302 engine knocking

[FASBAK]

Symptoms: After engine warms up, there's a knock coming from the top center of the intake manifold. Can only hear it when engine is at idle. It can be felt most noticable through the upper heater hose.

The rotating assembly on my 5.0/302 was recently disassembled and balanced. Before the engine was disassembled, there was no knock. Two engine builders advise there is nothing mechanically wrong and it's probably an air pocket in the cooling system. However, I've followed the standard protocol for bleeding the cooling system. Fill radiator to about 2-3 inches from top. Start engine and when T stat opens, the system burps. When burping stops, it appears all air is now purged from cooling system. I turn off the engine. Top off radiator. However, the engine still knocks. I've changed the T stat (180) to make sure that was not the problem. I've lifted the front end when filling the system to help get any air out of system. I've checked the highest point (upper heater hose) and it appeared to be full, with no air pockets.

1968 mustang, 5.0/302 (90's block), edelbrock performer intake manifold, griffin 3 row aluminum radiator, cool flex (ribbed) hoses, roller cam, roller rockers (hydraulic), ford racing valve covers (tall style)

Any ideas?

 

[Rapid]

I had a similar knock that turned out to be a sticking lifter. It eventually took out the cam. Make sure you have a proper valve adjustment first and then check for other problems.

 

[FASBAK]

We've made 3 adjustments to the lifters (1/2 turn, 3/4 turn, 1 full turn) all with same results.

 

[woodsnake]

Are you running a performance cam?
Did you verify the geometry?
The heads are the ones that came with it?

 

[FASBAK]

Its a mild roller comp cam with smooth idle and the heads were not changed, as it ran fine before the rotating assembly was balanced.

 

[1992MustangGT]

I was rebuilding a 3.8L V6 in a 91 Cougar last spring and after putting it back together I had the same noise. Long story short is....I had to hone the cylinders so much that I ended up honing it too much for the stock size pistons. The noise I was hearing was the pistons slapping against the cylinder walls. This engine had over 220,000 miles on it when the heads blew because of a leaky radiator (ran out of water & over heated). I know it's alot of work to tear an engine down, but you might want to consider piston slap and a possible problem. You stated you had a rotating assembly put in the block and I'm assuming that the cylinders in the block were honed, if not bored. If the block was bored, then you need to use oversize pistons. If the oversize pistons were not installed you will have piston slap, which sounds like it's coming from the top of the engine. I too thought it was bad lifters and I replaced them with new ones and the noise was still there. Anyway, hope this helps. Keep us posted on what you discover....

 

[FASBAK]

The block was honed and the cylinders are bored over .030. My engine builder is a drag car racer and says he has over 200 engines he has rebuilt and never had this problem. Another highly recommended engine builder in the area has been consulted and he says the only other time he's heard this type of knock/ping was when the head gaskets were installed incorrectly, this is of course after the possibility of an air pocket in the cooling system has been eliminated. However my builder says he is 100% sure he put the gaskets on correctly.

 

[1992MustangGT]

Well the bottom line here is that you're engine is making a noise and there are only so many possiblities it could be. And it doesn't matter if a professional engine builder has built a million engines, what you need to keep in mind is that even though a engine builder is very good at what he does, he is still human and still makes mistakes, even mistakes he's not aware of until the customer (you) has the misfortune of experiencing engine knock. It sounds to me like you have done everything you can do to rule out air in the cooling system. Now you need to continue your process of elimination and move onto the next possibility. Eventually you'll find the problem but it may take tearing the engine apart again to find it. I was just giving you another possibility on what it could be because what you're experiencing is EXACTLY what I was experiencing and all the symptoms were the same and my problem was piston slap. If there is no air in the cooling system and you have new lifters and the rocker arms are all adjusted correctly, then you have to look deeper. Start with the easiest and less expensive possible problems then move onto the more involved and costly possiblities. If I were you, I'd pull the heads off and check the head gaskets; however, in my experience I've never heard of putting a head gasket on backwards would cause a knocking sound that you're hearing. It would cause the engine to overheat, but make a knocking noise??? I'm not so sure about that. I could be wrong, but it's something you need to rule out. Then if it's not the head gasket, while you got the heads off check the piston to cylinder wall spacing. Push the piston to one side of the cylinder then take a feeler gauge and measure the gap between the piston and cylinder wall. Then take that measurment and go talk to a machine shop and ask them if your measurement is within spec or not. I guess my point is not to take someone else's word for something even though they are a professional. Like I said, bottom line is you have an engine problem and you need to look at every possibility.....Just trying to help.....

