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Classic Mustangs Tech Forum

Technical discussions specific to 1964-1967, 1968-1970, and 1971-1973 Classic Mustang. Discuss all tech related to in-line six cylinder and V8 powered Vintage Mustangs here.

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Unread 07-23-2011   #1 (permalink)
68coupe5.0 is offline Apprentice


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Default knocking engine noise with a horrible idle

So I recently just got my 1968 302 running and I ran into a problem almost immediately. At first when I started it, the idle was just rough and after adjusting the timing, idle speed and air fuel ratio it ran relatively well. About 10 min into the initial break in while it was idling, I heard a knocking noise. At first I thought that the timing was too far advanced, but when I attempted to adjust the timing again the noise didn't get any better or worse. Then I noticed the passenger side of the engine running very hot, much hotter than the driver's side of the engine. I let it run for a few more minutes to see if the condition improved or got worse and then it died on me. It wouldn't start again until it had cooled down. When I did start it again the knocking noise was very loud and as far as I can tell, it is just one cylinder. The knocking is consistent, roughly every 3 seconds. When the engine idled, the idle speed would increase for about 3 seconds and then the knocking or popping noise would occur and it seemed like that would just about shut the engine down, but then the idle speed would increase again for 3 seconds and then the knocking or popping noise would occur again. This pattern kept repeating itself for about 20 seconds before I shut it off, to try and not cause more damage. So I'm a little beyond frustrated and at a loss for whats wrong, the only thing I can think to do is to tear it down and find the problem. ANY and ALL ideas or suggestions are welcome
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Unread 07-23-2011   #2 (permalink)
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Was this a fresh engine, new cam, lifters....? When you say break in, what exactly did you do? Proper cam break in requires more than just idle speed for a while. I would start by pulling the valve covers off and turn things over and see if all the rockers move as expected. Unfortunately I have experienced the same thing on a fresh rebuild, I had some lobes on a cam go flat.
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Unread 07-23-2011   #3 (permalink)
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Everything in this engine is new with the exception of the crankshaft and block. The heads came off of a 1987 302 I believe. My local engine guy told me that they would work and the guy who sold them to me said his mustang ran great with them on. Well a better description would be that I tried to break the engine in. I had it idling, but since I heard the knock I didn't let it idle for long, or at 2000rpm (which I'm told is the rpm to break it in at for 20 min). I didn't start it again so I would avoid potentially causing any serious damage. Great a place to start! I'll go pull those covers off now and see how everything looks when it turns over and get back to you
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Unread 07-24-2011   #4 (permalink)
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As far as knocks go, is it a very deep knock, sounds like it is internal? Other potential sources are low end. Thrust not set correctly or possible rod issue ( one left loose?? ). These could be checked by dropping the pan.
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Unread 07-24-2011   #5 (permalink)
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good call on taking the valve covers off. Turns out that I hadn't tightened down the rocker arms all the way. A few of them had even fallen off. Its pretty embarrassing but fortunately there was no damage and it sounds great when it runs now! There was a little knocking but adjusting the timing took care of it this time. However the passenger side of the engine still runs hotter than the drivers side, about 30 degrees hotter when I used an infrared thermometer. All the heater hoses are on that side, is this temp difference normal?
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Unread 07-24-2011   #6 (permalink)
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If it is a fresh engine, I don't know that I would be too concerned yet. Does the engine over heat? Also, those IR thermometers can give different reads based on the surface you are measuring from. I also have one of those and the readings can vary a bit. I can't even get a reliable reading off my radiator ( aluminum ). Try being very picky about the surface you are reading from ( same color, clean.... ) and see what you get.
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Unread 07-24-2011   #7 (permalink)
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I bought an engine from a reputable company. It overheated badly the first time. Pulled the heads and saw both head gaskets were installed 180* out. You could have the gasket backwards on that one side.

The nice thing about screwing up the rocker arms is that you will never make that mistake again. Nothing teaches like screwing up.
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Unread 07-25-2011   #8 (permalink)
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So far the engine hasn't overheated, stays right around 170. That's good to know though, I didn't think it was too big of a deal but I wanted to make sure. I did notice that with the IR thermometer that if I moved it the slightest bit the temp changed, so I will try to be more careful with where I aim it. Very true about mistakes though! The screw ups I've made I won't be making again. The head gaskets are installed correctly, I was very particular about that during the build up and I can still see the tabs in the front of the engine though. good thought though.
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Unread 07-25-2011   #9 (permalink)
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Its not just the tabs on the head gaskets, but that the front that is stamped on them is to the front of the engine, which if I remember it makes one right side up and the other upside down. Good Luck.
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