New semi-loud knocking

[lego126]

In the last couple days ive developed a rather loud knock/ticking noise, only while under load, and only under 2200-2500 RPMS. It wo t do it while idling, wont do while running above 2500 rpms. It sounds like the dirty/low oil tick in my D21 only louder. Ive already changed the oil, and filter,1500 miles early. no change. Checked valve clearances. Small adjustment needed after 3+ years. Recent changes... my exhaust pipe has seperated somewhere below the manifold, but not too loud yet. Gonna correct my next weekend off. I inspected all the valves as best i could with the valve cover off, didnt notice anything. Just spitballing so ideas olease.

85' 720 2.4z manual. 2wheel drive. 204k

[flatcat19]

Buy a stethoscope from Harbor Freight. They're cheap.

 

Place on oil pan. Work your way up.

[distributorguy]

Since you have an exhaust leak, hold a large towel over the tailpipe while its running.  If the tone changes dramatically when you do this, its just a change in your exhaust leak size.  Get it fixed.  

 

You can use a huge screwdriver as a stethoscope too.  Put your ear to the end of the handle.  It works better than most crappy stethoscopes.  

[datzenmike]

Valve clearance doesn't get louder with engine load but it does speed up and down with the engine. Exhaust sound most assuredly does get louder under load. If you have an exhaust leak I would start by fixing it first.

 

Occam's razor... the simplest explanation is usually the right one. Also known as the Law of Parsimony or why may it more complicated than it is?  

[720inOlyWa]

Which is to say... I put my money on a rod bearing going south. It aint gonna fix itself, either. Better to rebuild now (after your stethoscope / screwdriver diagnosis, of course) than it is go  say “huh” after the rod has migrated through the block.  My Fudgecicle had a slight rod knock when I got it that would almost disappear when it warmed up, and it never got worse. But I knew it would, eventually, so it was restricted to around town use and never loaded fully. I also never grabbed the keys without making sure my,cell phone was in my pocket. This is an unacceptable limitation, so I rebuilt the motor which cost me $1,300 for everything, including machining. That much cost for huge ass benefit of knowing it is as solid as a Nissan truck can be seems like the best deal to me. Now, if need be, I have no reservations about jumping in this truck and driving to Chicago.

 

I like that feeling a lot and whole heartedly recommend it to you. Too many posters here are looking for a super cheap way to solve larger problem, which really ought to be dealt with thoroughly. A 720 truck is actually relatively inexpensive all the way around. I am in sympathy with the need to be budget minded, but sometimes you just gotta throw the long ball and get the job done right. Besides, with all the help around here, if is actually.....fun!.

[datzenmike]

Rebuild engine.... turns out to be the exhaust leak. Seems like the exhaust has to be fixed anyway and with luck that's the cause. Could very well be a rod bearing but only common sense to eliminate the simple easy things first.

[lego126]

Ive got a stethascope. Only knocking under load and only 2500 RPM. It does speed up and slow down with RPMS. Ive got valve cover off. I dont see any issues with the valves. Anything i should be specifically looking for valve wise? Bout to drain the 3 day oil and check the mains i guess.

[lego126]

Oh and its def. Metal in metal. Maybe piston slapping a valve?

[lego126]

The worst of the noise seemed to be coning from 3rd or 4th cylinder. But hard to tell at idle. I ended up have wife pull ebrake, put it in gear and start to take off. Useing ebrake to force load. Not enough to kill it but enough to "load" it.

[flatcat19]

Is the noise coming from the top or bottom?

[datzenmike]

Fix exhaust leak?

[flatcat19]

Tighten alternator bolts?

[distributorguy]

"Knocking" only at 2500 under load wreaks of detonation.  My guess is you burned an exhaust valve - bad!  A compression test would show a lower pressure on that cylinder.  A Leakdown test would confirm where the compression loss is.  A teardown will show the extent of the damage.  Or maybe its still an exhaust leak...

[datzenmike]

It's hard to diagnose with words. Ticking, taping, rattle, pinging, popping, knocking, banging, hammering, thumping, grinding...... all descriptors. A diagnostician must cast a wide net.

[lego126]

"Knocking" only at 2500 under load wreaks of detonation. My guess is you burned an exhaust valve - bad! A compression test would show a lower pressure on that cylinder. A Leakdown test would confirm where the compression loss is. A teardown will show the extent of the damage. Or maybe its still an exhaust leak...

[lego126]

Im leaning toward the valves. Ill have the head off morrow after work. I pulled plugs out and 3 and 4 cylinder plugs are toast. Not too old either maybe 12 months. Wish i could tell forsurr before head removal. Might as well.

[datzenmike]

'toast'is another word. A picture would help. Toast means different things to different people. You fix the exhaust yet?

[lego126]

Perfectly toast head gasket between 3 and 4 cylinder. Ill post a pic

[lego126]

'toast'is another word. A picture would help. Toast means different things to different people. You fix the exhaust yet?

Exhaust gettimg fixed with the head.

[flatcat19]

Do you mean combustion leaking into coolant?

[KoHeartsGPA]

Bad gas will make an engine knock under a load as well, happened with several vehicles I've owned in the past, most recently, my mom's truck did it, filled next tank with higher octane and went away, back to regular now.

 

Glad you found your issue.

[lego126]

Well my coolant was rather clean, same with oil, pretty clean. Can i not post pics from my android?

[datzenmike]

Has to go through a hosting service like Photobucket.

[lego126]

One last question before i start reassembling. The PCV valve tube goes around the block and into the block just underneath the exhaust. Couple years back i noticed it was starting to leak around the little baffle thing that goes into the block. Since heads off, should i try to remove this thing and reseal or something. With the head off ive got really good access, but im insure about the removal. Also the valves. Should i at least replace valve stem seals? It never smoked, or had any issues with em, but its an 85', and after the original owner died it sat for 10+ years before i purchesed it for 2 bags of whole corn for the guys deer feeders.. thanks again. Oh i tried posting on photonucket but hard to do from my phone. Ill try from desktop later on.

[datzenmike]

I would do the valve guide seals now while easy. They aren't very expensive.

 

The block vent just presses into place on the block. I had mine out recently because the head was off and just pushed it back in and hammered lightly downward to seat it. There should be a strap to support it on one of the bolts