I did a quick search and didn't find anything about this, so I decided I'd share...

I got my timing belt and water pump replaced today, and the mechanic found that my crankshaft sprocket was missing a tooth, thus causing it to have two gaps in it instead of one.

I'm no expert, but this is how he explained it: He said that the engine uses that gap so that it knows what position the crankshaft is in, which affects how the engine fires and what not, so he said that now that I replaced that defective sprocket my engine should run much smoother and much better in general.

My engine was running fine, but it got me thinking, about 3-4 months ago my gas mileage plummeted, and I couldn't exactly pinpoint why. I asked him if that would affect the mileage and he said it definitely would.

I just filled up today and we'll see how much it really affects it.

On a very positive note, I did notice that my engine is smoother and has a bit more pep to it

 

how many miles on the truck

 

The old, and streached belt, retarded the cam to crank timing which resulted in a decrease in performance and some mileage. The missing tooth would have no affect. The new belt restores the cam to crank alignment and will improve performance and mileage....some.

 

Well it had the first timing belt replaced at around 90,000 miles... and I just replaced that one now that it has 152,000.

 

maybe it was the missing tooth that helped you out who knows? or maybe they didnt really replace the old timing belt. lol

 

I'm going to have to disagree w/ you... A timing belt is not like your normal fan belt or any other belt on the engine for that matter. If one of those belts is loose, it'll slip, which isn't a huge deal, except that you get some excessive wear on the belt. A timing belt, on the other hand, CANNOT slip, for if it does it will totally screw up the timing.

Like I said before, the crankshaft sprocket has a gap in it (if it was totally dependent on the belt, why need the gap?) which the computer uses to determine crankshaft position. Mine had one tooth missing, thus creating a second gap, which would confuse the computer a bit.

I've been through about half a tank already, and I can say that mileage has increased dramatically, solely based on the miles I've driven, although I'll have an actual mileage for you all when I fill up again. The engine definitely runs better as well.

Another thing... if it was just a loose timing belt, wouldn't my gas mileage decrease gradually over time? Mine experienced a sudden drop, which to me I couldn't attribute to anything I had done different.

 

Not true at all. Missing a tooth will make a huge difference. I'm surprised it didn't have misfires or a check eng lite. The belt will not stretch so far as to retard the timing that much. If it stretched that much it would have broken

 

I don't get how the heck the computer knows where the gap is and if theres more than one.

 

the camshaft will be slightly out of position so the ecu will compensate that. you are lucky there wasnt any damage


hence, camshaft sensor

 

The results are in... Last summer I was getting around 20 MPG w/ about 50/50 city/ highway driving. Then around august or september my mileage plummeted to around 15. I couldn't figure out why. Then we started getting the winter blend gas and I was getting 12-13 MPG!!! The only thing that changed was that I was driving much shorter distances and more city... but I couldn't figure it out.

Well, I'm happy to report that this last tank (after getting the crankshaft sprocket fixed) I averaged 17.5, with winter blend gas.

Oh the joy.

 

X2

The timing belt has a ton of reinforcement in it so it won't stretch much before it breaks. You'd be lucky to stretch that belt 1/16" if at all. I totally agree as well, one tooth off will make a BIG difference in engine performance and fuel economy.

 

Guys...The crank shaft gear has 2 sets of teeth...the one the timing belt rides on, and another set for the crank position sensor. The crank sensor gear is SUPPOSED to have a tooth missing. Thats how it reads the crank position. If there was actually a tooth mission the crank gear, it would confuse the crank sensor and the truck would not run at all.



I bet the reason for the drop in MPG's and increase after the timing belt change is due to a loose crank bolt from the previous timing belt install. If the crank bolt was not REPLACED and torqued to proper specs, the bolt could have loosened up, causing the harmonic balancer to loosen up, followed by a loose crank gear, causing erratic signals to the crank sensor, causing poor performance and decreased MPG. Tightening things back up and realigning the crank gear would fix this. Was the timing belt previously replaced by the same shop? They could be covering up their previous mistake.

Did you hear any rattling or vibration noises coming from the front of the truck prior to this 2nd timing belt? That would indicate a loose crank pulley.


Something tells me your mechanic is a bit shady.

 

Why did you change the belt? Did it need it, or just routine maintenance?

 

I think you misinterpreted... the crank gear does have a gap on it (like you said) but mine had a second gap due to the fact that a tooth that was originally on it was missing.

And by no means is my mechanic "shady". It took me quite a while to find my current mechanic, and I have stuck w/ him because he is very good. I don't know everything there is to know about engines and what not, but I do know enough to know when someone is trying to screw me over or has no idea what they're talking about.

Apparently the missing tooth did confuse the computer a little, but not enough to cause any real problems.

Not tryin to start anything, but I find it very funny how many different answers come from people who "know" what they're talking about.

But that said, I appreciate everyone's input...

 

Just following the routine maintenance. People have different opinions on when you should or shouldn't do maintenance, but let's just say that I'm glad I did!

 

My bad. I must have read it wrong. Sounded like you said it was missing ONE tooth and they thought that was the problem. Then I put 2 and 2 together and it all made sense.

Again, my bad.

 

No problem man! I myself do that on occasion... Then I end up feeling pretty stupid after

Sorry if I came off a little harsh, but it wasn't directed just at you.