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Compression problem, need some advice

C

CaboJohn

Regular Contributor
I have been working for several monthsto determine the cause of a weak port engine. (2002 350 CaptainsChoice, a MEFI-4 injected engine). Above about 2500 rpm the throttlemust steadily be advanced more than the starboard to keep up. Atabout 3500 rpm or so it is maxed out and the starboard is at about athird. Since we don't run long distances (mostly local trolling andwhale watching) I have only worked on it when I had a chance. Boatstill fishes great and at 1800 rpm you would never know there was aproblem.
Fuel issues have been addressed(filters, etc.) finally culminating in a pressure test that showedthe correct 58 psi at WOT. So fuel starvation is not the problem. Ascan showed all sensors in range and Cam Retard correct. I alsochecked throttle opening carefully and switched the manifold airpressure sensor with the other engine.
Today I finally got around to acompression test (the port side of the motor is near the poop tankand water heater and a ***** to get at, so it is not easy). Anyway,all cylinders tested 180-190 psi except for #5 which was at 15.
So I know the problem but have aquestion. No way can I get the head off of that side of the enginewhile it is in the boat. I think I can (with some considerable effort) removeenough stuff to get the valve cover off. As interesting as thatwould be, in most cases I expect I would still have to have themarina pull the engine and then remove the head. Or would I? I am trying tothink of some reasonably likely case where I could fix the problemwithout head removal. Since the pressure difference is so drastic(and oil consumption is zero) I don't think it is rings. It must bea valve issue. The engine makes no strange noises or tapping. I canthink of two situations. First, I could have a valve not closing allthe way or severely burned. If the valve is dropped and bent orrusted wide open or burned then I need to pull the motor and take off the head. But if it is a broken spring (this seems unlikely) that is causingthe valve to stay open, then I can fix from above.
Second, I am also (hopefully) thinking one mightget a very low compression reading if the intake valve had failedshut. The exhaust valve stays open until near the top of its cyclemeaning there would be very little to compress if the intake neveropened. Could that give me the 15psi reading? (When that cylinderis tested instead of the whir sound, followed by a compression sound,followed by a whir [you do it four times to get an accurate reading]I just hear a constant whir, with no huffing or puffing). A nonopening intake could be caused by a broken push rod or a rocker armstud that backed out over time. Neither of those require headremoval, unless the push rod failed because of a frozen valve. Or it could be a cam lobe failure meaning the engine mustbe pulled.
We have a handful of experts on thisboard and I would very much like some feedback. If there is a highlikelihood I need to pull the motor in any case I will do so andavoid spending a day or so wedged over the engine trying to get thevalve cover off. But if you guys have seen this issue solved fromabove on occasion, it might be worth doing. What I do not want to dois pull the engine and then find a simple problem that could havebeen handled in the boat.
Thanks for your help.
CaboJohn
 
Actually, remove the valve cover, check the rockers on that cylinder(for looseness) and then loosen the rocker arms on that cylinder. Pump your air in(carefully the engine may spin a 180 degrees) and look for the leak. When you have 15 psi in one cylinder, you are really not concerned with the percentage of leakage. You are looking for the leak. If I remember from past posts, your engine is FWC. Don't forget to remove the cap on the heat exchanger and look for bubbles in addition to the intake, crank case and exhaust.
 
Docksidemarineservices said:
1.... Actually, remove the valve cover, check the rockers on that cylinder (for looseness)

2.... and then loosen the rocker arms on that cylinder.


3... Pump your air in(carefully the engine may spin a 180 degrees) and look for the leak. When you have 15 psi in one cylinder, you are really not concerned with the percentage of leakage. You are looking for the leak.


4... If I remember from past posts, your engine is FWC. Don't forget to remove the cap on the heat exchanger and look for bubbles in addition to the intake, crank case and exhaust.
Click to expand...
1... Ditto!

2... this can done in lieu of finding TDC C/S on # 5, as it accomplishes the same thing.
Whether the piston is up/down in the bore, will make little difference for a leak-down test....., other than the potential of the air pressure wanting to move the piston.
That being said..... rings may leak when near the top of the cylinder, vs perhaps not leaking the same when near the bottom of the cylinder.
The air won't care one way or the other.


3... Ditto again.

4... Ditto again.


Re: #2
If you do want to bring #5 up to TDC C/S, bring #1 up to TDC C/S, and then rotate the crankshaft an additional 450*, (or 5 increments of 90* each).
This brings #5 to TDC C/S where both valves should be fully closed.


