Working on the earlier 104.994 is a job full of fun. As the earliest Motronic it retains some of the original old-time vestiges from 80s compared to the later updated models… It was my first time working on it and I had a great time.
My first experience on M104 was the 2.8-litre on Persian Kitty. Although it was still called 104.994 after 5 years later, it was a very different design to Nonna’s original 104.994 from 1993. Everything had gone electronic, the mechanical connection between the right foot and engine was gone. Even though it was a deep depth work, there was nothing worth recording on this site. OK, I am an early model whiner, in fact I am happy to see people chasing after the later models, so that the early rare parts will be more plentiful…
Nonna’s engine serial number was 000603, literly the 603rd. Doesn’t sound that early? The first 80 were mostly installed in test and homologation vechicles, then Sindelfingen was producing 150 6-cylinder cars per day from June 1, 1993. Nonna was produced on June 3, 1993, the third day after 104.994 entered the production. This resulted in a MY1994 engine that Nonna should definitely be defined as a MY1994, although it still has many MY1993 main components. In a word, Nonna is another MY1993 & 1994 overlap example.
When Nonna first off the trailer, the suspension was a mess, later disassembly proved that there wasn’t even one reusable rubber piece. The driveshaft hanger was rotten, the dangling driveshaft was rubbing violently against the exhaust heat shield. The engine were always idle at 1200 rpm, but there were no engine fault codes… a quick vacuum test revealed a broken joint on the plastic intake manifold. There was no condition or desire to do a high-speed driving test before full scale repair, but the engine was considered generally healthy. After the initial external inspection, some tricky parts ordering started, that was 9 months ago.
Once again, Nonna’s restoration has its own philosophy as Nonna is a tribute to my grandma. Grandma lived a frugal life and she didn’t like to waste. So I reused parts that still had a future as much as possible. Of course, frugality does not mean using cheap low-quality parts, because my time or life is more valuable than that money. In terms of total utility, we should not replace parts that don’t need to be replaced, and use serious replacements in all places where replacement is needed. I define these works as major service, not complete restoration.
However the depth of the work depends on the actual condition of the engine, which is lacking in driving test, and driving test doesn’t tell everything… that can only be learned from the disassembly. As usual, the exterior of the engine was stripped down to the long block, and then the skin was peeled off. The valve covers definitely needed serious attention, which were severely oxidized and cracked. Then there were the front and rear covers, both oil seals are leaking. Based on the findings during the disassembly: poor repair and lack of proper maintenance, judging by the main oil seal that was never replaced and the sealant that was abused, I guess the chain guide is also in trouble.
Although the odometer has 197,755 kilometers (this has not changed since I received the car in December 2023), it is unnecessarily true. I am not saying that someone has lower the mileage down, but the plastic gears of the mechanical odometer can break and stop from time to time, sometimes for many years before it get repaired. The engine details are more informative, at least judging by the almost complete destruction of the attachment parts, this engine looks to have seen more mileage. Of course, the tropical island where Nonna lived for 30 years also credits, maybe it is just that the humid and hot climate accelerates the external aging.
Surprisingly the cylinder head looked pretty clean after the valve cover was removed. I mentioned that the car was owned by a single company before me, it probably stayed with that company owner for all the years, but it was not well maintained. That tropical island is one of the least developed parts of the entire country, the first and the only dealer in next decade was established in June 1994, but it is not even close to Nonna. There is a crooked handwritten maintenance sticky note in the glove box, which looks like it was written by the driver, some words are spelling wrong… It is also one of the least educated provinces there. All in all, it is not easy to maintain this car there, but the cylinder head told us that the oil change was fine.
Objectively, I don’t like to touch the timing. But combined with the other poorly maintained areas, I didn’t think the chain guides were going to be healthy. I envisioned driving the Nonna a lot in the future, so I couldn’t gamble on the next major maintenance on the 31 year old guides. After removing the chain, I was surprised to find that the guides are almost no wear, with a mark of production year of 2004. That’s unnecessarily the service year, but at least someone had replaced it in or after that. I had already ordered a new set in advance to reduce the potential wait for parts, I decided to use the new parts anyway. But the new parts seemed to be less quality in comparison, those 2004 guides looked like they would last me a lifetime… In the past, I might have still put new parts. But now thinking of my grandma, I decided to stick with the old parts that looked tough. I found that my grandma didn’t make me a cheapskate, but she made me a wiser person.
Because the hanger is on the front upper timing cover, the engine was lifted outdoors for dry ice blasting to clean the external sludge before the upper cover was removed. During this period, I also tried to insert the dry ice injector into the spark plug hole to blast the carbon deposits on the piston, at least on the straight six it’s theoretically possible. But after spending a lot of dry ice, the endoscope told me that they only left a clean circle on the center top of the piston, the rest of the piston surface and the top of the combustion chamber were all black. I have never seen a 140 with so much carbon deposits, so I decided to remove the cylinder head to thoroughly clean the cylinder head and piston. Although I must say that I don’t like to remove the cylinder head either.
