NIACAL4NIA

Sorry English is my second language and I'm going to get a lot of hate mail regarding this topic.

I agree with John Taylor who posted this article. It's the Timing Belt paranoia originated online and parts sellers. My 1986 32 valve original belt was replaced @ 100k miles because I too got paranoid reading online tech tips, however the belt looked okay. My second belt (Contitech ) + after market water pump + one roller and other belts were replaced @ 159k miles after 8 years in 2003 and before I replaced it the tension was very loose but no tooth was jumped.

My car now has over 180K miles it has been 7-8 years in 2011 since the second belt. I drive less than 2000 miles per year.According to the service record During the 25 years the car needed one clutch, one alternator , motor mounts and other maintenance items such as spark plugs, brakes, fuses,fuel pump, coils, caps, wires, MAF, bulbs, tires, coolant flush, cheap Mobile oil 5000....etc. I really think replacing oil and timing belt goes by hours of engine operating not months or mileage, however too cold or too hot, humidity and the way you drive effects oil and belts, a defective water pump or roller is the culprit but fail 2%-4%, . High Torque HTD tooth profile belts are good for 8000-12000 engines operating hours and regular oil is good for 200-250 hours for our cars and synthetic goes 300-350 hours regardless of mileage and months driven. So if driving a Porsche 928 makes you paranoid then this car is not for you. Buy a 1997 Toyota Corolla which is my other car. The best selling, most reliable, and the most boring car in the world.

Midnight blue 1896 Porsche 928 32v 5speed. Southern California

Speedtoys

Paranoia or not..would you drive your 32v 928 to Tbelt faiilure..or..just replace it every now and then as something to do in the winter between football games?


Your points are kinda valid, but there is no set TBO on a belt, and you really dont wanna do a cost comparison between belt replacement and engine replacement...so what're ya saying?


You should get tons of years/miles outta the belt, until it fails.


?

Landseer

Believe what you want with data set = 1.

928 has an atypical system. 6+ foot belt. Wear-prone guide busings. Oddball tensioner that can leak cushioning oil out. Close tolerances. Water pump that has been documented many times to fail into the block, as well as bind.

Nice theory. Glad you are fortunate. Whatever "John Taylor" believes for typical cars, right or wrong, doesn't translate well to this car.


Now, for me it manifests differently. Its so easy to replace the belt, once you've done it, I'll just do it once every two years or so. Like a medical checkup.

NIACAL4NIA

A friend of mine who replaced his 30k timing belt and water pump after 5 years on a 944, he got a defective water pump that caused the new belt to brake after 5000 miles. So what are the chances of getting a defective part? Most of these service intervals are marketing gimiks to get to the dealer to see the newer models.

stolarzj

What are the chances of a new water pump failure on a 928? VERY HIGH!!!!!!!

SeanR

Not much of a chance to get a defective part if you buy from those that have taken the time to ensure a quality product is being offered. Hence our vendors.

On the timing belt, I've replaced 20 year old ones, that looked ok, I've also replaced 20 year old ones you could see though. This is one item that is not worth taking a chance on. It is one of the reasons a lot of these cars end up in junk yards.

SeanR

Got something to back that up with?

stolarzj

Tons of pages on here about water pump failures due to the impeller moving on the shaft. Should be less now with the new plastic impellers and the other project that was taken on to make them bullet proof should the pump lock up. That was only recent though, so there are tons metal impellers out there in use right now just waiting to fail.

Bill Ball

I'm more or less on your side. The Porsche service schedule is based on miles. There is no good way to track hours of use. I try to do my belt every 60K miles, as Porsche prescribed, but I do monitor its condition by checking tension and looking at the condition of the belt and rollers about every 15K miles. If you only drive 2K miles a year, then you would have a 30-year belt change interval. Hmmm.... I don't subscribe to the 5-year change interval which some others promote, but I do think the belt needs to be examined at some intermediate interval. Also, I had a Conti belt lose tension at 25K miles. The tension warning came on but nothing more happened. I changed it immediately. These belts should not stretch beyond the inital settling period (1500 mile check), so I considered this later tension loss a very bad sign that the belt was on its way to failing.

In general, I'm not all that paranoid about the belt, but I check it over at intervals as I said. Since I put on 10-15K miles a year, that's fairly often. I think people who only drive these cars 2K miles per year are in a predicament when it comes to maintenance schedules.

We have had some problems with defective or short-term failure of water pumps, so I tend to just examine the current water pump and leave it alone if it is operating OK and there is no evidence of wear or leakage. My original pump was found to be absolutely fine at 95K miles. There is no prescribed interval for changing the pump. People often have changed them with the belt as a WYAIT procedure which seemed like a good idea, assuming the replacement pump is at least as good as the original.

hacker-pschorr

+1

Friend of mine serviced his first 928 in 1984, has owned at least a dozen since that time and performed an untold number of timing belt / water pump jobs.

He only buys OEM parts, never rebuilds (for 928's), and has yet to have a customer return with a locked up water-pump or one where the impeller came off.

