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Old 05-15-2019, 05:32 PM   #1 (permalink)
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Default Help diagnose engine knock on '91 VR4

I am looking for input from you guys. I have a 1991 3000GT VR4 with low miles (41K) that I have spent a lot of time fixing up. It was working great but then the engine suddenly developed a loud knocking sound.

I first thought it might be a bad/stuck lifter. I had installed 3rd gen lifters a while ago but they have always been really noisy, so I figured now was a good time to try putting the original lifters back in place. I did that and the lifter tick improved a lot, but the louder knock did not change at all. I am now worried that this is rod knock. But the knock comes and goes, and I don't know what to make of it. I took a few videos...

Here's the intermittent knocking before the engine is fully warmed up (1:47): https://youtu.be/jy_tROPRgqs

This is after the engine is mostly warmed up, you can hear the mild lifter tick plus the underlying knocking (0:50): https://youtu.be/EIORxiiyUtM

And this is after the engine is fully warmed up - the knock is still intermittent (0:31): https://youtu.be/WrLXKIHAHIY

Thanks for taking the time to help me figure out what's going on. I am so bummed, I did a total redo of the brakes and installed new wheels and tires too a few months ago, and I can't even drive the damn car
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Old 05-15-2019, 06:06 PM   #2 (permalink)
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Default Re: Help diagnose engine knock on '91 VR4

Rod bearing.... What you're most likely hearing, (and why it sounds like it's up higher) is the piston playing whack-a-mole with the head.

Another possibility that will make almost the same noise, is you dropped a foreign object (most commonly an intake lock washer) down the intake while servicing.
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Old 05-15-2019, 06:27 PM   #3 (permalink)
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Default Re: Help diagnose engine knock on '91 VR4

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Rod bearing.... What you're most likely hearing, (and why it sounds like it's up higher) is the piston playing whack-a-mole with the head.

Another possibility that will make almost the same noise, is you dropped a foreign object (most commonly an intake lock washer) down the intake while servicing.
Thanks Larry. I've had the oil pan off before so I can definitely get back to that point, just not sure about doing the rest of the job. I know everything has to be kept super clean. Aside from that, are the bearings a job that can be done by a do-it-yourself mechanic, without removing the engine? Would I want to do the full set of rod/main/thrust bearings? Or is my engine now damaged and needs to be removed and inspected?

Regarding foreign object...I removed the washers from the studs before removing the plenum (and then discarded the washers). I kept the intake covered at all times when working on the engine. Pretty sure that's not it, because I drove the car ~800 miles since the time when I last had the plenum off and there was no knock during that time.
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Old 05-15-2019, 07:05 PM   #4 (permalink)
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Default Re: Help diagnose engine knock on '91 VR4

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Thanks Larry. I've had the oil pan off before so I can definitely get back to that point, just not sure about doing the rest of the job. I know everything has to be kept super clean. Aside from that, are the bearings a job that can be done by a do-it-yourself mechanic, without removing the engine? Would I want to do the full set of rod/main/thrust bearings? Or is my engine now damaged and needs to be removed and inspected?

Regarding foreign object...I removed the washers from the studs before removing the plenum (and then discarded the washers). I kept the intake covered at all times when working on the engine. Pretty sure that's not it, because I drove the car ~800 miles since the time when I last had the plenum off and there was no knock during that time.
Can't hurt to pull the pan and diagnose the problem, but I'd be prepared for a rebuild. Crank usually gets pretty beat up when a bearing fails, and there's a good chance there's metal floating around that you don't want contaminating your repair,
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Old 05-16-2019, 09:33 AM   #5 (permalink)
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Default Re: Help diagnose engine knock on '91 VR4

damn, that sux! odd that a car with such low mileage developed a knock. Is your oil pan dented? I'd also suggest removing the top timing covers and have a look at the timing belt while the engine is running on the off-chance that the issue is a worn out hydraulic tensioner.
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Old 05-16-2019, 11:49 AM   #6 (permalink)
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Default Re: Help diagnose engine knock on '91 VR4

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damn, that sux! odd that a car with such low mileage developed a knock. Is your oil pan dented? I'd also suggest removing the top timing covers and have a look at the timing belt while the engine is running on the off-chance that the issue is a worn out hydraulic tensioner.
The oil pan was dented when I got the car but it wasn't terrible. I did remove the pan and hammer it back out. At that time I could see the oil pickup wasn't damaged. I did the 60K service about 1500 miles ago and installed a new OEM hydraulic tensioner. So it shouldn't be worn out but maybe it came out of adjustment?

There were a couple of vacuum lines switched back then (not sure if I did it or the previous owner) and that was causing pretty big boost spikes (leading to fuel cutoff) before I figured it out. Maybe that contributed to rod bearing failure?
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Old 05-17-2019, 09:20 AM   #7 (permalink)
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Default Re: Help diagnose engine knock on '91 VR4

Check the timing belt tensioner before taking the oil pan off. A failed one can allow the tensioner pulley bracket to knock against the tensioner body and sound very similar to a rod knock and be intermittent (Ive had that scare once). You can even run the engine with all the timing covers off to watch and put a stethoscope on the tensioner. Just make sure to retain the crank timing cog in some manner (such as put the crank pulley back on).
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Old 05-20-2019, 03:58 PM   #8 (permalink)
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Just drain the oil. If you see liquid gold in your oil, you have a rebuild or replacement in your future. If the oil is mostly metal free, check the timing tensioner as striker mentioned. Both are easy to check.
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Old 05-21-2019, 12:21 PM   #9 (permalink)
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Default Re: Help diagnose engine knock on '91 VR4

Thanks guys. I removed the upper timing covers and rotated the crankshaft...timing belt is tight and the timing marks line up fine. I drained the oil and I think it looks okay aside from being really dirty. There was just a bit of silver color draining out (see photo of oil pan drain). I didn't see any liquid gold color in the oil. Hoping to remove the lower timing cover tomorrow to have a look.



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Old 05-22-2019, 04:55 PM   #10 (permalink)
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Default Re: Help diagnose engine knock on '91 VR4

I removed the lower timing cover. There is almost no space between the tensioner and pulley bracket. Does this mean the tensioner has failed?

In order to start the engine to watch the tensioner, I assume I need to reinstall the alternator bracket, support bracket, motor mount, etc first?

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