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5.7 merc hard starting

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flatheadcat78

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Looking for some ideas. I have a throttle body fuel injected 5.7 mercruiser. Model 0L093978. Anyways when i hit the key it will crank and then die. It will repeat this over and over again. If you give it a little throttle she will crank and idle. Fuel system is good, pickup, lines, filter, injector pulse. I pulled the IAC out of the thottle body to look at it. It was pretty clean, but it was fuel soaked?? I was a GM tech years ago when the trucks had the TBI motors and I never seen a IAC that was wet. So what gives? I now these systems are similar but why is the IAC wet? TB problem? Fuel regulator problem?
And yes the boat runs great otherwise. Just the starting issue.
Thanks for any help! Don
 
flatheadcat78 said:
Looking for some ideas. I have a throttle body fuel injected 5.7 mercruiser. Model 0L093978. Anyways when i hit the key it will crank and then die. It will repeat this over and over again. If you give it a little throttle she will crank and idle. Fuel system is good, pickup, lines, filter, injector pulse. I pulled the IAC out of the thottle body to look at it. It was pretty clean, but it was fuel soaked?? I was a GM tech years ago when the trucks had the TBI motors and I never seen a IAC that was wet. So what gives? I now these systems are similar but why is the IAC wet? TB problem? Fuel regulator problem?
And yes the boat runs great otherwise. Just the starting issue.
Thanks for any help! Don
Click to expand...

when the engine is cold the injector open time is increased to give the cold engine more fuel less air...If the ECT is defective this can mess up the fuel mixture....causing poor start...

THIS IS TRUE ON MY VEHICLES 5.7L ...no sure on this boat engine...
 
Fuel delievery strategy is consistent across the GM line using the Delphi ECU's. The ECT cause and effect is the same in the marine apps.

The injectors had to dump fuel for a while or something is leaking for the IAC to get 'wet'. I've never seen a 'soaked' one either.

I'd check the computer's inputs with a scanner. ECT, TPS, and MAP, in particular. They may provide reasonable date and not set any codes - just check the values to see if they are accurate. (ECT provides 50 deg value to ECU but temp outside is 95 deg; end result is poor starting due to excess fuel.)
 
Thanks for the input. But with out a scan tool not much I can do. The thing that got me stumpped is the boat runs great otherwise. My best guess right now would be in the throttlebody itself. regulator maybe? Thanks again and ill keep up******. Don
 
Plenty you can do:

Put vac gauge on and make sure there is no minor leak. minor leaks usually effect idle. Makes MAP read low (for that RPM and condition), ECU thinks load is present and turns up the fuel.

Measure sensors' outputs with a good voltmeter - there are 'nominal values' in most of the service manuals
 
flatheadcat78 said:
Thanks for the input. But with out a scan tool not much I can do. The thing that got me stumpped is the boat runs great otherwise. My best guess right now would be in the throttlebody itself. regulator maybe? Thanks again and ill keep up******. Don
Click to expand...

the ECT is the engine coolant temp. this is a resistor that changes value with temperature. this costs less the 20.00 ...with the temperature resistance chart you with an ohm meter can measure the resistance of the two pins on the sensor with it in the engine cold , then repeat when hot . compare these readings to the temperature resistance chart ..with the GM Lt1 engine I own, the ECT at 70deg f reads 3500 ohms.

then at 140deg f 650 ohms

so when cold high resistance as it heats up lower resistance ..

if the connections are corroded and the resistance is very high it is possible too much injector open time is occuring flooding out the engine.. make sure the connector is clean pins not corroded wires not damaged ..
 
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