Has he seen this happen? Sure, from people who said he was too expensive and went else where. Usually doesn't take long for the car to show up on a flatbed.

It's not a marketing gimmick, and trying to prove this can get very expensive.

If a 928 is properly maintained, the belt should look like new when it's removed. Some could argue the belts can be re-used since the real importance of the job is inspecting the cam gears, tensioner and replacing the rollers and the pump. The belts are so cheap, might as well grab a new one.


Since I will never hit the mileage interval, my belts are changed every 5 years.

SeanR

The water pump issues stem more from people buying them on the cheap, you get what you pay for.

dr bob

The original belt life estimate was based on having all new gears, bushings, water pump, and all tensioner components in good condition. Great assumptions for a new car, and possibly or cars that are/were exclusively dealer-serviced.

I also suspect that driving conditions were considered, and the tendency is certainly to err on the cautious side with belt life estimates. Especially if there's a potential for warranty work getting charged back to the factory when a belt fails. In-town driving means more belt revolutions per mile. Higher engine speeds mean more wear per revolution. Higher ambient temps usually mean a harder worklife for a belt. An oil or coolant leak or spill onto the belt can make it trash in short order.

Many cars have the belts changed and a few already-worn (obviously worn) parts renewed. Enthusiast/conservative/informed/paranoid owners (that would be most of us cult members...) might be more religious about replacing wear parts as part of the service, even though the parts still appear to be OK. Then there are the rabid few, who decide that all things that the belt touches need replacement, and that there are improvements that can be made to the system to extend belt life. The Porken tensioner springs to mind.

The quality of the belts themselves has improved quite a bit sinc the first 928 engine was built with the extra-long belt.



So how does one rationalize or project the belt change interval with all these varying factors to consider? Who wants to do the run-til-it-fails test to predict belt life under one's own real-world conditions? Who can say that the failure data gathered that way would apply exactly to the situations of others? Hmmmm.

In the industrial world, maintenance and failure data is constantly shared among the users of particular equipment. A failure is looked at carefully, a root cause analysis is performed on the failed equipment. Other users compare the analysis data to their own circumstances, and estimate their exposure. We also carefully look at repair costs, loss-of-service costs, etc., and come up with a somewhat scientific estimate about ideal service and repair intervals. I suspect that the airlines and the aircraft maintenance folks are on the leading edge of cautious PM. The power industry isn't too far behind. I can generally tell you where loss-of-reveue insurance underwriters sit on these things too. We are all conservative.


So with all those factors to consider, it looks like I change the belt at about 40k intervals, at about 7 years spans. Yes, I could sell the old belts on e-bay, they LOOK that good. I'm due again sometime this year I think. I'll probably do the whole bearings and rollers set again, maybe a Porkensioner if I can figure out a reliable way to detect excessive roller movement, and new cam gears this time around. Everything will be clean and new again. And I'll be about 65 the next time this service rolls around. Two, maybe three more t-belt jobs on the 928 for the rest of my life. Doesn't sound too tough, especially compared to an old white-haired fart having to pull the engine to fix heads and replace bent valves. A few T-belt jobs sounds a lot easier to do. Especially considering I'd have to do the belt anyway with the heads refurbished.


So who wants to be the guinea pig on letting the belt go to 100k? NIACAL4NIA, you feeling lucky today?

NIACAL4NIA

Ok here is the math but please don't shoot me. My 1986 25 year old Porsche with around 180k has gone through 3 timing belts the last one done @159k in 2003. According to the forum suggest a 45k or 5 years interval, then 5 timing belts changed were needed on my car so I saved 2 intervals. 2x $1200-$1500 = $2400-$3000 if done by Independent Mechanic, which is the cost of a rebuilt engine on a 1986 with bent valves. What are the chances of getting a defective part? or an unskilled mechanic? It's kinda like playing poker. You should calculate the pot odds and outs? It's called risk management. A friend of mine was bragging about his 2003 Mercedes SL500 that uses timing chain and don't need service for over 100k but got an Airmatic light flashing and took his car to the dealer for a $4000 suspension job.

OOOOPS! My car actually went through two timing belts one replaced around 100k miles and the second belt around 159k miles and my car has around 180k on the second belt so I saved enough for bent valves.

Landseer

Good points. Espeically the roll of the dice with mechanics.

DIY, with knowledge backed by the sharing here online can yield completely different equations, though.

Top quality job w/ water pump is $600 and a long Saturday.

New belt --- $40 and a Saturday.

Belt tensioning / tracking inspection -- 60 minutes.

ANY Porsche 928 that makes it into my garage gets the covers pulled, BTW.
Lots of people think they can work on them by feel. Don't trust any that you bring home for the first time.

Rebuilding 928 waterpumps was supported by lots of knowledgeable 928 folks; chinese knock-offs were also a factor, both types failed, lots of them were apparently sold into the marketplace and using a rebuilt waterpump is not taboo outside in the broad world of cars --- so there are plenty on cars that will change hands. Beware.



Those Benz airmatics are ridiculous.

Mrmerlin

NIA are you a mechanic, how many 928 timing belts have you done?