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Old 11-22-2020, 09:01 PM   #1
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ONan vapor lock & burning smell

I’ve noticed that sometimes after I prime my generator and if it doesn’t start, then I try priming it a 2nd time, the starter half cranks and stops as if the gen gets vapor locked. Do these generators vapor lock? If not what am I experiencing?

And today it seemed to lock up twice, then we smelt a burning smell from the front cab after the 3rd cranking cycle. What device would make this odor? The smell dissipated, I’m definitely linking it to the way I started the generator.

I also would like to know how everybody is starting their generators? I hold my stop button down until the light goes on, then count five seconds and then crank. Should I be holding the stop button longer once the red light is on?
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Old 11-23-2020, 07:03 AM   #2
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Hello, I had tons of issues with our new 29MV onan at first , my symptoms were hard to start and it would overheat after using for 1 hour. Here is the list of items modified to get generator to run better. 1. Cut a hole in RV Door across from air cleaner to allow the generator to breathe when closed, I put some almond color 9x12 vent cover off amazon. 2. made appointment at local Cummins dealership to have carburetor serviced and all adjustments performed. NOTICE- they do all adjustments with door Propped open. 3. Downloaded the real operators manual from https://www.cummins.com/rv-generator-manuals and performed a complete plug/oil/air filter change on scheduled interval. remember 50 hours is first maintainace .
Possible smell could be coming from the internal auto switch in your RV. the autoswitch has a 30 second delay before power is supplied , you would have to smell closer to detect loose wire or poor switch relay contacts.
Yes I hold down the lower rocker (prime) only for 3 sec if the generator has not been ran in weeks or longer. if warm no need to prime.

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Old 11-23-2020, 07:16 AM   #3
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Well I haven't experienced a vapor lock since I drove my '52 Chevy across the Sonora Desert. Your genset first has an inline fuel filter that screws into the fuel pump and maybe a second inline filter in the fuel hose between the genset and the fuel tank. My guess is that one of those filters is your fuel delivery problem. The first filter cost about ten bucks and to change requires unscrewing the filter, gasoline compatable sealant on the new filter threads and care not to cross thread the new filter when installing it. You'll need a 9/16ths deep socket. The second filter cost about two bucks. It likely is secured to the fuel hose with small screw clamps or spring clamps on each end. The two buck easy to change filter doubles or even triples the life of the ten buck BE CAREFUL! to change filter. As to the "somethings burning" smell, that's serious cause for alarm. Stop doing what ever you were doing when when you smelled it and find out what was burning.
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Old 11-23-2020, 07:18 AM   #4
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I also would like to know how everybody is starting their generators? I hold my stop button down until the light goes on, then count five seconds and then crank. Should I be holding the stop button longer once the red light is on?
10 to 15 seconds won't hurt..
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Old 11-23-2020, 08:10 AM   #5
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My Gen kicks right over after the initial start when we use it daily on travel days. The only cranking issue I have is when it sits for a week or longer, but even then I don’t consider cranking as long as it does abnormal when it’s stone cold. And once it is running it never shuts down. It might surge a bit but that clears up too.

My only issue is that “clunk” and no spin of the starter unless I let go of the start button for a few seconds. The smell in my front cab only came when I cranked it 3 times and it did the “clunk” and no spin.

Any further thoughts and advice would be appreciated.
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Old 11-23-2020, 08:11 AM   #6
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10 to 15 seconds won't hurt..
And that’s after the red light is on?
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Old 11-23-2020, 08:20 AM   #7
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And that’s after the red light is on?
Yes..It doesn't start prime until light is lit.
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Old 11-25-2020, 06:38 AM   #8
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The ONAN manual says to prime for 30 secs, which seems excessive
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Old 11-25-2020, 06:55 AM   #9
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My only issue is that “clunk” and no spin of the starter unless I let go of the start button for a few seconds. The smell in my front cab only came when I cranked it 3 times and it did the “clunk” and no spin.

