Knocking under load/hard acceleration

[jux]

My engine just developed a knock coming from the engine area when I accelerate hard at the end of 1st gear and through 2nd gear, never in third that I can hear.

The sound does not happen when revving in neutral, and just started yesterday. Oil seems fine/full. It almost sounds like a rod knock. I was running on premium gas.

Engine is stock internally, three speed saginaw tranny.

Any thoughts?
I'll give more details later, just got home from running around listening to this annoyance. I'm not going to be driving it until I figure this out.

Thanks

 

[89Suburban]

You had a coolant loss issue and now you are hearing knocking. How does the oil look? Any discoloration? Any change in the oil pressure reading? I would think if it was a spark pre-detonation it would be worse in higher gears.

 

[austinado16]

Knocking under load is probably a rod. If it was spark knock (pinging) you would hear more of a rapid rattling.

 

[gpmorgan]

I think I would pull the valve covers and make sure the oil return holes are not stopped up or partially clogged. If they are, the oil pump is running dry because all the oil is in the valve covers at higher rpms.

 

[jux]

Thanks for the replies,

To answer all of you guys:

Paw paw: The oil looks clear, I'm about 2000 miles into my last oil change and it is getting a little darker. Smells a little like gas, I am running a bit rich. My oil pressure gauge is bad and fluctuates, but always stays between 30-60 psi.

Austinado: I have never heard rod knock in person (only on youtube), but it sounds like I would imagine a rod knock sounds like. I know what pinging sounds like, and it's not pinging. Although it will if I use under 89 octane.

GP: Thanks for the tip, I think I remember you mentioning that in another post a long time ago.

About my coolant issue: It has returned! The first night I drove it, the reservoir drained back into the rad as it should. The next day, it stopped draining back, leaving a pocket of air about a quart in volume.

I should mention that the night it began making the high rpm knock, it was cold outside (about 30d F) and for the first time ever, I did not let it warm up before driving. I drove it soft and easy until it was warm, but it didn't like it. Had a bit of stumbling and hesitation, not much but it should be considered because the truck pretty much never stumbles or hesitates.

The last thing I should mention is that the #8 cyl (passenger side closest to firewall) fouls spark plugs with grey ash deposits and a bit of oil, so I keep a spark plug non-fouler on it. I have tested for the knock without the non-fouler, and it was still present.

I wonder if I am looking at a head gasket issue. I can only hope that I can figure it out with the help of you guys, and that it's not extremely expensive to fix.

Thanks!

 

[gpmorgan]

Might need to do a compression test. Let the gauge stay on each cyl a few minutes to test for leak down of the pressure. Also do a pressure test on the cooling system. These two tests should show any problems with head gaskets or cooling system.

How many miles are on the engine? Does it run rich enough to put black smoke out of the exhaust? I've seen vehicles run rich enough to put gas in the oil and we know that's not good for the engine internals.

 

[kleedus]

take a wood dowl rod broom stick or something like that put it on the engine and to your ear and listen threw it you will hear the knock. you can put it in a couple places to tell maybe a water pump they can knock sometimes. I had a water pump knock that I would of sworn was a rod knock.

 

[jux]

Thanks, I plan on doing that Kleedus, it's helped me in the past.

GP, it does not blow black smoke at all. You can smell that it's a little rich, though. Mileage is unknown, as this engine is a donor engine that I used to replace a failed 4.3 v6. The 86 c10 I got it out of had a 4.3 from the factory as well, so mileage is unknown. I would guess somewhere between 100,000 and 160,000 mi.

Does anyone know if I can use a "Mity-Vac" to diagnose this?

Thanks again!

 

[austinado16]

A compression test gauge has a one-way valve (actually a tire schrader valve) that keeps pressure up in the gauge, so you can get an accurate test. So a compression test gauge can not be used for a leak down test gauge.

The quickest and most accurate method of testing for a blown head gasket is to take the vehicle to a shop that has an exhaust gas analyzer, or emissions test equipment. With the engine at operating temperature and the radiator cap removed, "sniff" the steam coming out of the radiator with the exhaust gas analyzer. If the machine picks up any trace of HC's(hydrocarbon parts per million), or CO(carbon monoxide %) the head gasket has failed.

As for the home made listening device, you can also use a long section of heater hose, garden hose, etc. or a long screwdriver. Have a helper bump the throttle just enough to create the knock. You can usually get pretty close to the general area, without the listening device, by closing your eyes and listening to the knock. Your ears will steer your face and when you open your eyes, you'll be looking at the vicinity. It's pretty accurate.

I had guy call me about a knock in a Ford 5.0 that he'd had installed in his '50's street rod. He'd had it to a local big name independent garage and they'd told him it was rods, and or pistons and on and on and the engine was junk (brand new engine). So he fired it up and made it knock. I closed my eyes and followed the sound. When I opened my eyes, I was looking at the front, center area of the engine.....ie, timing cover, water pump, crank pulley. Crawled under and found rust around the seam between the crank pulley and the front of the crank.......gee, you think maybe the pulley bolts were loose much? I tightened the pulley(harmonic balancer) bolts and the noise was gone. And then I pointed out the leaking hole in the back of the radiator where the other shop had slipped with a wrench while taking stuff apart to find the noise, and ripped a hole through his brand new radiator. Ka-ching.

My guess on the coolant not recycling would be that the new cap is defective, or the flange on the radiator is distorted/damaged, and not sealing.

Overall though, I'd say your engine is appearing to be junk. You've got one bad hole, and now this knock under load. BTW, rod knock is like a hammer striking a hunk wood, at about the repetition of just one cylinder firing.

 

[gpmorgan]

A compression test gauge has a one-way valve (actually a tire schrader valve) that keeps pressure up in the gauge, so you can get an accurate test. So a compression test gauge can not be used for a leak down test gauge.

Your are correct. I think I had a brain fart when I mentioned the leak down test with the compression tester. My bad.

 

[GTME94]

Another noise that can sound like an engine lower end problem is a loose clutch/flywheel or pressure plate or the torque convertor is not properly bolted to the pressure plate. The other symptoms seem to indicate it's something else but its worth checking.

 

[jux]

I don't know that I can re-create the sound without driving it. It won't do it in neutral.

 

[MrMarty51]

Drop that lower bell housing inspection cover, check that flexplate over for any signs of cracks and that the bolts/nuts is tight.
Try some upper cylinder carbon/soot remover, follow the manufacturers instructions on the cans label.
a container of water homogonized with some ATF will do to. Pour that stuff down the TBI while the engine is running at a brisk pace, about 2,000 RPMs.
Sometimes carbon can build on the pistons and cause these same symptoms.

 

[jux]

Hmm, I'll give that a try tomorrow, thanks Marty!