Compression test results

Marcoboater

Cadet
Joined
Apr 28, 2008
Messages
9
I am a new boater. I purchased my first boat in January and my family of 5 have been enjoying it tremendously since. Unfortunately it has become apparent to me that the previous owner PROBABLY switched out the hour meter prior to putting the boat on the market in an attempt to hide the true number of hours the boat has been used. I did have the boat surveyed but not the motor because at the time I believed there were only 130 hours on them therefore did not think it was necessary. My mistake. I did take the boat to a local marina in order to see if I could obtain the true amount of usage by having the ECU read by a scanner. However this was not possible because my engine is a model year 2000 which is the last year that Mercury equipped them with the "blinky" type of scan info as opposed to the full computer readouts that are available on later models. The engine seems to run fine and I have not had any issues with the exception of an overheating problem that happened when my seawater pump impeller completely disentegrated while a few miles offshore. Thank god the alarm worked otherwise being the "newbie" that I am, I wouldn't have had a clue. I don't think any damage was done but I can't be sure. I replaced the entire pump myself as the housing on the failed one was so corroded as to not be serviceable. Also the manifold and risers were shot which was picked up by the surveyor prior to purchase. These were replaced by previous owner as a condition of sale. The engine is a model year 2000 Mercruiser 7.4 Liter with EFI. It seems to run well. Starts every time. Burns very little if any oil. The level hasn't changed in approx. 30 hours of use. OK that's the history.
My question is about compression test results. Because I am suspect about the true number of hours the boat has been used I decided to do a compression test yesterday. The results were 6 cylinders came in at 160 psi with one at 175 and another at 152. The engine was warm the throttle was WOT and I had the kill switch turned off but did not ground any other electrical connections. Do these numbers sound OK? What might cause the descrepency between the two odd ones? There was a thin brown oily residue on a few of the plug threads especially on the cylinder with the high compression reading. Could this be unburnt fuel entering the cylinder do to WOT with no combustion during test? The cylinder with the low compression reading looked as if was burning extra hot. The plug wire was degraded due to heat and the plug insulator looked like it was cooked close to where it met the block. I havn't pulled the heads to inspect the valves (big job for me). Do you think I might have a problem here?
Any information you might be able to provide me would be greatly appreciated. I'm not an expert on these matters and don't want to cause any further damage if something is wrong. I realize without seeing the boat diagnosis is difficult but once again any and all advice would be welcome.
Should I just shut up and enjoy my boat?:)
 

Haut Medoc

Supreme Mariner
Joined
Jun 29, 2004
Messages
10,645
Re: Compression test results

Welcome to iboats.....:)
I'm not really an EFI savvy guy, bit if the two plugs from the odd high/low cylinders look different than the other 6, it does warrant investigation.....
I think I would be more concerned with the 'hot' cylinder, it sounds like an injector may be clogged & running to lean......;)
Stick around, you will get more input......:)
I do think that with numbers in the 160# range, that worrying about the actual hours is needless......;)
 

Bigprairie1

Commander
Joined
Jun 13, 2007
Messages
2,568
Re: Compression test results

As you look down at the engine facing towards the back of the boat, which two cylinders had the high and low readings? (ie: front left and front right?....left centre two?....back two left and right or?)
 

JustJason

Vice Admiral
Joined
Aug 27, 2007
Messages
5,319
Re: Compression test results

Numbers sound ok to me. Merc states that the lowest cylinder has to be within 70 percent of the highest cylinder, with all cylinders having a minumum of 100 psi.... so no problem there.
As far as the plugs.... I could care less about what the threads look like. I'd care more about what the tip and insulator looks like on the inside of the plug.
You never know what the previous guy may have slapped on the threads of the plugs. If you had oil on the threads on all plugs from 1 side but none on the other i'd be looking for a leaking valve cover gasket.... but that's all.
Since you have no idea how old the plugs are, I'd do a full tune up.
Cap, rotor, plugs, wire, all your fuel filters. And see how it runs and take it from there.
 

Marcoboater

Cadet
Joined
Apr 28, 2008
Messages
9
Re: Compression test results

I'm back and thanks so much for the help so far. I do know that a full tune up was performed in Dec. just prior to my purchase of the boat, I have the receipt. Sorry for not mentioning that. In response to Bigprairie's post the cylinder that was running higher than the other's was the forward most on the right when facing the engine looking aft. The one that showed lower was the second from the rear on the left when looking from the same perspective. Thanks again.
 

newport dave

Chief Petty Officer
Joined
Nov 21, 2004
Messages
458
Re: Compression test results

I did take the boat to a local marina in order to see if I could obtain the true amount of usage by having the ECU read by a scanner. However this was not possible because my engine is a model year 2000 which is the last year that Mercury equipped them with the "blinky" type of scan info as opposed to the full computer readouts that are available on later models.?

This is total BS, that engine will have an operating history available by connecting a scan tool

The results were 6 cylinders came in at 160 psi with one at 175 and another at 152.

These numbers are fine and the conditions found on the plugs are due to factors other than compression.
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,465
Re: Compression test results

The results were 6 cylinders came in at 160 psi with one at 175 and another at 152.

Ayuh,.........

Those #s lead Me to believe the Hour Clock is True.........

Those are Darn Good #s......
 

Marcoboater

Cadet
Joined
Apr 28, 2008
Messages
9
Re: Compression test results

Thanks guys. New boater appreciates help. As far as what I said about ECU being read, I was just repeating what the (junior) mechanic told me when I took it to the marina to be read. Senior mechanic was on vacation.
 

Marcoboater

Cadet
Joined
Apr 28, 2008
Messages
9
Re: Compression test results

I just called Merc.'s consumer hotline and Newport Dave is correct, I was given incorrect info by marina. The number of hours can be read by using appropriate scan tool. The combined knowledge available by this board to those of us that are not that well educated is great. Terrific forum. No sign of any of the egotistical bashing that permeates other forums either. Very refreshing.
Have a great evening and happy boating to all.
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,465
Re: Compression test results

No sign of any of the egotistical bashing that permeates other forums either. Very refreshing.

Ayuh,........

Give Us some Time,....... We're Not Angels.............:D
 

RCSConstruction

Chief Petty Officer
Joined
Mar 23, 2007
Messages
549
Re: Compression test results

Maybe a sea foam treatment is in order? That number 1 cyl with 170 may have carbon buildup on the piston, no?
 

Marcoboater

Cadet
Joined
Apr 28, 2008
Messages
9
Re: Compression test results

Don't know what sea-foam is. Please enlighten me. I'm willing to perform any preventative or corrective maintainance short of pulling heads. I plan to check injector on low (hot) cylinder as per Medoc's recommendation.
 
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