99 Caravan 3.8 loud engine grinding rattling noise near rear oil pan happens w/ac on

My 99 Grand Caravan with the 3.8 (97K miles) has developed a very odd noise--a loud grinding or rattle, like rocks in a KitchenAide mixer bowl. It happens only around 800 rpm when the ac compressor comes on. Once the engine speeds up, it's gone. It's usually quiet when the ac isn't running but not always, I can make it happen slightly if I load up the engine. Ocassionally the noise reduces, then comes back.

But it isn't the ac compressor! I located the noise and it appears to be coming from the rear part of the oil pan, from inside the oil pan! Very close to where the engine mates to the transmission. Boy does it get loud.

I'm thinking --ugh!--a rod bearing, but what would the ac cycling have to do with it? Does the load of the ac compressor just sufficiently alter the steady state of the engine and load up the crankshaft just enough at that particular rpm to allow it to fully bang? What would a bad rod bearing sound like anyhow?

Any ideas?

Mark W. in Philadelphia

Oh wow, de ja vu!

We, too, suffer from this strange noise in our 1998 T&C LX AWD with 3.8L and 94,500 miles. This noise has been there for the last two years to my recollection. It is loudest idling with the air conditioning on and then shifted into gear.

In all my attempts, I have not been able to locate this noise. It sounds to me like either some type of bracket either being cracked and broken or missing a mounting bolt, causing a rattling condition or one or more bearings going bad in a rotating element. Since operating the A/C effects the noise, this leads me to believe that noise is coming from a rotating element, either alternator, power steering pump, A/C compressor or water pump. Since this noise has started, I have replaced the water pump (it leaked) and both the idler pulley and the pulley on the belt tensioner (both of these pulleys are identical and replacable). Replacement of these items did not have an effect on the noise. Most recently, I placed my hand on the alternator mounting bracket (on top and behind the alternator) and could feel the vibration in sync with the noise.

A big problem with a vibrating noise such as this is that it can transmit throughout the engine, making it difficult to locate just by ear. I bet that both your van and my van are suffering from the same condition. If I knew just what was causing it, I would fix it and let you know the fix. Until then, however, I will keep monitoring this condition, waiting for something to degrade enough to help me identify the problem. I just hope that whatever the problem, it doesn't strand my wife and kids on the side of the road when it eventually fails.

If anyone else out there is suffering from the noise with their van, let us know.

Yes, it sounds like the very same problem. True, it is extremely hard to identify the noise, perhaps it is resonating through the engine, but I put my ear right up to the oil pan (almost on it) when it was running and it did seem to come from right there. But who really can be sure?

And yet the car keeps running fine. The AC works perfectly. I just drove it 100 miles today with the AC on, including stop and go rush hour big city downtown traffic. No problem. Like you, I am also very, very concerned about it stranding my wife and kids somewhere in the unknowable future.

Other thoughts I've had:

--motor mount--I jacked up the engine slightly, to take what I thought would be weight off the mounting. No change.

--removed and replaced the serpentine and tensioner, no change

--felt the power steering pump, seemed ok, no change

--tightened the center bolt on the crank pulley, it was already pretty tight, no change

-went to the Toyota dealer to look at an '06 Sienna!

maybe torque converter loose/damaged or likely(as was my case) flex plate cracked around the crank mounting bolts.But to see it the tranny had to be removed. And have you checked the timing chain for excessive wear?

Hey earlyrizr: Tell us about "your case" and don't spare any details. Thanks!

As KManiac requested here's my story in detail.I woke up one day and the van that is usually very dependable started to talk back to me..you see it's my daily driver and no one else drives it,so after a while you get used to it.So when any squeak/rattle or door not closing properly or anything unusual comes, up I notice it right away.I fix it myself unless it's something to do with the tranny(too time consuming) or the big computer related stuff.

So this one particular morning I started it up as usual,runs great,start to go down the block and it starts to make this noise(you know the kind that makes you turn off the radio,fan etc..so you can hear it better)I pull over to the side of the road open the hood and can hear this noise-like rattle.I look for small rocks lodged in the frame that are bouncing around no luck.I finish my day and return home for further inspection.I check the usual stuff, belts,pullys, w.pump (w.pump makes different noise than this when faulty)rack,power steering etc..all of it I checked and all was good.

