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Penta GSi Compression Head and/or Rings?

M

Maka10

New member
Hi, I am new to this forum!

I Just had a compression done on a 2008 Penta GSi 5.7 engine. The numbers were 200 - 190 in all the cylinders except 2 on opposite sides. One side was 150, the other was 140. The boat Tech is a very reputable Guy, well known for his 40+ years of experience, honesty and very competent work. His opinion was the engine would require new cylinder heads. I gave me a very fair price to install, with Volvo brand new cylinder heads.

I am considering having it done and want to, but had a follow up conversation with him about could the low compression possibly be the piston rings or combustion chamber compression leaking to result in the 2 low cylinders? The Boat is a 26 foot Cabin Cruiser and he knew the previous owner and has worked on the Boat for about 7 years. The Risers and Manifold were changed 3years and 4 months ago. He explained that likely being it was middle cylinders opposite sides with low compression that it was likely due to the old Risers failing that resulted in excessive heat and the valves going bad.

Due to his experience he was certain that it was the cylinder heads, and went on to explain that the motor was not hard starting, no smoking, nor any other problems and running smooth and strong despite the 2 low compression cylinder readings. He also advised me that the previous owner changed the oil and filter in proper time and did the maintenance very well. I do have some knowledge and hung around a engine performance shop a few years back that built engines from the crank on up.

Can you Guys give me some thoughts about if it's ok to just go the new cylinder head route? Thanks. Any experience in doing this work would be appreciated.
 
I think that is premature. The compression in the 2 center cyls is lower but that can be due to more than just valve issues. I would say that before taking apart a good running engine I would:
Do a leak down test, this will show where the compression loss is actually happening; intake or exhaust valves or piston rings/pistons/cylinder walls.
Based on what that shows you can determine what to do next. If it’s losing compression at the exhaust valves you can remove the exhaust manifolds & visually inspect the exhaust valves through the exhaust ports. If it’s intake valves the carb can be removed and using an inspection camera you can inspect the intake valves. If it’s not either of those then the engine would have ti come apart for more extensive repairs. However you have to take other indications:
Does the engine burn oil—> piston rings
Is there evidence of water in the oil? Can be caused by overheats
It’s very common to have lower compression on those 2 center cyls if the exhaust elbow gasket seeps water down into the bottom of the manifold. You can remove the rubber hoses & 90* exhaust tubes and check for that with an inspection camera. In fact it might make sense to do this first because it’s such a common problem. It is possible that you could replace the exhaust (if in fact it’s verified that it’s rusty exhaust valves caused by leaky exhaust) it might clear itself up just by replacing the exhaust & running it.
BTW I wonder if that comp test was accurate, those engines usually have compression test results of 150-175 when in good shape. 190-200 is uncharacteristically high.
Also yes defective exhaust will cause corrosion of exhaust valves but another issue that can cause burning of exhaust valves especially in those center cyls is the engine running over propped. This means that the engine never reaches max wide open throttle RPM as specified by the manufacturer and that caused the engine to labor every time you try to get the boat up on plane. That causes combustion chamber temps to skyrocket. On the V8 the exhaust valves for the center cyls are right next to each other exacerbating this problem.
So I’d say do some more investigation…
 
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Thank you for your reply.

It's hard for me to just be doubting a experienced Boat tech with 40 years who knows the boat and serviced it for 7 years. Who is also up to date on Volvo Certifications too. I must have respect for them, but then again it's not the end of it because I know that that 2 low compression numbers can very much have other variables than the cause being, Heads and Valves. I am really struggling with this but it could well be that it is just the Head and Valves also?

To correct what I posted the number were actually:

200
190
185
190
185
195

Then the 2 @ 140 and 150 on opposite sides. I did some research and did see where a few other Penta 5.7 Gi's did similar compression numbers in the 200 - 185 range also.

