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Old 09-09-2008, 02:33 PM   #1
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Default Down on power, diagnosis?

First off, yeah, I know it's a stock Miata, they're all slow but this one seems excessively slow

I just bought a '99 Miata, it's got a few little issues I'm working on patching up and was hoping to get some advice. The car was supposedly in perfect working condition. When I take it out for a testdrive it feels very sluggish, it gets progressively worse to the point it'll barely operate under it's own power. I pull over call to get picked by the lady at the dealership. Ultimately it's diagnosed as a clogged cat. We end up working out a deal on the car; they don't want to drop the money on a cat and I don't mind living without one. I buy the car, take out the midpipe and pour out the remnants of the cat and the car runs a 1000 times better.

I get the feeling the car is still a bit down on power though, especially at higher RPMS. I've got a friend with a stock '99 that I'm going to line up with tomorrow and find out for sure, but the butt dyno says the car is a bit slower than my '90 was when it was stock and I know that's not right. I've searched and come across a few possibilities; fuel filter, coil pack, plug wires (already did plugs) and just wanted to get some opinions before I started aimlessly throwing parts at it.

So anyone have any tips for me? What would you try first? Thanks for any (useful) advise you guys can contribute.
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Old 09-09-2008, 02:51 PM   #2
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Basic tune up? Plugs/wires. Definately an oil change, is it throwing any CEL's? Check that, possibly an 02.

How many miles? Maybe run some water/seafoam through a vac line and clean out the heads. (if you dont know how to do this research before running water through your intake manifold.)

After all that.. leakdown test? Unlikely but possible somethings not right internally.
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Old 09-09-2008, 03:03 PM   #3
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Well, if it wasn't throwing codes before, it will be now.

Seriously, though. The failed cat seems to me like a symptom, rather than root cause. Cats don't just die like that. When I pulled the (apparently original) stock cat off my '92 about three years ago, it looked just fine and dandy inside.

Something else is wrong with that engine. It's either running far too rich, or the ignition is massively retarded, or, well, who the hell knows. Could even be that whatever was wrong has been repaired, but that they failed to replace the cat which perished as a result.

First things first, though. Take that car down to PepBoys and have 'em pull the codes. I'd expect a P0420 / P0421 (catalyst efficiency), but what else does the ECU have to say?
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Old 09-09-2008, 04:33 PM   #4
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Forgot to mention, no codes at all. Which I thought was kinda odd for having no cat. Maybe someone disabled the bulb, I'm sure they'll think I'm nuts getting a code read when it's not showing a code but I'll get it pulled and get back to you guys. Ahhh, looks like I just bought another money pit :(

*Edit: Joe, just realized you're from Port Charlotte. Do you guys ever get together for any meets? Anyway to a local.

Last edited by pdexta; 09-09-2008 at 05:02 PM.
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Old 09-09-2008, 04:37 PM   #5
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Quote:
Originally Posted by Joe Perez View Post
Seriously, though. The failed cat seems to me like a symptom, rather than root cause. Cats don't just die like that. When I pulled the (apparently original) stock cat off my '92 about three years ago, it looked just fine and dandy inside.
True, my brother's car ran good when it was first boosted but about 1 month later the car ran horrible. I mean stock NAs could easily own it...he one day pulled the cat only to find a big wad clogging the exhaust. This was most likely due to him running cautiously rich at first...that motor has something wrong, my guess would be wires/plugs/coils...sounds spark related.

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Forgot to mention, no codes at all. Which I thought was kinda odd for having no cat. Maybe someone disabled the bulb, I'm sure they'll think I'm nuts getting a code read when it's not showing a code but I'll get it pulled and get back to you guys. Ahhh, looks like I just bought another money pit :(
My best investment for my 99 was my scanguage 2.
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Old 09-09-2008, 05:12 PM   #6
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CA car or Federal? Which cat was clogged? If CA, CA car does not monitor "main" cat, only precat. Possible both were bad, and easy to overlook the CA precat in that god awful exhaust mani if you're not expecting to find it.

Coil packs on those things suck, it's likely toast and the cause of the cat failing. Watch for a misfire code.
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Old 09-09-2008, 05:13 PM   #7
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Originally Posted by pdexta View Post
Forgot to mention, no codes at all. Which I thought was kinda odd for having no cat. Maybe someone disabled the bulb,
Easy enough to tell- does it come on for a couple seconds when you furst turn the key?

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Ahhh, looks like I just bought another money pit :(
Nah, just install a turbo and a MegaSquirt and all will be well.

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*Edit: Joe, just realized you're from Port Charlotte. Do you guys ever get together for any meets?
**** if I know. I've been living in SoCal for the past four years, just moved back here about a week ago, and I already hate the weather and miss the good roads back west. Florida sucks my *****.
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Old 09-09-2008, 05:41 PM   #8
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Failed cat is often caused by bad coil pack on a 99.

