New Timing Belt, How To Test Correct Synch?


#1

Please help. Mazda MPV, minivan, 1989, V6, 3.0L, 2wheeldrive, 133,000Mi, autotrans car runs great up 'til now.
Timing belt shredded, changed by local shop. Now when starting off there is low power, gradually builds up speed, automatic trans does not shift up as it usually did, then still pressing gas pedal trying to get up power & speed, RPM’s suddenly shoot up (4500rpm+) and there is a radically hard up-shift. Test drive done by a transmission shop (different place) says, he feels the transmission is not the problem; and that the engine has no power and the transmission is trying to compensate; he said it feels like a little 4 cylinder. Shop who did the work is slow to acknowledge a problem (they reset ignition timing; but no fix). A prior mechanic (unfortunately distant town) says symptoms sound as if timing belt is off by a tooth. He says he’s seen it happen; the car will still run, but poorly just as described. (Evidence is that this is not an “interference engine”.*)
(?)Without disassembling timing cover, is there a way to test/ confirm that the timing belt placement on cogs is correct or not? Thanks, TechKnot

  • Some say it is an interference engine, but found on web it is not. I heard no crunching noise when belt failed. Engine seems to run very well at 4500 rpm, once it manages to build up to that speed.)

#2

I believe that the covers must be removed to check the cam timing. And it certainly seems that the engine is out of time.


#3

The covers need to come off. And the it sounds like the belt needs ro be reinstalled correctly. I made the same mistake on my Camry. engine idled fine ran smooth but drastic reduction in power. And with a 1999 Camry 4 cyl. any power loss is very noticable.


#4

Yup…Tear it down…


#5

If the cam is phased correctly, the middle of the ‘overlap’, the part of the engine rotation where both the intake and exhaust valves are open at the same time should occur when the piston reaches the very top of its stroke.
I have used this technique many times to double check the phasing of the cam or to verify that the timing has or has not jumped.

Real world example, on my Kawasaki ZRX1200 motorcycle, the intake valves open 35 degrees before top dead center and the exhaust valves close 25 degrees after top dead center, that’s 35 + 25 or 60 degrees of crank rotation where the intakes and exhausts are open at the same time or “overlapped”. The center of the overlap period is not perfectly at TDC but if the timing is off just one tooth, it will be so off that the overlap will completely miss TDC.
If the crankshaft’s timing belt pulley has 18 teeth, each tooth is 20 degrees of rotation so one tooth off doesn’t make the timing a little off, it makes it 20 degrees off.


#6

Thanks: RodKnox, PvtPublic & Caddyman. The shop is willing to examine this again next week, but I was hoping it would be something simple, for their sake, and mine - maybe a vacuum hose. etc.

And thanks BLE. I am able to envision what you are describing. I gather there is still no practical way for me, or shop, to check that without the covers off this car. I found my old shop manual; I can count (small fuzzy photo) maybe 36 teeth on crankshaft; so 10-degrees per tooth(?). I presume it can still actually run with one tooth off in this case, ~~10degrees(?)
FOLLOW-ON QUESTION, anyone, please: If it were an “interference engine” and valves had hit pistons and been bent, would it be very obvious (engine seems to run smoothly still, in spite of its low power quirks). Should the shop test for impacted valvest also? (My first forum. Fantastic. Thanks All!)


#7

Yes. When an interference engine snaps a belt a competent shop should check for bent valves and this does not entail replacing a belt and then discovering the bad news.
Unfortunately, too many mechanics seem to think the belt has to be in place to discover this.

I’m in agreement with others about the timing being off. A tooth, or even two, can cause a dramatic loss of power. Not only the cam timing is affected but the igntion timing is altered quite a bit too.

What a shop should do after installing a new belt is rotate the engine through 3 or 4 times and recheck the timing marks before buttoning it all up.
The last step should be in inspection of the ignition timing. In theory, the ign. timing should not change dramatically but in practice it can change enough to make a big difference.
Unfortunately (again) many shops do not check the ign. timing and rely on it being a constant.
Hope that helps anyway.


#8

There seems to be an “off time” issue with this car. Something wasn’t lined up properly when the new timing belt was installed. The shop needs to redo the job and correct their mistake, don’t feel sorry for them. Someone doing your car either got sloppy or wasn’t qualified or improperly supervised to allow the motor to be put back together without being sure the timing marks were lined up.


#9

This issue has been RESOLVED. I posted the results on a new discussion titled:
New Timing Belt…Correct Sync? RESULTS
http://community.cartalk.com/discussion/2279272/new-timing-belt...correct-sync-results#Item_1
(Thanks, Car-Talkers!)