Mercruiser 7.4MPI excessive oil consumption

[alynnellis]

Recently purchased 1993 GW with a 2000 Mercruiser 7.4 MPI with less than 200 hours, Boat was little used last two years. Outdrive is Volvo original that was connected to Kad42 diesel. Original 3900rpm using B4 duoprop. Now have B3 duoprop but can only run 3800rpm out of 4000 4400 Mercruiser spec. Uses one quart an hour of oil. Starts and runs great, excepr for those areas. Mechanic tested compression at 112 to 115psi per cylinder.

 

[tashasdaddy]

Re: Mercruiser 7.4MPI excessive oil consumption

hi lulu don't know a thing about your problem but i'm a jax, fl native.

 

[Haut Medoc]

Re: Mercruiser 7.4MPI excessive oil consumption

Your compression #'s sound low to me....There are those who know alot more about newer models than I do ...... But again thoses #'s sound low....Does it smoke alot? You are most likely getting blow-by....What do the plugs look like?.....Wait for more feedback from those who are more knowledgeble on your specific set up......BTW, welcome to the forum at iboats.. .....JK

 

[Weirdwalt]

Re: Mercruiser 7.4MPI excessive oil consumption

Verify the props are stamped with the correct number.<br />What size is your boat?<br />What gear ratio is that drive?<br />Your tech should have checked the plugs and informed you of any that were fouled. A quart an hour will foul plugs. Compression is low.

 

[alynnellis]

Re: Mercruiser 7.4MPI excessive oil consumption

Thanks to all for the great response<br />I verified props and Volvo numbers, all OK.Plugs recently replaced after mechanic did compression checks. Most were oily on the threads, but light brown on the electrode end. Two were cracked, which the plug replacement took rpm from 3600 to 3800. Lucas oil added, no results yet. Is she safe to run at cruise 3200 rpm. Boat is a 25 foot Grady-White Sailfish(WA) Gears should be 1.78:1, cant verify easily.

 

[Weirdwalt]

Re: Mercruiser 7.4MPI excessive oil consumption

Something is definetly not right. That 7.4 should be using a B-5 propset minimum. Bottom clean? Gear ratio could be why you use the B-4 or 3. Still, you should have gotten an rpm increase with th B-3. Verify the tach, and you have an oil leak somewhere, or you are mistaken in the oil usage.

 

[Don S]

Re: Mercruiser 7.4MPI excessive oil consumption

The Merc spec for that engine is 4600 to 5000, not 4000 to 4400. If you are that low, and the bottom of the boat is clean, you have a problem other than prop size.<br />First off I find it hard to imagine a compression test that shows the cylinders within 3 psi of each other. That doesn't happen very often (If ever)<br />If the compression is actually that close, then the engine should run fine. Oil leakage is another thing. Being a Mercruiser engine replacing a Volvo Diesel engine is another big question. Why? and Why was a Merc engine installed to replace a Volvo diesel. Was the exhaust done properly, does the engine run right, has any troubleshooting in the fuel and ignition system been done?<br />I would be looking for a Merc serial number to verify what engine you have and get the proper manual for diagnosis of any EFI engine. Too many things in EFI engines to start guessing and replacing.... Way too expensive.<br />Also, get the compression test redone and do ALL the steps in the link below. Merc also had engine oil coolers on the big blocks. You might want to pressure check the cooler. Could be the source of you oil consumption.<br /><br /> Compression Test

 

[Luna Sea]

Re: Mercruiser 7.4MPI excessive oil consumption

I see you took my advise and found iboats! <br /><br />I sent him here from the Grady site, here's that thread: <br /><br /> http://www.greatgrady.com/forum/viewtopic.php?t=1415 <br /><br />He's commenting on "Gear ratio probably in boat and original is 1.78:1 recommended for V-8 power is 1.95:1"<br /><br />And another owner of similar boat is using 1.5:1<br /><br />I'm only guessing but could this engine have been overworked using the drive that was there when it was diesel powered?<br /><br />I'm thinking drive needs the right gears, then rings maybe?<br /><br />PS Lu, some good mechanics here...

 

[Weirdwalt]

Re: Mercruiser 7.4MPI excessive oil consumption

I agree with Don, oil cooler is suspect and exhaust should be checked. Something is definetly not right.