I would NOT stick a screw driver into a cylinder that is about to have it's piston moved!
Bad Ju Ju! :mad:


.
 
If you think you are going to get lucky, you might as well pull the rocker cover...

Sadly, my gut thinks it won't be so easy...

If the access is that bad and you honestly believe it will take all day to pull the rocker cover, let alone to do any troubleshooting, you may want to consider having the yard pull the engine...
 
Thanks for the advice but I guess I did not ask my question too clearly. I don't doubt I will find the problem easily when the engine is pulled and I can get at it....but is it worth a lot of agony to pull the rocker cover now just to see what's happening while it is still in the boat? I mean, it took me all morning to do a compression check that should take maybe 40 minutes. Things are really tight on that side of the engine. What I was hoping was for someone to say "yea, they saw only 15 psi on another job and it was a non opening intake valve due to a valve train problem" (that's about all I think I could cure without pulling a head). That might inspire a little hope. But frankly I am far from sure that the symptom of that unlikely problem would be what I am seeing. So in the absence of further information I guess the engine comes out. Then we will look under the cover and likely pull the head.

As for the suggested tests, if I manage to pull the valve cover and it is a non opening intake valve that will be obvious. Anything else means pull the head so the tests won't really help me decide what to do right now. And Mako (who did carefully read the post), no I don't feel that lucky. But maybe I should. Yesterday I took apart the motor on a broken wiper motor ($175 new) and found a screw had come loose in the mechanism. Simple fix, no cost. How many times does that happen on a boat?
CaboJohn
 
CaboJohn said:
Anyway, all cylinders tested 180-190 psi except for #5 which was at 15.
Second, I am also (hopefully) thinking one mightget a very low compression reading if the intake valve had failed shut. The exhaust valve stays open until near the top of its cycle meaning there would be very little to compress if the intake neveropened. Could that give me the 15psi reading? (When that cylinderis tested instead of the whir sound, followed by a compression sound, followed by a whir [you do it four times to get an accurate reading] I just hear a constant whir, with no huffing or puffing). A nonopening intake could be caused by a broken push rod or a rocker armstud that backed out over time. Neither of those require head removal, unless the push rod failed because of a frozen valve.
Click to expand...
John, rocker arms have been known to break, studs have been known to back out, and push rods have been known to fail.
If (key word "if") a rocker arm was to have broken (example only) this would prevent valve operation.

No valve operation would/may cause what you have described above.


CaboJohn said:
.......... but is it worth a lot of agony to pull the rocker cover now just to see what's happening while it is still in the boat?
Click to expand...
IMO, you either pull the rocker arm cover, and explore...., or you pull the engine, of which is considerably more work.

In post #5 I briefly explained how you can bring cylinder #5 up to TDC Compression Stroke by beginning with #1 cylinder TDC C/S.
I use #1 as an example due to how easy it is to achieve true TDC C/S.

............. bring #1 up to TDC C/S, and then rotate the crankshaft an additional 450*, (or 5 increments of 90* each).
This brings #5 to TDC C/S where both valves should be fully closed.
Click to expand...
If you'll do this, the #5 cylinder can be pressurized and a leak-down test can be performed.
No leak-down, then the valves are likely holding.
Leak-down, then you listen and learn where the leak originates.

Leak possibilities:
Failed intake or exhaust valve function (a likely candicate)
Failing intake or exhaust valve seating
Failed piston rings (although unlikely for this low 15 psi reading)
Failed piston crown/deck due to detonation damage (but this would cause excessive crankcase pressure)

No valve operation:
Broken rocker arm
Failed push rod
Rocker arm stud that has backed out


Use the tried and proven P of E...... ( process of elimination )
When used methodically and systematically, it won't let you down!


.
 
We all read the post carefully at least I did.
Unless your flooring is in the way, you should be able to get the valve cover off. Better to investigate now then to pull the engine and find something that was a simple repair.
 
