520 H headaches

[Mr. Knoop 5]

I've got a 96 520h with about 650 hours. I have owned it for 150 of those hrs or so. I am about to pull my hair out and buy something else. Please help. Main symptoms are that the engine will die randomly and quickly...no sputtering, no notice just dies. I then turn the ignition key off and right after, you can almost always hear and audible whoosh of air being expelled out the exhaust. There is also sometimes a loud backfire immediately after turning the ignition off. When I try to start it again, sometimes it will start up quickly and run fine, but will die out again. It sometimes starts hard after. I can smell gas so I am thinking its getting too much gas and when it does run it seems to use gas like crazy. I have changed all filters, fuel lines, cleaned/inspected the small fuel pump, cleaned/inspected carburetor, checked the Cylinder Heads out, new spark plugs, checked the wiring. I know the Onan p220g has a non adjustable carb. I am thinking about removing the glue holding the adjustment screws and trying to adjust it there, or checking the valve timing. Other minor likely unrelated issue is that my 15amp fuse for my headlights burns out everytime the headlight switch gets turned on. Anyone have any ideas? I'd really appreciate it.

 

[oliver2-44 9,796]

This is a very long shot, but does the muffler have a rattle like a loose baffle inside. If so it could be working it’s way to block the exhaust. Once it dies and the exhaust pressure goes away the baffle clears. 

[Mr. Knoop 5]

Nope the muffler seems fine, no rattling. 

[953 nut 55,465]

8 hours ago, Mr. Knoop said:

It sometimes starts hard after. I can smell gas so I am thinking its getting too much gas and when it does run it seems to use gas like crazy.

Check the oil, if your fuel pump diaphragm is leaking the crankcase could be filling with gasoline and diluting the oil.           

Some times an ONAN will have a bad valve and run fine on one cylinder but be pumping air/fuel mix out the exhaust, Remove a plug wire and see if it starts and runs, then replace it and remove the other plug wire, if  a valve problem is causing the symptoms then it will only run on the good cylinder.

[lynnmor 7,325]

Your carburetor has an accelerator pump and it can go bad, Onanparts.com may have the part.  When you had the heads off, did you check for loose valve seats?  You should check the compression to see if the engine is healthy, then adjust the valves.  You may have an electrical problem, check the 9 pin connector and the fuse block, that fuse problem may be a clue as the wires are bundled together.  The gas smell may be temporary loss of ignition.  Never turn off an engine till it settles down to a slow idle, if running too fast it will cause a backfire, this is especially true of the "emissions" carburetor that you have.

[FarmMechanic 3]

I have just spent 6 months fighting those same symptoms on my 1990 520H.  I adjusted the carb float, I replaced the ignition module, the stator (that turned out to be a self inflicted issue) and the voltage regulator.  In the end, the real cause of all of the intermittent behavior was the ignition switch.  It is the easiest thing to change, one of the least expensive.  I had been checking the spark and in the beginning, at the times I checked it, I had a good strong spark.  I would start it, run it for 30 seconds to 10 or 15 minutes and then it would die.  It finally would not start and I tested spark again and there was none.  I traced the wiring and then pulled and tested the ignition switch.  I got good contact in the start position.  In the run position the ohm readings would vary from 4 to 7 ohms and then up to mega ohms.  I put a new switch in and my 520H starts up immediately now.  It also runs as long as needed.  

Good Luck!

[Mr. Knoop 5]

Thank you all so much! I am grateful for your help. I will start with the ignition switch and checking the oil. I did not check the valves when I had the heads off recently. I had checked the 9 pin connector and fuse block recently as well and did not find any bad wires or anything suspicious. I will comment when I know more. God Bless 

[Mr. Knoop 5]

I forgot to mention that my oil pressure light on the dash blinks after the engine dies, and won't easily start backup. I ordered an ignition switch but I'm not convinced that's going to be the answer. It is using a lot of gas.

[FarmMechanic 3]

 

28 minutes ago, Mr. Knoop said:

I forgot to mention that my oil pressure light on the dash blinks after the engine dies,

 

The oil pressure light will blink any time the ignition is on and there is no oil pressure (e.g. engine is not running).  

[bo dawg 516]

The 520's are the worst when theres a problem, and the best when all is good. I have 2 and thats pretty much the same problem with one of mine. And my other one I believe has a bad valve seat. Dang machines.

[Rob R 966]

First thing I did with mine is replace the fuse block with all new H/D covered jumpers and a new starter switch solved all the electrical issues you mentioned. Also added an additional frame to engine ground strap.