Merc 7.4 EFI motor question ??

37 Dancer

37 Dancer

New Member
Oct 6, 2006
351
South Shore , Long Island
Boat Info
1995 Sea Ray 370DA
Engines
7.4 EFI's
Im having an issue with my starboard motor that is driving me nuts !!!

I recently changed all my risers and manifolds on both motors....replaced the 3" riser blocks with 6" riser blocks to give myself a little more pitch.....all went well.....motors started and ran fine....I used the boat a few times...no problems.

Then I let the boat sit for about 3-4 days......went out and started the motors....port started and ran perfect......but my starboard motor cranked and started on maybe 6/7 cyls...for about 30-40 seconds....the cleared up....and ran perfect. Right away I thought water in a cyl. So we pulled the spark plugs.....NO water. Then we pressure tested the risers and manifolds....with this special tool my mechanic had.....like a leak down tester......BOTH SIDES held pressure...we assume they are fine.

So we look toward electrical problems.....we do a complete tune up....Plugs....cap...wires....rotar....ignition module. Motor starts and runs fine.

OK....we let it sit for a few days.........same crap......cranks....starts to run on 6/7 cyls....for 30-40 seconds....then runs fine. Now we call a merc rep.....says sounds like .... the " cold start adjust sensor motor....
OK...we take the EFI throttle body apart...and change that littte bugger ( $$$$)..........starts and runs fine...........OK we figure thats it.

Let it sit for a few days..............SAME CRAP.............

Now we think fuel pressure problem ????




ANY help would be appreciated..........

Thanks
 
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My manuals don't mention a cold start adjust sensor motor. Which version EFI do you have?

The ignition module on the distributor is only for starting. Once the engine is runnning (over 300 RPM, I think) the ignition timing comes from the MEFI module. I'd check the fuel rail pressure next, then do an injector test with the scan tool. After that, I'd swap the MEFI modules between the engines. Then swap the distributors. It could be a pickup coil issue.

Best regards,
Frank
 
Do you have access to a bore a scope.I've seen a slight amount of coolant leak into the cylinders and cause a very slight misfire on cold start.You may possibly have an exhaust gasket leaking at a riser letting a small amount of water in when cold the as things expand will go away.You will need to remove the plugs and look at the piston tops with a bore a scope.if any look cleaner then the others there is the problem cylinder.
 
just curios,any smoke when the problem is happening?maybe try to see if it smeels like fuel,oil, or coolant.
 
Thanks Frank......Not sure what " version " EFI.....they are 1995 motors...blue water EFI is all it says on the motors. Maybe Im not calling that part by it right name.......its just inside the ( throttle body) past the 2 butterflys.......It adjust the air mixture on cold starts.

Whats the MEFI module ?

My mechanic is in the process of borrowing the Merc computer tester ??....to plug in the motor to run a scan test ???. He also mentioned he wanted to do a fuel pressure test......We will do it after the weekend.....Im boating this weekend...regardless.

Johngus.....We don't think its a coolant problem.....the motor has good compression.....and I loose no coolant from the exchanger....and have no leaks from excess pressure either. Also there is no smoke or smell of oil or gas out the exhaust. Just normal exhaust.
 
PS......My first thought was ....WHY DID I USE OSCO PARTS ( maybe to save $1500 extra )........but Im confident that they are not leaking....with that leak down test we performed....my mechanic says he has used Osco R/M for years and never had a problem with them. We put over 20PSI for about 15/20 mins with NO leak down.
 
The little "motor" will be the IAC valve. Controls the idle RPM's. Cold start air / fuel mixtures are set by the ECT sensor and the injector timing map data base stored in the computers memory.

You will need the scan tool to check all the computers inputs and outputs.
 
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2 more thoughts !
To check for fuel pressure, turn the ignition for 5 seconds, then off, then on again for another 5 seconds, then start. At pre start, the ECU will only run the fuel pump for 2 seconds, maybe not enough time to pressure up the system if the pump is bad. I doubt it is the pump as you say the problem disappears after 45 seconds, more than enough time to get fuel pressure.

Second thought, are your 7.4's RWC or FWC or the half setup where the block is FWC and the manifolds and risers are RWC. I did find once after fitting new risers to a 5.7L, one of the new cast iron risers was porous ! From the water jacket into the exhaust passage. It was a FWC 5.7, but only the block and the manifolds and thus the leeky riser didn't show up in the pressure test.
 
