Can A Timing Belt Being Off By A Tooth Affect Compression?

doug2060 · 01-08-2009, 09:49 PM

1990 Geo Metro
1.0 3cyl.
Manual Trans
162,000 mile

CAN A TIMING BELT BEING OFF BY A TOOTH AFFECT COMPRESSION?

I just replaced a timing belt on this car and now the compression is around 125psi on all cylinders. 2 weeks ago it was around 175 each.

It's too cold to go out to mess with anymore tonight so i thought I would ask...

Rooy · 01-08-2009, 10:21 PM

Yes it could.

doug2060 · 01-09-2009, 01:07 AM

The weird thing is that it doesn't seem to have much power loss. The compression just shows up much lower. 50psi per cylinder should be noticeable. Weird...

leonbentz · 01-09-2009, 01:32 AM

yea, it's a good thing that Metros are a non-interferance engine. Otherwise, you could bend a valve, by missing timing marks. Fortunately, they have a ten dollar belt and are quite forgiving

Gotta love the Metro. You're only running 175 pounds per hole? Must be an XFI model, right? I believe the LSI's are supposed to run at 199 psi.

doug2060 · 01-09-2009, 03:13 AM

Non XFI, just old.

I'll check it out later. I'm pretty sure the belt was right but who knows? It was cold and I really just wanted to finish up and get inside. I did warm it up before testing compression. I checked it because the idle seemed different but not by much. I just wanted to find out if it was off if it would affect compression and 50 psi is a lot. If it is off it's by 1 tooth.

DOCTORBILL · 01-09-2009, 11:48 AM

Could being off by one tooth ( ± 5 degrees ?) cause any of the valves to get burned ?

Wouldn't the valves be opening or closing before or after they should be?

DoctorBill

Woodie83 · 01-10-2009, 06:55 AM

Valves would be opening and closing either before or after when they "should be" and yes, that could lead to burned exhaust valves. "Should be" is not a firm number though, valve timing is a variable that can be moved around just like ignition timing to tune the engine and give it different driveability characteristics, the only difference is they decide at the factory and lock it down to a specific number. Advancing the valve timing will give you more horsepower and less torque, retarding it will give you more torque but it will run out of steam at high rpms. (possible I have that exactly backwards, but I think that's right) Problem is, one tooth is far too much to adjust it, it's about 20 degrees and 10 is as far as you'd ever want to go. There are adjustable sprockets available for this very purpose and the newer cars with the spoked cam sprocket already have two settings built in, one is marked "E" and the other "I". Can't remember which is which and what the effect is supposed to be, I've read it on TeamSwift.

Compression numbers when cranking the engine on the starter are not the same as they will be when the engine is running. Things are happening so fast that a cylinder can built up some compression even though theres a big leak. That's why cars with 100psi compression are hard as hell to start, but will run fairly well once started. That 50 pound difference is probably 20 when running.

leonbentz · 01-10-2009, 01:08 PM

162,000 and your compression is that low? Wow. I pulled mine down at 207,000 and it had much better compression than that. I only tore it down because all the seals went tits up and the front tranny seal did too.

Rooy · 01-10-2009, 01:11 PM

Quote:

Originally Posted by Woodie83

Advancing the valve timing will give you more horsepower and less torque, retarding it will give you more torque but it will run out of steam at high rpms. (possible I have that exactly backwards, but I think that's right) Problem is, one tooth is far too much to adjust it, it's about 20 degrees and 10 is as far as you'd ever want to go.

Yeah, you have it backwards. Advancing it will move the power band down, retarding it will shift it up.

+1 on the adjustable cam sprocket. One tooth is way too much.

doug2060 · 01-10-2009, 01:36 PM

175PSI is not that bad on a 19 year old 3cyl metro. 199 is what it is new and the minimum is 156. I've seen much worse!

leonbentz · 01-10-2009, 01:58 PM

Yea, I can believe that. Most people who drive these things don't use good oil in them and they wear out quicker. At 205,000 I was running abouut 187psi and better. Someone must have taken care of it. Mine is a 1991 LSI

doug2060 · 01-10-2009, 02:53 PM

You're lucky. I also have a 91 automatic. Before i put new rings and valves and all those goodies in it the compression was #1=148psi #2=128psi #3=157psi. Cylinder 3 would have been much lower but oil was coming into it steadily. It would foul a spark plug within 1000 miles. It's running great now though. I seem to be collecting these things now. LOL.

leonbentz · 01-10-2009, 03:21 PM

Yea, mine is an automatic too, plus it has air, which works good. I love the little shit. I bought it with 178,000 on it and it was quite peppy for the size engine it has. When I pulled it down, the ring end gap was still .016 and the clyinder walls were taperless. So, when I rebuilt it, it really didn't make any improvements on it, but at least, I know what I have and I put a jdm tranny in it.

Did you get your car running better?

doug2060 · 01-10-2009, 04:14 PM

I just got in there messing with it. I believe the belt may be stretched. No matter how many times I line it up, the notches just won't stay lined up exactly. I can line up the top but the bottom looks half a tooth off. I'm going to buy another belt later. The belt is less than a year old. Has anyone had this trouble. It's strange for a fairly new belt to stretch but even the alt/ps belts on these things have that problem.

Rooy · 01-10-2009, 06:44 PM

I'm not sure that a belt that has stretched would cause that. Has your head or block ever been shaved/decked? Or do you have a replacement head that could've been? That will change the relationship between the cam and crank and cause the timing marks not to line up exactly.

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