803 Bogs Down and dies at 85% Load

[USAMilRet]

I am having difficulty in finding that this broken rocker arm is responsible for the lack of either vacuum or pressure in the crankcase, What causes the vacuum in the crankcase (methology) and how do I restore it?

 

[Zed254]

As I understand it you do not want blow-by pressure in the oil sump. I do not think you have that. I've got an 802 that definitely has blow-by with only 450 or so hours on it. This usually means pressure is getting past the rings and into the crank case. I've been running it hard with break in oil to try and remove any cylinder glaze. Mine had some wet stacking when first fired up, so I suspect it was run under very low demand for most of those hours. I'm betting yours is in good shape....

And thanks for this awesome record of the work you've done on your machine. I almost fell out of my chair when I saw the pic of the broken rocker arm. I'm learning a lot and will be in a better position to attack my 802 if the blow-by does not diminish with run time.

 

[USAMilRet]

Well, I know why there is a vacuum. It is because of the diesel fuel particulates in the crank case. If there is a sufficient amount of vaporized diesel fuel in the crankcase and you get it ignited by either heat or spark, you can have a primary and secondary explosion (like a back draft). The primary is not very big compared to the secondary, but the secondary will blow the machine apart with the surrounding area.

So to prevent this, they build the engines so that a vacuum is in the crank case or they put some type of vacuum pump in to keep this from happening. My question is what causes the vacuum on the LP LPW4 engine/Onan DN4M-1/MEP803A engine and how do I restore it considering the rings are supposed to be new and installed correctly? There is definite cross hatching on the cylinder walls.

Guyfang indicated that if there was a lack of vacuum it would be very bad but he did not explain further even when prompted to in this thread.

In a gas engine you have PVC or positive crankcase ventilation which is usually re-burnt. In this engine, there is no PVC valve or anything else that I can find to explain how the vacuum takes place and was just wondering if anyone knew how the vacuum is created and how restore it.

 

[csheath]

The only engines I've ever seen with a crankcase vacuum either had a vacuum pump or exhaust scavenging system plumed to the valve cover. I haven't seen either on these engines. How an internal combustion engine can produce vacuum in the crankcase without one of these systems escapes me.

The TM MEP-803A-Onan-Engine-DN4M-TM-9-2815-253-24.pdf explains how much vacuum should be present and how to measure on pages 3-11 and 3-12.

 

[USAMilRet]

The only engines I've ever seen with a crankcase vacuum either had a vacuum pump or exhaust scavenging system plumed to the valve cover. I haven't seen either on these engines. How an internal combustion engine can produce vacuum in the crankcase without one of these systems escapes me.

The TM MEP-803A-Onan-Engine-DN4M-TM-9-2815-253-24.pdf explains how much vacuum should be present and how to measure on pages 3-11 and 3-12.

That I got. I just do not get the vacuum method to this engine.

 

[justacitizen]

i don't think the engine runs with a vacuum in the crankcase. but with the absence of blowby to cause a positive pressure the displacement of the pistons moving will cause a temporary sucking movement through the small dipstick opening. if you put paper there and experience this movement is indicative of neutral pressure in the crankcase thereby no blowby. if the paper was to be blown away constantly from this small opening you would have positive crankcase pressure so blowby would be present and indicate worn rings or some other .

 

[USAMilRet]

What is the 'some other'?

This engine is supposed to have a constant vacuum in the crankcase. There are 4 pistons and 2 move in sequence together as do the other two. Therefore, the piston movement cancells out any increase or decrease in pressure/vacuum, ie the internal space of the crankcase remains constant. I can see how worn rings causing blowby can cause a pressure increase but what is the vacuum mechanism?

 

[justacitizen]

What is the 'some other'?

This engine is supposed to have a constant vacuum in the crankcase. There are 4 pistons and 2 move in sequence together as do the other two. Therefore, the piston movement cancells out any increase or decrease in pressure/vacuum, ie the internal space of the crankcase remains constant. I can see how worn rings causing blowby can cause a pressure increase but what is the vacuum mechanism?

"some other" any thing that would allow compression out of the combustion chamber and into the crankcase. does the TM specify a vacuum in inches of water column ?

 

[USAMilRet]

Yes.

3-5. CRANKCASE VACUUM.3-5.1. General. The value depends to some extent on the type and size of air cleaner installed on tie engine.Regardless of type air cleaner used, vacuum with a clean air cleaner must not be less than a minimum of 0.79 in. (20mm) water gage (WG). The vacuum is measured with a manometer at the lubricating oil dipstick hole with the enginerunning. In engines in good condition vacuum increases slightly with engine speed but not proportionally. A fluctuatingvacuum may indicate faulty oil seals, valves, or piston blow-by troubles. Crankcase pressure can cause serious oil leaksand often occurs in engines which need overhauling.

The measurement is 1.37in which is considered normal. Minimum is 0.79in

 

[Demoh]

When I had a few engines apart I figured out how these get their vacuum. There are some passages in the head (Post #237, picture 1) that connect the tops of all the valve covers to the intake. Its that hollow dowel pin / roll sleeve that half are obscured in the photo by the springs. The valve cover continues that passage then IIRC it makes a sharp turn so the intake for that near the top point of the crankcase.

This is why the air filter plays a role in the vacuum. Basically it seems that the crankcase is under vacuum from the intake, but the passage size might prevent it from being excessive (thats a guess on my part)

 

[justacitizen]

well there you have it. Thanks that is interesting

 

[USAMilRet]

When I had a few engines apart I figured out how these get their vacuum. There are some passages in the head (Post #237, picture 1) that connect the tops of all the valve covers to the intake. Its that hollow dowel pin / roll sleeve that half are obscured in the photo by the springs. The valve cover continues that passage then IIRC it makes a sharp turn so the intake for that near the top point of the crankcase.

