4.6 cobra engine knock

[dimaiox1]

Hey guys boughtmy first mustang the other day. Been around them for years. Worked on my friends 6 mustangs over the last 10 years or so. I found this beautiful 97 Cobra convertable 5 speed. Anyway the engine was fine. By the time I got the car home as luck would have it I hear piston slap. From what i read the older cobras have some connecting rod issues. Seams like the bolts lossen up. It started happening as soon as i got about 1/4 of a mile from my house I drove it into my driveway and it hasnt run since. My question is Im going to drop the cross memeber and check the connecting rod bolts. Asuming One of them is comming lose I should be able to change out the bearings and put in new bolts with no issues. The engine runs fine at about 2000 rmp you hear a connecting rod knock other than that the car is fine. Anyone have experience in this? BTW Ive been a mechanic for about 10 years I know how to do the work I just like having some tips from people who have had these issues.

 

[trombonedemon]

I'm not sure, if it is piston-slap, then what does the Main-Bearing bolts have to do with Piston to valve contact?

If its piston-slap then your timing is off.

 

[Rick 91GT]

You really shouldnt just put new bolts in a rod especially if one has come loose, the cap may be out of round....that being said.

Piston slap occurs in the bore of the motor, and can go away as engine temp increases...a little slap at cold start would not be uncommon although those motors run a tight PTW clearance from the factory. Personally Id check the tensioners which are oil controlled, they get weak and you can hear it, very similar to what a rod knock would sound like.

The issues with the pistons and rods and the rods are powdered metal and the pistons are hypers both of which tend to have issues in the 500hp range.

 

[trombonedemon]

Piston to wall contact? That piston rattle/slap occurs in mostly forged piston heads/tops due to the metal needing to expand to the bore. The hyper's have a high silicon content so why would they rattle for a long time if it is piston slap.?

The mod motor heats up fairly quickly with in minutes of driving.

O.p. try switching to a thinner oil 0w-30, the w has nothing to do with viscosity but an ash point for the petroleum product, it acts the same as 5w or 10w. Synthetic that is.

 

[vertigasen]

O.p. try switching to a thinner oil 0w-30, the w has nothing to do with viscosity but an ash point for the petroleum product, it acts the same as 5w or 10w. Synthetic that is.

Ok, now you have me scratching my head. I know what the W stands for, I also know how it works, but how is 5w and 10w the same in ANY oil. The W stands for Winter and represents the way the oil acts at a specified cold temperature. Standard oil uses polymers in the oil to control the multiple viscositys, while synthetic uses viscosity improvers (VIs). The first number in 0w-30 means that in cold weather, it will act like a 0 weight oil @ -30C, but when it is warm (100C) it will act like 30 weight oil.

If I am missing something here, please, let me know because I am confused.

 

[dimaiox1]

I guess I meant to say more of a rod knock sound, But when i said piston slap I was refering too a worn bearing or if the bolts where comming loose the rod would move up toward the valves or head and hit something due to the connecting rod moving away from the crank shaft.

 

[trombonedemon]

Ok, now you have me scratching my head. I know what the W stands for, I also know how it works, but how is 5w and 10w the same in ANY oil. The W stands for Winter and represents the way the oil acts at a specified cold temperature. Standard oil uses polymers in the oil to control the multiple viscositys, while synthetic uses viscosity improvers (VIs). The first number in 0w-30 means that in cold weather, it will act like a 0 weight oil @ -30C, but when it is warm (100C) it will act like 30 weight oil.

If I am missing something here, please, let me know because I am confused.

A vehicle's oil has to be thick, but not too thick. It is important that in the winter, the oil be thin enough to allow for the engine to start. But when the engine is warm, the oil must be thick enough to lubricate properly. That's where the numbers come into play.

Neither number corresponds to an actual 'weight,' even though that is the term most people use when referring to motor oil. The viscosity (flow resistance) is tested by allowing a small amount of oil to flow through an aperture. The quicker the oil flows, the lower the rating numbers.

The first number rates the viscosity of the oil at a temperature of 0 degrees F, mimicking cold winter weather, which is why the 'W' designation is added at the end of the first number. The second number repeats the test at 210 degrees F., or normal operating temperature for a fully-warmed engine.