 

[alynnellis]

Re: Mercruiser 7.4MPI excessive oil consumption

Went to boat on Saturday with new compression tester,as I agree with Don S. about repeating tests. Mechanic now says he couldn't get busted spark plugout of cylinder 6 and didn't test that cylinder. I changed plugs after that and the plug had almost snapped in two at the fat part of the ceramic.Results for port cylinders (ran out of knuckles to do the starboard 4)<br />cyl 2- 125, 125, 122 psi<br />cyl 4- 110, 110, 109,<br />cyl 6- 78,75,75<br />cyl 8- 118, 115, 120<br />so that makes more sense now. A virus prevented download of comp test service bulletin until after Saturday tests. Will redo cyl 6 and the other 4 with adding oil to cyl for oil rings testing.Thanks for everyones help and advise.<br />LuLu

 

[rodbolt]

Re: Mercruiser 7.4MPI excessive oil consumption

so far those numbers are looking rather low. I would do a leakdown test from here. 78 psi on a 7.4 chevy is a bit on the low side. in fact its probably at least 60 psi lower than about any chevy I have ever noted.

 

[Weirdwalt]

Re: Mercruiser 7.4MPI excessive oil consumption

Tolya. Now go find another mechanic or buy a manual.

 

[Richard Petersen]

Re: Mercruiser 7.4MPI excessive oil consumption

Any engine that has to drink so much oil is bound to have a big carbon buildup on all the cylinder chamber parts. Sometimes so bad as to cause pre ignition from the glowing carbon pockets. Build up around valve seats. I would have the heads pulled. Unless someone there has a lighted minicamera to go in the sparkplug hole for looking.--------------How about a stuck ring due to all the cooked on oil? Engine de carbon cleaner? We all need some luck.

 

[alynnellis]

Re: Mercruiser 7.4MPI excessive oil consumption

Planning to decarb the engine according to procedure posted on engine FAQ. Any gremlins to look for?<br />LuLu

 

[Haut Medoc]

Re: Mercruiser 7.4MPI excessive oil consumption

You need more than a decarb....You need to find out why your #'s are so low.....I would start with Rodbolt's suggestion...& work from there...JK

 

[Don S]

Re: Mercruiser 7.4MPI excessive oil consumption

Not knowing any history on this boat, somehow I would bet there have been hard starting problems (Like not turning over) at times, poor running etc. I would bet your main problem is RUST. Probably has original manifolds and risers and they have been leaking water which is getting into the cylinderes and causing hydrolock and lots of RUST..<br />Decarb isn't going to help. You can try it, but you really need to pull the heads and have a look at this point. Rings rusted and stuck will also give you high oil consumption.<br />Unlike 2 stroke engines, big blocks don't suffer from carboning up like that.

 

[alynnellis]

Re: Mercruiser 7.4MPI excessive oil consumption

New mechanic at new marina will perform leakdown tests. I will finish compression tests this weekend and change marinas. Arranging to have heads pulled in next two weeks. <br />By the way, I know about lol tgif snafu fubar cya and even ychjcyaqftjb, but dont know tolya. Please inform<br />LuLu

 

[alynnellis]

Re: Mercruiser 7.4MPI excessive oil consumption

Forgot recent history. Exhaust manifolds and risers were replaced in September 2004, the last work done on the engine prior to my purchase. My guess is that the spark plug for cyl 6 was damaged then, and the problems started to add up then. Only my guess.<br />LuLu

 

[rodbolt]

Re: Mercruiser 7.4MPI excessive oil consumption

my guess is it had injested water someplce and rusted up. a leakdown will tell the story.<br /> but if its bad news dont sue the tech . i got sued 3 years ago cause the man said i broke his deal when i gave the prospective buyer the survey results. the owner actually tried to say i blew his motor up doing the leakdown test.<br /> would have almost been funny but it still cost me 150 dollars and some time off. after my attorney got done laughing he made a few calls and it all went away.<br />but he still charged me

 

[Don S]

Re: Mercruiser 7.4MPI excessive oil consumption

Originally posted by LuLu:<br /> Forgot recent history. Exhaust manifolds and risers were replaced in September 2004, the last work done on the engine prior to my purchase. My guess is that the spark plug for cyl 6 was damaged then, and the problems started to add up then. Only my guess.<br />LuLu

I think you're right. Are you sure it was a 2000 Mercruiser engine, or is that when it was installed? I just doesn't sound right for someone to go to all the work to replace a Volvo diesel with a Merc V8. Lots of changes needed there, and lots of parts from the diesel doesn't work with the V8.<br />When I see a Merc on a Volvo or a Merc on an OMC drive or any combination of the 3, I see red flags a flyin'. It was probably done on the cheap just so they could sell it....... Well, that's been my experience anyway. Hope I'm wrong.<br />Keep us posted if you will.