Engine Knocking. Could it be the Piston? heres what happend...

[Al from jersey]

I have a 1990 525 with an M20 engine...i took a 15 mile drive and when i got to my destination my engine was making a loud tapping sound...first i thought i might be a little low on oil, i wasnt. then i figured it was the VALVES so the next day i checked the clearences,with help from brunos page, to my surprise all were about .25 mm and everything seems normal in there ( im no mechanic) i put everything back together and me and my dad took a good listen and hes convincing me that its a bad piston? what do u guys think? and what do u think its going to cost me?

Thanks any advice welcome

[632 Regal]

if its a piston theres a good chance you will be able to tell with a compression test. Visually check your camshaft for any funny wear, it could be that the followers are worn funky but seem to spec out when you check the lash. Is the sound a heavy or light sound?

[Al from jersey]

the sound is heavy, i can actually feel the vibration from each tap inside the car. the car runs fine tho and the sound almost disappears when im on the highway giving it some gas

[632 Regal]

sounds like a rod or more likly main bearing, I wouldnt drive it until you take it apart to save money from replacing everything unless you plan on replacing the whole engine. What brand and kind of oil are/were you using?

[Al from jersey]

i was running mobil 1 ..i have always ran full synthetic in it...i wount be driving it anymore, tommorow im having a mechanic come over and take a look and i priced the piston with rings and rod and bearings from BMA and its not over $200...hopefully the mechanic wount charge me too much for the labor and maybe if im lucky i can get away with just replacing the rod or bearing and keeping the old piston.

[s.c.estes]

Sounds like a rod bearing spun to me.

[632 Regal]

well he will have to pull the head off, thats gonna cost some labor cause as far as I know you cant pull a piston from below. Once he gets in there you might want to replcace all the bearings since its not hard to do when your in there. With Mobil one I cant see any bearing related failure unless you ran dry on oil or the oil pump failed. Did you over rev this thing and perhaps forget to mention that fact?

[Al from jersey]

i didnt over rev...i am a spirited driver tho..my tranny is an auto ...i did however forget to mention that my car has about 205k miles

[Rory535i]

Sounds like a rod bearing spun to me.

I agree with this. My brother had bearings go in his mkII GTi and it sounded exactly like how you have described. It had similar mileage too.

[s.c.estes]

I agree with this. My brother had bearings go in his mkII GTi and it sounded exactly like how you have described. It had similar mileage too.

I've seen (experienced) this twice. Once in a Honda and once in a Thunderbird with a V8. Needless to say, the Honda was eaiser to fix. However, fixing it did not just mean removing the oil pan and locating the spun bearing and replacing it. The engine would have to come out, the block, crank, and connecting rods would have to be ground, new bearings inserted with the proper dimensions, and everything slapped back together. If it is indeed a spun rod bearing, you're going to need to examine what might have caused this too. In my case, I was screwing around in the Honda and managed to redline it drifting around a turn. The Tbird, the throttle stuck open because the cruise control had a defect. I managed to fix both cars, but I was quite a bit younger then and my time wasn't nearly as valuable to me as it is now.