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As noted in my build thread, an engine knock seems to have appeared recently in my truck. How do I go about diagnosing this? Is there anything simple in the lower end I can check by taking the oil pan out? I really have no desire to pull this motor apart, if I can avoid it.

What else could cause a knock? Fuel/air mixture? Timing?

It's intermittent, not very loud and goes away at higher RPMs.

Thanks for the help!!

A knock is nearly always a sign of a bad rod bearing. Loose valve tolerances will result in a "tick" and fuel/timing issues will result in a "ping." To the experienced ear, they are distinctly different sounds. To an inexperienced owner, however, they can be hard to tell apart.

As for the intermittent nature of the noise, a "ping" will normally disappear with increased rpm because the engine is getting more fuel (not running quite as lean). A valve "tick" will not disappear with increased rpm, but it may be hard to hear over all the other mechanical noise. A true "knock" may "disappear" when the engine is under heavy load either accelerating or decelerating, but will become more obvious when the engine is running free. It will also only get worse until the problem is repaired or the rod lets go completely.

My suggestion: get thyself to an experienced mechanic and have the noise properly identified. Quickly.

Well, it's definitely not a ping or a tick, as I'm familiar with those noises. Is there any chance at an easy solution here? Like something that could be tightened up without tearing the whole thing apart?

chicklin wrote:Well, it's definitely not a ping or a tick, as I'm familiar with those noises. Is there any chance at an easy solution here? Like something that could be tightened up without tearing the whole thing apart?

Highly unlikely that a simple fix will apply. Knocks are never a good thing, from what you have described, most likely is a rod bearing or piston pin bushing. Bite the bullet & check it out to be certain of what is going on, what you do now will look far better than a rod sticking through the side of the block.

Now if you said the knock goes away after it warms up I'll say just drive it. I had a 72 Montego with a 351C that knocked at 49K when I bought it. But the knock was only there on start up. Sold it after putting another hard 60k on it. But That was a cold start up knock, not what you mentioned. Follow Charles on this one and do it before you throw a rod.

Wayne64 wrote:Now if you said the knock goes away after it warms up I'll say just drive it. I had a 72 Montego with a 351C that knocked at 49K when I bought it. But the knock was only there on start up. Sold it after putting another hard 60k on it. But That was a cold start up knock, not what you mentioned. Follow Charles on this one and do it before you throw a rod.

So how does one go about diagnosing a knock? Although I don't necessarily want to tear this thing apart, I'm certainly not afraid to get my hands dirty if it's going to save me several hundred dollars. Are there some common things to look at first? Anything I can get to from underneath?

It's just a very light tap. And it's very intermittent. Not tap-tap-tap-tap-tap. More like tap......tap..tap.............tap.....tap......tap. Like I said, it goes away at higher RPMs.

This may sound dumb...and I am no expert here and haven't played with the old dodge motor, but that "rod knock" sound that did what you say "tap......tap..tap.............tap.....tap......tap" and goes away under load sounds like the water pump to me. On a M151 motor (an over head valve engine and my area of expertise for the past 20 year before I started my dodge project) when the water pumps start to go south they make a noise very similar to a rod knock sound, but not a stead tap tap tap, but the a-rhythmic tap.....tap tap......tap....tap tap tap.... kind of thing. IMHO I would look at the water pump. See if you can pull the fan in and out (engine off of course), see if it has any play in it. If it does try replacing or having that one rebuilt. If the pump checks out good, then I'm guessing that you'll have to pull the oil pan and see if anything is loose.

There is no telling what you may find upon a thorough inspection. I've never heard a water pump make a knocking noise, but certainly you should check for loose motion in the shaft, fix it if it's there or the next sight you may hate to deal with may well be fan blades protruding through the radiator core.

From what you describe, I tend to think more along the lines of a piston pin bushing failure. That will be intermittent & will usually sound off more after speeding up RPM's & letting off the accelerator during the coast down back to idle. Remember, long distance diagnostics may not be worth much, I have not heard the knock or tap. If it is indeed a tap, a valve could be flopping in its guide, or you may have a badly wallowed tappet bore in the block. Tappet bores can produce this sound consistantly or intermittently, it is a profound tap & not a knock however. No real help again as you have now changed the noise from a knock to a tap in your description. Have you done a major tune-up to this engine adjusting everything in sight? Let me offer this suggestion, do a complete compression test. Post the result here & I can advise you further. I say this because more than likely if you have major mechanical issues going on, the compression readings will be lower than in a normal, healthy engine. If you also have issues going on which compression readings show need attention anyway; no need in spending time & money to try & locate a single knock or tap source. If compression readings were to indicate it is at or near time for a mojor overhaul, then all your issues are falling right in line with each other. Trying to patch this or that issue in an engine that may need a major, well it's pretty much a sure thing you will never have a good reliable engine until it gets the major attention it may need. Not saying all this is an absolute as I say once again, I have not heard it. I can say that a complete compression test only takes a few minutes & will tell you more about the "real time" condition of that engine than any other simple test you can perform.

I'm getting the feeling you are possibly not really experienced enough with this to make a proper & sure diagnosis. Sounds as if you might need to seek professional help in getting a diagnosis. Go to someone local you can trust & ask them to help. Not trying to insult your intelligence, but you must realize this could be a death sentence for the engine if you loose a rod or other major component. You could easily find yourself sourcing a complete engine. At this point based on your post, obviously you are unsure about the whole issue.

MSeriesRebuild wrote:There is no telling what you may find upon a thorough inspection. I've never heard a water pump make a knocking noise, but certainly you should check for loose motion in the shaft, fix it if it's there or the next sight you may hate to deal with may well be fan blades protruding through the radiator core.