 

[1992MustangGT]

Oh, by the way...I forgot to mention this....not sure if you knew this or not but on a Ford head gasket there will be the word "FRONT" stamped into the end of the gasket that is supposed to be installed toward the front of the engine. If you decide to tear the heads off and check the gasket to see if it's on backwards or not, that's what you will be looking for. Keep us posted....

 

[FASBAK]

1992MustangGT, thanks so much for the helpful insight. Yes, my builder did mention the "FRONT" markings on the Ford head gaskets and he said he was sure about installing them right. This is the first reference I've heard about 'piston slap' as a possibility, and at this point, I'm inclined to believe that is the only other possibility. I'll mention this to my builder. I have one more step to try before I pull the engine...again. I'm going to try the vacuum method to ensure all air is out of the cooling system. I'll let you know how it goes. Thanks again!

 

[woodsnake]

You said it is most noticeable at idle, but is it RPM sensitive? I.E., does it speed up at all if you give it some gas?

I guess you have eliminated the possibility of "steam noise", from head gaskets that don't have the vent holes cut out. This sounds a lot like that.

Also, when you put the thermostat in, did you drill a 1/8th inch hole at the top to help the burping process?

 

[jcode68]

You probably have already checked this, but exhaust leaks from headers and or thermactor/smog ports on the cylinder heads will sound like a ping or knock commonly mistaken for valvetrain noise. I chased what I thought was valvetrain noise from noisy roller rockers for months, adjusted them 3 different times to no avail only to find out the thermactor port on the back side of one of my AFR heads had loosened and came all the way out. Wasn't noticeable when I checked for header leaks since it was on the backside of the head and I was checking the side where the headers bolt up. Worth mentioning and checking. Good luck.

 

[FASBAK]

OK, my engine builder figured it out. He made a human mistake. One of the head gaskets was put on wrong which prevented coolant flow.... And I thought it was the flux capacitor at 1.21 jigawatts.

 

[2+2GT]

However, I've followed the standard protocol for bleeding the cooling system. Fill radiator to about 2-3 inches from top. Start engine and when T stat opens, the system burps. When burping stops, it appears all air is now purged from cooling system. I turn off the engine. Top off radiator.

If you use the genuine Ford thermostat with the air bleed valve, you can avoid this entire song-and-dance.

 

[attasumaal]

THERMOSTS VALVE

Dear all,
Can anyone tell me the usage of T.Stat valve? I mean if you install it and don't install it... both way the engine will function fine... or is there something special to it?
Please clarify don't be specific as i'm a newbie... thanx.

AK

 

[rbohm]

Dear all,
Can anyone tell me the usage of T.Stat valve? I mean if you install it and don't install it... both way the engine will function fine... or is there something special to it?
Please clarify don't be specific as i'm a newbie... thanx.

AK

installing a thermostat allows an engine to get to operating temperature much sooner than one without a thermostat. this aids in fuel economy, and in cold weather getting heat to the passenger compartment. also, and this is where the arguments start, a thermostat can also prevent overheating, or cause it depending on how your cooling system is designed. i have had cars that when you installed a thermostat they over heated, and i have had cars that over heated without the thermostat. with a properly designed cooling system, a thermostat will regulate engine temperature, and keep it fairly consistent.