Dreaded engine knock

[Landon427]

Yep, another engine 'knock'... I've read as much as I can find on this site and others, I'm just looking for input before I do an 'exploratory'... Here are the relevant symptoms and data I have:
* No noise when engine starts (cold); as engine warms up, there is a hit or miss 'knock' at idle
* Noise goes away when accelerating
* Stethoscope revealed loudest noise in #3 position
* Noise goes away when #3 plug wire is pulled
* Compression test (cold engine) shows 130-140 psi on all cylinders; 2.5L motor has 240K miles
* Neighbor (retired mechanic) listened and thought it is a bad lifter; I don't have his experience (not by a longshot) but it sounds louder down low to me

Thinking my next step should be to remove the oil pan and see if any of the connecting rods look loose. I'm told you can remove the motor mount bolts, lift the engine by the vibration damper and get the oil pan off without removing the exhaust. I'd appreciate it if you could let me know if you see any other avenues I should explore, or any problems with this plan. Thanks.

 

[Chris]

Yep, another engine 'knock'... I've read as much as I can find on this site and others, I'm just looking for input before I do an 'exploratory'... Here are the relevant symptoms and data I have:
* No noise when engine starts (cold); as engine warms up, there is a hit or miss 'knock' at idle
* Noise goes away when accelerating
* Stethoscope revealed loudest noise in #3 position
* Noise goes away when #3 plug wire is pulled
* Compression test (cold engine) shows 130-140 psi on all cylinders; 2.5L motor has 240K miles
* Neighbor (retired mechanic) listened and thought it is a bad lifter; I don't have his experience (not by a longshot) but it sounds louder down low to me

Thinking my next step should be to remove the oil pan and see if any of the connecting rods look loose. I'm told you can remove the motor mount bolts, lift the engine by the vibration damper and get the oil pan off without removing the exhaust. I'd appreciate it if you could let me know if you see any other avenues I should explore, or any problems with this plan. Thanks.

Based on your symptoms, it does seem like you might be dealing with something in the valve train or possibly a rod knock, especially since you've identified the noise as being more pronounced from the #3 cylinder.

Here are a few thoughts and suggestions that might help you narrow things down further before you go into the exploratory surgery:

• Oil Condition: Check the oil condition and level; old or dirty oil can cause lifter noise. If it’s been a while since the last oil change, consider doing that first as it may help quiet things down if it's oil-related.
• Lifters: Since your neighbor suggested a bad lifter, consider removing the valve cover and inspecting the lifters. Look for any signs of wear or damage. You can also try to manually compress them to see if they’re sticking or if one doesn’t return to its extended position properly.
• Spark Plug: Since pulling the #3 plug wire made the noise disappear, you might consider replacing the spark plug itself if it hasn’t been done recently. A faulty plug can sometimes lead to odd noises due to misfiring or incomplete combustion.
• Exhaust Leak: Double-check for exhaust leaks around the manifold. Sometimes, the sound of a leak can mimic engine noise and may mislead you.
• Testing with Stethoscope: You might also want to use the stethoscope to listen closely to the lifter area and the oil pan itself to further localize the noise.

Best of luck, and keep us updated on what you find!

 

[ColoJeep]

Yep, another engine 'knock'... I've read as much as I can find on this site and others, I'm just looking for input before I do an 'exploratory'... Here are the relevant symptoms and data I have:
* No noise when engine starts (cold); as engine warms up, there is a hit or miss 'knock' at idle
* Noise goes away when accelerating
* Stethoscope revealed loudest noise in #3 position
* Noise goes away when #3 plug wire is pulled
* Compression test (cold engine) shows 130-140 psi on all cylinders; 2.5L motor has 240K miles
* Neighbor (retired mechanic) listened and thought it is a bad lifter; I don't have his experience (not by a longshot) but it sounds louder down low to me

Thinking my next step should be to remove the oil pan and see if any of the connecting rods look loose. I'm told you can remove the motor mount bolts, lift the engine by the vibration damper and get the oil pan off without removing the exhaust. I'd appreciate it if you could let me know if you see any other avenues I should explore, or any problems with this plan. Thanks.

I would bet on a more severe problem , like a cracked piston skirt , 2.5 have an issue with this. However the no noise when cold doesn't align with my theory.
I hope I'm wrong.

