99 Blazer LIM Repair Killed a Timing Chain
 

OK Blazerites, Here's one I could use a little input on. I just got done doing the lower intake gaskets on my 99 LT 4WD. Put everything back together, changed the oil and tried to crank her over. Turned over but wouldn't light. Started diagnosing the ignition system using the procedures in the 2nd gen tech sticky posted on this forum. Turns out my distributor wasn't turning. After checking the distributor, the problem is deeper into the engine. Here I'm thinking that a socket or similar small object fell into an intake port on the lifter valley, got wedged, and snapped the timing chain. If you're like me, you had visions of bent valves and mangled heads going through your mind at that point.

So after tearing everything back down and performing a THOROUGH sweep of the intake ports and the lifter valley, I found no foreign objects. Sure enough, the camshaft and balance shaft didn't move when I turned the crankshaft. Looks like I'm doing a timing chain too.

The question remains though. Has anybody out there ever seen a timing chain cut loose like this. I always thought that they were a little more durable. Either way I'll check out the valvetrain extra careful when I get that timing cover off, but what else should I be on the lookout for?

That doesn't sound too good. Timing chains are very reliable, but when the do decide to go, it can be for good. More than likely, your tensioner went bad which allowed slop in the chain and accelerated the wear.

Hey Swart:

Thanks for the quick reply. Additionally, I performed one last check. With the distributor in (run the oil pump as well, see if that's causing any additional drag on the valvetrain), I was able use the balance shaft to turn the entire valve train through 360 deg with relative ease (well, as easy as a non-roller rocker engine allows) and no interference. Along the way, I checked for loose pushrods as a sign of bent valves. Looks like my heads were spared.

Putting together a parts pull list to tackle this job as well. You mentioned a chain tensioner. Is that a dealer only part? I'm having trouble sourcing it.

There was a revision in the tensioner to cure a noise from the engine within a certain RPM range. CLICK HERE for the TSB concerning the tensioner. You should be able to get the tensioner from a number of different online sources, but check with your dealer as well. And don't take their first price quote as gospel. Heckle with them. Check out GMPartShop.com, RockAuto.com, and GMPartsDirect.com for price comparisons.

OK, finally got the timing cover off and I have a pretty good idea why the chain went. I was expecting to see something like this:

http://www.jegs.com/images/photos/20...220-C3004K.jpg

Instead, I came across this wimpy little thing that was smaller than a bicycle chain. No wonder it broke! A quick assessment shows two broken teeth on the cam sprocket, and all the teeth on both cam and crank sprockets pretty worn. Not to the point of saw teeth yet, but definately getting there. So it looks like I'll be doing the entire timing set.

Now, question time Blazerites: I'm not going to replace with OEM just to have it fail again the same way (Actually this time I got lucky. The chain blew on startup before the engine could come up to speed). Anybody know of a more robust aftermarket product that will drop in and still let me use the crank sensor, sensor disk, and factory timing cover?

Thanks in advance...

Use the one from a 95-97 engine. They will bolt up the same. And its a lot beefier.

These timing chains can last the life of the engine. Yours must have had a defect or other outside influence for it to fail like that. Replacing it with a new replacement intended for your engine is advisable. Especially when you consider the tensioner design revision that took place.

OK guys, My parts for the timing chain replace landed today and I decided to post some pics of what came out and what's going in.

http://i691.photobucket.com/albums/v...comparison.jpg

On the left is a SA Gear #73112 that I had delivered from my local advance auto. I double checked that this product is a drop in replacement for my 99 LT 4WD. Note the link belt design. On the right is the broken OEM roller unit that was in the truck when purchased new in 1999. Note the sprocket teeth almost worn to points. And this was on a truck that used synthetic oil on every 3-4000 mile change since it was a year old.

http://i691.photobucket.com/albums/v...hain_break.jpg

Here's a close up of where the original chain failed. See the penny for size comparison. At time of failure, my blazer had 62000 miles. If you have a 98+ blazer, your entire valve train is hanging on less than 1/4 sq. in. of material. It is beyond me why GM would have changed from such a proven and robust design, to something as flimsy as this. It isn't in the picture, but 1/3 of the chain length away from this break there was a deep nick in one of the chain links. Combined with the broken teeth on the cam sprocket leads me to believe that the chain wore enough to jump a roller link and the resulting tension spike tore the chain apart. My assessment is that the chain as installed at the factory was grossly undersized for the load it was expected to handle

http://i691.photobucket.com/albums/v...comparison.jpg

Side by side comparison of new link belt chain vs. old roller chain. The new one has at least 4 times the material in it's cross sectional area, and 3 times as many pin shear surfaces. Link belt chains also have a limited form of self adjustment. Link pin joint wear causes the width of the teeth to spread, thereby sitting higher on the sprocket, and taking up some of the wear slack. It's not perfect, but it is far superior to roller chains where ongoing wear is nothing but a downward spiral.

I read through TSB that Swart was kind enough to link in his reply. First thing I noticed is that it addressed a noise issue, not a performance or durability issue. Secondly, to use the tensioner requires the use of a roller timing set. The TSB even goes so far as to state that if so equipped, vehicles with link belt timing sets should be changed to roller type. Doing this I believe would be actually decreasing the durability of the engine.

I am reluctant to put a roller chain back in that truck in such a critical location. I got lucky on this one. Real lucky. That chain could have failed at highway speed. I don't know if the 4.3 L V6 uses an interference valve design and I have no intention of finding out anytime soon. As for adding the tensioner, that truck ran for 11 years without one using that timing chain of dubious engineering origin. I'm going to bet my money on noisy and strong instead of quiet and unreliable.

A lot of s-10 and blazer owners ask if and when they should get their timing chains serviced. If your truck is equipped with a roller timing chain, keep a close eye on it.

The pictures posted in this message are the property of blazerforum now, and will reside in perpituity on my photobucket account. Feel free to copy and use these as you see fit, as long as proper credit to blazerforum.com is given.

Somehow... i have the feeling that we haven't heard the end of this whole thing...

As said, I do not feel that a roller chain is any less durable than the link type you posted. The factory used roller chains with a tensioner due to the possibility of noise from the chain affecting the knock sensors. I hope that you do not have any issues, just filling in some info.

As I said earlier, your failure was likely due to a bad tensioner which is what was addressed in the TSB.

I just rebuild a 99 and I had the bigger chain. When I ordered the new timing set from rockauto.com for a 99 the little chain and gear set came. I went and ordered a 98 timing set and installed it and the motor seems fine.