The C4 has a weird system where they were trying to be fancy by incorporating a pre-OBD2 type system. There's actually a couple of computers... not sure if you included those in the swap, but there's the dash computer which is kind of like a body control module. It gives you codes in a different manner, and communicates directly with the ECM.
The ECM does not require any of that stuff... but just asking in case they included it. Are you using the C4 gauge cluster?
Otherwise, the ECM itself can still be accessed the old fashioned way with a paperclip in the ALDL connector. I would check to make sure you don't have any codes. I always run cats on my cars... but even with them off, the fuel smell shouldn't be overpowering... it's likely doing that because your oxygen sensor is starting to go bad (or something else). Sometimes, the O2 sensor can have premature failure on OBD1 systems, but still be within range and not give improper readings. You'll notice because fuel economy will drop significantly, and you'll start smelling fuel more.
So... check the codes if the install was done properly... and report back.
The C4 has a weird system where they were trying to be fancy by incorporating a pre-OBD2 type system. There's actually a couple of computers... not sure if you included those in the swap, but there's the dash computer which is kind of like a body control module. It gives you codes in a different manner, and communicates directly with the ECM.
The ECM does not require any of that stuff... but just asking in case they included it. Are you using the C4 gauge cluster?
Otherwise, the ECM itself can still be accessed the old fashioned way with a paperclip in the ALDL connector. I would check to make sure you don't have any codes. I always run cats on my cars... but even with them off, the fuel smell shouldn't be overpowering... it's likely doing that because your oxygen sensor is starting to go bad (or something else). Sometimes, the O2 sensor can have premature failure on OBD1 systems, but still be within range and not give improper readings. You'll notice because fuel economy will drop significantly, and you'll start smelling fuel more.
So... check the codes if the install was done properly... and report back.
Did you keep the factory ECM from the C4? I didn't do the swap myself; but I do have a reprogramed factory C4 ECM. I have paper work for it.
Are you using the C4 gauge cluster? No, I have the factory Fiero dash gauges
Gas milage sucks.... 8-10 miles per gallon
I have an ODB1 port located at the rear for the C4 ECM. However, I am not sure if it's wired into the Fiero's Engine Check Light. I haven't tried the paper click hack yet but I feel it will not lightup the fiero dash. I was thinking of purchasing an ODB1 reader. But I am not sure which one to buy. I've seen some GM readers for $50 on ebay.
The oxygen sensors are Bosch with lessen than 1500 miles on them but the car has been running rich since I got the car 4 years ago. When I got the car, the cat's (universal) were bad and blowing out chunks of honey comb thus I cut them out and ran stright pipe.
Amazon has universal CAT's for $80, do you think these will work?
The CAT's have on oxygen sensors bong on them. Do I run the oxygen sensors here or keep them on the hearders?
Did you keep the factory ECM from the C4? I didn't do the swap myself; but I do have a reprogramed factory C4 ECM. I have paper work for it.
Are you using the C4 gauge cluster? No, I have the factory Fiero dash gauges
Gas milage sucks.... 8-10 miles per gallon
I have an ODB1 port located at the rear for the C4 ECM. However, I am not sure if it's wired into the Fiero's Engine Check Light. I haven't tried the paper click hack yet but I feel it will not lightup the fiero dash. I was thinking of purchasing an ODB1 reader. But I am not sure which one to buy. I've seen some GM readers for $50 on ebay.
The oxygen sensors are Bosch with lessen than 1500 miles on them but the car has been running rich since I got the car 4 years ago. When I got the car, the cat's (universal) were bad and blowing out chunks of honey comb thus I cut them out and ran stright pipe.
Amazon has universal CAT's for $80, do you think these will work?
The CAT's have on oxygen sensors bong on them. Do I run the oxygen sensors here or keep them on the hearders?
Thanks for the help.
JM
Can you show me how close on the headers the O2 sensor bungs are? They shouldn't be directly in the fire, so to speak, but should be at the Y-pipe / collector pipe. The 4 cyl Fieros have them further down just above the down-pipe. It's still in the exhaust manifold, which is why it works so well, but it shortens the life if they're getting direct flame from the exhaust ports.
I would keep them at the header, rather than putting them on the catalytic converter.
I definitely agree with going with cats... it'll improve some of the around-town driving... but I don't think it'll fix your problem. If you can manage it... figure out which leads come off the ECM and which is supposed to go to the check engine light. You really want to be able to know what your ECM is doing.
There are SOME scanners that work, but for your year... they should be compatible. It gets iffy on cars like the early ECMs... but you've got one of those ECMs that's more modern and meant to sit inside the engine bay with the newer connectors. It's like a neutered OBD-2 system almost. Almost any OBD-1 compatible scanner should work on your ECM.
Do that first before you get new cats, otherwise you're just masking a symptom (in my opinion).
...the car has been running rich since I got the car 4 years ago. When I got the car, the cat's (universal) were bad and blowing out chunks of honey comb thus I cut them out and ran stright pipe.
It's quite possible the cats went bad because the engine was running way too rich. If so, new cats will be destroyed in the same manner. Fix the engine problem before replacing the cats.
I purchased small ODB1 plug in reader 'INNOVA 3123 GM OBD1 Code Reader ‘ hoping that would trigger the check engine light on the fiero’s dash. Unfortunately , this was not the case. I don’t think Check Engine Light is wired into the VETT ECM. Is there a way to wire a light to the factor VETT ECM?
