A few weeks ago I helped a friend drop a "new" engine with known good compression into his 2000 miata. While it was out we did a timing belt..
It's quite sluggish after about 3000 rpm compared to the "baseline" (another stock 99 miata).. I'm thinking the exhaust cam is off by one tooth, but it "counts" correctly. What do you think?
Recheck the timing, for sure
Sounds to me like it is off a tooth as well. I bought a 1995 a few years back that was off a tooth when I got it. It ran smoothly but was slow. I changed the belt and it fixed the problem.
Timing looks OK... CAS issue?
VICS or VCTS manifold? If VCTS, are the butterflies opening properly?
BTW, is this the Wonko that used to be in Prescott?
Thanks for looking at the pics, Rodan! No, I'm not that Wonko, though I did live in Phoenix for a bit.
This one is the VICS, I'll look into checking to see how that is activated and check whether is working.
Did you check it after pulling the pin on the tensioner (or however the tension is set) and rolling it over 2 revolutions? It's possible to lose a tooth to slack.
Good idea, but the pics above are from today, so it's driven about 50 miles since we did the job. That's what it looked like when we pulled the covers a bit ago.
Huh, it does look right in the pics, and as you said, counts correctly.
Looks like the vics is activating properly, but I'm guessing the cat is shot.. We did a run with some 0-60 launches, and they were averaging around 11.5 seconds. According to the intartubes, it should be about 7.5.
When we got back to base, we quickly popped the hood and the exhaust manifold was glowing right in front of the cat.
I'm thinking when the previous engine lost its head gasket on the highway, it plugged the cat some. Luckily, I have a spare cat for this engine, so we'll try swapping it tomorrow.
Use a vacuum gauge attached to the intake manifold to confirm the plugged exhaust.
Check ignition timing too (CAS still sets timing, right?) as overly retarded timing will kill power and make heaps exhaust heat.
The belt may have been advanced a tooth in its previous life and the CAS cranked around to get it in spec. BTDT on other vehicles.
noddaz wrote: Use a vacuum gauge attached to the intake manifold to confirm the plugged exhaust.
What's the procedure for this? Do you have a good write-up on what I'm looking for? I do have both a vacuum gauge and a vacuum generator..
Knurled wrote: Check ignition timing too (CAS still sets timing, right?) as overly retarded timing will kill power and make heaps exhaust heat. The belt may have been advanced a tooth in its previous life and the CAS cranked around to get it in spec. BTDT on other vehicles.
I don't think it's adjustable on a 2000.. It is triggered by both a cam and a crank angle sensor, both of which are fixed. The car stalls immediately if either is unplugged..
WonkoTheSane wrote:noddaz wrote: Use a vacuum gauge attached to the intake manifold to confirm the plugged exhaust.What's the procedure for this? Do you have a good write-up on what I'm looking for? I do have both a vacuum gauge and a vacuum generator..
I measure backpressure directly.
Get a spare Miata rear diff or transmission drain plug, drill and tap 1/8" pipe thread from the outside in, get a 1/8NPT to 3/16 brake line adapter from the auto parts store as well as a short length of brake line and 6 feet or so of vacuum hose, remove the precat O2 sensor and thread this conglomeration in its place, run the hose up to a pressure gauge in the car and go for a drive. You can just tie the O2 up out of the way, you're not going to be driving it for very long.
A good cat won't see more than maaaybe 2psi under any conditions. Usually less. A bad cat might see significant pressure at idle and peg the gauge on snap throttle, no need to drive it.
Cool idea, Knurled!
Just be careful because I know that a hanging downstream O2 sensor will get hot, hot, hot even when it's not installed. The upstream one may too.
Any 4 wire one should get hot (and subsequently fragile), so I'd unplug any o2 sensor I did it with..
That is the best way to do it, of course, but apparently it is impossible to clear a check engine light with a scan tool with a Miata since everyone says to do it by disconnecting the battery. Unplugging the O2 should set all sorts of heater circuit and O2 sensor circuit codes that may not self clear after plugging it back in and cycling the key on three times. (Which is generally enough to put out an "Oh crud I forgot to plug something in" check engine light, but not always)
If you leave it dangle, the worst that happens is it might run a little poorly in closed loop, might skew the long term fuel trims a little bit. Nothing that won't immediately get better by putting it back into the exhaust. But yeah, the business end gets to about 600F.
I rarely actually drive anything with a suspected plugged cat. A free-rev in the shop is generally enough to tell me what I need to know and no need to go further than that. It is still a good idea to put the tester in the rear O2 position also. It'd suck to replace the cat when the problem is a muffler with a folded baffle.
I've had no problems resetting an obd2 miata with torque.. The older ones are the "pull the battery plug" ones.
When you took that photo of the belt timing, was that after you turned the engine over once or twice to verify? Everything you describe screams one tooth off. And remembering the last bp I did a timing belt on, I swore it was dead on until I rotated the engine over and low and behold was off a tooth.
Those pictures were taken after about 50 miles of driving.. It totally feels like when I had an exhaust cam off a tooth once on an xterra, but we didn't adjust it since it seems correctly lined up.
Is the distance between the cam bolts the same as the distance between the timing marks? (with the belt tight)
I'm not sure what their measurements were, but the marks on the teeth of the pulley were vertical.
Do you have a shot of the cam lobes while in that position? There was one case on miataturbo.net recently where the pin on the exhaust cam had come off and so the cam was rotating relative to the cam gear.
You can build exhaust pressure testers using NPT-to-compression adapters and copper tubing from the hardware store as well. I expect you'll see more than 2 psi through the 99/00 pre-cat, though. I never ran the test naturally aspirated, but with a ~ 10 psi turbo on the car, the pre-cat was something like 5-6 psi of back pressure.
A VICS that isn't working right (the nipples on the solenoids often get broken when junkyards pull motors) will make it slower, but not 11.5 seconds 0-60 slower. Did you do a leakdown test on it before installing?
Did you drop a screwdriver down the spark plug hole just to confirm that TDC on the crank pulley is actually TDC on #1?
No, I didn't grab a pic of the cam valley. I'll check into that though. That would be a tough problem to solve.
The vics it's working correctly (rev it up past 5700 rpm and you can watch it activate) but this is flatter well before that should be affecting it.
I did verify that #4 was at tdc when the crank mark was lined up.
I did not do a leak down on it, just a Compression check before purchase. It was in a reputable local sources' daily driver before we got it, so I'm going with the assumption that it is in good shape internally.
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