Engine Revving Problem

winner · 03-26-2002, 08:28 AM

I have developed a rather mysterious problem with my engine! I have a 1984 Airstream Excella 270 Motorhome with an LE8 484 Chev Enginer w/power heads, and a 4 BBL carburetor.
The problem develops spontaneously - As I drive down a long stretch of interstate - rather level country, constant speed, constant throttle, abt 45-60 MPH, the engine will spontaneously slowly start reving up - and will level at a higher rpm for an indefinite distance before spontaneously reving back down, and will do this sporatically over a considerable distance several times in an hour. On a recent trip from Greenville to Charlotte, it did this constantly for the entire trip both ways. No other problems evident, gas consumption remained constant and close to a normal 8 minus GPM. One Airstream dealer suggested that perhaps this involved the choke in the carb. Have had transmission checked and serviced, no problem with transmission Anyone else experiencing this aware of it?? Appreciate inputs.
Dr Joe

ALANSD · 03-26-2002, 10:22 AM

My 454 would sometimes idle too high for no apparent reason, until I bought some carb cleaner, and cleaned out the varnish and stuff. It seems to have helped the starting also, and I made sure I cleaned the choke area, then started it up, and sprayed some slowly..while it was running to clean further in the throat.

winner · 03-26-2002, 10:36 AM

Thanks Alan - appreciate your response, but Im not having any problems with starting up or running for that matter - It just seems to come about spontaneously and after running at "high" as I call it for a few minutes (perhaps 10 miles) it will slowly die back down.

thenewkid64 · 03-26-2002, 11:50 AM

I Will defer to Frank or Alan on this but it sounds as if you are experiencing a leak on the intake side after the carb or a vacum leak that is affecting ignition timing. Does it happen when the engine is cold? or only after 20-30 miles?

Does the overreving affect your speed? or power?

More info will help us help you.

ALANSD · 03-26-2002, 12:18 PM

I wasn't having a starting problem really either, just the idle would go up all of a sudden, and a carb cleaning cured it. I would say just take off the air cleaner, and look at the carb you could see if it is faiirly discolored and looking like it needs a cleaning.
Not sure....maybe Frank will jump in on this one.

winner · 03-26-2002, 12:38 PM

It doesn't happen when the engine is cold - It only happens on the road - 20 to 30 minutes after running. There is no effect on speed - I can maintain constant speed and no throttle action. I cannot detect any difference in fuel consumption, etc. There is not a reputable Chevrolet Truck place in this area, and I would have to travel to Greenville to the nearest. It was an Airstream Dealer in Delaware that told me he thought the problem was a choke problem. But it is eerie - just going down the road at a nice even speed and it just starts reving up like someone was pushing up the throttle, but the throttle is in one place and speed stays the same - no transmission activity at all -= after running for a few miles that way, it just very quietly moves back down, and the fuel consumption meter on the dash moves back into the green.

PeterH-79MH · 03-26-2002, 01:37 PM

Are you describing the fan clutch engaging?
If you maintain the same speed, but the engines sounds like its reving up, your fan clutch is kicking in and you should be glad it is.

thenewkid64 · 03-26-2002, 02:16 PM

Based on info I have read on other sites the cooling fan clutch when fully engaged can eat up a lot of horsepower. And I have experienced the change in engine noise on other vehicles.

You may be on to something!!

winner · 03-26-2002, 03:52 PM

Had not considered a cooling fan clutch kicking in to cause the noise = that is certainly something that I need to discuss with the Chevrolet Mechanic when I get one - I did not detect any overheating with the engine - It always runs at about 220

FrankR · 03-26-2002, 04:01 PM

After reading the first post, my view is that the trans is locking and unlocking the converter. I have had similar event while pulling a load in overdrive. If ya had a tach and could give more info. Ya may also be hearing/feeling the fan clutch engaging and releasing. I would say to check the temp gage but factory gages are always about 25-30 percent too hi or low. It is very difficult today to monitor an engine with out some non-factory gages. With some quality gages, no failures come as a surprise. Ya get to see what is happening as it occurs. All hard working equiptment needs good gages. The gages I always install are engine temp gage, auto trans temp gage, volt meter, oil pressure, and pyrometer with or without turbo. Frank

winner · 03-26-2002, 05:56 PM

Yes, I do believe we must be talking about the fan clutch. When is happens, it is very subtle - rather slow build-up of noise, and it fades out similarly. I looked at the engine manual - It isn't a big operation - just remove cooling cowl and unbolt the assembly, replacing with a new clutch assembly. I believe my regular mechanic can handle that problem. I think there would be a disruption of speed if the latter case were involved, and as it is, there is absolutely no differential in speed or vibration, etc.

FrankR · 03-26-2002, 06:47 PM

The noise from a fan clutch does not mean it is defective. In fact the opposite is true.The noise is proof the unit is working and probably correctly. With some gages the temp rise and decline may be observed. And this is a very easy task, if needed to change the fan clutch. No water or fluids, just careful work... Spend the money on fun stuff.......
Da ole guy....... Frank

PeterH-79MH · 03-26-2002, 06:58 PM

Dr Joe,
Hearing your fan clutch means that it is operating properly. You may have a heavy duty one.
Rather look into getting your operating temperature down a few degrees.
The engaged fan clutch on my 98 chevy van sounds like driving a tractor. Normal, according to the dealer.
On my 79 MH, I am running at 180, peaking at 210.
220 and above is too hot for my taste.
The fan clutch on the MH rarely engages during Interstate travel, only during traffic jams with high outside temperatures. It will disengage, after I resume speed and get the airflow going.

Lets hear from the other MH owners about their
operating temps.

Airstream402001 · 03-26-2002, 10:12 PM

I too think that the sound that you are hearing is the fan coming on. I live in Indiana, and in the cold weather here, the fan does not engage. But, when I go to Fla., the warmer temperatures cause the fan to engage. I hear the noise, but there is no change in the speed or rpm.
From what I understand, this is the way it is supposed to work.

Cliff in Indiana
GO HOOSIERS !!!