Johny, I would Prop outside door open , sit on ground infront of generator, take off outside cover, use the rockerswitch on the GEN and listen close. The clunk might be a starter loose or not aligned (maybe?) If i remember correct left is prime and right is starter . Mine sounds just like a electric fuel pump while pushing prime only, start will engage the starter . Ted
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Old 11-25-2020, 07:09 AM   #10
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My dealer service manager told me years ago to prime for a few seconds and if it doesn't start with a few seconds of cranking, prime some more and try again. My typical prime after sitting for a month is 8-10 seconds and then virtually instant start the first time.
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Old 11-25-2020, 07:20 AM   #11
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My only issue is that “clunk” and no spin of the starter unless I let go of the start button for a few seconds. The smell in my front cab only came when I cranked it 3 times and it did the “clunk” and no spin.

Johny, I would Prop outside door open , sit on ground infront of generator, take off outside cover, use the rockerswitch on the GEN and listen close. The clunk might be a starter loose or not aligned (maybe?) If i remember correct left is prime and right is starter . Mine sounds just like a electric fuel pump while pushing prime only, start will engage the starter . Ted
Thanks Tgroves,

When I say “clunk” , it’s not an unusual noise, I get a brief spin of the starter, then it binds up, doesn’t crank the motor, just like cars use to do back in the day when they would vapor lock.

Funny you should mention a loose starter, in my younger years as a mechanic, I had a car that had one starter bolt completely out and one hanging on by a few threads. The starter gear was close enough to the fly wheel that when the Starters Armature flew out, it grabbed the fly wheel, propped up in place and started the car! What a site to see and hear!
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Old 11-25-2020, 07:43 AM   #12
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My dealer service manager told me years ago to prime for a few seconds and if it doesn't start with a few seconds of cranking, prime some more and try again. My typical prime after sitting for a month is 8-10 seconds and then virtually instant start the first time.
Thanks Edatlanta,

I will try priming longer. But I still would like to know what is causing this cranking issue that I described. It almost like a one crank with a dead battery, but will crank again if left for a few seconds.
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Old 11-25-2020, 07:47 AM   #13
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Had this same thing happen to a friends rv Check the Aux Battery connections , and the Batteries he had 2 bad Batteries ( had no Cranking amps ) Do this first . If in doubt have a professional check it ...
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Old 11-25-2020, 07:56 AM   #14
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Let me explain, the engine will spin fine Until it hits the compression stroke, a weak or low amp battery will spin it over until you hit compression ,a weak or low battery does not have enough cranking amps to get past the Compression ,so after a few seconds the compressing that was in the cylinder has gone and you can spin again .
If in doubt have a professional check it out ..
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Old 11-25-2020, 07:57 AM   #15
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Had this same thing happen to a friends rv Check the Aux Battery connections , and the Batteries he had 2 bad Batteries ( had no Cranking amps ) Do this first .
Thanks herewego,

I was thinking that. Did he have a load test put on the battery to find out what Cold Cranking Amps he was getting?
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Old 11-25-2020, 08:00 AM   #16
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Let me explain, the engine will spin fine Until it hits the compression stroke, a week or low amp battery will spin it over until you hit compression ,a week or low battery does not have enough cranking amps to get past the Compression ,so after a few seconds the compressing that was in the cylinder has gone and you can spin again .
Makes perfect Sence! Now have have an excuse for my DW to upgrade my battery and possibly add solar!
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Old 11-30-2020, 09:38 AM   #17
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Yes, those symptoms would indicate to me a weak battery. herewego explained it perfectly IMO.

On my own generator (cold), I will prime for 10-15 seconds, try to crank for a few seconds, then prime again for another 5 seconds, and it fires right up. Of course, like others, if it's warm or has run recently, it doesn't need much prime.

It might help to have the chassis motor running when you start the generator to give the house battery a little "assist".

I have a known weak house battery (haven't gotten around to replacement yet, and we almost always have shore power anyway so it's low on the priority list), so when I walk up to my cold rig, I will start my chassis motor first, let it idle for a bit while I secure/make ready other items in the rig, which gives the alternator a chance to put a little charge on my house battery. Then I'll go through my generator startup process. Works every time.
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Old 11-30-2020, 09:49 AM   #18
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Yes, those symptoms would indicate to me a weak battery. herewego explained it perfectly IMO.

