j3studio

Another question by a C4 near-newbie...



I'll try to describe this condition as best as possible:

Engine idles fine (about 550 rpm), performs decently under full thottle. Sometimes, under partial throttle (and, I believe, when going up hill) the engine will go:

"bap-bap-bap-bap"

and the check engine light comes on. It doesn't sound like the end of the world, but it's not particularily reassuring...after about 5 minutes the check engine light will go out. This happens about every 25 miles or so. It does not seem to happen during highway driving.

I have not yet purchased any diagnostic code equipment (there's so much to buy with Corvettes ) so I don't know what codes it might be throwing. I haven't yet narrowed it down to what RPM range or gear it is in when this behavior happens, but I believe we're looking at about 900 to 1,400 RPM. Also, and most pathetic, I really don't remember what knock sounds like and couldn't tell you if this is what I'm hearing.

I use only name-brand plus gas (I've been told by folks I respect that premium won't help the engine any) and the transmission has new fluid. I am running the standard A4 with the G92 performance axle, so I'm pulling more revs than your average automatic 1985.

Any ideas?

j3studio

Any thoughts would be helpful - I'm wondering if I should really be worrying about this...

mikey whipreck

I'd like to hear a solution to this... My '85 has a similar condition. Car seems to run fine. All the gauges report normal readings, The engine doens't make any noise like you describe. But every once in a while, I get the check engine light (really makes my stomach turn). But it almost always goes out after 5 mintues or so.

flyersfan1088

I know of a cheap diagnostic tool. A paper clip. A short piece of wire works too. Underneath the dash on the drivers side is the ALCL(Assembly Line Communications Link) Jumper the two right most terminals on the top row with the wire or paper clip. Turn the ignition to on(not run, you don't want to start the car) The check engine light will flash once, then a short pause, then twice quickly. This translates to code 12 which means the diagnostic system is operating. It will do this 3 times. Then it will flash any stored codes. After any stored codes are flashed, it will again display code 12 3 times, indicating the diagnostic is complete.
Post your results here for translation of the code(s)

Judging by your symptom, I would guess you will see either a code 33 or 36, which is MAF related

j3studio

Wow! Thanks, Steve - results coming tonight...

Corvette0096

There is some good info in here,

http://www.c4vettes.com/ecm.htm

j3studio

Very informative...thanks!

j3studio

Update, update, update!



Using the fancy paperclip method suggested by flyersfan, I got a code of 43 - an electronic spark control fault (I at least have the service manual). Anybody have experience with this problem?

More updates to follow...

Here's an update: looks like it might be the knock sensor (makes sense, I guess). A question: should the harness detach easily from the ESC module? I'm having a bear of a time getting it off...(just want to check the connectors).

CFI-EFI

Interesting that the manual deciphers the codes, but doesn't tell you how to pull them. Mine does. Have you spent much time with that book? My book has trouble shooting charts and procedures for every code. Once you disengage the lock on the connector, the plug should pull right off.

RACE ON!!!

j3studio

Not enough, obviously - I didn't read closely enough to figure out that it was saying you could just connect terminal A to terminal B. I was too locked in on fancy diagnostic equipment...

...still such a newbie, but learning.

CFI-EFI

Spend some time studying. You may be amazed at the vast amount of available information. Take the trouble to locate and follow the trouble shooting charts. They will save you much time and a lot of money, "throwing parts" at a problem in HOPES that will cure the trouble.

RACE ON!!!

jackdaroofer

The ESC Module is a very simple plug connection. It should be located on passenger side mounted to the (I Forget ) ESC and knock sensor (in the block on passenger side) tell the ECM of "knocking noise in the engine" and the ECM retards spark to eliminate the knock (weakness of this system is it assumes noise is timing related when it could be headers or other internal engine sounds and not timing related)

The driving condition you describe (your choice of words) doesn't sound like the engine experiencing spark retard, though spark retard will definitely affect acceleration and power under a load.

Some model years ESC modules on the Vette were very sensitive and a nearby firecracker could trigger spark retard. To solve this, Gorden Killebrew recommends going with an ESC module from a mid 80's Malibu

NAPA number is ESC 312

Again, the connector should just slide off after you have applied a little relief presure to the plastic lock catch.

Since it sounds like you are serious, you really should invest in an Easy link scanner $ 200 or some laptop software. It will save you big $ and make life easier

j3studio

It's on the list - there are so many ways to spend money on this car...

CentralCoaster

Annother way to make the knock sensor less sensitive, is to wrap the threads with some teflon tape before threading it in.

j3studio

Hmm...

good input.

j3studio

Update:

I followed the troubleshooting procedure in the 1985 service manual until I got to the point where it says to replace the knock sensor. So, now there's a new knock sensor in there. Drove it today and it exhibited the same behavior as before and threw both codes 43 and 42.



What do I do now? Might I have an ECM issue that is causing all these problems? What am I missing?

STEVEN13

Hey,

I am chasing a similar problem on my 1992 (false knock counts?)
I just ordered a scan tool (autoxray 3000) hopefully I can monitor the circuit and fix it! If you want follow my threads.

Good luck,

Steven

j3studio

Thanks, Steven - good luck to you, too!

bogus

John - for some reason, I doubt it's the ECU... if so, you would have a lot more problems, and more than likely, it would not return a code 12.

Check the harness. I would not be surprised on an older Vette that you have a bad wire somewhere.

Some where in 1990 or 91, SAE required that wiring be updated to last longer. The insulation was made to a new standard that would withstand heat better. Obvously, yours is older than that.

j3studio

Gotcha, Andy. You are so lucky you are not in Delaware anymore...I would already be on the road heading down!