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Engine diagnosis, rod knock.

13K views 35 replies 20 participants last post by  MoanaMojo 
#1 ·
Seeking your advice in diagnosing my engine. The truck has a new battery, cap and rotor, spark plugs and wires and an oil change. The engine turns over fine, sometimes I need to hold the gas a little when I start her in the morning. It idles fairly smooth expect for a repeating knock in the engine. My buddy said it was rod knock. No vacuum leaks that I was able to detect and it is not throwing any codes. How would I go about in diagnosing the problem? The oil levels are good.
 
#2 ·
Install a Mech oil pressure guage and check oil pressure if you suspect any mech knocking. Can also get a 2' long heater hose, have motor running and see if you can isolate the sound with it. A stethescope is easier but i'm cheap and heater hose I have.
Have you pulled and checked plugs, see if one is discolored differently.
Look at manifolds for black soot if there is a leak or cracks.
 
#4 ·
Check for cracked manfiolds for sure then. How many miles? Mine they where both cracked at the back near the collector and I was missing two bolts on each side as well.

Exhaust coming into cab? Do you have any holes in exhaust pipe, driving with rear window down sucks all that sleepy gas inside if any leaks.
 
#7 ·
X3 on the hose.
Garden hose works just fine too.
Great way to isolate where the knock is coming from.
Sometimes the old school ways are still the best.

I would recommend you isolate the noise first,
May save you a lot of unnecessary time and effort.
 
#11 ·
being an 95 you have efi. pull codes should be your first step. if your hitting the gas peddle to get it started that also is an issue being you shouldnt need to do that. check your fuel pressure as well and timing
 
#13 ·
the hose is a good method

also if it comes rite down to it and you cant isolate the rod/crank knock youll have to drop the oil pan and try to wiggle each rod journal to see which one is knocking

if a rod is found to be knocking hopefully it wont be slaping the head and all youll have to do is replace that rod/bearing/piston rings for that cylinder......hone the cylinder before reinstalling the piston so the rings can seat properly

parts needed would be a complete engine gasket set, connecting rod, rod bearings, piston rings, fresh oil, and a cylinder hone with instructions on how to hone a cylinder properly :thumbup
 
#14 ·
I've heard manifold leaks that sound like knocks. the fact that you have exhaust IN the cab, and this sound, i'd thoroughly inspect the exhaust system.
 
#15 ·
The classic method of finding a rod knock is to run the engine up to about 1000 RPM, and then pull one plug wire at a time. The knock will go away when you pull the plug for the offending cylinder.
 
#21 ·
As there are two rods on each crank throw, how can you differentiate the sound you hear through the stethoscope to determmine which rod is knocking? The stethoscope, when placed near the main bearing adjacent to the offending rod, would "hear" the knock, but which cylinder? Of course, if you have a stethoscope, when you pull the plug of the offending cylinder, you may be able to better hear it. A loose wrist pin (or upper rod bushing) and a rod bearing have a similar sound, but the wrist pin will often be heard twice per stroke (when the piston reverses) and is a lighter "click" than a rod bearing (wrist pin noise is very rare in modern engines).

What method of non-intrusive rod knock diagnostics have you found more reliable? Please share your findings, all of FSB will be grateful.
 
#22 ·
Mikey350 said:
What method of non-intrusive rod knock diagnostics have you found more reliable? Please share your findings, all of FSB will be grateful.
No need to be a dick.

It doesn't seem a good idea to me to have the recommendation to reach in and pull spark plug wires on a running engine, for all the noobs who may search and read this thread. That'd be an unpleasant experience for them if they're not extremely careful.

A stethoscope will help to determine exactly WHAT the knocking sound is, as in the OP's post, he is not even sure yet that it is a rod knock, and often times what sounds like a rod knock could simply be an exhaust pipe steadily banging into a crossmember (I had this situation once, not kidding you at all, it replicated a rod knock so well I almost **** myself when I heard it), so we're jumping the gun a little to start powering down individual cylinders. Also, if the bearing is bad enough, removing spark from the cylinder will not ALWAYS prevent the knock from being heard.

Above all, if it is determined that it is in fact a rod knock, why should I then care which rod it is? Once pulling the engine, oil pan, etc. why would you only repair the offending cylinder? Would it not be more prudent to restore all 8 cylinders at that point? Not to mention once you are this far into it, it should be pretty apparent which was the offending cylinder.