After the engine was disassembled, bolts and nuts were immediately packed and shipped to the supplier for processing. Despite this, four batches were sent because some pre-processing required the bolts to be fixed, such as the aforementioned brackets for lifting the engine must be fixed on the timing cover. The first batch was sent immediately after the initial disassembly. The second and third batch was sent after the engine was fixed to the brackets and the front cover and oil pan could be removed. The fourth batch was sent much later, actually waiting for the first batch to return and then found the missing parts, and then removed from the donor. In order not to delay the progress, some bolts were directly replaced with the same specification replacements instead of waiting for the donor. There are three types of treatments for fasteners. The first is yellow zinc plating, which is the most common. The second is chrome plating, which is intended to replace clear zinc plating because chrome is more durable. The third is black paint, which is a repaint after galvanizing. The purpose of the above colors is to restore the original color of the car.
Hopefully the depth stops there. I turned the crankshaft to check the condition of the cylinder walls, and it looked like only 2 cylinders had minor grooves, which was acceptable. So we found our way back, starting with cleaning the pistons in cylinders, repairing and installing everything…still called a major service in my mind. It took a long time to clear the carbon deposit, but it was very relaxing, I have to say that the action of cutting things with a blade is very satisfying. Judging from the scratches on the contact surface between the cylinder block and the cylinder head – it was definitely not the deep grooves caused by my little scraper, this engine had the head gasket replaced before, and the cylinder head was knocked during transportation.
Further cylinder block treatment included replacing the coolant plugs, there are 7 embedded ones and 1 spiral one. The spiral plug is the absolute weak point of all the M104, its threads are usually corroded, especially in tropical areas where water can be abused instead of antifreeze. This one was not spared, although it was not rusted through, it could not be rotated off intact. When you rotated it, it broke by socket. The black paint on the cylinder block had peeled off a bit, so I decided to paint it first, and then replace all the plugs. This way, you will be left with shiny plugs. All plugs are replaced.
Then the valves on the cylinder head were removed for cleaning. The cylinder head was thrown into ultrasonic cleaning. The carbon deposits on the exhaust throat were very stubborn, after a day of soaking and a day of cleaning, there were still some residues. Well, dry ice blasting was used again, but dry ice blasting could not completely exterminate them either, just weakened. Another day of ultrasonic cleaning, plus some brushes, they were completely removed. The carbon deposits on the exhaust valves were also very tough. Dry ice blasting could completely remove the carbon deposits on the intake’ valves’s, but it was almost powerless on the exhaust valves. They could only be sanded and polished off, and then the valves and cylinder heads were re-grinded to make them seal. After a few hours of grinding, the 24 valves and spark plugs were installed back on the cylinder head, gasoline was poured for sealing test, the job was completed in one go.
The valve seals were actually the only part of the cylinder head that was replaced. Even the valve tappets were flawless, which is impressive for a poor looking engine. Cleaning the sludge, which was not much, was not too much of an effort. The main annoyance of the cylinder head was carbon deposits, which are caused by low fuel quality. We are back to the island in the 90s before unleaded gasoline was popularized, there was a lot of RON 85. By the way, like other cars without three-way catalysts, the Nonna also has a low compression ratio engine, compression ratio of 9.2. Luckily not as low as the M119 or M120.
While working on the engine restoration, I pushed back the “dead work” as planned, such as sludge cleaning and glass blasting. Blasting the valve cover takes a lot of time, which was scheduled after we got stuck on the engine. Generally parts are not a problem, but from time to time we have to wait. This time the schedule was actually quite good, we had the cylinder head installed before the first batch of fasteners came back, and started waiting for the bolts to install the chain cover… In the meantime, we can start working on the chain cover, valve cover and oil pan. The ignition coil cover over the valve cover is interesting, the stamp indicates that it was made in August 1994, Nonna was built in June 1993… You may wonder why this unimportant cover was replaced, but the valve cover in poor shape wasn’t. Even if this part was replaced, it was not in good condition.
While waiting for the galvanized fasteners, the cylinder heads received new copper alloy screw rods. I made the mistake of tightening them too far inwards, so there was not enough length for the exhaust manifold to be installed later. This restoration was indeed a bit hasty, and there was a lack of photo documentation before installation. I was a bit rushed, desired to do as much as possible in each day, very soon I had disassembled everything before I could take a photo. I don’t take off my gloves after each part, take a photo with my mobile phone, and then put on gloves to continue disassembly… Disassembling a car is a frustrating process, the sludge and rust will reduce your judgment. That is, some parts are so completely damaged that you don’t know it is beyond repair until you spend some time to clean it thoroughly. Then when you find out that it must be replaced, you have already invested a lot time in it… Such is the torture of car restoration. No matter how well you prepare, you have to compromise with the actual condition of the car.