EDIT, Whoa, I see Ben already caught this one.
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Old 09-09-2008, 07:38 PM   #9
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OP I actually have two working 99 coil packs, if it turns out you need one lmk.
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Old 09-09-2008, 11:20 PM   #10
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Originally Posted by 99NBMia View Post
OP I actually have two working 99 coil packs, if it turns out you need one lmk.
PM'd. Thanks for the offer. I'm assuming that's what it is. I swapped some new plug wires off my other car and it seemed to help a bit but there's definitely something else going on.

I'm not sure if my car is a cali car or not, I found the emissions sticker under the hood that says: "Conforms to US EPA regulations". I don't see any canister off the exhaust manifold but I guess it's hidden under the heatshield anyway. I'm kinda thinking it is a Cali car since the check engine light isn't on (definitely comes on when i turn the key though) and the cat is definitely out. Maybe that heat shield needs to come out to get a better look.
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Old 09-10-2008, 09:13 AM   #11
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What is the VIN?
Does the CEL illuminate with the other lamps when you roll the key to IGN?
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Old 09-10-2008, 09:38 AM   #12
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The VIN is JM1NB3533X0131132.

The CEL light does come on with the other lights when you turn the key.
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Old 09-10-2008, 09:42 AM   #13
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Congrats to buying a -99, the best year Miata ever made!! It is easy to spot if your car is a Cali or a standard. The Cali has an extra small cat just below the manifold, while the standard car just has a descending pipe. Judging from the sticker, your car probably is a standard.

The lacking of CAT should make your car produce a CEL. To start with regular maintenance, such as oil change, plugs and filters is a good advice. Mobil 1 10x30 or 10x40 is recommended for the Miata engine.

Also as mentioned: The coilpacks on our cars are known to be weak and fail and this often causes a CAT failure. But they typically either work or don’t, not slowly failing and causing the engine to loose power and if they fail, a CEL for misfire is typically present.

Did the previous owner doodle around with the intake? I have seen a car with a failed PVC valve in combination with a breather on the crankcase (instead of the normal line to the intake) suck so much oil into its intake that it ran terrible on higher rpms. PVC valves are about $6. Change it to be sure.
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Old 09-10-2008, 09:54 AM   #14
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Thanks for the advise, I'll pull the intake off tonight and see what it looks like inside. The car is completely stock and I haven't seen any signs that it's been modded, but it is a 9 year old car, no telling what it's actually been though.
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Old 09-10-2008, 10:35 AM   #15
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Federal car
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Old 09-10-2008, 12:22 PM   #16
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Quote:
Originally Posted by pdexta View Post
Thanks for the advise, I'll pull the intake off tonight and see what it looks like inside. The car is completely stock and I haven't seen any signs that it's been modded, but it is a 9 year old car, no telling what it's actually been though.
No need to pull the intake manifold before you answer: does it have a aftermarket intake?
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Old 09-10-2008, 04:01 PM   #17
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Quote:
Originally Posted by Torkel View Post
No need to pull the intake manifold before you answer: does it have a aftermarket intake?
Nope, she's all stock.

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Originally Posted by Ben View Post
Federal car
Woo hoo, some good news! Thank you sir.


99NBMia is sending me a coil pack so hopefully that takes care of the problem. Oh and the car has 105k miles on it, just noticed it was asked earlier and I overlooked it.
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Old 09-21-2008, 05:17 PM   #18
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Figured I'd go ahead and bump this with an update. Still trying to diagnose the problem. I'm getting around 19-20mpg, car is noticeably slower than it should be, especially over 5500rpms. In a similar commute I use to get 24-25mpg in my stock '90 miata. So far I've done: oil change, new spark plugs, new coilpack, new plug wires, and new fuel filter. I also seafoamed the intake and gas, it smoked after I did the intake but not too much and certainly nothing like I've seen some cars do. There was no change in the car from anything I've done to it. The air filter looks brand new. I pulled the intake off and it's perfectly clean.

I did a compression check and got 150, 145, 145, 150. Those numbers seemed a bit low, so I tested it on another car that I'd done previously with another gauge and numbers were ~25psi lower than the previous test a couple months ago, I'm guessing the gauge just reads a bit low.

I'm pretty much out of ideas at this point. I thought about hooking up a wideband to it to see if it's running abnormally rich or lean, but beyond that I have no idea what to do. I'm about ready to go buy some Vtec and type R stickers to see if I can pick up some power from those.
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Old 09-21-2008, 05:55 PM   #19
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Quote:
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I have no idea what to do. I'm about ready to go buy some Vtec and type R stickers to see if I can pick up some power from those.
You just made my sig
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Old 09-21-2008, 06:31 PM   #20
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Maybe crank related... I have a 99 engine in my car that had a bad crankshaft...Loses a lot off power...

http://www.miata.net/garage/crankshaft.html

http://www.miataclub.org/orlando/94crank/

http://www.miata.net/garage/hsue/LoctiteCrank1.html
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