The 'W' rating can be 5, 10, 15 or {20;} lower numbers mean the oil is thinner in cold temperatures, necessary for icy climates. The second number rating (meant to represent normal operating temperature of an engine) can be 20, 30, 40 or 50. Warm-weather spots usually require oil in the upper end of that range that can handle extreme heat.

I'm saying the W was used in a equation for ash point, like a variable. Maybe I should have said 0w-30 reacts the same after it is heated like the other 30's

 

[vertigasen]

A vehicle's oil has to be thick, but not too thick. It is important that in the winter, the oil be thin enough to allow for the engine to start. But when the engine is warm, the oil must be thick enough to lubricate properly. That's where the numbers come into play.

Neither number corresponds to an actual 'weight,' even though that is the term most people use when referring to motor oil. The viscosity (flow resistance) is tested by allowing a small amount of oil to flow through an aperture. The quicker the oil flows, the lower the rating numbers.

The first number rates the viscosity of the oil at a temperature of 0 degrees F, mimicking cold winter weather, which is why the 'W' designation is added at the end of the first number. The second number repeats the test at 210 degrees F., or normal operating temperature for a fully-warmed engine.

The 'W' rating can be 5, 10, 15 or {20;} lower numbers mean the oil is thinner in cold temperatures, necessary for icy climates. The second number rating (meant to represent normal operating temperature of an engine) can be 20, 30, 40 or 50. Warm-weather spots usually require oil in the upper end of that range that can handle extreme heat.

I'm saying the W was used in a equation for ash point, like a variable. Maybe I should have said 0w-30 reacts the same after it is heated like the other 30's

Op.. sorry for interrupting the real discussion here.

I know how it works, but I was just confused by the use of the ash point. I know what the sulfate ash percent is in relation to oil, but not the ash point. I am guessing it is generally related as in the point where the oil begins to burn and leave ash residue....but again, that has nothing to do with viscosity.

Now to the Op. If it was worn and the rod bolt was backing out and creating a gap between the rod and crank......well, you would know that without hearing a tapping sound, it would just be one loud bang and done. The gap created would increase the loads placed on the rod 100x. It would create a microsecond of time where the crank would lose contact with either the top or bottom of the rod until it contacted the opposite end of the rod. Think of it like this, put a brick down, and put the hammer on it and try to push down and break it. You cant. Pick up the hammer about 6" and try to slam it down on the brick....it will probably break.

Rod knock is typically a byproduct of bad bearings. You just bought it, so it is going to take some searching. If we knew how it started, we could say check this and check that. The knock could be any number of things that can cause the modular engines to knock. Bearings, timing chains/tensioner, lifters. I had a really nice tick for the first 20 minutes after I started mine....it almost sounded like a diesel....and it was the lifters. I could hear it from behind the plenium and in the valley, but it didnt sound like it was coming right out of the head.

 

[trombonedemon]

Op.. sorry for interrupting the real discussion here.

I know how it works, but I was just confused by the use of the ash point. I know what the sulfate ash percent is in relation to oil, but not the ash point. I am guessing it is generally related as in the point where the oil begins to burn and leave ash residue....but again, that has nothing to do with viscosity.

Ashe percentage/ashe point

Motors do burn oil in the cylinders, right. If viscosity is too low with really high sulfated ashe percentage (detergents), lots of deposits on spark plugs and valves. Hence putting additives in gas tends to ashe up spark plugs.

My original suggestion was changing the oil to a less viscous number like 0 rated oil to see if the thinner oil can get in the bearings quicker to see if the noise in the engine goes a way, if so, he has his answer.

 

[vertigasen]

Now you are making sense. Sure, the engine will always burn some oil.....but since the new standards, the sulfated ash content has been reduced in normal car oil by the API for emissions. Unless you are running oil from 5 years ago, or a C spec oil, there shouldnt be enough ash in it to cause any significant issues. In your original post, I though you were trying to say that the sulfated ash % has an effect on the viscosity.

I have no arguement for changing the oil viscosity, it is a good way to check a few things.