There is no water in the Oil, No smoking or any other indicators other than the 2 low compression reading. This boat was well taken care of oil changes, Outdrive fluid oil changes, wash downs, new Bellows, seal and much other, more done regularly. It's like a 8.5 to 9.0 overall on a scale of 1 - 10.
At this point being I am out of state I am currently looking for someone experienced to go out to the Fernandina Beach Fl to get a 2nd opinion and leak down test. I googled and called around a bit to see but have not found anyone as of yet. I may put up some post on some of the forums looking for referrals down there to get a 2nd check out. Again Thanks!!!
 
Well here's the thing, while this guy may be good, and experienced, but the reality is this:
Your marine engine, adapted by Volvo Penta, is a small block Chevrolet 5.7 straight from General Motors. There is nothing internal to the engine that came from Volvo, the only Volvo specific parts are the transom assembly, outdrive, steering actuator, that's about it. In some years, they may have specified a particular EFI system, but it is likely that they don't actually build it, they probably buy it from GM's own suppliers.

And Chevrolet/GM small block engines usually do not have compression readings that high, unless the engine has been running very rich, and has a lot of carbon build up on top of the pistons. The other possibility is that the gauge is somewhat inaccurate. The bigger issue is you don't know why the compression is lower in those 2 cyls, but there are common causes as were mentioned above. I would like to see more evidence for the cause of the lower numbers before taking apart a good running engine. The other thing is this, if the compression is being lost from the piston rings, etc, changing the cyl heads will not fix that. So that's why you want to know the acutal cause. And remember there are no Volvo Penta cylinder heads for these engines, they are 100% GM. Meaning that you do not have to pay what Volvo dealers charge for these parts which is beyond obscene.
Have a leak down test done, if it is really losing compression on those 2 cyls due to valve problems, you will then know is it the intakes or exhausts, or even if it is something more, like the piston rings/pistons etc.
If it is the exhaust valves, as I said the exhaust manifolds can be removed to inspect/assess. If the intakes is the problem, the EFI intake manifold would have to come off to get a better look at them. If there are problems in either area, the usual way of repairing it is to simply take the cylinder heads to an automotive machine shop, they will check the heads for cracks, and perform a valve job, like they have done on probably thousands of small block Chevrolet cylinder heads, and re-assemble the engine with new gaskets, exhaust, etc. It's actually not a hard job, I did the same job on my '88 Chevy/OMC 4.3 V6 about 8 years ago.
4.3 new cyl heads installed.jpg
 
I'd second Lou's recommendation on the leakdown test...you can always ask the "experienced" guy to execute it...if he waffles, he may just not know about it...
 
Yes I understand that the Volvo engine is a GM and so is the Merc. Yes I also understand there are different options if it's the Valves to correct the Heads if that is actually the problem and the compression numbers are resulting in low compression on 2 cylinders.

The Boat is in Fernandina beach Fl right now out of the water. Now I need to try and find a Tech that will go to the Boat and do a leak down. At this point I would rather get a different look with a diff Tech to confirm if the compression is correct, and low, due to the Head, or combustion chamber and rings. Guess I will have to put some post up a few places to ask for a referral to a diff Tech in Fernandina Beach, who is competent to get accurate test results and determinations?

Like I said in my 1st post I hung around a Performance engine building shop some years back. This is the reason I am questioning the Head replacement only route. I never intended to go forward without more substantial test results. Wanted to hear what others might say about this situation, just to check my own thoughts.
 
makomark said:
I'd second Lou's recommendation on the leakdown test...you can always ask the "experienced" guy to execute it...if he waffles, he may just not know about it...
Click to expand...
I agree you both, you and him. I think the Guy just has seen so many engines that he feels kinda confident that it's not having signs of combustion chamber pistons, rings, cylinder walls, signs. There are things the engine will show if it was the chamber.

Its also normal for a 400 plus hour motor 18 years old to have some small ring leakage a bit. If I remember it's 5 - 10%. Then again I am still going to try and get a referral to a diff Tech for a 2nd opinion and test results before I do anything at all. Including the Oil in the cylinders to see a change or not in compression?
 