OK, now we are getting somewhere. Does Chris, or anyone else, concur with Ricardo (and me) that a non opening intake might cause a reading as low as 15 psi? In my case I am just speculating since I have not had experience with anything other than worn out engines (in my misspent youth). If it is a real possibility that the issue is valve train related, then I really must pull the valve cover before removing the engine. I "know" it is not rings as the engine has been running this way for some months and the oil is clean and it has not used a drop.
Pulling the valve cover sounds easy but access is the issue. Last year, chasing the overheat issue on the starboard engine I did pull the intake manifold, valve cover and head on the left side of that engine so I have done the job. But as I recall, since I needed to set the valve lash, and had the engine together and running before I buttoned it up. And getting the valve cover back on with the exhaust manifold installed was very difficult, and I did not have an access issue on that side of that engine. In this case I would be leaning over the port engine (actually laying on it) and reaching with extended arms to do the work. I figure I might be able to loosen the exhaust manifold, put it on some extended bolts, and push it away to get a needed inch or so clearance to remove the cover. But if the manifold gasket tears (don't they always?}, I will be scraping and changing it blind.

But Chris raises an interesting point. It is in fact the flooring that creates most of the access issue. And the flooriing removal is the first step to pulling the engine (so it would have to be done anyway). So I will get a quote on having the marina use their forklift to pull the flooring. Once it is out of the way, it would be much easier to get the valve cover and take a look. Then I will know if the engine must come out.

Looking forward to any further comments/thoughts.
CaboJohn
 
Had the flooring unit over the engines removed this morning. Found the access pretty good so it looks like whatever I do, I will not have to pull the engine. I took off all the electical stuff that covers the top of this MEFI-4 350's and finally got to the valve cover. Pulled it and turned the engine over. Both valves on #5 go up and down as the should so that does not seem to be the cause of the 15 psi reading. I will have the head off tomorrow and then we shall see.
If it is a burned exhaust valve as expected, there are a couple of issues that some of you with more experience may wish to advise me about. My assumption right now is that I take the head to a competent shop (we do have one down here) and have a new valve, seat if needed, guide and seal installed. Since the engine burns no oil and has 190 psi in the other cylinders doing any additional valve work is a waste. Agree? Have it checked for flatness and if all is OK put it back on.
But should I be looking for what caused the issue in the first place and if so, what do I look for? I see that the valve cover blue paint is pretty well burned looking next to the exhaust manifold (but then the boat is 11 years old and they come very close to touching so that may not mean anything). And the boat had new Barr manifolds and risers put on 2 years and 200 hours ago so they should still be OK. With the engine off there is a little play in the valve train for that cylinder (as there should be) so it does seem the valve is not being allowed to fully close. Any ideas or do I just chalk it off as "one of those things"?
Thanks for any help or suggestions.
CaboJohn
PS Just as an aside, I noticed that the stud for #5 exhaust has a dab of yellow paint on it. Highly suspicious. Do you suppose someone has changed it out once before??
 
You need to loosen the rockers and pump air into the cylinder before you pull the head. DO NOT PULL THE HEAD UNTIL YOU DO THIS! You need to know where it is leaking from.
 
CaboJohn said:
...With the engine off there is a little play in the valve train for that cylinder (as there should be) ...
Click to expand...

What exactly does this mean?

Being as the boat was used when you bought it, unless you have a detailed log book, you can only speculate on what happened prior...I'd suggest not worrying too much about the history and concentrate on doing the necessary repairs so you don't have to repeat the exercise anytime soon.

It would be prudent to determine where the compression loss is occurring before you pull the head but i wouldn't deem it "essential".
 
OK, will do. How about my other two questions? Things will move pretty fast if it is a valve. I should be taking the head to the shop this afternoon.
CaboJohn
 
As suggested I put air on the bad cylinder after loosening the rockers and I could hear the air roaring out of the exhaust. Pulled the head and yep, the exhaust valve on #5 was burned. Took the head to my mechanic who took it to the local head shop. Needs a valve and a seat and he recommends a light facing (even if it measures flat) and an inspection and "reseat" of the rest of the valves. Said OK. He also said we now have a shop (they call it a "laboratory") that has the equipment to clean and test injectors for flow and pattern so I pulled those and sent them off. The other possible cause of lean running would be the clogged "pancake" filter I found several weeks ago. I did my fuel pressure test after the cleaning and it was good, but perhaps would not have been if I tested it earlier. Timing is not adjustable (set by the computer) but I scanned it earlier and it is on spec.
I should be my pieces back tomorrow late and can assemble on Friday. Had to split the manifold and the riser to get it out. Wish I had a new gasket but this is Mexico and we learn to make do.
Mako, I was trying to point out that the valve train had some slack so the valve had not worn in a way that left is cracked open and doomed to failure. Maybe someone can correct me, but when I adjust lifters I crank the nut down until the pushrod will not freely turn and then give it another half turn on the nut. (Some folks do a complete turn). But if I go back a few minutes later I find things have "adjusted" to their new state and the push rods are once again a little loose. Years ago I chased that looseness until it went away and then the valve would not fully seat, so I don't do that anymore. The looseness does go away with oil pressure but that is what I felt and described as "a little play". Poor choice of words.
Thanks for the responses.
CaboJohn
 