Now I start to get concerned that it might be internal like a bearing or such,but in asking around the concensus was that the engine was fine and that it might be the tranny or related components but it still shifted fine,because it would do this stationary and driving.As mentioned before it ran great and still did,except for the rattle and yes the vibration.So the only other logical place that was not yet checked was the TC and flexplate,but I did remove the inspection cover to see if it was loose,no luck.

So I pull the tranny and what i found really surprised me,the flex plate has 5 bolts in about a 4in radius in the centre but the plate is about 18in in diameter.So what happened was the constant turning of the plate was enough to completely crack all around the outside of the bolts in a octagon pattern.(remember it`s not thick metal) The octagon pattern was key in that it was still able to turn the motor over and keep the crank sensor on place,but it was able to move back and forth causing the rattle.Once the crank bolts were removed it fell into 2 pieces.I replaced it with a new one and did the crank seal aswell as the external tranny seals(it was out so I did it now,but there was no prior leaking)put it back together and no more noise and no more vibration.

Not to say that is your problem,but when all logical items have been checked there`s not that many places left to go.So you have to start thinking of the unusual.As a side note I got the new plate from a dealer(wreckers wanted more than the dealer)and the parts guy said they sell alot of them.So now I know why.I hope this has been helpfull.

Thanks for the details. Sounds like your noise was louder and more persistent that what I notice with our van. Still, if this is a common problem, mine may be on it's way to what happened to you. By the way, what year and mileage is your van?

Does anyone else out there have any experience with these symptoms or this repair? Please let us know.

Intriguing, very intriguing. Yes, I bet that's it. But I do wonder why the AC cycling affects it so? Yes, the year, engine and mileage of your car would be good to know.

Now, a question, just how difficult is it for an advanced home mechanic to pull the transmission on a 3.8? '99 Grand Caravan? With no lift? With no helpers other than 7 year old twins and their 9 year old brother. Obviously I have ramps, a few jacks and hundreds of other tools. I've done it on a 240 Volvo and a Fiat Spyder in the past. I also removed and replaced the power steering rack on a '99 Volvo S70 once and did a timing belt on a '98 Audi A4 twice. I suppose enough unbolting (axles undone, connectors, linkages, subframe, engine I suppose somehow supported with a crossbar maybe and the thing pulled back and down) and it comes out. A transmission adapter for my floor jack or a dedicated transmission jack can be obtained or built. Any suggestions on this? Thank you.

99 3.0 and about 75k. For the advanced home mechanic with the twins getting you lunch and coffee and the 9yr getting needed tools it shouldn`t be to bad if you get the tranny adapter.As the AC could be any load maybe.Its not the ideal situation without lift(done both ways)lying on your back isnt great,but it can be done.

Had the very same problem on my 98 GC w 3.8 motor. I replaced the tensioner pully and that did the trick. You can get it at Advance auto for around $20. Have fun though the serpentine belt is a bear. Make sure you have a buddy to help with the belt.

one way to isolate if it's a pully,tensioner or w.pump is to briefly disconnect the serp. belt and run it for a few seconds to see if any change.

I too have a 99 grand caravan 3.8 AWD with around 98,000 miles and am having the same problem the noise only happens at idle or when the a/c compressor kicks on,if I give more rpms it goes away. On the first start of the day its really loud but after bringing up the rpms it's fine for the rest of the day unless I turn on the a/c and then it gets extremely loud. After trying to find it I am suspecting that it is probably the belt tensioner but am not 100% sure.

Wow, it looks like there are others with the same problem.

The first thing I am going to do is remove the serp belt, start it up and see if this effects the noise. I have a handy-dandy, 3-foot long, belt tensioner tool which makes removal and installation of the serp belt a quick, one-man job.

If the noise continues without the serp belt, I'm going to drive it up on jackstands, remove the torque converter inspection cover, have a second person in the drivers seat with the engine running, the A/C on, transmission in gear and standing on the brake pedal, while I look and listen to the torque converter and even flash a timing light on it to see if I can visually detect any adverse or irregular movement.

