Courtshark

The TB light recently came on in my 88. The timing belt was replaced about 4k miles ago, and retensioned about 3k miles ago. I figured the problem was electrical-related because the light would go away when I shut off the car and refired it; the light would eventually return though, typically under heavy load. Then recently it came illumniated on deceleration, which seemed very odd. I still thought it was electrical, or maybe a low oil level in the tensioner system.

Turns out that the belt was actually loose. No damage done to the engine as far as I/we can tell. It was way out of spec after only 3k miles. I don't yet have the tool to check the tension myself; I will be investing in that soon. My mechanic (Porsche specialist, but not a 928 specialist) had no explanation for why or how it could get out of spec like that, especially so quickly; at least he didn't charge me for the re-retension (he did the work the first time). Any ideas? TIA

Jim R.

If the belt was re-tensioned already, and is loose after another 3,000 miles i would be concerned that something else is amiss. A belt should hold tension well after the initial stretch. Look at the whole tensioner and roller system for any bending/bent or loose bolts/hardware, specifically where the main tensioner roller arm attaches at the water pump.

I had to retension my belt once after 1200 miles (not loose enough for the light though) the last 2 yearly checks have been spot on with no need to adjust. I'm pretty sure most belts behave about the same as that after that first re-tensioning.

Better to look closer than to just tighten it up again, but that's just me........

Jim

the flyin' scotsman

What else was replaced when the tbelt was swapped out?

As Jim says there are other components that may be wearing now that you've got a new belt on and properly tensioned. Time for a PorKen tesioner solution?

Courtshark

The entire engine! It's a freshly rebuilt long block from an 89. Engine swap done by a 928 specialist, too.

rixter

check to make sure the tensioner is flat against the block, if it isn't it'll leak and you'll lose tension...

the flyin' scotsman

Specific to the tbelt:

tensioner, rollers, water pump, crank, oil pump and cam sprockets.

Were all these new with the long block or swapped from the original?

Mike Frye

I read a post on here where someone (Bill Ball?) found that a belt he'd replaced needed adjusting more than one-time the initial wear-in period and he decided it was a bad belt and just replaced it. If everything else looks good and the belt is tracking true, I'd swap out the belt.

Vlocity

+1

I am looking at a similar problem and hope it's just the belt.

Ken

Mrmerlin

I would take a very close look at the water pump, this part needs to be checked with the belt off of it, you want to look for a leak , probably not, also how smooth the shaft turns and finally if there is any top to bottom play . It stands to reason that if your install was done correctly and the tensioner was rebuilt and isnt leaking and you replaced all of the rollers and the cam, oil, and crank pulley gears are all in good shape the only part that can lose its strength is the H2O pump, the H2O shaft axis will start to bend in an upwards direction, and the impeller will start to grind away on the front of the H2O relief in your block

Mrmerlin

as a reference you can take the old belt and measure it against the new one , if you still have the old one, The belts are not rubberbands, they are made from very strong materials and meet very specific tolerances. As such if you think that the belt has streched get a new one and make sure you buy the belt from a different vendor , one of the our online supplyers is the best choice. The belt will see a bit of stretch in the first 1500 miles or so but after that it should stay tight and so should all of the the rest of the belt run , Now if the belt is overtightened this will possibly damge the rollers,cam/oil/crank pulleys and lastly the H2O pump shaft

Courtshark

I'll have to check the records from the job; the engine rebuild/replacement was done by the PO, but at a nearby mechanic who I know to be a 928 expert (he drives one, too). I'm at work, but I'll check tonight and post what I find.

Rick Carter

It is risky to drive it until you are certain everything in the tensioner system is OK; best with all new parts, cheaper than bent valves BTDT.

JHowell37

Personally, I would get a new belt to install since at this point you're going to have to remove a lot of stuff to check the condition of each pulley. I guess I've lucked out with mine. I've got about 2000 miles on my belt since I replaced it last year and haven't seen a warning light for the tensioner. That actually concerned me so yesterday I unplugged the wire and grounded it out while the engine was running to make sure it worked (it did.) So I haven't tightened mine yet. I'm kind of torn about doing it because I don't really want to mess with a good thing.

mark kibort

the tensioner oil has nothing to do with static tension.

mk

Courtshark

Fortunately (?) for me a mechanic did the work on all fronts, so if something goes wrong, at least it won't come out of my pocket. I'd love to replace the belt now, but the mechanic retensioned the belt today and he says it's good to go. I'll drive it around for a bit, including to the 928 Frenzy this weekend, and see how it goes. I'm guessing it will be good for another 2k miles before the light returns. Probably just a bad belt... I hope.