mb230sl

Hi guys,

I've been helping a friend register his S, having imported it to South Australia when he moved out from England. Yesterday was the big inspection day, and it passed with flying colours.

I'd previously driven the car several hundred km over the course of a few months - it felt very happy indeed, with no strange noises or any hint of issue.

So, having completed the inspection, we were idling along in gear, exiting the car park, when a subtle bearing type noise was heard, followed by stalling. It sounded like an engine accessory bearing. We attempted to restart - turning over smoothly, but laboured, and unwilling to start. I removed the accessory belt and tried again. It started, noisily, and I switched it off straight away.

The car was towed back to my workshop and this morning I began investigations. Brought it up to TDC, and removed the exhaust cam plugs - bank one was out by I would guess a tooth. Plugs out - leakdown test fail.

The car has 120,000km, and has been sitting doing little for 18 - 24 months. It had been well serviced before then, and showed no signs of distress whatsoever.

Based on the above, it seems a safe bet the IMS bearing has failed. Is this a fair assessment? My real question is - given the sequence of events, and the happening occurring at idle, how likely is it that we'll be able to get out of it by doing the bearing and pulling the head/s to replace valves?

I'm a mechanic, but specialise in classic MB and RR stuff. The world of Porsche is certainly a new one to me !


Thanks
Davide

Aspen Autosports

Pull the oil filter and inspect it for metal debris. If you have debris in the oil filter then chances are pretty good the entire engine needs to come apart to be properly cleaned to prevent issues down the line.

paulofto

Drain and inspect the oil and oil filter for debris. Pull the sump cover and do the same. This will tell you if the IMS bearing material has spread through the engine. If there is nothing, then there is a good chance that the failure, if that is what it is, was caught before significant damage was done. Doing forensic work is the only way to decipher what happened here.

This forum as well as http://986forum.com/forums/ have a ton of information and search is your best friend.

Good luck!

Schnell Gelb

The inspecting - don't just look at it !If there is debris in the filter, drop the pan also.
Wash any pan debris with petrol(gasoline). this will reveal the actual colors. Separate the debris into colors. Then use a magnet to separate the ferrous. Take a photo. We'll tell you which bits came from where.
If there is a significant amount of ferrous, it is probably the IMSB and unless you have the spin-on upgrade filter ,the engine will require a completer dismantling to remove the distributed debris.The debris will have gone through the bypass ! Yes, bad design- hence the spin-on upgrade comment.

mikefocke

You may find some help at http://porscheforum.com.au/. They may be able to refer to you to someone down under who knows the internals of the M96 engine and can diagnose. Cleaning an M96 engine is always a task for experts and even then there is risk. Oil passages especially in the heads are hard to get free of debris.

See if you can find someone who went to a "Raby" class.

Most replace with a motor from a wreck. Sorry I don't know the P-car wreckers down there.

mb230sl

Hi guys,

Thanks for the replies. The suggestion of oil/sump inspection is an excellent one, and my next step. I'm determined to diagnose this properly and if possible, repair rather than replace. The replacement thing seems to be the fashion, but I do wonder how many times it is needless, and if perhaps the recommission route might've made for a better and more cost effective outcome. Certainly, in the case of a MB V8 with timing chain/guide failure, my first instinct is top end rebuild rather than a used engine, which will need a new known timing chain etc to be safe, anyway.

So is there anything else that could have caused this? If a chain guide breaks, or a tensioner fails, can it introduce enough slack to cause a skipped tooth at low speed?


Thanks again

Schnell Gelb

Once we see the debris from the pan it may be possible to identify which components are disintegrating.Yup ,it could be a failing chain tensioner.
I fell into the trap of thinking that because I could rebuild engines like the MB V8, the M96 would be within my skill set,tools and equipment. I had a humbling surprise. The M96 was like stepping up to aeronautical standards .Yes you can slap one together with 1980's standard mechanics but it is inadequate. And the process is too expensive to mess up. I spent $7k on new/extra tools+equipment and upgrade parts for a total rebuild.

mb230sl

Unable to resist, I made it my first job today to drain the oil. Very little to report. One sliver of plasticy stuff that presents like shellac style sealant. One tiny bit of chain rail. Nothing caught in the folds of the filter. Very light bronzing in the filter housing, which may be an illusion of the plastic. But certainly, no discernible pieces of metal. Next up is valve cover off.

mb230sl

Rocker cover off. Chain and sprockets look ok. Bent exhaust valves, which was already known. That head's tensioner looks/feels ok. Tensioner guide pivots as it should. Piece of plastic found at the bottom of the scavenge port, as pictured. Some light scoring from the chain on the valve cover. Happily, no stray metal apparent, though.

mb230sl

Hi again,

Dropped the sump, and found these two (one in the baffle and one in the strainer), along with a few more specks of guide. Otherwise, very clean.

Given the relative lack of debris, could it be that the bearing stud has snapped?

mb230sl

apologies, but bump!

Macster

The lack of ferrous metal debris in the oil filter housing oil and filter element suggests the IMSB didn't fail the usual way, if that is any consolation.

All you know at this time is apparently a cam jumped time and some valves got bent. Doesn't have to be the IMS bearing, could be a problem with one of the IMS to exhaust chain drives.

To know for sure what happened you'll need to remove the engine and open it up to determine the cause of the problem and the extent of the damage.

You might find the engine is damaged beyond repair.

mb230sl

Hi again,

Today we removed the trans to inspect the bearing, which turned out to be just fine. We were quite disappointed as it at least would've been a definitive diagnosis.

Cams out, chain guides out - all looks fine. The upper rail material is black, as seems to be the IMS to crank guides. The origin of the piece of chain guide found remains a mystery, but it seems a safe bet it got stuck between the chain and sprocket and ended up where it did.

Other than bank one chain guides, and the guides for the IMS at the rear, are there guides other than those for the other bank?

Macster

No. There are the guide (or guides) and tensioner for the crankshaft to IMS chain drive, then each IMS to exhaust cam chain drive has its set of guides (and a tensioner) then each exhaust cam to intake cam drive has its set of guides which are manipulated by the VarioCam actuator located inside each exhaust cam to intake cam chain.

Doesn't have to be a failed tensioner/guide. A piece of hardware, a bolt (a failed rod bolt for instance), can work loose and get between the chain and sprocket and cause the chain to skip and the cam to jump time. If there is sign of the chain touching say the inside of the camshft cover this suggests then something got caught in the chain.

To find this piece of hardware can require further engine disassembly. While it might end up in the sump it might not.

mb230sl

Thanks for your thoughts. The piece of chain guide was in the scavenge port, so it seems likely this caused the timing to jump. There are corresponding chain marks on the cover, as you suggest.

It is a later engine with three chains, which makes the failure seem even more unusual.

I would love to be able to identify the origin of the plastic. Are the IMS chain guides white/yellow or black plastic? I notice the fixed cam chain guide is black, while the pivoting one is white/yellow.