DE_Bag

Hi guys, I'm stumped on this one. 2012 GL450 with about 82,000 miles. This started about 1,000 miles ago and at first was so faint I thought I imagined it. At first it only seemed to happen when car was cold and first time accelerating. Now it is happening regularly.

Here's the symptom: On acceleration at about 26MPH, I hear a small thump from the passenger side front floor area. Sounds as if it comes from under the car or the right side of the engine bay. Almost sounds like a vent flap opening, it is that faint sometimes, but lately seems louder. The for-sure time that it always happens on first time accelerating after engine start. Now it also seems to happen in normal driving too, maybe not every time but only when the car hits 26 MPH, no other speed. Not road related and it doesn't matter if I accelerate hard or lightly.

Is there some event that always happens at about 26 MPH, like an electronic motor mount switching? Anybody had something like this? Car has been great otherwise, no major issues. Thank you in advance for any ideas!

RileyRRC

I have the same issue on my gl350. Mine happens between 25-30mph. At first I thought it might be something in the transmission but I have ruled that out. Mine happens at ever stop light when coming from a standstill.

did you get the issue sorted?

DE_Bag

Hey Riley,
No, I still don't know what it is. Since then though I also noticed it happens on deceleration sometimes at about 40 MPH.

This thing seems to be some type of computer-controlled event, given that it happens always at the exact same MPH.

kombifan

DE_Bag

It is kind of a thunk sound, but not heavy. More like if somebody tapped on the firewall with a rubber mallet if that makes sense. I don't feel anything at all come through the steering wheel or the drivetrain, but it does seem to come through the body of the car a little bit. As I mentioned before, it happens at exactly the same speed every time whether accelerating briskly or slowly. A couple of times I am pretty sure I have noticed it on deceleration at around 40-41 mph.

RileyRRC

kombifan

DennisG01

You can sometimes get lucky by pushing/pulling/wrenching on the end link and look/listen/feel for any play (with front end off the ground). Even the tiniest bit means it's done. Another way to check is to simply disconnect them and go for a drive to see if the sound goes away, or not. Just keep in mind that your front end will be sloppy when you drive so be careful and don't take any fast/tight turns.

DE_Bag

Riley, just want to clarify: is your issue just one single thump when it occurs? On acceleration, mine happens exactly and only at 26mph as far as I can tell. On deceleration, I have only caught it around 40mph. Do these symptoms match yours or do you get multiple thumps at different speeds?

In my case, I believe it has to be tied to a computer-controlled event, something that switches modes at 26mph.

DennisG01

What computer controlled event do you think it is?

This is just "thinking outloud", but... what about a vent/flapper for fresh/recirculating air? Maybe it's not closing/opening properly and the incoming wind is affecting it? Just for giggles, try with the HVAC off and then in the various modes. This may not be a 100% guaranteed way to find the problem, but it could provide the answer.

DE_Bag

Hi Dennis, I'm really stumped about it. Not sure what computer controlled event it could be but the reason I think it is is because of these conditions:

A. the thump happens at exactly the same speed each time, regardless of how hard I accelerate
B. When it happens, it happens only one time until I slow down or stop for a while
C. Road surface does not matter, it can be billiard smooth and I still get the thump
D. It is not apparent in the steering or drivetrain

My ideas so far are:
1: an active engine mount [on further investigation it appears our cars don't have this]
2: any active suspension components, like a stablizer bar that stiffens at higher speeds [I don't think we have this either]
3: HVAC - I have tried it with the system shut off, still happens. Maybe I should try it with the recirculate mode on too - I'll try that this afternoon.
4. Intake flaps sticking - but it seems like this would be more RPM dependent than MPH dependent
5?

I think tonight I'll try holding it in 2nd or 3rd gear as I go past 26mph. Any other ideas you have are appreciated!

DennisG01

I don't believe there are any computer controlled events that would explain this. The only suspension/undercarriage related thing is when the car lowers at highway speed. But that's just letting the air bags lower.

I think if it was the end links, you would notice clunks going over bumps. However, the end links are SO easy to diagnose by disconnecting (and they are very common item to fail), that it may be worthwhile to check them out... if for nothing else than to eliminate them as a variable.

I would defintely check out the area in the passenger-side firewall. Assuming this vehicle is like every other one I've worked on, that's where the fresh air is brought into the cabin, through the firewall. I'm "assuming" that there is some type of flapper/vent there.

RileyRRC

Mine only happens one time on acceleration. It is not rpm nor transmisson gear dependant. HVAC ob or off makes no difference. Air bags were replaced 15Kmiles ago. Total mileage is 84500, 2011 GL350. I'm leaning owards the sway bar ends. As mentioned previously, I do get slight clunking on speedbumps. From my experience with Range Rovers bushing start wearing out and mysterious suspension sounds start to emerge. Will investage the sway bar links and bushings this weekend.

thanks

91stealthes

I have a 2012 GL350 and have had the truck to the dealer regarding this. Unfortunately, I told them it was the transmission, because I notice the thump around 25 MPH on upshift and around 35-40 on downshift. I too only notice this when it is cold and during the first few minutes. Dealer said everything with the transmission is fine.

I rarely drive the truck (it's my wife's), so I don't experience it much. Glad I'm not the only one...now to figure out what it is. I'm still under extended warranty, anything you think I should have the dealer check to see if we can figure out what's going on here?

RileyRRC

I fixed the mystery. It happens due to the bolt holding the compressor bracket is loose or missing. Compressor turns on at 26 mph on certain acceleration conditions. My bolt on the bracket was causing the compressor shift when starting. Check the compressor bracket and bolts. That should fix the 26mph mystery.

DE_Bag

Riley, thank you so much - I think you nailed it! I have been systematically trying different things with HVAC and transmission shifting, but was probably on the wrong track and never thought about the airmatic compressor.

Here's an excerpt from the airmatic function description in Alldatadiy:
"If the system detects insufficient pressure in the AIRmatic central reservoir during driving operation, the AIRmatic compressor unit and the AIRmatic central reservoir charge valve (Y36y5) are actuated by the AIRmatic control unit and compressed air is supplied to the central reservoir.

The function is active as of a speed of v greater than 40 km/h (during acceleration) and up to a speed of v greater than 20 km/h (during braking)."

So I went out and opened the hood just now and reached down to the compressor. It's quite wiggly on its mounts, so I bet this is the problem. I will probably not be able to get to fixing it for a week or so but will confirm it back to this thread when I do.

C_R

De Bag,
Was this indeed the problem? Did you fix it? I know it's an old thread but I am resurrecting it because I came across it after trying to search for this problem after a long time. . Others who've had this problem, I'd greatly appreciate if you could chime in.
Kind regards

DE_Bag

Hi CR
Yes, The problem was definitely worn or loose isolators on the air suspension compressor mounts. I checked my records and unfortunately there is no indication of what parts I bought for the repair or where I got them. I traded the car a little over 2 years ago - my memory of the actual repair is a little foggy.

C_R

Thank you so much for the reply..! I'll definitely start looking into it. I did have my compressor changed and the mechanic most probably either forgot something or didn't tighten a bracket. While he was doing that, I had him relocate the compressor air filter into the engine compartment from beneath the fender liner. A very good decision. Easy access and won't be overlooked and that $5 part can be changed at every oil change.

mglsite

Mine would make a popping noise coming from somewhere in the passenger area in the first 1/4 mile after starting it, just one loud pop and nothing more. I looked everywhere and underneath it too. I found the culprit, it was a plastic water bottle in the storage compartment behind the passenger seat. When the temp would change it would pop. Cheapest repair I ever made.