From what you describe, I tend to think more along the lines of a piston pin bushing failure. That will be intermittent & will usually sound off more after speeding up RPM's & letting off the accelerator during the coast down back to idle. Remember, long distance diagnostics may not be worth much, I have not heard the knock or tap. If it is indeed a tap, a valve could be flopping in its guide, or you may have a badly wallowed tappet bore in the block. Tappet bores can produce this sound consistantly or intermittently, it is a profound tap & not a knock however. No real help again as you have now changed the noise from a knock to a tap in your description. Have you done a major tune-up to this engine adjusting everything in sight? Let me offer this suggestion, do a complete compression test. Post the result here & I can advise you further. I say this because more than likely if you have major mechanical issues going on, the compression readings will be lower than in a normal, healthy engine. If you also have issues going on which compression readings show need attention anyway; no need in spending time & money to try & locate a single knock or tap source. If compression readings were to indicate it is at or near time for a mojor overhaul, then all your issues are falling right in line with each other. Trying to patch this or that issue in an engine that may need a major, well it's pretty much a sure thing you will never have a good reliable engine until it gets the major attention it may need. Not saying all this is an absolute as I say once again, I have not heard it. I can say that a complete compression test only takes a few minutes & will tell you more about the "real time" condition of that engine than any other simple test you can perform.

I'm getting the feeling you are possibly not really experienced enough with this to make a proper & sure diagnosis. Sounds as if you might need to seek professional help in getting a diagnosis. Go to someone local you can trust & ask them to help. Not trying to insult your intelligence, but you must realize this could be a death sentence for the engine if you loose a rod or other major component. You could easily find yourself sourcing a complete engine. At this point based on your post, obviously you are unsure about the whole issue.

Thanks for the ideas. I will definitely do a compression test and see what I find. Like many of us here, I am very much a do-it-yourselfer, so I am going to lean towards fixing it myself before taking it to someone (for better or worse). That's how you learn. While I am certainly no expert on the internals of these motors, I know enough not to go hauling arse around town when the engine is making a noise, so I'm not too worried about a rod through the block.

I will look into the water pump, as well. I had just replaced the fan belt when I noticed the noise, so maybe with a little bit different tension it could've changed something. Who knows. The noise sounds like it's coming from the back of the motor, though, but I could be wrong.

If the sound is definitely coming from the rear engine area, it could be the flywheel loose on the crankshaft flange, something coming apart in the pressure plate assy, loose rivets that hold the clutch facings onto the disc, or something in the transmission. Here again, a trained ear would be much more likely to correctly diagnose.

If you are certain this started immediately after the belt installation, it could be a generator bearing on its way out, or the front gen bearing gear train if yours has the early style generator with the oil reservoir in the front. The front end crank hub loose on the shaft or the pulley loose on the hub. The front crank pulleys are easily warped or bent, if not running true to center, it could easily be bumping something as it goes around making a noise. Many times we've seen generators loose on the mounting bracket making a noise, it is very typical to find them loose & the bolt holes wallowed out in the bracket.

MSeriesRebuild wrote:If the sound is definitely coming from the rear engine area, it could be the flywheel loose on the crankshaft flange, something coming apart in the pressure plate assy, loose rivets that hold the clutch facings onto the disc, or something in the transmission. Here again, a trained ear would be much more likely to correctly diagnose.

If you are certain this started immediately after the belt installation, it could be a generator bearing on its way out, or the front gen bearing gear train if yours has the early style generator with the oil reservoir in the front. The front end crank hub loose on the shaft or the pulley loose on the hub. The front crank pulleys are easily warped or bent, if not running true to center, it could easily be bumping something as it goes around making a noise. Many times we've seen generators loose on the mounting bracket making a noise, it is very typical to find them loose & the bolt holes wallowed out in the bracket.

Could it be valve noise? Let's say the oil pump is not working correctly and things are getting dry, would that result in a tapping noise? Maybe some passage are partially blocked and oil is not flowing freely or consistently. I'm showing oil pressure on my guage, but I don't know if I trust it. Also, could the valves just need adjusted? I had to do this on a Chevy 235 in another truck. I realize the design is different, but do these engines suffer some of the same maintenance issues?


EDIT: duh...read back through the posts above....it's not a "tick", it's definitely a "tap", so you can probably disregard this one.

If you have not adjusted the valves, they likely need to be, however the way you have described this tap doesn't really sound like an adjustment issue. That produces a ticking noise if adjustment is loose. It won't be intermitent, but usually is continuous. Worn tappet bores produce a knocking or defined tapping. Pray that isn't the issue as it's a tough problem to fix. The valve train isn't oiled by pressure lube, simply splash, puddle, & drip.

MSeriesRebuild wrote:If you have not adjusted the valves, they likely need to be, however the way you have described this tap doesn't really sound like an adjustment issue. That produces a ticking noise if adjustment is loose. It won't be intermitent, but usually is continuous. Worn tappet bores produce a knocking or defined tapping. Pray that isn't the issue as it's a tough problem to fix. The valve train isn't oiled by pressure lube, simply splash, puddle, & drip.

Had my dad stop by who has worked on old trucks for years and has heard loose or broken rods before and he's convinced it's outside the engine. We held a couple things (2x4, breakover bar) against the block to see if it was transmitting any vibrations with each tap, and it wasn't. Probably not a definitive test since it's not a real loud tap, but a little encouraging, at least. I'm going to loosed the fan/generator belt and see if that makes any difference.

Didn't you say in another thread that you broke a mounting "ear" on the generator when you changed the belt. There's a good chance this is somehow related.

Brett

Get yourself a mechanics stethescope and start listening for the location of the noise.