Tensioner makes a knocking sound, common problem? what is the fix?

[RmvBfrFlght]

Part of the part number is the length of the belt:

Part Number: 640K6

Which is 64.0 inches. A 635 is 63.5 inches long.

RemoveBeforeFlight

 

[bison]

Bison, dang well I appreciate the knowledge!

Part Number: 640K6
Weight: 0.35 lbs
Warranty: 1 YR
Notes: Multiple accessory
Belt Construction: K-Section
Belt Material: EPDM
Belt Rib Count: 6
Effective Length (in): 63.98
Effective Length (mm): 1625.092

Get a 635. The 6 part of the # is the number of ribs. You want 635k6

 

[Ttype6]

The belt that works almost every time is the 635. Every now and then a 630 will go on. The stock replacement is 640 and 99 times out of 100 it will be too long after a short run time.

This is what I've found to be true. When this symptom occurs,I recommend looking at the top of the AC compressor for evidence of the generator hitting it.

 

[bison]

This is what I've found to be true. When this symptom occurs,I recommend looking at the top of the AC compressor for evidence of the generator hitting it.

The tensioner pulley will intermittently hit the stud that is on the right side of the AC compressor also.

 

[sakudog]

Exchanged & testing this evening, hoping the KNOCK KNOCK goes away!

 

[87gninpa]

Ive had great luck using 630K6 belts, a bit shorter that stock, but keeps the tensioner dead quiet.

 

[sakudog]

635K6 seems to have solved the problem!

 

[bison]

635K6 seems to have solved the problem!

Note the location of the tensioner pulley with the new belt. Now pop the old one on and see the difference. This has been a problem for about as long as I can remember and can cause KR. I always remove the accessories to check for loose fasteners. Anything rattling against the engine and accessories could cause KR.

 

[sakudog]

If I get time this weekend I will snap a picture and post it to continue to share the knowledge. Love this board and thanks Bison!

 

[liv4gnz]

i searched and saw this post, mine was doing the same thing and when i blew out a plenum gasket the other night, it was idling crappy and i c0uld hear the noise i thought was the alternator, i was thinking the belt was too long and not enough tension on the tensioner so when it would loosen the spring inside would hit 0 tension sometimes and hit the housing, as if it was released fully, i think i might have figured out my other false kr issue lol, gonna try a tighter belt tomorrow and check the bolts to the plate just to make sure, it seems to do it only at rough idle but when its floored im sure anything is possible, and it could be still hitting the housing

 

[Joeld]

I just went through the belt length issue myself, put in new belt a 40, within 10 minutes of idling I hear a rap in the motor. I'm like oh man my car just broke, pulled out the stethoscope and sure enough belt was too long and tensioner pulley was hitting AC compressor stud. Put in a 35 and perfect fit. Not too short and not too long.

 

[liv4gnz]

ill throw it in tomorrow, hopefully this solves my other false kr issue the more expensive other issue was my posi, when i floorit, it gets kr but in 3rd when i place my foot gracefully against the rubber sole of the gas pedal and ease into it slowly i get 0 kr lol ill try the 35

 

[liv4gnz]

i checked the belt out i already had a 30 on it, but it sat for 3 years with tension on it, i got another dayco in 30 since i have a ac delete pulley also and it was stretched about an inch! i have only a real slight kr issue now, but it definately took the slack off and tensioner wont bottom out now

 

[TT/Ameasap]

I think I just had this issue with my new setup. I swapped to an aluminum pulley and "threw" a used 630 belt on it. Was getting audible knock at 2000rpm and was thinking my converter bolts were loose. After thinking real hard about it, I decided to swap back to the old system I had. Now it is gone. I still havnt went WOT with it to see if my false kr is gone, but the ols system always had a high degree of KR on the initial hit and then went away. I think the belt I have on there is the culprit.

Now, my question is... Do you want the tensioner in the middle after the belt is installed?

 

[TT/Ameasap]

Ok. Got another belt on it that is tighter. Brings the pulley about a 1/2 from the coolant hose now. Before I had an 1''. I wrapped the throttle a few times and could see the pulley tighten each time. So, I must have been too loose and the tensioner was hitting the limit or something. I will report back and see if this fixes the false KR I get on the hit tho.