The method of preventing a specific cylinder from firing would be helpful to diagnose a cylinder misfire issue, so that in the case of a dead cylinder it would help to find which cylinder is not firing, but even then it would be beneficial to include the safety concerns involved with removing live plug wires. And although admittedly I cannot think of any, I am sure their is a safer way of blocking spark to a specific cylinder without manually removing the plug wire from it's contact.
 
#23 ·
@blackbronco92 - I didn't diagnose the engine, I don't know if it has a rod knock or whatever. What I do know is a method of determining which cylinder is noisy IF it is a rod knock.

The danger of pulling a plug wire from a running engine? Well, the OP said he replaced the plug wires in his first post, so the boots should come off the plugs rather easily. If not, for about a $10 investment, a spark plug boot puller should work fine. (or $100 for some rubber gloves) Pulling the plug wire off the distributer instead of the plug is an alternative, and just as effective. Or just pull a plug wire off (either end) and then start the truck, if you leave the plug boot close to the plug (or terminal), the plug might even fire until you move it far enough from the terminal (or plug).

A method of disabling the ignition of a specific cylinder on an engine that doesn't use a distributer (think large 2-stroke outboard motors): They also have a coil and carburetor for each cylinder.
Take a paper clip, bend it in a "U" shape and put one end of the "U" in the spark plug boot and put the boot back on the plug. Attach a test lead with an alligator clip to a good engine ground. Start the engine. Wave the end of the test lead near the exposed part of the paper clip. The spark will jump to the test lead, and stop that plug from firing (electricity taking the path of least resistance). I never tried this on an automotive engine, but I'll bet it would work if you put the paperclips under the distributer terminal boots (maybe just 4 at a time).

I do find it a little humorous that you believe that the OP isn't competent enough to pull a plug wire without an "unpleasant experience", yet competent enough to rebuild the engine.
 
#24 ·
@blackbronco92 - I didn't diagnose the engine, I don't know if it has a rod knock or whatever. What I do know is a method of determining which cylinder is noisy IF it is a rod knock.
If it IS a rod knock, then why do you need to go any further in figuring out which cylinder if you are going to have to pull the pan anyway? I didn't say that you did diagnose it, but in trying to help the OP it would be pertinent to focus on his issue step by step without jumping the gun.

The danger of pulling a plug wire from a running engine? Well, the OP said he replaced the plug wires in his first post, so the boots should come off the plugs rather easily. If not, for about a $10 investment, a spark plug boot puller should work fine. (or $100 for some rubber gloves) Pulling the plug wire off the distributer instead of the plug is an alternative, and just as effective. Or just pull a plug wire off (either end) and then start the truck, if you leave the plug boot close to the plug (or terminal), the plug might even fire until you move it far enough from the terminal (or plug).
Thanks for clarifying. If you read my post you'd find that I was referring to the thousands of users who are inexperienced, or in general unfamiliar with an engine, and we shouldn't be recommending people to simply grab live plug wires without specifying what precautions to take. I assume the OP knows this, I don't assume that 100 other users who may search this topic and find this post over the next few years might.

A method of disabling the ignition of a specific cylinder on an engine that doesn't use a distributer (think large 2-stroke outboard motors): They also have a coil and carburetor for each cylinder.
Take a paper clip, bend it in a "U" shape and put one end of the "U" in the spark plug boot and put the boot back on the plug. Attach a test lead with an alligator clip to a good engine ground. Start the engine. Wave the end of the test lead near the exposed part of the paper clip. The spark will jump to the test lead, and stop that plug from firing (electricity taking the path of least resistance). I never tried this on an automotive engine, but I'll bet it would work if you put the paperclips under the distributer terminal boots (maybe just 4 at a time).
I would think that this would work the same.

I do find it a little humorous that you believe that the OP isn't competent enough to pull a plug wire without an "unpleasant experience", yet competent enough to rebuild the engine.
Once again, no idea where this comes from. I wasn't referencing the OP's skillset.

Let me summarize.

I think your idea of singling out the offending cylinder via removing a spark plug wire, when finding an engine knock, is unnecessary. Because ultimately the pan will have to be removed for the bearing repair anyway, and you will quickly find which cylinder was bad at that time. Not to mention it would be wise to replace all while accessible, and recondition other related components while the space is open. Something caused that bearing to fail, you'd need to figure out what that was, and assume others are affected. Anything less and it was a waste of time.

The important part is finding WHAT the sound is. Once that's established, the rest is pretty self explanatory, no real need to continue diagnosing from there, move onto proper repair.

