C4 Problem?

[cajunboyla]

Check engine light on... Already changed electronic spark module, should I change the ECM as well?

[mastertech1964]

you need to have it scanned and find out what codes its got rather than just throwing parts at it

[vettemaan]

Bring it to autozone. they will scan it for free

[ASEMAT]

Why replace something that is just doing its job?
Check engine light is just an indication that codes are stored. Those codes will indicate the problem area, and so you can go directly to the area to diagnose the problem.

With C4's, the only inheritance issue I can recall at the moment is that you have to keep the ventilation valve kind of clean, or, in other words, sort of keep after it so it is clean, and then it will not chatter. The fix is a larger vent line diameter. I had an 86 anniversary before I sold it for my C6. C4's are great after 84 I think.

[eric58]

you can pick up a grounding code key at any auto parts store, the key plugs into a module under the dash (you manuel will give more details on the location under the dash), once plugged in you start the car and watch the check engine light (once again, check the manuel), the number of times the light flashes indicates the first digit, followed by a short pause then additional flashes indicate the second digit (example: the light flashing 6 times, then a pause then 3 times will indicate trouble code 63, check your manuel to see what 63 is and go from there. Good luck

[Rimmer]

Originally Posted by cajunboyla

Check engine light on... Already changed electronic spark module, should I change the ECM as well?

What year is your c-4?

[batvette]

Eric58 gave you a good tip, though technically you don't need a code key you just need to put a jumper wire across two of the terminals of the assembly line diagnostic connector- one is ground, can't remember the other (I painted the two in my '86 till I got my auto Xray)
Here's a couple things you gotta know-
1. counting the flashes will give you the trouble code. Of course you will need a manual to decipher it. If you intend to keep your C4 for the long term don't mess around with those <$20 rags at autozone, spring for the ~$90 factory book from Helms- that's a bit over 1 hour of labor at the dealer and is money well spent. Full diagnostic flow charts on every sytem with schematics and functional block diagrams AND parts placement charts.
2. sometimes you may replace the bad part and fix the problem BUT the service engine light will remain on until XX restarts- can't recall but it's 50 or 100 IIRC UNLESS you pull the negative battery cable for 60 seconds then it resets.
3. It is suggested by another poster (he implies) that if it's running you should ignore the light. WRONG. It might go into closed loop mode and run, sure. If you have a faulty part you have a lot of problems that can really get expensive, such as the TPI running really rich to compensate for a problem. I ignored the check engine light for about a month and while it was getting poor mileage it was running okay.
The rich mixture situation clogged both the cat converters on the front Y pipe from the unburned fuel, cost $1300 to replace.
4. A check engine light is cause for failure on the visual systems check for California smog check. Even if your car runs perfect.
Even though my 86 was running perfect and not storing any codes, the light kept coming on intermittently, and I was due for the biannual. Chevy finally said "your injectors and ECM are out of sync, there's a tech bulletin on it" and even though no fault was indicated I was facing about $1000 just for PARTS on this and I said "**** this" and I recalled a local surplus place was selling a remote control setup for 12v applications with 2 outputs.
Pulled the center indicator out and removed the connector to the light and mounted a panel lamp inside the center console, and hooked the remote to another bulb in the original socket in the dash.
So when I get a smog check when the guy pops the hood and disconnects the spark advance connector and gets back in and restarts, I have to be standing line of sight and activate the light momentarily from my keyring.
My real service engine soon indicator is inside the console under my elbow.
Hehe.
5. According to Mid America Corvette, ECM's almost NEVER go bad but are always diagnosed as so by dealers and private owners/mechanics alike. Often it was another part or they just needed to be reset or like mine they were "out of sync" with the injectors. MAC will be happy to sell you a "rebuilt" one with the core returned- which they will check, 99% of the time it passes, and will sell to the next guy. Got that from a tech rep of theirs on the phone, don't expect them to divulge that often. BTW MAC has an awesome selection, good support and has always been my first "go to" in the 24 years I've owned my '86 coupe.