loss of 600 rpm..

Timdogge

New Member
Jun 17, 2019
5
Boat Info
2001 Sundancer 270
Engines
383 mag
Hello everyone! im new here.. hope you fellas can help me out. I recently bought an 01 sundancer 270 with a 383 mag w/208 hrs on motor. did all the normal maintenance before running it. took it out several times, put about 40 hrs on it with no issue, boat ran awesome! couple weekends ago i took it out and everything was good. came into a wake zone while at about 3400 rpm. went thru the wake zone which took about 15 minutes. went to take off and i got an alarm. temp was good, oil pressure was good, drive oil good. turned key off started up with no alarm. went to take off again and got the alarm again. i pulled the dipstick and i was low on oil. had a quart onboard, put it in and took off with no alarm, but it took me forever to get out the hole, which wasnt normal. so for the next couple hours same thing, took forever to get on top and when i did WOT would only get to 4200 when normally it would top out at 48-4900. soooo, changed sparked plugs, pulled injectors and cleaned them, which they really needed. changed the oil, 40lbs of fuel pressure. took it out same thing... so then i do a compression check. in random order 140, 175, 175, 170, 135, 165, 170, 160... anyone have a clue of what happened? if the cylinder numbers are important i can provide that. any help would be greatly appreciated!!!
 
forgot to mention, before the issue started, when i was dumping the throttle once i reach a certain amount of throttle the engine seemed to jump in rpm and she would get up on plane quick.. almost like a boost of fuel or something. it doesnt do that anymore either..
 
Which cylinders are the 135/140 from? Those are a little low. Are those numbers on a hot engine?
 
135 & 140 no bueno - down on power so won't reach max rpm.....are the two cylinders next to one another? on the same bank?
 
According to Mercruisers compression checking procedure, everthing is within spec here (I don't agree). They say the lowest cylinder (135) should be within 70% of the highest cylinder (175) which in your case is 77% AND no cylinder should be below 100psi. BUT, there is a reason those two cylinders are low, something is not right. A common thing when one or two cylinders are down is they are the cylinders right under the riser on each side of the engine - beginning of riser failure that is letting seawater drip down into those cylinders - boat sits unused with a little bit of water on the exhaust valve or even in the cylinder and rust starts. Eventually that cylinder starts having problems.

I would pull the risers and have a look.
 
Last edited:
thanks for the replies guys, these numbers are from a cool check. i did one last week on a hot engine and all the numbers were higher with one of the cylinders coming back at 205, which really seemed weird. numbers when hot were 6 of the cylinders between 180 & 185, one came back at 160 and the high one at 205. the two low numbers on the cool check are not across from each other. 140 is 2nd cylinder on the right looking at the engine. 135 is the last on the left side of the motor, looking into engine compartment from the bow to stern..
 
The hot numbers are a better indicator, as that is the proper way to perform a compression test.
 
Could be. So you pulled injectors and they really needed a cleaning ? So who did the cleaning ? And if that's the case, your entire fuel system probably needs to be cleaned. If you send those injectors out without doing that, you'll probably plug them up again in short order.
 
scoflaw, thats what i was thinking. local mechanic put them on a beacon machine.
 

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