While waiting for the first batch of bolts, I also started repairing the power steering pump and the generator. I prepared a NOS power steering pump to save time, but that one was made by VICKERS, and Nonna’s original pump came from LuK, so I started repairing the original LuK. The repair of the generator started with determining whether the generator was worth repairing, and fortunately only two bearings and a plastic ring needed to be replaced. However, both parts had screws that needed galvanizing, which joined the fourth batch, and then a long wait.
The first batch of bolts came back so the timing cover and oil pan were installed. The top cover came from the donor, the original aluminum cover from Nonna was corroded a hole in the o-ring donut where it connects to the block, which would have definitely caused a coolant leak. Then came the new NOS fan clutch bracket, which was an expensive part (€400) but not worth the money. When installing the generator and power steering pump hanger, I discovered that the EPC catalog had some bolts positions and sizes marked incorrectly. Some bolt lengths were mixed up with vertical bolts, and since the bolts had been sorted and packaged beforehand, some time was wasted rechecking them. We had almost no bolt missing, but EPC had written the wrong lengths in at least 3 places, and some bolts didn’t actually exist on Nonna’s early 104.994…
The other parts went in quickly. The oil filter housing was original, disassembled, cleaned and assembled. There is a coolant pump from GEBA, a brand I have never dealt with, and it looked somewhere between Genuine and Chinese, maybe closer to the latter, at least the price is… Unlike the later models, early 104.994 retained the belt tensioner from the 104.990. The replacement part from URO was installed on Nonna before, which was the wrong one. Because the belt width on the earlier 104.994 was 6PK, but the tensioner from the URO is 8PK. It seems that since the tensioner and pulley are one URO assembly, the tensioner was also replaced… What a waste, only a new pulley was needed, is a Genuine pulley more expensive than the entire URO assembly? So the tensioner with the plastic pointer was removed from the donor and the new pulley was installed. The shock absorber on the tensioner is also updated.
The wait for the chrome fasteners was particularly long, they were in the second batch, but chrome takes longer than zinc plating. Fortunately, for the engine, the chrome fasteners were limited to the valve covers. The valve cover bolts and sleeves are clear galvanized steel (the sleeve became alloy in later versions), there were signs of rust, and they were located in the outermost areas. So chrome was chosen to ensure rust prevention. The chrome plating may be too shiny, so we considered polishing it with 800 grits of scouring pad. Before the chrome bolts were in place, I went to work on the brakes and front and rear axles.
Then the worst thing happened, the chrome parts were sent back without a single valve cover bolt! After that the supplier didn’t find any for a few days, we gradually found that the loss was not only that, but it was the most fatal to the schedule! There are 12 valve cover bolts per engine, to prevent thread damage, I also sent the donor’s, a total of 24, now they are all gone. If the bolts can be replaced by 316 stainless steel or titanium alloy counterparts as some kind of upgrade, the sleeves for are also missing 3 of the 12. On earlier 104.994 they are steel, the later models are aluminum, which of course cannot be chrome plated, so the donor 1998 S320’s can’t be used either… In order to keep the schedule and solve this problem later, I had to install 3 polished aluminum sleeves in the most accessible place, that is, the most conspicuous front of the engine. In the future, when the steel bushings are found again, they will be chrome plated and installed.
As the saying goes, when you see a cockroach in the room, there will be a hundred. Since our attention was entirely on the valve cover, fasteners elsewhere were bound to be lost, but there was no way to count the specific losses because I also sent in redundancies. The lesson from this incident? I don’t know, I thought I had done the best on my side, but it still happened. The supplier was more confident than I was, saying they would find a replacement soon, they had no idea what they were talking about. I could only suspend engine work and turn to the chassis so that I wouldn’t wait in vain on the way for the replacement bolts to come. We delayed the progress of the engine, but not the entire project. Three threads on the cylinder head that fixed the valve cover were identified as damaged and re-threaded. After getting the replacement M6x50 and M6x65, we finally put the valve cover on.
With the final fasteners in place, we finished the power steering pump, the alternator and installed the valve covers. The engine became pretty much complete. Many times special attention needs to be paid to the order of installation, and I am glad that I did not make mistakes in the absence of photos. This is the benefit of doing it all yourself, I once did some rework on the Octavius ​​because my employees provided some help in my absence and I could not keep up with it. Some items that need special attention include the interconnection between the belt tensioner and the power steering pump, and the oil cooling line under the intake manifold. The closer you get to the end, the more careful you need to be.
