Brinkley

Hey guys, I have a difficult question and hope some of the tenured mechanics or shade tree guys can give me some thoughts on where this problem might be coming from.

Here is the scenario: The motor is a '05 996 3.6L. It is mated to a 2000 Boxster S 6 speed. When the motor is running, either in neutral or with the clutch in and in a gear, the motor sounds normal. Then when you put it in 1st and start going under load there is a tapping sound coming from the passenger side of the engine compartment. At first it sounded like a rotational noise from a half shaft. As you go faster the tapping speeds up as you would imagine if a nut/bolt/part was hitting something as you role down the road. However when you are rolling and put the gearbox into Neutral and get off the gas the noise goes way. So its not something in/on the half shafts as we originally thought.

Put the car in the air and ran it up through the gears and NO tapping. The tapping only occurs when the car is on the ground and when you are in gear and on the gas.

The tapping occurs in every gear as long as you are on the power. Noise goes away when your coasting and not on the gas.

I know it might be impossible to diagnose a car you have never seen or heard but I am hoping someone might have an idea of where to start looking.

Any thoughts?

jrgordonsenior

I would try running it up on a elevated dyno. There you might be able to indentify the tapping noise under load....

Brinkley

JR,
I don't believe there is an elevated dyno anywhere near Tampa FL. but I'll search it out. someone thought it might be a lifter?
Do they make noises as they go bad?

Larry Herman

I'll take a flyer on this one; check your motor mounts. Could be that the engine is torquing enough that something rotational is hitting under hard accleration.

jrgordonsenior

A bad lifter would be noisy all the time especially when cold.

Dynojets are sometimes elevated as they use 44 or 48" rollers. Many operators out here use a 4-post lift to bring the cars level with the rollers. Here's there link for Florida Dynojet operators, start calling.....
http://www.dynojet.com/dyno_centers/....aspx?state=FL

PS: Also check your tranny mount....

MUSSBERGER

I think Larry is on the right track.

d15b7

hi. I agree with Larry very strongly -- broken motor mount, or broken mount bolt, or collapsed/disintegrated rubber in motor mounts. it's happened to me (not with a P car).

Todd
ReidSpeed

Brinkley

guys, thanks for the thoughts. I'm still searching for "strike marks" around anything and everything that rotates. I've looked searched each half shaft on both the trans side and wheel side and no marks. I've looked at the belt and pulleys and see no marks that would indicate hitting under torque. That's all the rotating parts I can think of.

How do you check the front motor mount to see if its good/bad. Is dropping it out the only way?

Spent the entire day under the car and no marks. Any other areas I should look???

Thanks everyone.

Larry Herman

One thing that you didn't state but it was assumed, the frequency of the tapping increases with engine RPM, meaning that it is before the ring gear/pinion connection and not with road speed, right?

J richard

Sounds like you've potentially got something going on in one of the rear cv's. The load/no load is suggestive of that. I'd first check to make sure the allen bolts are all tight and the joint isn't wobbling/slapping against the output flange, then check the rear axle hub nut and make sure the axle isn't loose in the hub. Also the rear squats under load, could you have a tire thats catching something like a fender liner under accell but is fine coasting along...

Brinkley

Larry,

"One thing that you didn't state but it was assumed, the frequency of the tapping increases with engine RPM, meaning that it is before the ring gear/pinion connection and not with road speed, right?"

Answer: tapping increases with engine RPM but only when the engine is under load. When you are sitting on the ground and in Neutral and rev up the rpm's, no tapping. If you are under power going down the road and on the gas, tapping but as soon as you come off the gas, tapping goes away.!.

J,

"Sounds like you've potentially got something going on in one of the rear cv's. The load/no load is suggestive of that. I'd first check to make sure the allen bolts are all tight and the joint isn't wobbling/slapping against the output flange, then check the rear axle hub nut and make sure the axle isn't loose in the hub. Also the rear squats under load, could you have a tire that's catching something like a fender liner under accell but is fine coasting along..."

Answer: both cv's are brand new units from the dealer, zero minutes on them, All allen bolts on the inner joint have been re-torqued and marked to confirm location. Re-torqued axle hub with four foot torque wrench to, what 300 lbs, they are both tight. regarding tires, they are Hoosier slicks and show no rub or scoring so I don't think its a tire rub.

The noise is very metal on metal sounding. But at the same time I was wondering if its the sound of a cracked header?????

Thanks again for all the thought and input. I'm at my wits end.

Van

How about reverse?

(I expect it would do it then, too...)

Does it happen only under full load, or any time you're in gear?

sbelles

If it was the motor mount, you would most likely hear grinding from the CV joints on cornering. Remember our experiment?

Viperbob1

Your motor will still be moving and torquing over with or without a load. If you were watching from above or underneath on the lift a broken mount should be evident when revving. But in your car the rear mounts are right there so you should be able to do a visual.

Might want to check the axle bearings, and also the tranny. I have seen the bearings in the tranny go bad causing a reduction of the preload. In essense the diff inside could literally move and bounce around. You only heard it under load though. Just a thought. Do you still have the factory diff in there, or do you have a LSD? Pulling an axle and the side cover is pretty easy. You have to pull the right axle to get the side cover off. Also make sure that there is no scaring inside the upright/wheel carrier or on the axle.

onefastviking

Lots of good advice already given, here is the approach I would take. But first, ask yourself what was the last thing that was repaired, maintaned, removed and reinstalled, or upgraded BEFORE this noise started happening. - Usually this is the best place to start looking. I seem to remember you discussing exhaust headers and pipes not too long ago, did you change anything there ? Many times the exhaust pipes rub on the inside cv joint under load, turns, etc. After that......

A real good visual inspection. Look for any rubbing from the exhaust around the axles, pull the wheels and check for any rubbing marks from tires or brakes, etc. Then check the two transmission mounts off the triangle brackets of the transmission. Presuming you have stock transmission mounts I would first check for any leakage from the mounts, then check to see how much they move if the transmission is jacked up a few inches, is all the movement in the mounts ? Then the front engine mount, this is important to check the rubber mount itself (kinda hard to see) as well as the metal brackets and bolts that hold it to the engine and the body.
If everything checks out ok there and still no signs of rub or noise then I would pull the oil filter, engine oil, and drain plugs for transmission and engine. Closely inspect these for any metal particles, catch the first quart of oil from engine and trans oil, let settle and then check for metal, cut open the oil filter to see inside the folds of the filter real well. See any metal anywhere ? If so then inspect where you see the metal MUCH more closely.
You stated you replaced the CV's with new units. A few things here, did you get the left and right axles in the correct positions ? They are different left and right and I don't remember off hand if you could put them in wrong and cause one to bind. If you are lucky you can read the part# labels on the axles easy and this will be an easy check. Also, the CV's can get in a bind and when the cage rotates under a load they can cause noise and eventually crack the cage. If you repacked them with fresh grease and removed the ***** and cage the cage can be put back together clocked off from the center of the joint and this could cause noise also, not to mention a quick failure. When you replaced the joint did you get the retaining clip back in the proper position ?

Well there are the simple obvious things that come to mind. Pulling the LSD is a thought to check as well. So is a potential clutch spring, pressure plate drive strap issue, but these require trans removal for further inspection.

Let us know what you find, and also what was done to the car just before this noise started happening, it may help rid the gremlin from your ride !