Next, I'm going to go talk to my transmission man. I want to see if he has found any cracked flexplates and how often. It would be worth the money to me to have him pull the transmission and inspect the plate, if only just to ease my mind. I don't even want to attempt to remove the transmission in this van myself, especially since I have an "all wheel drive" unit attached.

Speaking as a mechanical engineer, the resonant frequency of vibration of the rotating engine assembly consisting of the crankshaft, rods, pistons, flexplate, torque converter and vibration damper, is somewhere between zero rpm and idle rpm. If the flex-plate has a fatigue crack in it, it will vibrate the most at the lowest rpm of engine operation, which is cold idle, in gear, with the A/C on. This is when my van rattles the loudest.

If there is anyone else out there who wants to tell us about there experience with this problem, please add to the post.

I want to thank all of you for bringing up this subject and sharing your thoughts. It has motivated me to get my ....hands, and start diagnosing the cause of the noise.

This morning, I removed the serp belt and started the van. The noise disappeared! So I started to check the belt driven accessories for any signs of binding, noise or looseness. Everything seemed good until I got to the power steering pump. Although the shaft and pulley spun freely, without any wobble, I noticed that I could push and pull the pulley a small, but noticible distance, in and out of the pump. I don't think these pumps were designed to have noticible end play in the pump shafts. Once I reinstalled the serp belt, the noise was back.

Later, while driving the van today, and making a U-turn at the end of a cul-de-sac, I noticed that I could get the noise to increase in volume by pushing the steering wheel against the extreme turn stop while making the turn.

I think the problem here, at least in my case, is a power steering pump that has degraded a certain extent. I am going to look at replacing this in the near future. I will let you know what happens once I do.

After I removed the tensioner pulley I ran the bearing on my finger and it felt perfect. But as I mentioned before that was the cause of my noise problem because the noise is gone now after replacing it. You could only here the noise at a stand still with the A/C on or have the tranny in gear. Hey where do you get that handy dandy 3 foot one man belt tool from?? I really could have used it.

Guys, well I replaced the tensioner and idler, no change whatsoever.
Next I'll remove the belt entirely and run the car a little bit.

I have a trick for the serpentine installation. Take a 15mm box-end wench, attach the old serpentine belt to the unused end of the wrench (typ. the 13mm end), engage the wrench on the tensioner bolt, then run the old belt so it hangs over the front bumper. Put your right foot in the belt and step on it to hold the tension off the tensioner while you install the belt. To install, use a yard stick to push it down around the various lower pulleys, then attach an elastic bungee cord to provide temporary upward tension (so it doesn't fall off the lower pulleys; attach the bungee hook to the hood latch above your head) while you finish up on the upper pulleys. Ha!

Guys, I took it to Cottman trans. and I forced them to take the transmission off. They didn't want to, because, guess what?? the grinding noise stopped!!In place of the grinding noise, the car now bucked and misfired under load and could barely climb a hill. They said it was a fuel control issue and I should take it to a motor mechanic, they said it would be waste of money to take the trans. off it were just a fuel issue.

I pondered that and I drove the car around. Indeed the noise went away, but this horrible misfire which wasn't there before was really bad, then a trouble code popped through the OBDII, finally! At that point the power of the engine dropped off so much I could barely get up to 30mph on level pavement. Code P1391 which is a crankshaft timing sensor failure/anomaly code. Then I knew I was right. Why?

Because the flexplate has slots in it that are read by a hall-effect transducer (the crank sensor) which tells the main computer, the PCM, where the engine crank is. This is compared to the readings from the cam postion sensor. An anomaly will cause a fuel control/ignition problem since the PCM apparently depends on input from those sensors to time the fuel and spark. When it detects a misfire, the offending cylinder is shut down!! This is to protect the catalytic converter.

What happened was the flexplate was cracked, I believed, in an annular fashion right around the flywheel bolts. I hypothesized that it rotated around that crack, and eventually caught tight allowing torque to be transmitted but also causing a gross mistime for the PCM which in turn caused the improper ignition and fuel delivery, and which finally chimed through the OBDII.