I didn't mean to put sand in your vagina. I simply said I didn't think that was a reliable method for diagnosing a rod knock, and asked you to tell me how it was better than a scope. Not sure why it offended you.
 
#25 ·
... I simply said I didn't think that was a reliable method for diagnosing a rod knock, and asked you to tell me how it was better than a scope. Not sure why it offended you.
OK, then tell us your method of determining a rod knock with just a stethoscope. I said to pull a plug wire and listen to hear if the sound goes away (with or without a stethoscope). You told us that pulling a plug wire was too dangerous for anyone but a professional to attempt. this was to protect "all the noobs who may search and read this thread."

I offered some suggestions on how to mitigate the chance of getting an "unpleasant experience" in the process of diagnosing the offending cylinder. You offered nothing.

...but in trying to help the OP it would be pertinent to focus on his issue step by step without jumping the gun.
and
...The important part is finding WHAT the sound is..
You said my diagnostic method wasn't a "reliable" diagnostic method and when I asked you how you would diagnose the OP's suspicion of having a rod knock, you called me a dick.

So, come up with something, before the OP tears apart his engine because he couldn't tell the difference between a rod knock and an exhaust pipe hitting a crossmember.
Otherwise, it appears you have nothing to offer in the way of a diagnosis, just noise.
 
#26 ·
Tl;dr


It takes five seconds to pull a plug wire off. IF it IS a knock, then if he pulls plug wires and the knock stops on a certain cylinder, not only does he know which cylinder it is, but he also has verified that it is a fricken knock.

Why make things difficult?
 
#27 ·
OK, then tell us your method of determining a rod knock with just a stethoscope. I said to pull a plug wire and listen to hear if the sound goes away (with or without a stethoscope). You told us that pulling a plug wire was too dangerous for anyone but a professional to attempt. this was to protect "all the noobs who may search and read this thread."

I offered some suggestions on how to mitigate the chance of getting an "unpleasant experience" in the process of diagnosing the offending cylinder. You offered nothing.
I didn't need to offer any safety tips because I still think it's a pointless idea, but again, thanks for clarifying in your previous post.


and


You said my diagnostic method wasn't a "reliable" diagnostic method and when I asked you how you would diagnose the OP's suspicion of having a rod knock, you called me a dick.
No, I called you a dick and then answered your question. I will answer it again.

So, come up with something, before the OP tears apart his engine because he couldn't tell the difference between a rod knock and an exhaust pipe hitting a crossmember.
Otherwise, it appears you have nothing to offer in the way of a diagnosis, just noise.
ISOLATE the sound, with proper tools, IE; a stethoscope (Are you actually asking me to explain how to use one?). Start in obvious, and easier areas. Is it a loose exhaust? Is it an exhaust leak being mistaken for an internal knock? If it is, is it upper or lower engine noise? Do you have good oil pressure? Is the oil in good condition, at proper level? Have you pulled codes to help determine ANY other possible engine performance issues that may or may not contribute to possible knocking sounds (I'd like to first get to the bottom of why he's holding down a fuel injected throttle to start the truck)

It just sounds to me like everyone is joining the internal engine knock bandwagon before the OP has even had a chance to respond to what others have asked him to do. Pulling spark plug wires one by one seems to be starting at the end of the line and taking steps back until you figure it out.

Either way, to each their own. I'm done cluttering the OP's thread arguing about what I think is better and what you think is better. We've both made valid enough points. Carry on to fixing the OP's rig.
 
#29 ·
Blackbronco- the pull the plug wire method has been around for a long long long time and used by proffesonal mechanics soo.... I can also verify it is effective.

On another note the chance that there is some carbon loose in the cylinder is possible also. Fogging[using a liquid sucked thru a small vacuum hose] while holding the engine at about 2500 rpm can sometimes clear carbon from the combustion chamber- if you dont feel confident in your skills for this get a pro.
 
#31 ·
That is typically from flywheel bolts backing off. I once bought a truck super cheap as the owner thought it was about to throw a rod (at the time, so did I)

As it turned out, the truck had a brand new motor installed, and after 15k miles, the bolts had backed out enough to knock. Easy fix.

As for the spark plug wires, I've heard of it being done before. That's fine. Professional method or not I just don't care for it. :shrug
 
#32 ·
Oooor you could pull a the power lead to the injector a much easier and safer method for the fuel injected motors. I never liked pulling the plug wires either those things will about make your piss yourself if you get the opportunity to become the ground.
 
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