[John Rush]

Hello to all,

I'm a bit stumped as to what is causing my problem. As always everyone on this forum is beyond more than helpful.

I'll jump right into the issue:

I have a 1996 Mariah Shabah with the 5.7L EFI throttle body injection with vapor Separator tank.

When first firing up the motor on a cold start, it starts and runs fine while in the high idle stage for the first 20 seconds or so until motor is warm, once the motor settles down it starts to get a noticeable vibration almost like a skip. (no ticking or knocking sounds) just sounds like the cylinders aren't firing correctly.

This first started after a 30 min trip back to the dock running at 3,500rpms, once throttled down to the no wake zone it started this act. Soon after it started it cleaned its self up, and idled correctly. (assumed i just sucked up a piece of junk in the fuel filter) next trip back out it started again after a little journey up the lake and hasn't stopped since.

The boat will run beautifully anything over 2,000rpms and doesn't skip a beat.

So far this is what i have done:
First: Pulled spark plugs, the plugs from the right ride of the motor were just about soaked with fuel, left side were nice and dry. Replaced, ran a little better, no major change.

Second: metered the TPS sensor, metered correctly ( 0-5v from close to open)

Third: Connected a stethoscope and watched spark coming from the distributor (looked good, with a normal trend)

Fourth: Metered impedance on the coolant temperature sensor ( read a little now )

Fifth: Removed the MAP sensor, plugged open hole on throttle body with finger, and started motor (sensor still plugged in) motor seemed not to change idling at all, which really threw a red flag to me.. I then blew and sucked on the sensors opening port to try trick the motor, no change in anything.

What really stumps me is that it runs like a chapm anything over 2Krpms, and when the motor is cold high idling. The motor will never stall or cut its self off, just runs rough.

Something is telling the motor to continually dump fuel when not needed..

Thank you in advance!

[finleydj]

Clueless in St. Louis, good luck though. I feel for ya

[Oregondunes]

two things come to mind:

1. Cool fuel. Does your motor have it ? If so, the cooling system may have stopped working.
2. Fuel pump. Pump maybe experiencing some loss in voltage or is just worn to the point it cant provide the adequate pressure necessary at idle when the PSI is lower then at full / open throttle.

Fuel pressure is easy enough to check. Find the test port on the front (top) of motor and connect a manual gauge to verify.

Cool fuel is not so easy to test.

[John Rush]

Just in case anyone was following this thread or has the same problem in the future it has been resolved.

I replaced the MAP ( Manifold air pressure sensor) As well as the coolant/ water temperature sensor.

ran like a champ all holiday weekend long.

Thanks Guys

[ColoradoDiablo]

Thanks for posting the results John!
If you don't have a scanner diagnostics on the EFI/MPI engines can really be a bear.
Congrats!

What did the MAP end up costing you?

[John Rush]

Sure thing!
I have a friend at NAPA that got me the house brand one for $34.99 and the water sensor was like $15 if I remember right. As for that tool I ended up designing one with a 9v battery, LED and a resistor.
I'll post a write up on how I built it.

Thanks Colorado.

[ColoradoDiablo]

Cool John ,
That will definitely come in handy to a lot of the fuel injection members.
Might think about posting Napa part numbers.

Thanks again!

Might think about doing a play by play , if your up to it.