "lope" or "surge" we had. if you remain in Neutral, and bring up the RPM some. IDLE is fine at 6-700 with no surge. But if you just bump up the throttle a bit, there is a very noticeable fast surge up and down. This occurs while bringing the throttles up to about 1200-1600 I believe. After that the Surge seems to settle out. The same exact scenario occurs on Both ingines.

While still tied up, we put it in gear (under load) and cannot replicate the Surge...Seems to be only in Neutral without load.

SO THE QUESTIONS ARE THIS:

1) Is any of this normal. (other posts seem to indicated it "can-be") My feeling on this is that those that believe it is, have just not got to the
root cause. CURIOUS.

2) What have others done, or looked deeper into to solve similar described Surge issues?

The motors only have 500 hours on them, are very very maintained (we have all service records). All hoses and clamps are good. Throttle plates appear fully closed at idle. (but have not examined their motion at different throttle positions).


NOTE:
This boat was brought from its purchase location, to home-port on a rough trip through New England open-water 113mi, in April and ran flawlessly. This doesn't seem like a usability issue, as I cannot see many scenarios where you would be much above Idle in Neutral. Again, Doesn't seem to happen In-gear under load. But it is annoying. Why doesnt a fuel injected automobile do this if it is a normal behavior here?

 

[Bt Doctur]

Why doesnt a fuel injected automobile do this if it is a normal behavior here?

Because cars dont float

Check the fuel pressure at the rail ,the surge could be caused by a vac condition ot a bad pressure valve
check/change the fuel sep filters and also check the anti-syphon valves

 

[rothfm]

HAHA, Thanks BTdoctor...

I Can look at the rail pressure next visit, I've looked into vac lines/leaks. Can you or someone elaborate what pressure valve/location? and same for anti-syphon valves? Again, this is the exact same issue, on both motors to a "T". (very well maintained motors)

Im a decent outboard guy, trying to help him out!

 

[Bt Doctur]

 

[Bt Doctur]

The anti-syphon is the very first fitting at the gas tank outlet. Its a spring loaded check ball that sometines stays halfway closed restricting the fuel flow

 

[rothfm]

Thanks DOC...

Will look into that anti siphon valve at the tanks...I Would think if they were restricted, our 113 Mile open-water trip at 3400 rpm, sucking LOTS of fuel, wouldn't have gone too well. I'm Not sure if these have the Cool Fuel option. Will have to look. We will try and get a fuel pressure reading at the rails at several stages of RPM's, next visit. That should give me an indication of the state of the Fuel Pressure Regulator, I imagine.

Thank you!

 

[alldodge]

This occurs while bringing the throttles up to about 1200-1600 I believe. After that the Surge seems to settle out. The same exact scenario occurs on Both engines.

There is a vacuum line which goes from the fuel pressure regulator on the cool fuel module up to the intake and/or Tee into a fuel damper. If you have the L29 model motors then there could be other components (could use serial number). Check to see if this vacuum line is intact. The way the hose is routed it gets brittle over time and will leak or break apart.

On top the motor under the intake plenum look for a shrader valve. The valve is where fuel pressure is tested. Pressure should be 43psi at 1500 or so rpm

 

[rothfm]

Thanks, when im at the boat again, will examine that line mentioned and get a rail reading.

So im assuming now that this surge is NOT typical as some posters seem to believe.

The serial number off one engine is:

L008277

 

[alldodge]

Look forward to hearing, the motors are not the L29 models so that helps.

 

[rothfm]

We did both IAC and TPS's on both motors and the pvv's for good measure. Seems a tad better but still does it noticably at the dock not under load. Away from dock its hardly noticable.

The fuel rail pressures were verified by family member with MC Support and acceptable.

Almost like the ECM is searching at that range and compensates up/down repeatedly.

 

[alldodge]

It's not normal for it to surge, good thing it doesn't do it in gear, only in neutral.
Since both engines are doing it is puzzling. They use a common ground between both motors, and corrosion can be an issue, same as if alternator voltages are erratic. Other then that I would connect a scanner to the motor and watch the readout to see if something could be found.

The manual troubleshooting guide indicates injector harness testing along with injector balance testing.