This is why the air filter plays a role in the vacuum. Basically it seems that the crankcase is under vacuum from the intake, but the passage size might prevent it from being excessive (thats a guess on my part)

You are 12 hours ahead of me. I was looking at the TM's, diagrams, and parts list to see what the sleeves and holes were and finally figured that they had something to do with the lack of vacuum as the rings are supposed to be new. I am missing one of the sleeves. Also because they did not want to buy a new gasket set, the gaskets (those that were found anaways) set on the first rebuild were used with lots and lots of RTV cement on them for the second build. Even the head gasket......go figure.

So I think I was burning oil at first, not very much, but some (blue smoke in exhaust). With a sleeve missing, that would allow the oil to enter the intake manifold and burn off. This would have a definate impact if the sleeve port was filled with oil. Also, due to the amount of RTV cement used, I have been finding ports partially filled with this crap and am wondering if the other ports are blocked as well. Time to snake the ports.

As you are 12 hours ahead of my thinking process, please provide next weeks winning lottery numbers to me. I'll share a handshake and beer with you....... We are after all next door to each other. You in St Pete and me in Seffner (I know.....where the **** is that?). Between I-4 exits 10 and 12 just to the east of Tampa. Kingsway & US92

So it appears that the missing vacuum problem might be solved. I am going to take a piston out to verify ring status as well. Trust but verify. I trust the rings are new and installed correctly, but I am going to verify.

Thanks Demoh. I'd kiss you but I don't lean that way.........You wouldn't happen to have an extra sleeve lying around that I can gat from you?

 

[justacitizen]

i think you have mostly an OK engine. to bad you have to disassemble it check and reassemble but you are almost there anyway. when you are done at least you will know for sure what you have and not have to take anthers word for it.

 

[Guyfang]

I have been gone a few days. You guy have been up to all kinds of stuff. The long and short of it is, you got reamed. And who ever did this to you, should be known to the world. So it might not happen to someone else.

 

[Demoh]

...You wouldn't happen to have an extra sleeve lying around that I can gat from you?

The roll pin sleeve thingy for the vacuum that I was talking about or something else?

I have a trashed engine (water in exhaust) that ive been sitting on however I dont know if I want to start parting it out (I havent pulled the cover or head to see if its salvagable yet, its last on my list). Either way if thats the part Id just go to a fastener shop and grab a roll pin (not the split type but the multilayer rolled type)

If I end up parting out Ill let you know but in watching your threads you are working faster than I would even consider tearing into my spare dead engine. I am in the middle of moving shops and have 2 gens clients are waiting on for me to go through.

 

[Demoh]

I have been gone a few days. You guy have been up to all kinds of stuff. The long and short of it is, you got reamed. And who ever did this to you, should be known to the world. So it might not happen to someone else.

I full agree with this. I dont like spreading bad words about people but it would likely do the community a favor (esp here in florida where since Irma everybody wants a generator) to not get burned.

 

[USAMilRet]

I have been gone a few days. You guy have been up to all kinds of stuff. The long and short of it is, you got reamed. And who ever did this to you, should be known to the world. So it might not happen to someone else.

How do you figure? I knew the motor worked because it ran before they rebuilt it. I knew the generator worked because it made power before they touched it. They rebuilt the engine and put a bunch of new parts into it. They sent the head out and had the valves reground and the head surfaced, replaced a couple of valves and all the springs. Because they could not figure out a problem does not mean I could not. So I used this to my advantage and got a $1500 refund plus about $500 of other stuff.

So for $3500 and a little elbow grease, I will have a newly rebuilt engine and a newly rebuilt head. A very nice and clean machine that only had 212 original hours on it. Yeah I have put about $500 into it, so we will say $4000, but wait.....the $500 free stuff. Now back to $3500.

So, yes I agree this was a very incompetent rebuild from what appeared to be a legitamate business, but maybe you can explain to me how I got reamed for a basically a 'what everyone really hopes to get genset' in this condition for $3500.?

I just saw a scrap parts MEP803A sell on GL from Jacksonville for $3200 plus taxes and fees, so I make it about $3800 and is all but garanteed NOT to work.

 

[USAMilRet]

I have been gone a few days. You guy have been up to all kinds of stuff. The long and short of it is, you got reamed. And who ever did this to you, should be known to the world. So it might not happen to someone else.

If you really feel I got reamed......nothing I can do about it except disagree.

Your replies in this thread are not at all what or how you post in other threads. They have not been of much help and I had been hoping to tap your knowlledge. Are you aware of something I an not about the seller of this unit or his business? I did have his info out for a day before I removed it but it is not hard to figure out where it came from.

 

[csheath]

In my opinion you got reamed. Throwing an engine together with no gaskets, a clogged air filter, and gobs of RTV is a far cry from being a competent rebuild. If you are happy with it then great but I would feel cheated.

FYI, be mindful that those gobs of RTV can get into oil and water passages and ruin an otherwise okay engine.

 

[strycnine]

In my opinion you got reamed. Throwing an engine together with no gaskets, a clogged air filter, and gobs of RTV is a far cry from being a competent rebuild. If you are happy with it then great but I would feel cheated.

FYI, be mindful that those gobs of RTV can get into oil and water passages and ruin an otherwise okay engine.

I don't have a dog in this fight but I agree with this.