Absolutely frustrated with this Mercruiser engine

[aerobat77]

Congrats mate , you just really moved forward !!

Ok , we can really close any fuel delivery discussion because now we know for sure its not the case .

Cap , rotor , pickup coil : do it only if you have the old parts . Very unprobably since the engine starts and idles fine and does not misfire or similar . You also told the plugs look normal.
Ignition advance module : worth a shot but your diagnosis scan tells us the advance works .

You,re sure you have no leak / false air on the intake ?
Your manifold reading at idle is indeed odd . Spray at idle some carb cleaner around the intake ( not into the flame arrestor ) - if the engine reacts you have an intake leak .

Finally : I know its not what you want to hear but it more and more boils down to a mechanically shot or at least damaged engine . A good cylinder leak down test is now really in order to judge mechanical condition.

Put the boat for sale on ebay etc. regardless what the broker predicts and keep an eye if you somewhere can get an used block . It will not hurt whatever further work you perform on it.

 

[km1125]

Fuel pressure floated between 40-45 psi while running with the pressure regulator off and then maintained about 37 when I reconnected it.

If it was 45 at full throttle and 37 at idle, then I'd say your pressure regulator was OK. However, if that 37 was measured at anything other than idle (or a very low speed) then that indicates a problem, as would a 40-45 PSI reading at idle or low speeds.

A big problem with trying to troubleshoot any problem (and THIS one in particular) is getting decent data out of the tests. When the test data varies all over then the test data is pretty much useless in trying to use it to troubleshoot. Getting consistent test data should be a priority especially when things aren't making sense.

 

[km1125]

...You,re sure you have no leak / false air on the intake ?
Your manifold reading at idle is indeed odd . ...

Why do you say that? 14-17inhg should reasonable for a well-idling engine unless you have a big cam. No?

 

[aerobat77]

Why do you say that? 14-17inhg should reasonable for a well-idling engine unless you have a big cam. No?

I expected it a touch lower but after thinking it twice you are probably correct - the reading is ~ within expected range and a non issue

 

[rustybronco]

Do you know or can find someone with an inductive timing light that has a dial advance feature?
One, you could check the lights flashing on each cylinder and two, you could get a basic idea of the engines total advance capability at 2000rpm and above.

Personally I wouldn't continue throwing parts at it at this point.

 

[Pirate Lady]

I certainly ain’t no mechanic. But throwing this out there. 500+ posts focused on engine and nobody can find anything wrong.
What if; just what if there is nothing wrong with engine, and the problem is something in the drive line is slipping at 2k rpm? Anyone consider that?

 

[Hoplite808]

I certainly ain’t no mechanic. But throwing this out there. 500+ posts focused on engine and nobody can find anything wrong.
What if; just what if there is nothing wrong with engine, and the problem is something in the drive line is slipping at 2k rpm? Anyone consider that?

If the drive were slipping the. It would rev high and then not go faster. Plus I paid to have the whole drive replaced in July of 2022.

The drive was replaced because the old one had a bad gimbal bearing and the bellow was leaking slightly. I also found some damage from corrosion and some strike damage to the bottom of the old leg. I figured better safe than sorry since I operate in the open ocean at times. Since the leg needed a full rebuild I just had it swapped one for one with a freshly rebuilt one done by a certified mercruiser mechanic. We triple checked the gear ratio. It is the correct one matching the OEM one exactly. Sorry can’t remember the ratio off the top of my head but I have it written down in my logs on the boat.

Photo of the drive in July 2022 while in dry dock.

 

[rustybronco]

The MAP pressure seems high for an idling engine.

17.22 would indicate an elevation of around 5,000ft. It looks like the manifold gauge connected shows around 15 inHg


Informational purposes only...
https://forums.iboats.com/threads/mercruiser-7-4l-compression-test-after-valve-job.510486/

 

[Scott215]

That's what I thought. Don't know if it's a mercruiser factory sensor

 

[techmitch]

Low manifold vacuum and low engine compression usually walk hand in hand. I believe I have question both of these readings more the than once in this thread.

 

[techmitch]

After a hot/up to operating temp., dry and then a wet compression test, a proper cylinder leak down test will tell us where we are losing it.

 

[rustybronco]

Any questions?
https://www.thegsresources.com/_for...ompression-tester-to-use-as-a-leakdown-tester

 

[techmitch]

Any questions?
https://www.thegsresources.com/_for...ompression-tester-to-use-as-a-leakdown-tester

Run what you brung.

 

[aerobat77]

I Compression in all 8 cylinders is 120 psi and testing for spark at all 8 cylinders showed no problems.

Low manifold vacuum and low engine compression usually walk hand in hand. I believe I have question both of these readings more the than once in this thread.

He claims he says he has 120 in all 8 holes . If this is true what the hell are we debating the compression ?
Theoretically you can with these numbers skip a leak down test since its for judging if it looses pressure via the valves or the piston rings . Here all looks fine.

