Timing belt tooth sheared/hesitation when warm

[griffincox]

Hey guys,

I am very frustrated right now. My M20 in my '90 325is developed a hesitation when warm not too long ago. It was basically out of the blue. After reading a bit I figured a tune up, valve adjustment, new valve cover gasket, new timing belt and water pump is a good place to start since it needed all of that anyway. Fast forward to tonight. Completed everything. Fired it up to start bleeding. Hesitated at first (changed fuel filter too) then ran as expected. Got warm and the hesitation was back so thats a little frustrating but as I pull it back in the garage I find a timing belt tooth on the ground. Brand new contitech belt from Pelican. So either I screwed up or the belt was defective (anyone heard of that?). I followed everything to a T and went over it 3 plus times. I pay close attention to detail. I am not an expert but did my buddies 944 belt recently and am savvy. Using google plus Bentley for help. What did I screw up? Any help is greatly appreciated. :(

[UserError]

From the fact that the engine runs, a sheared off tooth would be unlikely. If a tooth shears off, then usually a bunch of teeth shear off at once, and the engine makes expensive noises. To be sure of it, pull the alternator, fan, and shroud off of the engine, open the inspection cover, and turn it over by hand with a 22mm ratchet wrench to check for missing teeth. As for the hesitation, the first two steps would be the classic jlevie check for vacuum leaks and fuel pressure. Once those two are certain to not be a problem, I'd suggest an oxygen sensor, if that has any large number of miles on it.

[paperplane94]

My warm hesitation was caused by a leaky FPR bringing in too much fuel ti the mix.

[griffincox]

I have the tooth (in the trash now). The old belt was good, so unless it was from 2 changes ago and miraculously fell out now its definitely sheared. I will confirm before disassembly but I am 99% sure. It was making a few expensive noises at rev haha but couldn't tell what.

As for the hesitation, 02 sensor came to mind (190k miles) as well as fuel pressure or vacuum leak. The fuel pump makes some funny noises. It happened rather rapidly, does that help narrow it down at all?

[griffincox]

Quick question on belts. Should you be able to see the fibers on the belt edges of a contitech belt? The one that came off was all rubber everywhere including the edges. The new contitech has exposed/visible fiber around almost the entire edge of both sides. Clean with no fraying of course, but it is something I noticed then and want to confirm now what is normal. I will take pictures tomorrow or saturday when I inspect is the question is not clear.

[TurboJake]

Quick question on belts. Should you be able to see the fibers on the belt edges of a contitech belt? The one that came off was all rubber everywhere including the edges. The new contitech has exposed/visible fiber around almost the entire edge of both sides. Clean with no fraying of course, but it is something I noticed then and want to confirm now what is normal. I will take pictures tomorrow or saturday when I inspect is the question is not clear.

Yes, this is normal and nothing to worry about. I've done 4 belts now and each one has the core threading visible on the edges.

As for one tooth suddenly shearing/missing.... That would be a first for me hearing about it.. Either they tear a chunk, snap, or teeth get marred without actually coming off.

You can check to see if one sheared by just taking the top cover off, turning by hand and watching the cam sprocket. It will be easier than taking everything off.

[cabriodster87]

Remove intake/AFM, alternator bracket, and rubber cover, Voila.

[jeffnhiscars]

and what have you discovered ?

[Cabriolet]

Remove intake/AFM, alternator bracket, and rubber cover, Voila.

Dude, your bolt that holds the tentioner and the cover is on backwards. the 12mm side goes into the block and the larger flat side goes out to hold the cover...

[griffincox]

Discovered nothing yet. I may have time tonight to look. If not tomorrow morning. I will inspect it first. Hopefully find a belt in good shape. We shall see. I will update then.

As for the bolt/stud in the pic, Cabriolet is right, it's backwards. I will use that window to inspect though.

Thanks for the help everyone!

[cabriodster87]

Oh well next time, what that bolt doesn't know wont hurt. Thanks for the heads up, or backwards. lol
(kudos to previous installer)

[griffincox]

Well. Everyone was correct. I dragged the old man into the garage to watch the belt as i turned it by hand. It all looks good. The tooth must be way old. No clue what recesses it was hiding in but all is well there.

I also got a new O2 sensor, air filter and spark plugs. Will continue to diagnose the hesitation. Would you guys do a smoke test next? I have the intake system apart and all is normal looking except one hose of the idle air assembly has a small crack at the end, not enough to leak, but will replace it anyway. The rest looks normal. The AFM move freely. Have not gone much further yet. Read about potentially a temp sensor causing issues like this?

Any tips are appreciated and thank for all the input!

[jlevie]

Definitely have a smoke test run. But replace any iffy hoses first.

[jeffnhiscars]

Well. Everyone was correct. I dragged the old man into the garage to watch the belt as i turned it by hand. It all looks good. The tooth must be way old. No clue what recesses it was hiding in but all is well there.

Would have been easier to just carbon date the old tooth :)

I was thinking of starting a "Weird Crap I have Found in my Car" thread after finding a snake skin nested n my blower cover...right near the mouse nest that no longer had a mouse in it :shock:

[griffincox]

Been busy, but an update. It hesitated after plugs still. Then I put a new air filter and O2 sensor in, warmed it up again, finished bleeding, and the hesitation is almost entirely gone. It is drivable again is the main thing. There is still a little lumpy idle and an occasional hesitation and it turns over a few times before starting. This is all pretty much how it has been since ive owened it so not a big deal, but anyone have guidance on where to go next? Maybe fuel? Maybe vacuum? Maybe both?