 

[Landon427]

Thanks for the input! If I was better at writing, I would have included that the noise stayed after changing the oil (10W30 non synthetic) and filter (Fram ultra), and that using the stethoscope made me think bottom end but my neighbor thought top end. I changed the spark plugs, wires, cap and rotor yesterday (old ones didn't look bad, though #1 was loose and a Champion while the others were Autolites) but the noise persists. I was thinking the noise seemed different than an exhaust leak, since it goes away when accelerating (and sometimes randomly goes away even at idle), but it wouldn't be hard to check.

 

[macleanflood]

Could run a can of Seafoam through it and watch for exhaust leaks. Pull the vacuum line off the brake booster and dip it in the can. Run the throttle body so it doesn't die. Have folks watching or I use a couple GoPros for smoke coming out of places it shouldn't... basically it all should go out the back.

-Mac

 

[jeepguy03]

I would bet on a more severe problem , like a cracked piston skirt , 2.5 have an issue with this. However the no noise when cold doesn't align with my theory.
I hope I'm wrong.

I'm also thinking valvetrain. I've seen lots of 4.0s with a similar noise as the OP, it can range from lifter tap to piston slap. It will basically sound like a diesel at idle. All common, all pretty insignificant in the scheme of things and they run like that for years. Pistons with cracked/broken skirts knock all the time, hot or cold, and at all RPM. It's one of those "oh wow something is definitely broken" noises.

With no noise when cold being a factor, I'm thinking one of your lifters isn't pumping up all the way, and the thicker oil when cold causes it to work temporarily.

 

[ColoJeep]

I'm also thinking valvetrain. I've seen lots of 4.0s with a similar noise as the OP, it can range from lifter tap to piston slap. It will basically sound like a diesel at idle. All common, all pretty insignificant in the scheme of things and they run like that for years. Pistons with cracked/broken skirts knock all the time, hot or cold, and at all RPM. It's one of those "oh wow something is definitely broken" noises.

With no noise when cold being a factor, I'm thinking one of your lifters isn't pumping up all the way, and the thicker oil when cold causes it to work temporarily.

A good way to diagnose the potential lifter issue would be if you start it and is clicking , shut the engine off / wait 10 seconds then restart it.
I've had this issue occasionally on our 04 4.0 on a cold start , after shutting off waiting 10 seconds and restart it is fine and nice and quiet.

 

[Strizzo]

Yep, another engine 'knock'... I've read as much as I can find on this site and others, I'm just looking for input before I do an 'exploratory'... Here are the relevant symptoms and data I have:
* No noise when engine starts (cold); as engine warms up, there is a hit or miss 'knock' at idle
* Noise goes away when accelerating
* Stethoscope revealed loudest noise in #3 position
* Noise goes away when #3 plug wire is pulled
* Compression test (cold engine) shows 130-140 psi on all cylinders; 2.5L motor has 240K miles
* Neighbor (retired mechanic) listened and thought it is a bad lifter; I don't have his experience (not by a longshot) but it sounds louder down low to me

Thinking my next step should be to remove the oil pan and see if any of the connecting rods look loose. I'm told you can remove the motor mount bolts, lift the engine by the vibration damper and get the oil pan off without removing the exhaust. I'd appreciate it if you could let me know if you see any other avenues I should explore, or any problems with this plan. Thanks.

I helped a buddy chase a similar sound and issue on a ford smallblock. It turned out to be a spun rod bearing and the knocking sound was the piston making contact with the head when there wasn't enough pressure to take up the slack. engine still pulled strong, compression and leakdown tests were good, oil pressure was strong (over 50psi) but as soon as you let off the throttle it would knock like something was trying to get out.

When his mechanic pulled the plugs before pulling the motor out, the spark plug gap was smashed closed on that cylinder. no idea how it didn't have a miss, it ran fine other than the knock.

 

[Landon427]

I took the oil pan off and all of the upper con rod bearings are worn down into the copper (roughly 0.001" of wear according to my dial caliper). I plastigauged them with each piston halfway down and all the way down and all were about 0.002", which was surprising. I thought they would be more like 0.003. The piston skirt to bore clearance is about 0.003" by feeler gauge, out of 'new engine spec' (0.0008" - 0.0015") for sure but it doesn't seem terrible. Thanks for all the help. I'm planning to replace the rod bearings and see if the noise goes away.