On my own generator (cold), I will prime for 10-15 seconds, try to crank for a few seconds, then prime again for another 5 seconds, and it fires right up. Of course, like others, if it's warm or has run recently, it doesn't need much prime.

It might help to have the chassis motor running when you start the generator to give the house battery a little "assist".

I have a known weak house battery (haven't gotten around to replacement yet, and we almost always have shore power anyway so it's low on the priority list), so when I walk up to my cold rig, I will start my chassis motor first, let it idle for a bit while I secure/make ready other items in the rig, which gives the alternator a chance to put a little charge on my house battery. Then I'll go through my generator startup process. Works every time.
Thanks CamperBob, yes same symptoms and process as you explained. I will try and run the coach when I crank the generator next time.

I highly maintain everything in/on my rig, and always keep an eye on my water level in my battery. I know of 1 time the service center I was using, (not anymore), killed my coach battery when I picked it up once. Who know how many other times they killed it! My battery’s days are numbered for sure! I might upgrade to AGM and a little solar, or sell my rig and trade up in the near future.
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Old 11-30-2020, 10:29 AM   #19
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Thanks CamperBob, yes same symptoms and process as you explained. I will try and run the coach when I crank the generator next time.

I highly maintain everything in/on my rig, and always keep an eye on my water level in my battery. I know of 1 time the service center I was using, (not anymore), killed my coach battery when I picked it up once. Who know how many other times they killed it! My battery’s days are numbered for sure! I might upgrade to AGM and a little solar, or sell my rig and trade up in the near future.
Yeah, when I took my TT in for warranty service, they bricked the battery. Luckily I was able to plug it into my truck to get it hitched up and plugged in back home. AFAIK, that was the only time that battery had been killed as I babied it the whole time I owned it, and it never could get up to a "full" charge again.

My MH is a different story. I know for sure it has been bricked AT LEAST once at the dealer, and likely more than that. I can probably "repair" it with a smart charger in my garage, but it'll never perform very well and it's small in the first place. I'll have owned my rig for 3 years this coming April, so that's not too bad for a battery that was marginal to begin with.

I will probably go with dual 12V AGM dual purpose setup with a panel up on the roof at some point, but like I said, we almost always have shore power, and I'm not shy about running my generator. The only time we run on battery is when we're stopped for an over-nighter on a long trip, and all I need the battery bank to do is run a couple CPAPs for the night. So I don't need anything exotic, and running the generator in a rest area parking lot is no big deal in those instances. In fact, I intended to get that battery/solar project under way this year, but I had other, more pressing issues crop up and had to "table" a bunch of stuff for a while.
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Old 11-30-2020, 11:25 AM   #20
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Yeah, when I took my TT in for warranty service, they bricked the battery. Luckily I was able to plug it into my truck to get it hitched up and plugged in back home. AFAIK, that was the only time that battery had been killed as I babied it the whole time I owned it, and it never could get up to a "full" charge again.

My MH is a different story. I know for sure it has been bricked AT LEAST once at the dealer, and likely more than that. I can probably "repair" it with a smart charger in my garage, but it'll never perform very well and it's small in the first place. I'll have owned my rig for 3 years this coming April, so that's not too bad for a battery that was marginal to begin with.

I will probably go with dual 12V AGM dual purpose setup with a panel up on the roof at some point, but like I said, we almost always have shore power, and I'm not shy about running my generator. The only time we run on battery is when we're stopped for an over-nighter on a long trip, and all I need the battery bank to do is run a couple CPAPs for the night. So I don't need anything exotic, and running the generator in a rest area parking lot is no big deal in those instances. In fact, I intended to get that battery/solar project under way this year, but I had other, more pressing issues crop up and had to "table" a bunch of stuff for a while.
In regards to AGM’s, I’ve read that some of them are not for intended for cold cranking use. I’m guessing these certain ones have a very low CC capacity. Something to consider if they are used to start a generator.
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