But than theoretically this engine should run.

We can only question if the numbers are true since its strange he has 120 exactly on all cylinders but if indeed , the compression ia well above manufacturer minimum acceptable for a 7.4 and surely a non issue .

 

[PlayDate]

The latest test by the OP seems to rule out a fuel, spark or ECM related issue. What we are left with is a mechanical problem. You don't make horsepower and torque without compression. While you may accept the 120 numbers ......I don't. This entire thread started due to a water intrusion issue, poorly rebuilt heads and an unknown engine history.

574 posts later.....all we have done is eliminate possible causes. Low compression fits what we see the engine doing in the video.

Get a new compression gauge and test it again. I'll bet two or more cylinders are closer to 100 psi than they are 120. I also suspect the heads might have a problem based upon the Frankenstein valve job. There is no way an engine with a few low cylinders will push 8,500 pounds of boat on plane.

 

[rustybronco]

I don't think anyone is debating compression. What I do think, at least in my opinion, the OP needs to start again with all the basics. Cylinder pressures, timing events etc. so they can rule out mechanical issues and move onto any external causes.

There is a reason for the issue and as we cannot inspect with our own eyes we have to rely on accurate data provided in order to help. Cross checking information is just one way of doing that. I provided the leak-down tester information as a just in case needed scenario. They will have enough on their hands doing the actual work.

The two compression ratios for the 7.4L MPI that I found on the web were 8.6:1 and 8.9:1 That equates to approximately 126 and 131 psi at normal atmospheric measured at sea level.

 

[PlayDate]

 

[rustybronco]

How can two different C-R's have the same theoretical cylinder pressures @ the same atmospheric pressure?

 

[Hoplite808]

@rustybronco and @Scott215 I am at sea level and I ordered the replacement sensor via Amazon so it might not be a quicksilver part.

@PlayDate For the umpteenth time there was no water intrusion. I have found zero evidence to support it having occurred prior to owning this boat or since. The Frankenstein valve job as you put it was because mercruiser and GM in general for that matter decided to be cheaper in their manufacturing for a bit with V8’s and decided that rather than having tried and true valves that you adjust and are good they’d save ten cents by making ones the factory could mass assemble slightly faster by just torquing them in place. Which is fine when the geometry is perfect. However, part of the valve job I had to have done required a bit of grinding to get them to seat again. This changed the geometry and caused the #7 cylinder valves to hold open. I bought the jegs kit that converted the mess back to adjustable valves and they began to close all the way again.

For those questioning the compression test I have done I don’t know what to tell you. I got a janky old gauge that gives me readings close enough for government work. I take all the plugs out and go one by one to each cylinder dry and cold and get a reading of 120psi +/- 2-4 on all 8 cylinders at sea level. The repair manual I have says that’s within the tolerance limits. The cylinders all looked like the photo below showing cylinder 7 when I had the engine opened up and when it’s running there is no signs of excessive engine blow by. Per the ECM there is all of 567 hours of run time on the engine. There is just absolutely no indication that this issue could possibly be a mechanical problem with the block. If it were the block shouldn’t I have noticeably lower compression on one or more cylinders, knocking noises, metal flakes in the oil, low oil pressure, or something indicating such a problem other than it won’t rev all the way up under load? The only thing I can think of that’s left to try now is an electronic issue with the ignition. I did replace the advance module but with some off brand from Amazon. I did not change the pick up coil however but I had changed the ignition coil early on because it was arcing like a taser to itself. My thought process now is what if the defective coil I had to replace damaged the pickup and the off brand module just isn’t right. I know the data from the ECM says it’s advancing but I can’t test that with it running due to it needing to be in maintenance mode which negates the electronic advance in order for the timing light to give me a reading from the crank.

 

[km1125]

....
The two compression ratios for the 7.4L MPI that I found on the web were 8.6:1 and 8.9:1 That equates to approximately 126 and 131 psi at normal atmospheric measured at sea level.

What you're not accounting for in those measurements is the difference between gauge and absolute pressure nor the temperature changes when the gas (air) is compressed.
You can take 14.7 PSI of atmospheric pressure and multiply it by the compression ratio and get 126 or 131 PSI, but that would be absolute. What you measure with a gauge would be 14.7PSI less than that on a "normal" gauge.

And that's if nothing leaks out and if you wait for for the compressed air to cool back to room temperature!

When the cylinder is compressed, the temperature inside the cylinder goes up, which further compresses the gas(air) inside. It's this temperature increase that causes a diesel to start combusting, and what helps a gas engine with combustion as you need a lot of energy to break all the molecular bonds. The instantaneous temperature in a gas motor can easily get over 500F during the compression stroke. You can calculate a maximum, but what is unknown is how much of the heat is quickly absorbed by the piston, cylinder walls and head.