 

[Landon427]

I replaced the rod bearings with Clevite aluminum/silicon standard size bearings. Plastigage now shows 0.0017"-0.0018" clearance as best I can tell. No excess wrist pin wear (no 'clicking' when I moved the pistons up and down). No broken piston skirts, no cracks in the piston skirts that I could see. Piston to bore clearance is about 0.003" using a feeler gauge. Put it back together and I have the same noise. It was never super loud, and I believe it may be quieter now. Hot oil pressure is 45-55 psi according to the dash gauge. Not sure whether I should just run this as is or if there is something else I should check. The noise is not there on cold startup, and it gradually becomes noticeable as the oil/engine heats up. Let me know if you have any further advice. I guess I should have put in new crank bearings while I was in there, but I've heard the main bearings are very stout and I guess I talked myself out of it. I can't find much info about the sound of worn crank bearings, but one friend that was a mechanic thought this was rod knock (before the new bearings) and another thought it was a lifter (also before the new bearings).

 

[Landon427]

Moving on to the top end - Valve cover is off. Valve assemblies look very clean to me, especially for 240K miles. Maybe they were replaced at some point? Nothing catastrophic is evident, based on my limited knowledge of these engines. With the lifters on the cam base circle, I can rotate all of the pushrods. Nothing is loose when I wiggle the rockers. I guess as a last ditch effort I will replace the lifters, check to make sure the pushrods are straight, and check the rocker assemblies for obvious wear.

I did find some very fine copper colored 'silt' in the oil pan and on the heads by the valves, and I think it came from the oil additive the previous owner was running (Restore - Engine Restorer and Lubricant), which contains copper, a little silver and some amount of lead. He said it definitely quieted down the 'knock/tick' noise the engine was making.

Question for the gurus - What is the best way to break in new lifters? I'll put assembly lube on them, and on as much of the cam lobes as I can.. Should I use a break in oil? Run the engine at 2,000 rpm for 15 minutes? Thanks for any advice. I'll be running Melling JB2011 lifters and hoping for the best. Thank you.

 

[Richard Mccullough]

Before dropping the pan, you might try swapping the #3 spark plug and wire with another cylinder, just to rule out a funky plug or wire. If the noise follows the wire, you’ve got ignition issues. If not, pulling the pan to check rod play makes sense. Just watch those motor-mount bolts when you lift, support the engine well.

 

[Landon427]

New lifters were installed. Pushrods looked straight, and I didn't see any unusual wear on the rockers.

I used Lucas 30 wt break in oil and pulled the fuel relay to build oil pressure, then installed the fuel relay and ran the engine for 23 minutes at 2000-2500 rpm for break in. No change to the 'knocking' noise.

Not sure what to think - maybe I have piston slap - when I moved the pistons around (while changing the rod bearings earlier) there was visually more gap between the piston and bore than what I was able to measure (0.003"). That doesn't explain why the engine was quiet when cold, and the knock gradually gets louder as the engine / oil heats up. Maybe the "Restore Engine Restorer" that the previous owner had installed was masking the noise when cold? I know you have two things going opposite directions during warm up, ie pistons expanding more than bores (aluminum vs cast iron) while oil is thinning out. Not sure what to think..

 

[Landon427]

Just continuing with my saga.. I'm pulling the engine to rebuild it.. It does have 240K miles on it. One thing that might help someone is that I was able to remove the top two clutch housing to engine bolts by using an 18mm box open wrench, with the box end on the bolt head and a short 1/2" extension on the jaw and facing up vertically. That gave me enough leverage to loosen the top two bolts. I didn't have to drop the transmission to get any of the clutch housing to engine bolts out.

 

[Landon427]

Got the engine out late last night; thankful for talented friends willing to help. I had M12x1.75 80mm bolts to mount the engine to the stand, which were a good length for the bottom two holes but needed some washers to fit the upper two. Also, the driver's side bottom bolt hole in the engine skirt had an alignment sleeve in it to align the engine and trans, and it took quite a bit of convincing to get it out (Liquid Wrench for the win, after heating with a torch).