Found out today that two of the injectors were partially clogged. That may explain the burnt valve. They tested good after cleaning but an additional injector was "stuck open" and must be replaced. Not so sure about that since I did check for pressure leak down when I did the fuel pressure test. Anyway my mechanic (actually my car guy, I do the boat) found a replacement on a junker and it is off to the injector shop to see if it will work. Hope so as new ones are $$$.
CaboJohn
 
Chris, thanks for the response. I have a copy of the 337 page PCM MEFI 4 manual. It says for my engine (2002 MP 5.7L) I should have 57-62 psi @WOT. So unless I am reading it wrong I should be good. This manual is an incredible help and with very little looking is available for free on the internet. The bigger engines have lower pressure specs (don't know why).

Latest phone call on the injectors: Mechanic says he cannot find any Chevy injectors available and the junker he found was actually off of a Dodge. It tests good except that in flow tests it will do 73cc in the time the others do 75 cc. He assures me that difference is too small to matter. I guess that makes sense to me, unless these engines are so finely tuned that the computer has them right on the ragged edge of too lean in an effort to save fuel. In any case it will be a whole lot better than the clogged ones, but I really do not want another burned valve. I may throw the question out to the hot rod community but if anyone here has some knowledge please weigh in.
CaboJohn
 
CaboJohn said:
Chris, thanks for the response. I have a copy of the 337 page PCM MEFI 4 manual. It says for my engine (2002 MP 5.7L) I should have 57-62 psi @WOT. So unless I am reading it wrong I should be good. This manual is an incredible help and with very little looking is available for free on the internet. The bigger engines have lower pressure specs (don't know why).

Latest phone call on the injectors: Mechanic says he cannot find any Chevy injectors available and the junker he found was actually off of a Dodge. It tests good except that in flow tests it will do 73cc in the time the others do 75 cc. He assures me that difference is too small to matter. I guess that makes sense to me, unless these engines are so finely tuned that the computer has them right on the ragged edge of too lean in an effort to save fuel. In any case it will be a whole lot better than the clogged ones, but I really do not want another burned valve. I may throw the question out to the hot rod community but if anyone here has some knowledge please weigh in.
CaboJohn
Click to expand...


Glad to here. I didn't know what the spec on your engine was.
 
I agree with what the shop guy told you...unless you have a set of matched injectors, they are likely to be in a +/- 5% type of tolerance band for flow rate...I'd also be sure to check on the electrical characteristics of the 'dodge' injector to make sure it is compatible with the ECU.

Was one of those "bad" injectors feeding the cylinder with a burnt valve?
 
@Mako I mixed up the injectors after removal so I don't know what was where. It is logical that the burnt valve was on one of the clogged injectors that was running the charge lean. The other clogged injector likely was not as bad, or was on an end cylinder where the exhaust valves (not being next to each other) run a little cooler.
I spent some time yesterday researching injectors and I also came up with that +/- 5% number as typical for many, but not all installations. The correct injectors are GM12567905 and as far as I can tell are unique to marine use. They are made by Bosch. The nominal flow rate is 24 lbs. per hour. I say "nominal" since all injectors are rated at 43.5 psi for comparison purposes, even if they will see use at higher pressures (58psi in my case) and therefore will flow more than that. As near as I can tell (data is sparce) the Dodge injector, 53031740AA is rated at 23.2 lbs. per hour and that is about what it tested. It is bright red, but otherwise looks identical. I measured the coil resistance and it is exactly the same.
One big difference however, is the Dodge has a one hole pintle and the GMs have 4 hole pintles. Most sources say the 4 hole gives a little softer pattern and cleaner burning but I have not found anyone that believes the change will harm the engine.
Still, I think that when I return to the States I will pick up the correct injector and put it in when I come back. In the meantime I will run the Dodge in one of the end cylinders (cooler) and enjoy the water.
Got the head back yesterday (with the new valve seat) and will start reassembly Monday. Also will pull the injectors on the other engine and have them bench cleaned and flow tested. (It cost $10 a pop down here). I have a new fuel fiter for that side as well.
CaboJohn
 
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