Today they removed the transmission and my suspicions were confirmed. It will be reassembled next week and it should be fine.

glad you found the problem.diagnosis is always tricky,we know that there is a problem ,but the shops usually dont think so.frustrating at best.And while it's out you might aswell change any external seals just to be safe

the noise started about 2 months back, the van has 41,000 kms on it. and it has the same noise you guys talking about. One time there was a 01-04 GC by my side and heard the same noise my van has. Only happens with a/c on and idling. sometimes it does it too without the a/c but not too loud. van still have warranty on it so i will bring it to the dealer soon.

Well folks, I finally corrected the rattling problem with my Town & Country. Since I contributed to this thread earlier, I will post my solution here, too.

A guy I know who works at a local, independent auto parts store, got out his stethoscope and started searching under the hood while the van was running, in gear, with the A/C on (of course, I was in the driver's seat). He found the rattle at the serpentine belt tensioner. I promptly bought a new one.

When I attempted to replace this myself, I became very frustrated because I could not see or access the mounting bolt for this part. Mine has All Wheel Drive, so the PTU blocked my view of the bolt from underneath. Not wanting to get in over my head, I took the van to the local Chrysler dealer and paid them to replace it, making sure to get the old one back. The rattle has gone away. I am happy that this problem is solved and it wasn't as severe a problem as it could have been.

I hope this experience will be helpful to someone else out there.

Thanks to KManiac for bumping this thread up. I started a post on what sounds like the same issue and this thread sheds a lot more light on it especially if my first mechanic's word is correct that the knocking persisted with the serp belt off. Based on what I'm reading and what others are telling me, I now doubt that guy's word.

Someone above mentioned twins...mine are about to turn two so I'm more worried about them eating my tools then asking them to deliver them to me so I'm leaving the diagnosis to a second professional opinion. I will post what I find as soon as I get it.

Thanks everybody...

Hi, I have a 99 Voyager 3.3 148K miles and based on all the posts here sounds like my problem is the flexplate, too. Runs fine, and has that awful rattle usually when engine is cold and AC on. It has been doing this for months -- usually stops once car has been running for a while. So rather than wait for the failure I was just going to have this done -- especially before a big out-of-state road trip with the family!
I recently had the GATES Conversion Kit for the serpentine belt done (an EXCELLENT solution, by the way!) and it was still making the noise for a while after that. The mechanic that did that kit was concerned about the noise and when I told him about this flexplate suggestion he was able to put it up on the rack and confirm the noise was coming from the rear of the engine...
Well, now the noise has stopped!!! But does this mean the problem went away? I see some post from Woll1234 that mentions his stopped too -- of course I don't have the bucking and such, but that might be my lucky coincidence... Just wondered if anyone else had the experience of the noise going away, or advice if I should just proceed to get the flexplate replaced as I had planned. Thanks!

As I posted a while back on another similar thread, my case ('00GC 3.8, 90k) turned out to be the flex plate and it was diagnosed by my mechanic who said he could "see the plate striking the pan" as he turned the crank. I'm quoting him because I honestly don't know the first thing about that end of the drivetrain or how you make such an observation w/o pulling the transmission. I also don't know how he noticed it and a tranny guy didn't, but it turns out he was right. This came after replacing the tensioner, by the way, to no effect. Seems when it's all said and done, if you pull the serp and the noise persists, smart money's on the flex plate.

i had the same problem on a 99 caravan at 90,000 miles that only happened with the ac on and when you turned the wheels (put extra torqe on the engine with the ps pump). it turned out to be a cracked flexplate.

Just an update -- I went ahead and had my mechanic replace the flex-plate. It was confirmed! I wanted to see the old one so I could know what this was all about. It was cracked all around the bolts, in a circular pattern and would have failed soon! Now it runs nice (actually, it always did), quiet and smooth (overall vibration reduced significantly). I was especially perplexed about why it had stopped making the noise about 2 weeks ago... Apparently it had just gotten jammed temporarily in a certain position. So while they had the tranny out they went ahead and replaced the rear seal - an added benefit... The total cost was $425, including tax. The flexplate itself could only be bought at the dealer and was $65. So now I'm ready for the roadtrip...