Is there a way to wire a light to the factor VETT ECM?
Having a functional Check Engine Light would be nice... but now that you have a reader, is having a CEL really all that necessary? Your reader should be able to tell you if there are any stored codes.
Looks like I plugged in the code reader upside down.
I was able to pull 2 codes:
Error Code 12 Meaning: Code 12 indicates "No Distributor Reference Pulse" or a normal condition when the engine is not running. In General Motors’ OBD-I systems, Code 12 is often a default code displayed when the diagnostic mode is activated but the engine is off. It confirms the diagnostic system is functioning but doesn’t necessarily indicate a fault unless it persists while the engine is running.
Error Code 24 Meaning: Code 24 indicates a "Vehicle Speed Sensor (VSS) Circuit" fault. The VSS informs the ECM of the vehicle’s speed, which is critical for transmission shifting, speedometer operation, and other functions.
I don't think either code would be responsible for the engine running rich. Right?
[This message has been edited by str8maxn (edited 05-22-2025).]
Looks like I plugged in the code reader upside down.
I was able to pull 2 codes:
Error Code 12 Meaning: Code 12 indicates "No Distributor Reference Pulse" or a normal condition when the engine is not running. In General Motors’ OBD-I systems, Code 12 is often a default code displayed when the diagnostic mode is activated but the engine is off. It confirms the diagnostic system is functioning but doesn’t necessarily indicate a fault unless it persists while the engine is running.
Error Code 24 Meaning: Code 24 indicates a "Vehicle Speed Sensor (VSS) Circuit" fault. The VSS informs the ECM of the vehicle’s speed, which is critical for transmission shifting, speedometer operation, and other functions.
I don't think either code would be responsible for the engine running rich. Right?
Yes, Code 12 is totally normal, and it means your ECM is "aware" and alive.
Code 24 means the VSS is disconnected, or failed. THis is definitely important for making sure that it's operating properly at the correct speeds and loads. I can't tell you where the VSS "connector" would be located since it's an LT1 from a Corvette... but if you have a manual transmission, it should be the ONLY thing that plugs into it, other than the reverse connector.
I didn't do the swap myself; but I do have a reprogrammed factory C4 ECM. I have paper work for it.
When I got the car, the cat's (universal) were bad and blowing out chunks of honey comb thus I cut them out and ran straight pipe.
It sounds like the programming may never have been right to begin with and why the car was sold. Maybe contact Sinister http://www.gmtuners.com/eprom/index.htm
You'll need a scan tool to see what the engine is doing. An ALDL cable with Tunerpro on a laptop will do.
Code 24 means the VSS is disconnected, or failed. THis is definitely important for making sure that it's operating properly at the correct speeds and loads. I can't tell you where the VSS "connector" would be located since it's an LT1 from a Corvette... but if you have a manual transmission, it should be the ONLY thing that plugs into it, other than the reverse connector.
I have a manual trans 4 speed in it . Where would the VSS most likely be located on a 4 speed?
Code 24 means the VSS is disconnected, or failed. THis is definitely important for making sure that it's operating properly at the correct speeds and loads. I can't tell you where the VSS "connector" would be located since it's an LT1 from a Corvette... but if you have a manual transmission, it should be the ONLY thing that plugs into it, other than the reverse connector.
I have a manual trans 4 speed in it . Where would the VSS most likely be located on a 4 speed?
The standard Fiero Muncie 4-Speed?
It would be this sensor:
Keep in mind, this is a 5-Speed Isuzu, and an Iron Duke... but the VSS (circled) is a two-pronged connector that's located in basically the same place on the 4-Speed... it essentially tells the ECM what the speed of the car is, based on a combination of factors and measuring it at the transmission. Does your speedometer work?
EDIT: Also... that other connector in the middle that's hanging off to the left, that's your "reverse switch" to turn on the reverse lights. You should have one of those too... though that has nothing to do with the ECM and merely connects into the relay that controls the reverse lights. But with your 4 speed, those are the only two things you should see.
Also... if you remove the sensor... be very careful that the gear does not fall off into the car. I've never seen this happen personally, but I've heard that it does, and then you have to fish it out of there, or hope the transmission gears grind it up and it goes away... haha. But they are basically unobtainium.
[This message has been edited by 82-T/A [At Work] (edited 05-23-2025).]
Does your speedometer work? It is currently not working. Stopped a few weeks ago.
Also... if you remove the sensor... be very careful that the gear does not fall off into the car. I've never seen this happen personally, but I've heard that it does, and then you have to fish it out of there, or hope the transmission gears grind it up and it goes away... haha. But they are basically unobtainium.
LOL- this has happened to me already. This would be the second speed sensor on the Fiero that has failed. The first time the gear fell back into the transmission distroyed it.
Does your speedometer work? It is currently not working. Stopped a few weeks ago.
Also... if you remove the sensor... be very careful that the gear does not fall off into the car. I've never seen this happen personally, but I've heard that it does, and then you have to fish it out of there, or hope the transmission gears grind it up and it goes away... haha. But they are basically unobtainium.
LOL- this has happened to me already. This would be the second speed sensor on the Fiero that has failed. The first time the gear fell back into the transmission distroyed it.
I don't think they sell new replacements.
JM
Well, you can get used gears. Your speedo will be off, but not enough to make things too bad. If you remember what the color is... go looking for a replacement one on eBay. They do pop up.
