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Hey guys, no matter what I try I just can't seem to get my recently acquired 94 GS to pass emissions. I knew I had to do a bit of engine work but I didn't think it was this bad. It's failing HC and NO and is running slightly lean actually. I've put in good O2 sensors, good crankshaft position sensor. Plugs and wires are good. Seafoamed the engine, fresh oil change. Checked ignition timing, it's like 10.5 BTDC so that's fine. VAF is good. I'm using stock intake so EGR and everything seems to be hooked up, is there a possibility it's not working without giving me codes? The catalytic is apparently like 1 year old but it's not OEM, however I checked the integrity by measuring the exhaust pipe temp before and after the cat and it's like 170F hotter after the cat so it seems to be working fine. I'm wondering maybe it's a high flow? I've even readjusted the TPS a little to compensate for running lean, helped the A/F ratio a bit but I'm still failing. I know coolant sensor plays a role as well but there are no signs that it's bad. The guy suggested that an aftermarket muffler might cause the cat to run hotter since backpressure cools it down (I think that's BS). Another thing: I know the timing belt hasn't been changed for a really long time, would it be possible that it maybe slipped by a tooth or something so that the performance change isn't noticeable but the slightly offset valve timing is indirectly messing up the ignition timing? Like maybe some of the mixture slips by the exhaust valves or something? (I'm just hoping I don't have burnt valves)

Anyways I just don't know what else to do cause it seems like I've tried everything. The funny thing is the car runs great unless you stick a probe into the tailpipe and get the readings, lol. I'm getting great fuel economy too. This is so strange.... any suggestions?

When I had a bad cat I used a bottle of RXP and put in premium. I passed but the next year I used failing emmissions to get a high-flow cat and a custom exhaust.

Anyone...?

Maybe the EGR valve is a bit stuck or gummed up. I've heard of this happening, you have to take out the valve and clean it out, but don't ask me how to do it because I don't have EGR. You would notice if the timing belt skipped a tooth, that's enough to make it run badly. You should still try swapping the coolant temp sensor, mine didn't give me a code but it made the car run pig rich and it would actually flood after a few minutes of idling.

And did you do the full seafoam treatment?

After checking the EGR, try 2¾ Oz. pure Acetone, and 1 Oz. 90% Isopropyl (rubbing) Alcohol in a full tank. It has worked for several MX-3s, including mine with a hollowed out cat, and for several of my other cars. Run the car for at least 5 miles after adding them, and be sure the car is hot when you go into the test.

fowljesse wrote:After checking the EGR, try 2¾ Oz. pure Acetone, and 1 Oz. 90% Isopropyl (rubbing) Alcohol in a full tank. It has worked for several MX-3s, including mine with a hollowed out cat, and for several of my other cars. Run the car for at least 5 miles after adding them, and be sure the car is hot when you go into the test.

I'm guessing this will dilute the emissions?

Try a litre of methyl hydrate into the gas tank...

Nd4SpdSe wrote:

I wish it was that simple...

_-Night-Shade-_ wrote:I wish it was that simple...

Maybe is it?....

Catalytic is fine, no vaccum leaks, O2 feedback is good, timing is good... and no codes. That's like almost everything from that chart.

You could always try swapping in a KLDE IM and telling them to test it under hot rod standards because it has the motor from a 626. But that pretty much only works in canadian tire, and not every store.

it would probably help if you posted the numbers the car is currently getting, that can help with a diagnosis.

_-Night-Shade-_ wrote:

fowljesse wrote:After checking the EGR, try 2¾ Oz. pure Acetone, and 1 Oz. 90% Isopropyl (rubbing) Alcohol in a full tank. It has worked for several MX-3s, including mine with a hollowed out cat, and for several of my other cars. Run the car for at least 5 miles after adding them, and be sure the car is hot when you go into the test.

I'm guessing this will dilute the emissions?

It basically would clean your fuel system, and burn more cleanly. Acetone loosens the molecular bond in the gas molecule, so it atomized better, and burns more completely.

Could it be that your cat is simply rubbish, and isn't actually cleaning anything? It's not hollow, right?

And EGR might not throw a code if it's something that's just dirty...