96 5.7 Mercruiser missing/compression issues

[Limited-Time]

My 96 5.7 starts and runs but misses. Oil pressure is normal, no backfiring though carb or on the exhaust side. Oil looks clean and clear, Motor runs at normal temp., But knocks loud when loaded. A compression check reviled 2 cylinders with no compression #3 & #5. The compression gage barely registers on either cylinder. The cylinders are side by side for those who don’t know. Any thoughts or help would be appreciated. Thanks.<br /><br /> L-T

 

[rodbolt]

Re: 96 5.7 Mercruiser missing/compression issues

most likly a blown head gasket. is this a boat ya just bought or do you know the maint history or lack of?

 

[Limited-Time]

Re: 96 5.7 Mercruiser missing/compression issues

rodbolt,<br />Thanks for the reply. Am the original owner. Never had a problem till I had the water pump impeller replaced. first outing noticed the temp climbing at idle or slow speed. Did not think the temp ever reached critical mass. Took the boat back to the dealer they claimed I'd sucked up some debris and that was the cause of the temp issue. Noticed the miss right off the trailer next time out<br /> <br /> L-T

 

[Don S]

Re: 96 5.7 Mercruiser missing/compression issues

Alpha or Bravo drive?<br />Raw water or Fresh Water cooled?

 

[Limited-Time]

Re: 96 5.7 Mercruiser missing/compression issues

Don,<br />Alpha 1 (sure) gen 2 (I think), Cooling is open system fresh water only.<br /><br /> L-T

 

[TilliamWe]

Re: 96 5.7 Mercruiser missing/compression issues

I have a very similiar problem, and have had it for two straight seasons. I am surprised to hear Rodbolt say blown head gasket. I would have thought that water would get into the oil if that was true.<br />On my 1997 5.7 EFI, it's #2 cylinder (is the the one the farthest forward on the right?) that has NO compression. I actually adjusted the intake valve, a lot, and got 60psi. Is this a different problem, or the same one?

 

[Bondo]

Re: 96 5.7 Mercruiser missing/compression issues

L-T's Problem is that it Burnt Thru between the cylinders,.... Not into a waterport.......<br /><br />Bill,... I think Your Issues is Totally Different.....<br />With the Valve Adjustment making that kind of difference,.... I'd pull the Head,... I think that's Where Your Problem may lie........

 

[Don S]

Re: 96 5.7 Mercruiser missing/compression issues

TW, yes your problem is completely different. <br />Would be best if you start a new thread instead of tacking on to this thread. It'll get real confusing with 2 seperate problems going on at the same time.

 

[TilliamWe]

Re: 96 5.7 Mercruiser missing/compression issues

Bondo & Don thanks for the knowledge. I will not be starting a new thread, however, as I have ksuspected I have a valve issue.<br /><br />So L-T's problem is most likely a head gasket just because it's two adjacent cylinders? Head gaskets can blow just from "over heating" it once??

 

[Bondo]

Re: 96 5.7 Mercruiser missing/compression issues

Head gaskets can blow just from "over heating" it once??

There's MANY Reasons that'll cause a Headgasket to Give Up the Ghost............<br /><br />Over-Heating is Only 1,..... And,.. Ayuh, Once Can Do It...........

 

[Limited-Time]

Re: 96 5.7 Mercruiser missing/compression issues

Next step is a upper end tear down. Any thing spicific I need to keep my eyes open for?

 

[Limited-Time]

Re: 96 5.7 Mercruiser missing/compression issues

Additionally the boat is set up with an Audio warning system Oli pressure, temp, and drive fluid. which never sounded so I still dont think o/h was an issue. What other things could cause the head gasket failure?

 

[achris]

Re: 96 5.7 Mercruiser missing/compression issues

Slack bolts could cause head gasket failure, or just a faulty gasket.

 

[rodbolt]

Re: 96 5.7 Mercruiser missing/compression issues

how many chances ya need ?<br /> but yes its time for a head removal and a careful analysis of what failed and why. a leak down test will positivly ID it as a blown head gasket or not.

 

[waterone1@aol.com]

Re: 96 5.7 Mercruiser missing/compression issues

I think that an examination of the head would be in order, could just be a head gasket.....could be a head problem. If you find that you have valve seal or valve problems in the two adjacent cylinders under the center of the elbow's/risers...it could be a water ingestion problem from the maniflod/risers. One way or another the head must be removed...I would have it looked at by a good machine shop.

 

[Limited-Time]

Re: 96 5.7 Mercruiser missing/compression issues

Thanks everyone for the insight. The way I see it, it does not much matter now what’s really wrong. The top end has to come off. My concern is I do not want to treat a symptom i.e. blown head gasket, and later find out some thing else caused the gasket to fail. Is there anything else I should do before the tear down? Aside from the leak down test rodbolt mentioned. FYI am a gear head from way back have wrenched on cars, trucks, motorcycles, snowmobiles, 2 stroke, 4 stroke gas and diesel, just something about these marine motors has me a little intimidated. Thanks for the help.

 

[trog100]

Re: 96 5.7 Mercruiser missing/compression issues

head gaskets do wear out.. they are subject to frictional movement each times the engine warms up and cools down plus corrosion factors.. they dont last forever..<br /><br />check that the two mating surfaces on block and head are flat and not corrosion pitted before u fit the new one.. make sure u fit and torque the new one correctly.. not doing these things can cause premature head gasket failure.. not a lot else assuming the engine has never been well and truely run too hot.. running very hot can warp the head and lead to gasket failure. <br /><br />no compression on two cylinders next to each other and the rest okay is almost always down to a head gasket failing at the narrow spot right between the two cylinders that lack compression.. check that area on block and head extra closely when u take it all apart.. if the gasket has gone it will be pretty obvious when u look..<br /><br />trog100

 

[waterone1@aol.com]

Re: 96 5.7 Mercruiser missing/compression issues

I would also look at the spark plugs from those cylinders to check for any signs of water intrusion.