Dieseling

[fleetside72]

Howdy Boyz,,,,i got 2 problems with my 72 chev,,,1st,,when i'm going uphill she knocks(dieseling),,what gives???,,,,,,2nd,,,when i'm going down the road,oil pressure is good,,,right in the middle,,but when i;m sitting still,,,the oil pressure go's down a little bit,,,when i take off,,it go's right to the middle again,,,The truck do's smoke when i start it up right away(needs a valve job),,but the smoke quits,,,Could this be the problem to the oil pressure thing??????Sincerly Confused:

[bigjimzlll]

pre-ignition and detonation are very bad... Has it been doing it long? could of just got some BAD gas...try putting some good stuff in..91 octain...see if it cures it..check timing...maybe mechanical/vacuum advance is sticking..could be a build up of carbon...check one thing at a time..but fix it..its not good

[Hooter]

back your timing off two degrees, sounds too advanced to me

[Southpa]

Diesling is actually a condition where the engine runs on a bit after ignition is turned off. Thats not your problem. Rattling noise from up front when under load is a timing issue and could be even as bad as backfiring out the intake and severe power loss. The timing chain could be stretched, timing needs adjustment and/or dist. cap components are worn.

Your oil pressure is fine. Mine goes down to just above 1/4 when idling and up to 3/4 when moving. Your pressure gauge is connected to the engine by a small plastic tube. When the engine is revved oil is pushed up the tube which registers at the gauge.

Blue smoke on startup is a result of worn valve seals. They allow oil to leak down into the cylinder which ignites on startup. Valve seals should be replaced about every 5 years, they WILL be crusty by that time. Replace the valve seals and you won't believe how much your oil consumption is lowered.

[bigjimzlll]

Causes of Detonation
Trying to isolate a detonation problem in a highly modified or even slightly modified engine requires you to determine if the combustion problem is being caused by a change you made to the engine, is inherent to a design flaw, or is derived from an external cause such as carbon deposits or poor fuel. Obviously the area with the largest variable is the fuel quality. Octane determines a fuel's ability to resist auto-ignition, or in lay terms, its ability to wait for a spark to initiate a flame. For our purposes we will explore knock independently of fuel octane rating.

When knock occurs, there is uncontrolled combustion with multiple flame fronts, with only one of them being initiated by the spark plug. During knock, cylinder pressures experience an extremely rapid rise and spontaneous burning and release of the end gases' energy. It is not uncommon to see a pressure rise of up to 8 bars per degree of crankshaft angle, or up to 50,000 bars per second. Anything that will either increase heat or retain heat in the combustion chamber will promote auto-ignition.

Common problem areas are sharp edges of metal either on the piston or in the combustion chamber. For instance, if the piston has a valve relief cut into it, there is usually a very defined edge that the cutting tool leaves. This sharp edge is greatly prone to super heating and will actually retain enough heat that it will start to glow. If the fuel should hit this glowing edge either prior to the lighting of the spark plug or even after ignition, it is very likely that another flame front will initiate. If this unintended ignition occurs very early in the compression stroke, then the piston will be forced up against the increased pressure of the burning gas and will result in a form of abnormal combustion referred to as preignition. When this happens, the end result, if severe enough, is that the connecting rods bend. Turbocharged engines are specifically prone this. Carbon deposits that build up in the combustion chamber or on the piston top will have the same effect, along with the decreased volume at TDC, which in turn raises the compression ratio.

As stated earlier, anything that will raise temperatures in the cylinder will help promote detonation. This rise in temperature could occur many different ways: very lean air/fuel ratios, heating of the charge air prior to entering the combustion chamber, compression of the air molecules from forced induction, or poor heat dissipation through the cooling system. Increase of cylinder pressure will also prompt detonation. Camshaft profile, compression ratio and quench area all come into play. Design criteria as to combustion chamber shape and mixture motion, spark plug location and piston design can all make an engine more or less inherently prone to abnormal combustion. Naturally, spark timing and fuel quality round off the list.