My 78 Eldorado has the funniest noise when the engine is cold. The easiest way to describe it would be that it kind of sounds like a loose exhaust. When I start off, I can hear it from the exhaust area, tuk tuk tuk. Only at a certain speed though. If I drive slower, I don’t hear a thing. Then after a few minutes the noise goes away. If I turn everything off, and start the car while everything is still hot, no noise. I checked the exhaust, its tight. Any ideas?
Does it have a heat riser valve? The valve could be rattling.
Does it have an AIR pump? Could be a leaking diverter valve, or one of the ports. The manifold that goes on the front of the head on our 68 had a rust hole in it, causing it to chug a bit. I fixed it, and it's stayed fixed for more than 10 years, with a little dab of JB-Weld over the hole.
crack in your exhaust manifold? if it hasn't gone completely through (like mine just did) it sounds loud when you start it, but then as the exhaust heats up it closes itself.
I too am with the manifold crack suggestion, as it doesn't take long for a crack to close up when heat is applied.
What you need to do is have someone start the engine whilst you are outside with an end of a 6 foot piece of garden hose up to your ear, and the other end moving around the engine and manifolds listening for the noise.
It also could be at the Manifold to Head joint, and Manifold to Pipe connection.
A crack in an exhaust pipe itself will take a long time to close up due to heat.
The trouble with trying to locate this type of noise that soon goes is because once it has gone, then you have to wait so long for the parts to cool down to cold. before starting again
Bruce. >:D
Will it do it not moving? If you just rev the engine? Rev with it in gear?
Thanks guys. Going to do some research on this in the next few weeks and report back. At least have a place to start now.
First thing going to look at the AIR pump like Gene suggested. Work my way up from there. Thinking of just cutting the air pump belt. Get rid of any problems that might be there. Then check manifold.
Without modifications you cant just cut the air pump belt. Its the only belt that hits the water pump and besides the water pump being sort of important it also drives the alternator.
Oh man. Good catch. I was thinking of the Seville. There the previous owner just took out the AIR pump. The AIR pump runs OFF the water pump, so there it made sense to take the AIR pump out. The ps pump tensions the belt to the water pump.
For the Eldorado, looking at the belt diagram again, that is obviously not going to work :- )
That would have been one of those "oh crap" moments right after I cut off the belt.
What if I just take off AIR pump hoses? See if one of the diverter valves are bad? If there is no pressure, the valve should stay closed?
The diverter valve normally pumps air into the heads. When the valve operates it diverts the air to the vent built into the valve rather than into the heads. This usually happens when you lift off the gas. When you lift off with a carb you get a rich burst and when that most likely not completely burnt fuel hits the exhaust and then gets a burst of fresh air from the AIR pump it finishes burning which most of the time just gives you a popping sound in the exhaust. The valve is supposed to shut the air off during those rich deceleration conditions.
You can just remove that elbow hose and see what happens.
What usually kills the valve is the check valve up on the cross over pipe goes bad and lets exhaust get back to the valve. It eventually kills the pump too. The check valves fit a lot of engines well into the 90's so they are not hard to find or expensive.
Quote from: TJ Hopland on August 02, 2012, 10:22:10 AM
The diverter valve normally pumps air into the heads. When the valve operates it diverts the air to the vent built into the valve rather than into the heads. This usually happens when you lift off the gas. When you lift off with a carb you get a rich burst and when that most likely not completely burnt fuel hits the exhaust and then gets a burst of fresh air from the AIR pump it finishes burning which most of the time just gives you a popping sound in the exhaust. The valve is supposed to shut the air off during those rich deceleration conditions.
You can just remove that elbow hose and see what happens.
What usually kills the valve is the check valve up on the cross over pipe goes bad and lets exhaust get back to the valve. It eventually kills the pump too. The check valves fit a lot of engines well into the 90's so they are not hard to find or expensive.
Thanks. I might just replace them either way. On my list of stuff to do when I gather the courage to do the timing gear soon.
The 'stethoscope' idea is a good one, and a very useful tool. I usually use a length of 3/8" or 5/16" fuel line, and probe parts of the engine for the offending noise.