How's your oil consumption and blow-by? If it doesn't use oil and blow-by is minimal then it's likely the rings/pistons/cylinders are in good shape. The more common cause is the leaky exhaust elbow gasket issue, also because Volvo never changed to a dry joint exhaust like Merc did. When installing those exhaust systems, it is important to measure the flatness of the mating surfaces of both the exhaust manifold and elbow, they have to be flat and level to less than .003" and Volvo revised the gaskets a few times, use the updated ones and follow their torque specs. I use the Barr Marine aftermarket version of Volvo exhaust on my OMC (actually Volvo just copied OMC's design for exhaust) and with their gaskets they sealed fine. But if you see any sign of leaking on the outside of that joint, it can be leaking on the inside....
The other cause of valve issues is if the boat is over propped and not reaching max WOT RPM, this will over heat the combustion chambers and the valves that suffer are the exhaust valves on cyls 4/6 and cyls 3/5 because the exhaust valves are right next to each other on those cyls for the Chevy small block V8. The little 4.3 V6 doesn't have this issue as much because the center cyls are eliminated to make the V6 from the V8 casting.
Always verify that the engine reaches its correct max WOT RPM, and I also use at least 89 octane fuel if not higher, despite what the manufacturer might say about using regular. You will never hear detonation on a marine engine, till it's too late....
 
The lakdown test should be included, but a quick test to see where it's leaking can includepulling the spark plugs, fitting an air hose (like the one on a compression gauge into each cylinder and turning the crank manually. If it's leaking from a valve, you'll hear air escaping whether the other valve is supposed to be open, or not. Mine had a bent exhaust valve, so I heard air escaping through the exhaust regardless of the position of the crank,
 
I have had 2 other of these engines. A 5.7 GS and a 7.4. They both use just a very, very small amount of oil, as normal for these. This is about the same and actually far less. Even when the motor has sat for months, on start up there is zero smoke coming out. I asked about the gasket seal already, seems no signs of it leaking. The Prop allow it to WOT redline just fine. I try to just use premium fuel as much as I can. I am aware of the ECU Timing controls.

I am in the process of trying to find a 2nd Tech to go out and do a 2nd compression check and leak down to get a 2nd verification. I am also considering pulling the motor to have new rings and honing done, and the heads and valves done too. Just to be sure the motor will be trouble free and in good shape the next 10 years or so give or take. All depends on what this 2nd test out come is?
 
Maka10 said:
I have had 2 other of these engines. A 5.7 GS and a 7.4. They both use just a very, very small amount of oil, as normal for these. This is about the same and actually far less. Even when the motor has sat for months, on start up there is zero smoke coming out. I asked about the gasket seal already, seems no signs of it leaking. The Prop allow it to WOT redline just fine. I try to just use premium fuel as much as I can. I am aware of the ECU Timing controls.

I am in the process of trying to find a 2nd Tech to go out and do a 2nd compression check and leak down to get a 2nd verification. I am also considering pulling the motor to have new rings and honing done, and the heads and valves done too. Just to be sure the motor will be trouble free and in good shape the next 10 years or so give or take. All depends on what this 2nd test out come is?
Click to expand...
Was the cmpression test done at normal operating tempoerature, or cold? Wet or dry?
 
cold compression tests can be inaccurate, engine should be hot, ignition disabled and throttle wide open.
 
It was done warmed to operating temp with all plugs removed.

Got to thinking about the results? Even if the 8 reading showed some were on the unusual high side? The 2 low ones based on the high grade scale were still lower. Also could be a exhaust or Gasket leak?

Getting a Leak down done next.
 
The things that causes lower than normal compression typically are:
Valves not seating
Worn piston rings
Leaking head gasket
Cracked cyl head

Your lower ones are in the normal range for a typical Chevy small block, the majority are somewhat higher than normal. Why not check in a Volvo shop manual? I know from my OMC shop manual the 4.3 that I have is supposed to be about 160 psi, I think the V8s, about 150, but these are much older than yours (late 80s pre Vortec engines). If it were me I would pull the exhaust & inspect the exhaust valves. If it’s just light rust install new exhaust if needed and run it. As long as it runs well and had no water in the oil I would not be concerned. Light corrosion will clear on its own with regular use. The worst thing for a marine inboard is sitting for months with even a slight water leak from the exhaust. Are you in salt or fresh water? If salt then in Fla you are going to have inspect and if needed replace the exhaust at 3-5 years or so.
 
