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Old 01-23-2014, 12:34 PM   #1
C350AMG's Avatar
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Intermittent issue – engine has no power (7.4 Vortec)

I have an issue with my engine that is intermittent and also temporary and it only happens in very specific conditions and can be repeated easily.

When the RV is parked overnight and it rains (can be a very light rain) the next morning when I start it, it starts on a dime no issues, after 2 minutes engage drive and when you hit the gas pedal you feel the lack of power and some misfires (I think). Then drive down a boulevard and the max speed I can get is maybe 30kmh (very slow). Floor it and release it a few times (maybe for 1 minute) and the giant wakes up (full power now) and the issue is gone. It will not happen again today.

I can drive it in the heavy rain with no issues, I can park it 4-5 hours in the rain and it won’t do it again. Start it again it has all the power, perfect idle, no misses.

My wires and sparkplugs are new. If I open the engine cover in the morning there is no evidence of water on the engine or anywhere. And every time it happens when I start it from cold, it starts immediately and the issue appears 2 minutes after.

It’s not a big think but it really gets to me ;-(

Ideas anyone? (Cap, rotor MAF, coil, …..)
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Old 01-23-2014, 01:21 PM   #2
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issue appears 2 minutes after.

guessing but could be a gas cap ..wrong psi in the gas tank

prug valve not working right for the tank

take the gas cap off see if you get alot of air pushing out where is some what trys to blow the cap out some in your hand

you could have to much back prasher

fuel filter sounds like some thing to do with gas

if it was a 8.1l i could help alot more but you have the 454 i have the 496
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Old 01-23-2014, 02:56 PM   #3
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It might be "carb ice" do to a cold engine and that the heat riser/warm air intake to the air filter is not up to temp.
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Old 01-23-2014, 08:13 PM   #4
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I believe 87Rockwood has a very good point. Years ago, I had an engine with the same issues. The problem was icing.
You have nothing to lose by investiigating a little further. Anyway to operate the rig with the engine cover off, and breather off to visually confirm or deny?
As 87rockwood pointed out, make sure the engine warm up system is operating correctly. Are there any vacume lines broken or disconnected, etc? If the breather has a snorkel, does the door close when the engine is cold?
I am not familiar with the way your rig is set up, but it is worth a look.
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Old 01-23-2014, 08:25 PM   #5
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Interesting points but this issue is happening regardless of the outside temperature. It will do at 60F as well as 80F or 100F it just needs to rain a bit and sit for 8 hours.

I am more thinking of the MAF sensor being affected by humidity or the distributer cap and rotor .
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Old 01-24-2014, 07:37 AM   #6
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Your MAF sensor could be bad. While the engine is running, gently tap on it. If the engine acts up problem solved. If the tap test does not work, then get the diagnostic check for your engine.
Another comment on the icing. It is the humidity in the air working with the temp of the air plunging in the venturi that can prompt the ice build up, either at the opening of the air horn or down in a little further in/on the butterfly.
Might as well check your MAP sensor/connections and EGR, EGR solinoid/connections while you are at it.
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Old 01-24-2014, 07:48 AM   #7
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Distributor, plugs, wires, rotor, all can do strange things. If you have not replaced all of these very recently, I suggest it.

I had 2 gassers, an 84 C abd a 96 A, and the older they are the worse the air flow in the doghouse is. I fried many wires and plugs and such, some in just a few days of hot weather driving. A cracked exhaust manifold or leaking exhaust gasket can fry stuff real fast.

I have suggested this to several folks that go on to diagnose all kinds of other parts before they try the ignition parts. Often, it was simply that.

Best wishes!
Dave and Nola, RVM1
The Journey is Our Destination!
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Old 01-24-2014, 07:58 AM   #8
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I believe that C350 has a multi port fuel injection system. In another post he mentioned a lot of engine work done. So when did this problem begin? Might be something with the throttle position sensing to the ECM.

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Old 01-24-2014, 08:25 AM   #9
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Correct I have the last generation of the 7.4 a multi port Vortec engine.

This issue has been there since I purchased the unit. Even after all the repairs which included:

EGR valve
Banks headers
Rebuilt heads
8 new injectors
Fuel pressure regulator

I need to investigate as mentioned above the MAF.
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