97 Camry 2.2L -- timing belt set right?

[Paul Milenkovic]

My 1997 Camry has 160,000 miles, and I had a shop replace the timing belt. Taking it home, it started downshifting on hills where it didn't before, and the idle fuel flow on my Scan Gauge was reading .4-.5 GPH where it was reading a solid .3 GPH before.

Took the top timing belt cover off, set the notch on the outside-the-bottom-cover crankshaft pulley ("harmonic balancer") to the 0-deg mark, and the cam pulley was back from its mark. Took the car back, but they claimed that "harmonic balancer mark is not the right crankshaft mark" so they would not change anything.

I checked the harmonic balancer mark as OK two ways -- a timing light showed the same idle advance of 5-7 deg as the Scan Gauge, and I pulled the spark plugs out and put plastic rods down the holes to find the #1 cylinder TDC aligned with the harmonic balancer mark. Next, I advanced the cam pulley 2 full belt notches to align with its mark, and I followed the procedure for resetting the belt tension and turning the crank a full 2 turns before rechecking.

The car has more power, and the idle fuel is back at .3 GPH. But something has changed on the car. I don't have a lot of tank fills to be certain, but it appears though that the Scan Gauge tank-fill correction has changed from -11% down to -4.5% (In other words, with the -11% correction and my change to the timing belt, the Scan Gauge was giving optimistic MPGs, and changing to -4.5% gives more realistic, lower MPGs).

The idle manifold pressure, transmission in neutral, reads 4.1 PSI at 700 RPM on the Scan Gauge, and a "steam gauge" vacuum reading is 21" Hg. With the transmission in Drive, the MAP is 5.1 at 650 RPM. I tested the compression and got 185, 190, 187, 185 PSI where the factory spec is 178.

I have a theory that a another shop that changed the camshaft timing belt, at 85,000 miles and 13 years ago when I bought the car used, set the belt back one notch, accounting for the -11% correction. After my second timing belt change, it was set it back another notch (each notch is 15 crankshaft deg so the valves were a full 30 crankshaft-deg back). I wish I had written down more Scan Gauge readings, but I am thinking the Scan Gauge was being pessimistic about gas mileage because the power-robbing retarded intake valve was reducing manifold vacuum but reducing pumping losses? Shop #1 having the belt back one notch had the Scan Gauge somewhat less pessimistic but still requiring the -11% adjustment (meaning the tank fill is 11 percent below what the uncorrected Scan Gauge thinks). And now that I set the cam to the factor spec, I am getting low MAP, high vacuum at idle?

I don't have enough tank fills to be certain, but it seems that I am trading power for gas mileage, and maybe I want to try 1 notch back for more MPGs? The thing that is unexpected is that I can vary the cam timing over a 30-deg range on this car without making it hard-to-start, throwing a code, or cause and alarming symptoms in the engine.

Do people have experience that the cam timing can be varied through a full 2 belt notches (30 deg) without doing anything crazy to the car? Any thoughts about the different Scan Gauge tank-fill calibrations at what are 3 cam settings -- how does the Scan Gauge calculate MPG? Doesn't it work from the fuel injector on-time, and why would the cam change that calibration? Anyone experiment with cam timing?

[user removed]

If your cam sprocket has 36 teeth then one tooth off is 20 degrees. (two revolutions of the crank per rev of the cam). The cam sprocket would have to have 72 teeth to have 10 degree changes moving 1 tooth since the camsprocket would rotate only 180 degrees for each revolution of the crank.

regards
mech

[California98Civic]

How do you have compression *higher* than factory spec in all four cylinders on a 1997 car? Are you sure of factory spec 178? Do you have a factoy service manual?

[Paul Milenkovic]

The 4-cyl 2.2 L Camry engine has 24 teeth on the crankshaft wheel and 48 teeth on the camshaft. Each timing belt tooth indexes the crankshaft by 15 deg (15x24 = 360 deg), the camshaft by half as much or 7.5 deg.

Typically engine cycle events -- spark, valve opening and closing -- are given in terms of crankshaft angle. For example, when the spark is timed at 10-deg BTDC, this is with respect to the crankshaft.

