low revs on mercruiser EFI

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Cadet
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Aug 21, 2003
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I own a chaparral 26ft cruiser fitted with a Mercruiser 5.7L EFI engine.<br /><br />I am experiensing a very unclear fault as follows. I take the boat out and start giving it throttle to get the revs up. The boat planes at 2800rpm and everything seems fine. I continue giving throttle until I reach a cruising speed at 3300rpm. After a few seconds the rev counter starts going down slowly for no apparent reason and the speed reduces. I give more throttle, but mothing happens and the revs keep going down. The only way I can get back to cruising speed is by pulling back on the throttle for a few seconds and applying the throttle again. This gets the revs up again but only for a very short while because this thing repeats itself indefinitely.<br /><br />I have replaced the high pressure fuel pump for nothing, I have checked the EFI system with the digital scan tool but everything seems OK. I have checked fuel lines and fuel vent but everything seems normal.<br /><br />Can anyone help me??
 

denniz

Senior Chief Petty Officer
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Re: low revs on mercruiser EFI

Whats your fuel pressure?
 

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Re: low revs on mercruiser EFI

I don't know the fuel pressure but I have replaced the high pressure fuel pump with no success.
 

Northern Eclipse

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Re: low revs on mercruiser EFI

Did you check the throttle position switch/sensor, this sends a signal back to ecm to control injector fuel delivery,also look into Mass Air flow could be dirty, these will not always give a code... But the most common issue related to fuel is a rich/lean condition, you may be leaning out, causing your power loss, if so more than not its a faulty O2 sensor....I love my FI for the pavment.. but I'll stick to Carb for the water.
 

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Re: low revs on mercruiser EFI

You are right, it can be a TPS problem but I don't think there is anything wrong with the air sensor because at idle, the engine runs smooth and perfect at 800rpm.<br /><br />I was hoping someone out there had encountered the same problem on a similar engine. I do not want so called mechanics dismantling my engine for nothing.
 

Northern Eclipse

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Re: low revs on mercruiser EFI

Joseph I understand your concerns about just anybody wrenching on your boat... My family is in the auto business and believe me its hard enough to find a mechanic that knows how diagnose a problem correctly, never mind trying to find a marine Mechanic that is up on the tech. Another thought I just had is 'do you think the antiknock sensor' is kicking in to hard maybe due to poor quality fuel, and retarding the timing. I have seen this with some GM products over the years. Anyway just a thought.. its hard to trouble these things if you cannot pull a code.. even if you can sometimes the codes are not correct either i hope you find your problem, just keep one thing in mind...no matter how much junk is on a motor it still only needs the basics to run right Fuel Air Spark and Timing it just a matter of finding out what controls F.A.S.T notice the result if these conditions are met..........Fast.
 

denniz

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Re: low revs on mercruiser EFI

If you dont know your fuel pressure your just guessing, waisting time and money. Merc had a problem with fuellines swelling causing low fuel pressure. Id suggest finding a good tech.. How much was that fuel pump???? --- =money down the drain. What years your boat. they only had a 2500rpm rev limit for a few years. Ill bet its low fuel pressure. wont set a code. D Wheres Malta anyway?
 

denniz

Senior Chief Petty Officer
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Re: low revs on mercruiser EFI

could be exhaust restriction (broken shutter)
 

boat doc

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Re: low revs on mercruiser EFI

I do not think it was mercs problem with the fuel line, it was good year's, if we are talking about the same thing. You can tell this by at the end of your rubber fuel line, it pukes out the glue that is used to hole both layers of your fuel line together. Typically what happend here is you did not notice a problem unless your boat was used quite a bit.<br /><br />I would check fuel pressure (is this TBI or MPI?)<br /><br />What year is your engine, mefi 1, 2, 3<br /><br />what scan tool software are you useing, if you do not have the right software, it will not properly read trouble codes off the computer. If I am not mistaking early models were 8 bit, and now we are up to like 64 bit computers. Also if it is a sensor, it might not be giving a hard fault yet.<br /><br />I would tend to lean towards a t.p.s. at the moment without being able to see the boat. I doubt it would be a knock sensor, unless the boat is going into limp home mode<br /><br />one other thing, will this happen under a load only, or will this happen in neutral as well. If you can get your hands on a merc manual, it has a great trouble shooting guide. If not let me know, and I will look to see if I have this engine on disk, and will try to post it.
 