Add to your top list also Exhaust Mani/Riser Gaskets.

I have tried to find the Volvo Spec for compression on that Gi motor. Searching around I saw a few 185's as the top number unofficially. But I also saw many say that Volvo never gave a specific number even in the shop manual. I have a new Guy going with me on Sat, to do leak, and Oil squirt, Bore scope Cam, and more. I'll get to the bottom of it. Wish for me that it not as bad as I was thinking.
 
Here are the actual specs from my 1988 OMC Cobra manual for the GM engines:
4.3 V-6: 160-175
5.0 and 5.7 V8s: 155-175 psi
they also say that the indicated compression pressures are considered normal if the lowest reading is within 75% of the highest.
if you take the exhaust off post up pix of what the exhaust valves look like
 
So the engine I am talking about is 2008 Vortec Volvo Penta Gi 5.7. I got to doing some more research and found this:

The Volvo Penta 5.7 GI engine, which is a marine version of the GM Vortec 5.7L V8, typically has a compression ratio of around 9.5:1 with a .0315" (.8mm) gasket and 8.4:1 with a .098" (2.5mm) gasket. A typical compression test should yield around 150-200 psi per cylinder.

Here's a general guide to performing a compression test on this engine:
  1. Prepare the Engine: Throttle should be wide open, spark plug wires disconnected, and spark plugs removed.

  2. Install the Compression Gauge: Screw a compression test gauge into the first cylinder's spark plug hole.

  3. Crush the Engine: Crank the engine several times (3-5) until the gauge shows the maximum compression reading.

  4. Repeat for All Cylinders: Repeat the process for each cylinder.

  5. Compare Results: Compare the readings from each cylinder. If the compression readings vary significantly (more than 15 psi), it may indicate a problem with a specific cylinder, such as worn rings, head gasket, or valve issues, as discussed on the Volvo Penta thread.
So if my 1st Guy was correct about the reading it just may be in line the Results I posted in post # 3.
 
Here are another 2 test done on the Penta Gi 5.7 motor:

compression test. Here are the numbers. #1 - 180 psi #2 - 190 psi #3 - 165 psi #4 - 190 psi #5 - 180 psi #6 - 180 psi #7 - 180 psi #8 - 190 psi.


2000 5.7 GSi - How low is too low compression?

Had an overheat due to blocked portside riser (two years old, but that's another issue). The engine began to sputter, shut off immediately but major exhaust elbow and rubber overheat. I now have compression readings all around 190 except for cylinder No. 5 which is 150. It goes up approx 20 psi...
forums.iboats.com forums.iboats.com
I now have compression readings all around 190 except for cylinder No. 5 which is 150.
 
FWIW, GM redefined the compression test thresholds for their engines a couple decades ago...

100 PSI minimum in any cylinder and variation across all cylinders between max observed reading and 70% of max reading.And with the engine at 'room temperature'.

I would suggest getting the leak down test done before taking another step...
 
Maka10 said:
Here are another 2 test done on the Penta Gi 5.7 motor:

compression test. Here are the numbers. #1 - 180 psi #2 - 190 psi #3 - 165 psi #4 - 190 psi #5 - 180 psi #6 - 180 psi #7 - 180 psi #8 - 190 psi.


2000 5.7 GSi - How low is too low compression?

Had an overheat due to blocked portside riser (two years old, but that's another issue). The engine began to sputter, shut off immediately but major exhaust elbow and rubber overheat. I now have compression readings all around 190 except for cylinder No. 5 which is 150. It goes up approx 20 psi...
forums.iboats.com forums.iboats.com
I now have compression readings all around 190 except for cylinder No. 5 which is 150.
Click to expand...

150 wouldn't seem catastrophic, but it probably reveals a problem. My boat's engine, which had overheated before I bought it, had a bent valve- that's the reason I recommended using compressed air through the spark plug hole to listen for leaking.

When I worked for a boat dealer, a boat needed a valve because that cylinder's spark plug lost its center electrode and the valve had tried to close on it as it passed through.
 
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