The early 104.994 is a cross between classic and new era, with fancy plastic parts added on top of the traditional design. Plastic isn’t the part I appreciate, because Nonna has a damaged plastic intake manifold, which luckily we found on the updated version donor. Updating the plastic parts is a must, and this time I added a little creativity. The traditional matte finish makes the plastic look dirty or lack vitality, so this time they are paiinted with semi-gloss black. The normal state of black plastic like this is that it looks like some sealant has been applied, not completely original but more attractive.
As for the finish of the aluminum cast parts, we were somewhere between doing nothing and the best . The cylinder head was just polished to make the alloy look fresher, but the other cast aluminum parts were sandblasted and painted with aluminum powder because they were heavily oxidized. Weicon’s A100 aluminum spray claims to have 35% aluminum, so the picky people could say we did 35% of the work… It is possible to make alloy brighter and duller by magnetic polishing, but they are not permanent. Generally speaking, a new generator carriage will develop indelible oxidation after 6 months of outdoor contact. I plan to drive the Nonna for a long time, so I protected the aluminum surface with non-natural paint. Of course it doesn’t look very original, but like I said, somewhere between doing nothing and the best. Appearance is always the most important part of restoration, for a daily driver I strive to achieve a balance between authenticity and longevity.
I worked 6 hours a day, when the engine stopped I was busy with the front and rear axles, and I was satisfied with the progress. If the chrome fasteners could be delivered more promptly, I could have finished all the work in a month, but that was difficult to happen. In the end, I didn’t fall behind too much, some part of it were even ahead of my schedule. According to the plan, we can complete all the remaining work before November and take Nonna to take some amazing photos.
The following is the main schedule:
08.13 Remove engine.
08.15 Disassemble engine.
08.16 Disassemble engine. Ship zinc plating parts out.
08.18 Disassemble engine.
08.21 Dry ice blast engine. Disassemble engine.
08.22 Disassemble cylinder head.
08.23 Clean cylinder block. Dry ice blast valves and oil pan.
08.27 Paint engine cylinder block. Ultrasonic oil pump, timing cover, exhaust manifold.
08.28 Polish valves. Assemble oil pump. Replace block coolant plugs. Ultrasonic cylinder head. Disassemble donor engine. Ship chrome plating parts out.
08.29 Clean cylinder head. Assemble throttle mechanism. Ultrasonic cylinder head, timing cover, attacment parts.
08.30 Grind valves and cylinder head. Paint timing covers, attachment parts. Ultrasonic attachment parts.
09.01 Assemble cylinder head. Install cylinder head, oil pump. Ship chrome plating parts out. Install timing chain. Disassemble donor engine.
09.02 Disassemble generator, power steering pump. Install exhaust manifold screws.
09.03 Glass blast valve cover, exhaust manifold, attachment parts. Paint valve cover, exhaust header, attachment parts.
09.04 Service injectors. Assemble valve cover. Glass blast attachment parts. Paint attachment parts. Ship zinc plating parts out.
09.05 Clean clamps. Remove donor’s intake manifold. Glass blast attachment parts. Paint attachment parts.
09.07 Sort returned zinc plating parts.
09.08 Paint oil pan. Install timing covers, rear cover and oil seal.
09.09 Glass blast attachment parts. Paint attachment parts.
09.10 Install oil pan, exhaust manifold.
09.11 Install coolant pump, intake pipe, oil filter housing.
09.12 Install belt tensioner, throttle body, hrottle mechanism, sensors.
09.19 Assembly the generator, power steering pump. Install the power steering pump
09.24 Paint intake manifold.
09.25 Repair oil cooling lines. Install generator, oil cooling pipe, intake manifold, injection system.
09.26 Repair cylinder head valve cover mouting thread.
09.27 Install heat shield, A/C compressor, valve cover, ignition system.
Our donor for this restoration, A129828, a 1993 300 SEL, made a great sacrifice. This car was acquired in Shanghai after I got Nonna. It was neglected as collateral in a warehouse for 8 years, had a mileage of under 120,000 kilometers. In terms of condition, it has more potential for restoration than Nonna, and even has more monetary value than Nonna at the beginning. Maybe others will choose it, but Nonna is my car. This car has a 104.990 engine, but many parts are shared with the earlier 104.994. The lower mileage retains a lot of original parts, such as the belt tensioner and plastic pointer that the Nonna lacks. Let’s remember its merits, it will still serve other cars in the future.
At the end are some camera photos, I didn’t record the process well this time due to tight schedule. If you look closely at the photos there are still some parts missing or not tightened. We are still looking for parts that the supplier screwed up…