Markbon

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Aug 24, 2003
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Re: low revs on mercruiser EFI

Joe,<br /><br />This may be of interest to you, as my Searay has had the same problem, same engine as yours, 1996 5.7l EFI TBI very similar symptoms. <br />Ours occurred after changing the exhaust manifolds, part of that problem was due to the (mechanic??) out here not sealing the inlet manifold properly when he replaced it (had to replace two valves that were heat stressed as well) We have had the scan tool out, no fault codes, ALL the parameters within book limits at idle. <br />A partially blocked exhaust manifold has also been mentioned as a possible cause. Our Fuel pressure good, up around 28 - 34 psi (book figure again) even at about 3200 rpm, which is also max for us, under load. <br />Interestingly at "full throttle", ie as far forward as the lever will physically go TPI is only saying 55%, so I'm wondering about this also. Plugs appear to be very dry and clean, which I believe is a lean mixture indication.<br />Three months after having the manifolds changed, we are getting extremely frustrated with no solution in site....<br /><br />Hope this may have been of some assistance....
 

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Re: low revs on mercruiser EFI

Thanks all for your suggestions. I like the one about the broken exhaust shutter. I will check it ASAP.<br /><br />By the way, Malta is a very small island in the centre of the Meditterranean Sea just below Italy.<br /><br />Another point I wanted to mention ... Since I posted my first letter in this forum I spoke with the local distributor for Merc in Malta and he sent his tech to have a look at the engine. The tech says its my fuel pump again. I told him I replaced it already and he said that the one I got was probably a defective low pressure one which Merc have issued a service bulletin on.<br /><br />Now I'm really going crazy !!! <br /><br />My next step now after chacking for exhaust restrictions will be to check the fuel pressure.<br /><br />Thanks to everyone once again!<br /><br />Joseph
 

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Re: low revs on mercruiser EFI

Just wanted to add something for the attention of boat doc ... The engine performs great in neutral, it is only on load that the fault occurs and it occurs everytime.<br /><br />I just remembered that the symptom started from the start of this boating season and curiously enough this season was the first time I filled the tank with unleaded gas. During the past I used regular leaded gas but since the start of 2003 its been banned from this territory.
 

denniz

Senior Chief Petty Officer
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Re: low revs on mercruiser EFI

If you would have checked the fuel pressure as I first posted you would know its a fuel problem. Chances are the dealer sold you the wrong pump. there are only 2 different inline pumps one is for carburated(low pressure) and one is for efi(high pressure) there are a few different kits containing one of these 2 pumps. Malta sounds great, why dont you post some pics???
 

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Re: low revs on mercruiser EFI

For all those interested and especially for Rod Knocking, you can go to www.maltamedia.com for all the pics and history about the island of Malta and its sister island Gozo.<br /><br />Thanks for your help guys!
 

boat doc

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Re: low revs on mercruiser EFI

Rod Knocking is correct on the 2 fuel pump diffs. Both fuel pumps are identical in appearance, and will fit in the same area, that the other was placed. Let me tell ya, it sucks when you use an efi pump on a carb.........lol
 

denniz

Senior Chief Petty Officer
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Re: low revs on mercruiser EFI

I was thinking about this today. If he has COOL FUEL there is only one pump available. I got a 502efi in today with a similar problem, but he had good pressure....... and a gas tank full of water. Malta looks like a great place thanks for the link. :)
 
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