Bi-annual Compression Test - Merc 7.4 MPI

[tpenfield]

I reminded myself yesterday why I don't do compression tests on my twin engines, but every 2 years

The engines are so close together that is makes the 'inside rows' of cylinders nearly impossible to reach. 4 hours to do both engines.

Anyway, here are the results of the one I did in 2018 and this year (2020), the engines have 1,030 hours on them. I also have a couple of questions for the group here . . .


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It looks like cylinders 4 & 5 on both engines may have some valve leakage. I was going to do a cylinder leak test to verify, but I did not have the proper hose connections to do it (seems like I lost some stuff from my leak test kit :noidea: )

Anyway, the spec for the engines is 150 psi (red line) the test done in 2018 is in BLUE, this year is in GREEN.

A few questions . . .

1) I was thinking the engines may need a rebuild at some point, but it looks like they are hanging in there. Maybe just a valve job ??? The port engine is a bit higher on the compression . . . maybe by about 5 psi.

2) The spark plugs are 8 years old (ACDelco MR43T) . . . They look OK, but thinking it may be time to replace? (280 hours on the plugs) Are there better plugs to be running? Ignition is the HEI / EST type . . . Engine controller is the MEFI-1

3) During my winterization and testing over the past few days, the starboard engine would occasionally not want to crank (turn the key and nothing - no noise, nothing). I initially thought it was the solenoid, so I replaced it (Mercruiser OEM part). Same thing. Then I checked the voltage to the solenoid and it looked fine. After a while, it magically started working again . . . then the next day it would not work for a while . . . later on it started working again - all by itself Thoughts? :noidea: I am thinking a faulty ground connection, but no time to chase it.


BTW - 'we' often talk about engine cranking speed . . . It seems like the engines are cranking about 180-200 RPM during my compression test. (12 compression strokes in 8 seconds). I think this is about the normal cranking speed, but it is often discussed as needing to be 300 RPM, but not sure most V-8 engines actually reach that speed during cranking :noidea:

Also another thought - does having the outdrive attached during a compression test slow down the cranking speed much? I wonder if the 150 psi spec is without the outdrive, similar to a car being in neutral :noidea:

 

[Lou C]

It would be hard to tell if it’s actually valves without doing a leak down test, but from experience we know you usually have trouble with valves before rings. If oil consumption is normal then rings are likely fine. What about checking the exhaust elbows; how long has that set been used in salt? If they are starting to leak there might be some leak back to the cyls. If they have been in use more that 5 years I’d probably check that first.
As far as the starter issue do you have the older style Delco direct drive ones or the newer PMGR starters? Does Merc use an assist solenoid between the ignition switch and the starter? These can get corroded internally and cause this problem but so can a corroded negative battery cable. I just changed my old Delco for an Arco pmgr starter and it cranks much faster; they draw much less amps than the old direct drive starters. I’d try to check all the connections and assist solenoid if it’s used on that engine; if you have the old DD I’d think about changing to the PMGR style ones...

 

[Lou C]

PS with center riser style exhaust the center cyls are likely to be affected first by any exhaust leak back and your #5s seem to be the lowest which might point that way...

 

[Lou C]

PPS

I see you have the stainless elbows and changed the gaskets and block off plates like 2 years ago; so what I said above probably isn’t relevant lol unless it’s past leakage....

 

[tpenfield]

Yup the exhaust manifolds are new from 2012 and have never seen salt water, as they run coolant through them. The riser/elbows are stainless steel. and yes I replaced the gaskets a few years ago with the solid block-off type along with the squishy types.

Usually any water that sneaks into to the manifold gets to the middle cylinders first, so that is why I was thinking valves for the lower cylinders, since they are in the middle of each row.

I assume that I have the old style starter motors. . . heaven help me if I have to replace them . . . impossible to reach. I would probably have to take the exhaust manifold off on that side.

I'm going to clean up the electrical connections in the spring.

What's the norm for spark plug replacement? Years, hours, other?

 

[Lou C]

Ted I give you a lot of credit for being able to do what you do with the twin engines! What I do with plugs is I have an old set I use for fogging, I put them in for that and save the clean plugs and reinstall after burning off the fogging oil burns off. This also gives me a chance to see if I had any water in a cyl. So I’ve used them at least 4 seasons worth. I like the AC Delco plugs in marine engines they seem to perform well and don’t corrode in salt water.
I would go over all the connections in the starting circuit before changing a starter. My old DD was cranking every time but it was starting to drag and labor even with a good battery. Doing the starter on my little 4.3 was easy just move the battery out of the way.

I did some checking on a Merc parts site and 96-97 was the year range when they changed over form the DD to the PMGR. So it’s possible you might have the PMGR and they are like half the weight and easier to install. 9 vs like close to 20 lbs. Holding that thing up under the engine while trying g to get the bolts started was a job calculated to make you mean. The lighter PMGR units are so much easier!

 

[Lou C]

Here's a size comparison of both types.

 

[tpenfield]

The part number for the starter on my engines is 806964A3, which is superceded by 50-863007A1.

So not sure which type of starter those are . . . :noidea:

Not sure if there is a spec'd RPM for which to run compression tests at :noidea: 180 RPM is probably a bit slow though . . .

 

[Lou C]

I Looked it up and it looks like the lighter more compact PMGR unit....
even if you had the older style direct drive ones you can replace them with the PMGR unit; the main thing is to use the correct bolts because if the are slightly too long the starter can shift under load and break the bolts. So if you use a Merc starter use Merc bolts but if you use an aftermarket starter use their bolts because they housings may be dimensionally slightly different. I used an ARCO so I had to buy their bolts..

 

[tpenfield]

Yes, if it ever comes to that, I'll consider the PMGR starters. I guest the difference is in the size of the body of the starter vs. the mounting flange.

I've also checked out a few gearhead articles about starters . . . it seems that they should turn the engine at about 200 RPM, which is certainly enough to get an engine to fire.

Once the engine starts to fire, the MEFI is supposed to take over at 300 rpm. When starting my engine(s), I can tell when it starts to fire and I release the key - the engine controller says 'I got this' and the engine continues to fully start.

 

[Lou C]

PS
I did a compression test on my engine after I replaced the cyl heads back in 2017, this was done in Sept of that year...

1 160 2 160
3 175 4 165
5 160 6 165

Here are results from 8/5/2013, I had a bad overheat a few weeks before this:
1 170 2 160
3 180 4 180
5 165 6 165

and from about 10 years back, 10/30/2010:
1 165 2 170
3 180 4 185
5 165 6 175

 

[tpenfield]

It is interesting to see how compression fluctuates over time. The valve job that you did seems to have made the cylinders more consistent.

 

[tpenfield]

I also found (in 2018) that the test gauge that I was using was reading 10 psi low. I was initially concerned that the psi had dropped so much in a couple year's time. When I found out it was the gauge, then it did not seem so bad.

Here are a couple of graphs of my 2 engines compression over time . . . the trend is somewhat down, but a couple of cylinders have shown improvement (probably deposits on the valves at certain times).
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Overall, the trend is down slightly ( -8 psi over 8 years ) and there is more variation among the cylinders.