Some history on this.
85 corvette with 134,000 miles.
Intermittent misfire that sounds like a popcorn machine. When Vette goes into closed loop it starts to misfire. Also very strong fuel smell.

Things that have been replaced.
IAC, CTS, Knock sensor, O2 sensor, spark plugs, Spark plug wires. Cap and rotor, Plenum air temp sensor, all intake manifold gaskets, Valve stem seals, Valve cover gaskets, Alternator, Belt, Belt Tensioner, Gas tank, Gas tank sending unit, fuel pump, fuel filter. oil and coolant with a flush.

On top of that its had a Smog pump delete, EGR delete as well as all the Emissions stuff removed. 2.5" exhaust from headers back with Race Pro mufflers.

Things that have been checked. Removed Distributor and tore down all components for testing. ICM passed check at O'reilly and AutoZone. Pickup coil passed with being in the correct Ohm range. Ignition coil tested good. Set timing at 6TDC after reinstalling and no change. Car runs great up until closed loop then starts making a puffing noise like crazy. Disconnecting EST totally eliminates misfire. Runs like a dream.
I've spent so much money trying to chase this down but I'm no where closer it seems to figuring it out.

edit:

Also I forgot to mention that FPR diaphram is replaced and the Bosch injectors have been rebuilt and flow tested. Leak down test shows that fuel pressure is held all night.40 PSI before key on, losing about 20psi by morning if I pressurize the rails before bed.

 

Two things:

1/ Which CTS was replaced? The coolant temp sensor in the head, which supplies the dashboard gauge? Or the one in front of the intake manifold, which supplies to the ECM? The intake manifold one is important here.

2/ If disconnecting the EST solves it, have you checked/replaced the EST module itself? Right side of the engine compartment...

 

Both have been replaced. And for the electronic spark control module, I think that's what you are referring to, I thought that was only responsible for sending the signal from the knock sensor to the ECM. I'll see if there's a way to test this.

 

My 86 had a problem that sounded very similar to this. It turned out to be collapsed lifters. I have since rebuilt the entire motor with all high performance parts and am now running a 383 stroker. The only original part now is the block

 

I would get the car diagnosed cause throwing parts you never get lucky, what you are paying for parts, I recommend taking it someone that knows how, I did the same when I owned C4, and it turned out ECM malfunctioning.

 

Pop the connectors off and ohm check the injectors. Should be around 15 ohms. Check them when the car is cold and again when hot. If they're not within a couple ohms of 15 ohms, this is nearly always the issue based on your symptoms. It's rare for them to barely fail....ie "13 ohms" sort of thing. Usually they'll test as 0 or very low single digits.

In a nutshell usually what is happening in this situation is that during Open Loop the engine fueling is off but not so much to run bad (although you may not realize you're down on power). As soon as it goes Closed Loop, the ECM starts trying to adjust fueling but the readings are skewed by the bad injector(s). The ECM makes adjustments based on that information which then throws everything else out of whack.

 

Thank you. This makes a lot of sense. I'm pretty much at my wits end with this. I was this close to going and blowing money on a new ECM and D.U.I. distributor to try and sort this out. I'll ohm the injectors when I get home and report my findings. It's weird because these are rebuilt Bosch injectors but I didn't ohm them when I put them back in the car so it could be that.

 

Ohm'd my injectors this evening.
1- 16.3, 2- 16.2, 3- 16.1, 4- 16.1, 5- 16.4, 6- 16.1, 7- 16.3, 8- 16.2

They all look good which is great news but it's also another dead end.
Anyone have a spare working ECM they could loan me? Lol

 

Did you check them cold and at operating temp? They can read good one way and not the other.

If you're smelling fuel, I'd also check the fuel pressure regulator. If the diaphragm goes bad, a bunch of extra fuel is getting sucked in. Pull the line off the pressure regulator. It should be bone dry. Any fuel in it means it's bad.

Do you have a way to log the fuel trim? If yes, what are you seeing? As I recall, early C4s used a single o2 sensor and it was on the driver's side. If my memory is correct, that narrows the issue to the cylinders on that side of the engine.

 

Besides the above....How did you set timing? Did you disconnect the EST wire first?

You say timing is 6 "TDC". Should be BTDC....Before Top Dead Center. Is that what you did?

 

Oops, that's what I meant. I disconnected the EST wire and with my timing light set it at 6 BTDC. The devil is in the details.

And just to reiterate the engine runs smooth until it warms up then starts making that popping sound in the exhaust. (It sounds like a misfire in the video I posted) Unplugging EST wire makes the sound stop. Also it seems come from the driver side downpipe after headers and the O2 sensor.
I'll post another video today of it running from headers to exhaust tips, then unplugging the EST wire.

 

Tearing my hair out now. New symptoms appeared over the past week. Now when it starts it runs until warm and dies. Doesn't run until it cools down. Runs great after unplugging EST bypass wire.