OK. My mistake. You have throttle body injection, not MEFI that I was describing. Not sure how GM's TBI works.

You might want to pull the plugs before your first cold start and power off the fuel injector. Then check that nothing blows out of the plug holes when cranking the engine as a base sanity check. Then do a cold compression test looking for a stuck valve that unsticks when the head gets a little warm. Rare, but happens. Then go to the EFI system.
 
Thanks Chris.....that IAC valve.... sounds like the " motor " thing I was trying to describe. I wasn't aware of the ignition on for a few secounds routine.....but I will try that at cold start-up today. My motors are FWC....with RWC risers and manifolds. We did pressure test the system.....but if it does the same ruff start after I try the ignition on/off thing.....Like Frank suggested....I will try pullin all the plugs...crank the motor.... and check for water out of the cylinders....although we did pull the plugs and just check the plugs for water.....and all was fine....with a nice dry tan burn pattern. I hope thats not the problem.

I used Osco parts on a previous boat.....with no problems...........


Thanks guys..........
 
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OK.......just tried the ignition on for 5 sec....off...then on for 5 sec....I did that 4X.....cranked the motor......same ruff start......but this time the exhaust had a rich fuel smell........so now Im thinking...maybe a bad injector ?.. stuck slightly open ???...dumping fuel into a cyl ????......I give up !!!
 
Is this throttle body or MPI?With a throttle body you can put a paper towel under the injectors after shutting off and let sit to see if the 2 injectors are leaking down.With a MPI you would need to pull the injectors from the intake,leaving the fuel line connected and presurized the put a paper towel under each injector and let sit to see if any leak down.Did you leave the fuel pressure guage on after shutting down to check for pressure drop.I know its too late now but you could have bought original merc parts for almost the same as the aftermarket Osco's.MarineMAte had them for a steal early in the season.I think small blocks were $495 manifold and risers original Merc.
 
Thanks John.....Its EFI....its an older version ( 1995 ) It does have a throttle body....thats where the IAC valve is located.

this morning I was messin around....and I tried the ignition on/off test again. But his time I did it with more time in off possision. I turned the ignition on for 5 sec ( heard the high pressure pump pumping up fuel pressure )....then off for about 20 sec....then on 5 sec....off for 20 sec.... 5X. Hit the switch to start the motor.........and it started right up....and ran on all 8 ....like a dream.


SO.........now where do we start to look ........pump.....hoses....fuel rail..........????


Anyone have a suggestion ?????


THANKS guys.
 
OK Don, that's good, you've got the problem isolated. Now it's either the pump or the back pressure controller. Both mounted in the cool fuel box in behind the port engine mount, a pain in the A to get to.

First, I would check the fuel pressure at the test point. There is a schrader valve located on top of the motor, just near the thermostat housing. It's a piece of pipe with a cap on it. It is just like a big tyre valve and you connect the special Mercruiser fitting to that with a normal pressure gauge. If you don't have the special fitting, you can take the valve out with a tyre valve removal tool and clamp on a rubber hose.

Fire up the pump without starting the engine and the fuel pressure should get to between 40 and 50 psi. in the 2 seconds. If it doesn't the pump is sick or there is a fuel blockage.
 
Chris......... I know what your talking about when you say cool fuel box.....but this is a weird fuel system.....its unique. It doesn't have a cool fuel box....what it has is fuel coolers.....similiar to an oil/tranny cooler ... its actually cut into the raw water side of the cooling system.....with fuel lines going in and out of the cooler. I had them re-worked last season ( no longer a merc part ) and they are in as-new condition.....so I know that they are not the problem....plus my fuel filters ( after the coolers ) are always water free.

My high pressure pump is mounted at the rear of the motor....right near the distributor.....Where else could they have mounted the back pressure controller ??.... how big is it ??....and what does that look like ???

I seen that valve by the thermostat housing....and wondered what it was for. OK....so after the weekend partying.......I plan to get my mechanic here to track this down I hope he has all the test equipment we need.......and we can still get parts ....that we may need.

Im assuming that the port pump ....being about the same hours.....should also be checked....Im sure its not far behind in its service life.


Well.....I will sleep better tonight.....and enjoy my M/D weekend


Thanks for the info !
 
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I don't know this TBI system. It sounds all different from MPI. But I still think you need to instrument the fuel pressure at the throttle body.

Best regards,
Frank
 
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