What knock is this?

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jim45r

jim45r

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Feb 27, 2004
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Location
Bay Area, CA
Just finished replacing the cracked head on my '87 4WD - the old one had gone sudden-like and I drove about 10-15 miles on a nice frothy coolant/oil milkshake before I discovered it. I towed it to my mechanic, who put on a new head, and it sounded great, except for the "creeping idle" stuff that I've read about cropped up. I'm trying to work that out, when I notice a knock that started after it's warmed up. It only knocks starting up in 1st and 2nd from 1800 to 2500 rpm, then it quiets down - even back down to 2k, lugging. I can even drive up a grade and it's quiet, until I come to a stop and then start off again. I though I'd munched a rod bearing, so I dropped the front diff and put in a new set with the engine in the truck- none of them was to the babbit but they were pretty worn. I checked the rear main while I had the pan off and the main looked fine, and plastigaged within new spec, tho it still had the oil film on the engine half so it's not exactly correct, but the bearing looked fine. I didn't check the others after that - anyone who's done bearings with the engine in the truck on their back in the driveway can relate.
Well, I crank her up with my new bearings, and I have better oil pressure, but glory be - I still have the knock. Could it be a wrist pin? those get splash-oiled, and I can't imagine one getting that screwed up for the short time I drove it. of course, I guess the crankcase was getting a nice steam cleaning with that water in there....Any ideas? It doesn't sound like a main to me - not a heavy sound... it just sounds like a rod. I've never heard a bad wrist pin tho.
Also, is it normal for the idle to go up when you take out the dipstick on a running engine? I don't know how the PVC ties in with the computerator...
Thanks
 
if it was an intial morning sound, i would say its a mis adjusted valve, thats extra noisy before it gets oil.

but its a operating temp noise. i had a simalar noise on my 20R. i noticed it while i was at work, building the smoke stack.

i thot it was mains. the other 2 mechanics listened to it. one said rods, one said mains.

i pulled out the 20R and put in a 22R, and kept the 20R head.

a few days later, it turned out to be the #3 main bearing.

its somthing that a trained ear has to hear. but somtimes even that can be wrong.

i suggest pulling it out and re doing the whole thing.

and yes the idle changes when pulling the dipstick.

like jerod said, you make a vacuum leak
 
Thanks, Wristy, dang, I thought that the rear might show the most wear. I drove it a bit more today, and found that I can drive it in a way that doesn't knock, taking off with the Rs up and shifting crisp while keeping them up. Not my usual old lady, clutch-protecting ways.
So maybe that would point more toward a main - spinning faster might give less chance to bottom on the cap? It's just strange that I get it in 1st and 2nd, and not when I lug it in third... I've read of ways to do the mains in the truck, but ugh, it's a drag working on the back with that crap dropping on my face...I kinda knew I'd regret not checking the others...
 
generally main bearing noise gets worse with more rpm,and especially underload when you put pressure on the crank.piston pin noise is worst when cold and asthe motor heats up the aluminum piston expands and the noise gets less audible,same with piston slap.from your description it doesnt sound like any of these,although it could be the thrust bearing on the main causing to much crank endplay,or connecting rod side clearance,causing the connecting rod to walk back and forth either of these would make noise dependant on rotational force and rpm.the question i have is did
you do the timing chain when the head was off.timing chains can cause all kinds of funny knocks,especially at lower rpms when oil pressure is lower(the tensioner works off oil pressure)
 
Well, I was tight for time so I had my mechanic do the head - and asked him to check the timing chain. He said it looked fine. The PO had a gotten new cover and guides (I found th old ones in the pan!) and I would imagine a chain, too. The knock wasn't there before the head replacement, and they didn't have the front cover off to do the head. The oil pressure is better with the new rod bearings....
I didn't check the thrust bearing or rod endplay, the motors got 116K on it but was quiet and running like a charm B4 the head let loose, so I didn't expect that kind of problem. Ya, never assume anything....

It's running hotter now under load, too, (he did replace the thermostat with a toyota upgraded one - I assume with the bypass valve) and I found the timing wasn't correct when I got it back - it was 1* with the jumper shorted, I set it to 5* but that didn't solve the hot motor. It was really busy when I took it over to him, so it must have gotten overlooked. He's got a really good reputation, but you never know about the help. Since the timing was off, and I had the idle surge problem, maybe I'll just take it back over to him and ask him to troubleshoot it. it's very weird that it wouldn't be making a helluva noise under all loads if it was a bearing.
 
my engine kind of knocks too but it's definetly overhead knock and i'm pretty sure it's just valves although i keep on adjusting them but it never goes completley away. replacing the mains wouldn't be too bad from below. just be sure to let the oil drain completley before you get under or wear goggles or something. i took out my pistons to inspect the cylinder walls and change the rings when i got a head job and it was really easy. my truck didn't start knocking until after my head job as well but like i said it's definetley overhead noise not main or pushrod bearing noise.
 
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