Seems like it's has gotten worse.


First video is with EST plugged in. (Starts and dies now whereas before it was just a random misfire from both banks.) Second video unplugged the tan wire, bypassing the EST.

 

I'll bet it's the ICM. They often test "good" when cold (ie at an auto parts store) but fail when installed since they pretty quickly heat up.

If it's not the ICM, then you have to start thinking about something like the timing chain being stretched. You can *sort of* check that by pulling a valve cover and then turning the crankshaft by hand while watching the rocker arms. If there's a lot of movement before the rockers move or you feel a lot of slack, that's a potential indicator. It's unfortunately one of those things based on feel and experience since there's always a little slack and the trick is knowing what "good" is.

You really need a scanner to see the dynamic data.

Last thought....What o2 sensor did you buy? I don't recall the older cars being sensitive to brand however on the C7s, those cars absolutely hate aftermarket o2 sensors. About 50% of them experience engine run issues unless the OEM sensors are used even though there are no codes and no check engine light. The o2 sensor is ignored until closed loop so that may be another indicator in your case.

 

The auto parts store ran the ICM test a 5 or 6 times. Maybe 7, I can't recall and it definitely got warm but not exceptionally warm. The O2 is a Bosch single wire. I'm not sure what brand is stock but I hear a lot of people tend to have better luck with the Denso O2s.

I am starting to think it is the ICM as that's the only thing I have yet to replace. (Because of the passed test at AutoZone) I guess if that doesn't fix it, it could be the O2.

EDIT: I do need to get a scanner. Or at least an ALDL cable that I can run tunerpro on.

 

""Things that have been checked. Removed Distributor and tore down all components for testing. ICM passed check at O'reilly and AutoZone. Pickup coil passed with being in the correct Ohm range. Ignition coil tested good. Set timing at 6TDC after reinstalling and no change. Car runs great up until closed loop then starts making a puffing noise like crazy. Disconnecting EST totally eliminates misfire. Runs like a dream.
I've spent so much money trying to chase this down but I'm no where closer it seems to figuring it out.""

CODES ???????

1. The C4 is controlled by a computer and sensors. You have to get on it's level to see what is happening. Your parts canon is not working and you are clinging to it. WINALDL at least....

2. If it has the problem immediately upon closed loop there's your sign !!! This is question I do not have an answer for, you deleted smog stuff, do you need to delete them in the ECM programming??

3. Understand the EST wire and bypass timing mode.
.

 

Thats the procedure I use to set the Base timing. And the only code I get is the EST bypass code from unplugging the tan wire to set the timing. It goes away after a key cycle.
1. No more parts cannon. I just now bought an ALDL cable from OBD diagnostics Inc. Admittedly I should've done this a long time ago.
2. The only thing that would need tuned out of the ECM is the EGR delete. Most people don't bother with this but it was something I was planning on doing. The 85's tend not to throw codes for an EGR delete anyway.
3. I understand the Electronic Spark Timing. The disconnect is there to prevent the ECM from trying to advance or retard timing when setting the base timing.

Edit: One more parts cannon part. But I swear thats it. I bought a new Ignition Control Module.

 

Just making sure of one thing, you probably did this but just making sure. You time it as you did by disconnecting the EST and set timing, then you shut the car off and plug in EST.

 

Correct.


Once I get the new ICM in I'll report back any changes. I'm also interested in what my O2 readings will be once I get my cable. Hoping by next week.

 

The next time you get a no start, test for spark and re-test the pickup coil resistance quickly while it is warm.
Have you tried running with the EST wire connected and the ESC knock module disconnected ??
Did you use a specific torque value when you replaced the knock sensor ??
.
.

 

Pickup coil is new. It starts just fine. My old one tested good as well but since I was dropping in a new DUI distributor it got replaced anyway. The distributor didn't have a new ICM with it so I just swapped my old one over. I'm being told that I most likely have a failing ICM that tests ok but is actually not. It's the only thing ignition related I haven't changed yet and could be the root of all my issues. I just ordered a new one. It should be here today. I'm really hoping that that's it.

I don't recall setting a torque value on the knock sensor when I changed it and did a coolant flush before stowing it away last fall. I'm thinking I must have since I'm pretty methodical about these sort of things. I know that torquing it too much can damage it or cause it to read too high of knock counts thus retarding timing. Would backing it off and retorquing to double check pose an issue?

I haven't disconnected the ESC module since it either works or doesn't. Correct me if I'm wrong, It's only job is to send the knock signal to the ECM and then the ECM sends the signal to the ICM/pickup coil to either advance or retard the timing. I would get a code if that failed.

I can disconnect it today and see what happens.


I'm getting my ALDL cable next week but I'm a little nervous about whether the 85's 870 ECM is able to interpret knock counts, O2, and other such relevant data.