What I reported above is considering how the shop installed the belt and how I changed it, I operated the engine with valve timing over a range of 2 belt teeth or 30 crankshaft degrees. I didn't even know that you could change it that much without preventing the engine from starting or even doing damage, but the Toyota has a "non-interference engine" where you won't hit anything, and maybe the ECM is able to compensate for this wide range.

The performance was off the way the shop had it, the performance was back and the gas mileage was great the way I set it, but the gas mileage wasn't really that good because the way I set it changed the Scan Gauge readout in relation to what went into the tank.

Has anyone experimented with the cam timing with respect to idle vacuum, engine compression, MPG? I am tempted to experiment, but it takes me 3 hours to get at the top timing belt cover, change the camshaft alignment, and put everything including the cover, engine mounts, and alternator belt back together.

[Paul Milenkovic]

The value of 178 psi is quoted from Web sources for that "generation" Camry. This is the "factory spec", higher is OK provided the cylinder don't vary that much, below 142 is too low.

Maybe the compression tester is only that accurate -- I heard that low-end models can read low. This one was a loaner from O'Reilly Autoparts -- it had a check valve at the bottom of the rubber tube, which is good, but it required a screw-on fitting over the bottom of the tube to match the spark plug hole on this car, which slightly increases to "clearance volume" and could lower the compression reading.

Can a tooth-ahead timing belt increase compression beyond what it should be?

[California98Civic]

I don't know, but you also might want to look at valve lash. That's pretty high compression for such an old car so maybe there is not much overlap between intake and exhaust valves, upping your compression test result numbers abou what you say is factory specification. Normally, compression drops over the life of the engine, if I understand correctly. Somehow you have really significantly defied history. Here is a link I found to some dudes discussing why compression test results might be higher than spec: too high compression test results? - Team Integra Forums - Team Integra ... carbon deposits is something they suspect. Hmmm... your car might reasonably be down 20-50 psi from 178 after all those miles, but instead you are above spec?

Also, Old Mech (above) is right: one tooth off on your engine would be quite a large change and I think you'd have wrecked it in seconds, with a piston crashing into valves.

Last, it is July 4th and lots of EM members are in the USA, so ... I expect your responses might be a bit few today. I'm leaving for a cook-out right now.



good luck!

[Paul Milenkovic]

In pondering what to inspect on this car, I had read the Federal underhood decal with the valve lash specs and though, "Mechanical lifters? Valve lash adjustment? This can't be real, the last car I owned with valve lash adjustment was a 1978 Ford Fiesta with the 1.6 L "Kent" engine."

Then I thought, this is a Toyota, not one of those British or Italian sports cars where you had to adjust valve lash with every (frequent) oil change. But the Camry is 18 years old at 160,000 miles, maybe I need to check this.

What are the "stages of taking on auto maintenance" -- first grief, then denial, then bargaining, then . . .

Valve lash. Fantastic suggestion -- I need to look into that.

[Ardent]

Originally Posted by California98Civic

Also, Old Mech (above) is right: one tooth off on your engine would be quite a large change and I think you'd have wrecked it in seconds, with a piston crashing into valves.

Nope, this is a non-interference engine.

[Ardent]

Originally Posted by Paul Milenkovic

Can a tooth-ahead timing belt increase compression beyond what it should be?

Advanced cam timing can cause higher cranking compression as the intake valve closes sooner allowing for less reversion as the piston ascends from BDC.

[Paul Milenkovic]

I think I am back to the grief/denial stage.

The valve lash adjustment is nothing like on a vintage Ford Fiesta with a Ford-of-England pushrod motor or a British sports car -- i.e. feeler gauge to measure the clearance, turn the adjuster screw on the rocker arm.

The arrangement on the overhead cam engine involves shims. You need to measure the clearance, remove the existing shim with a special tool, measure the thickness of the shim with a micrometer, go to the Toyota dealer to get a factory approved shim of the required thickness to correct the clearance to spec . . .

The lash adjustment is supposed to be a "lifetime" affair with the shims instead of adjuster screws that can "back out"?

The linked article emphasized the carbon deposit theory and suggested either in-tank or in-the-brake-booster-vacuum line engine treatments.