Hi,
1985 with a new engine (3000 miles). headers without a warp, new fuel pump, new fuel filter, Bosch III injectors (up to 15K miles),Dynamic EFI ECM (MAP, not MAF), new MSD spark wires, new spark plugs, new ignition module, external ignition coil (Jacob's).
Fuel pressure is fine. 43 PSI when start a cold engine without the vacuum connected. about 38 up to 45 PSI at WOT.
From a cold start up to 20 mins driving and/or (190F engine and oil temp), everything runs great. No hesitations nor pops or backfires. It seems the issue begins when the engine is reaches working temperature.
From that point it begins with a little hesitations up to a loud backfires from the exhaust. In severe cases, it simply shuts off when I go for a stop. I can start the engine immediately after it shuts. I can eliminate a tune problem, I checked it with two different fuel and spark maps.
As I mentioned, I already replaced the ICM but not the pickup.
It all began a three months ago. I replaced the normal 1 wire O2 sensor with a heated one. the positive was taken from the distributor and the ground from the body. After that, I went to a 300 miles trip without any issues at all. I can also add that my 700 transmission was rebuild a 4 month ago (the shop might move the headers a bit).


Any help will be appreciated

 

Attaching to the "Heated" Oxygen sensor to the Distributor Power might have been a mistake. Online it suggest that a heated Oxygen sensor can draw anywhere from .2 amps to 8 amps. If your O2 is pulling 8 amps from the distributor power that would clearly make problems for you. I might suggest you wire it in with a relay that turns on when the car is started so it is not overloading any other circuit with it's current draw. This is very likely "linked to your problems I would guess.

You should also do a quick look at your MAP sensor to be sure it is attached to the wiring harness. That popping in your exhaust might be from air leaking in around the exhaust manifold but could be a sign of a bad air fuel mixture as well.

Have you looked at the Coolant Temperature Sensor to be sure it is seeing the proper temperature? That is an important sensor on any Fuel Injected vehicle but very important on out TPI Corvette. It's output is used in a variety of ways to help the Corvette run better.

 

Thank you,
From the logs, the coolant doesn't reach 200. when the engine what shut during a drive, The coolant was 190 by the log.
I'll check the heated O2 sensor. Actually, I don't need it with the current tune because I'm running OL only. I'll check it out. Thanks for the tip.

 

Doesn't running around in Open loop cause your system to run too rich? That would be hard on the catalytic converter (IF you have one). Why are you using a heated O2? Open loop doesn't use any O2 at all from my understanding of how it works. What are you using to verify the air/fuel ratio in your open loop system?

I would measure the current used by the heated O2 sensor to ensure it is not overloading the distributor power. I can't imagine the Distributor parts liking a 8 amp load added to the circuit, it would likely cause something to happen.

There is a "magic" number at which your ECM will shut down the engine for you if it senses too much heat. In a warmer environment it is that much more important to verify the CTS's accuracy.

My new Holley Sniper Fuel Injection system uses a heated O2 and it amazes me at how fast the engine goes into closed loop. All I wait for is my coolant to hit 120* (F) and it goes right into closed loop. The heated sensors are really good at reducing emissions from what I have read.

 

I installed heated O2 because back then I tuned a CL. the problems I got with my headers, is bad reading with one wire O2 sensor. I made the same check with other one wire O2 sensor. I know with OL, I don't need a O2 sensor. I'll run a check by disconnecting the heated O2 ground. I don't have a cat on my Vette. I don't think the CTS is related because as soon as you shut the engine, The coolant becomes warmer until the fan kicks again. As I mention, If the engine shut I can start it immediately again.

 

I was curious about the use of a heated oxygen sensor in a C4. I have seen people do it to improve on their emissions and warm up times are reduced. I am not familiar with people running their C4 Corvette in Open loop all the time. Is the other way of measuring the air/fuel ratio while in open loop?

Is the problem with the O2 that you have it mounted too close to the engine to get a good mixture of cylinders? I have never had any issues with single wire O2's acting wacky. On my C3 with it's 427 I put the O2 about 14-16" rear of the joint of the headers to my exhaust system. This works well for my Holley Sniper EFI.

Pulling the heater circuit off that Distributor should make the voltages higher in the distributor circuit and that might help make it run better.

You are probably right about the CTS as long as you are aware. I defer to your knowledge as you obviously know more about Electronic fuel injection than I do.

Best regards,
Chris

 

Hi,
I haven't went to a drive yet. I just disconnect the ground of the heated O2. I hope that do the trick and disables the heat feature of the O2 sensor. I already checked with the EBL software and noticed there is still a reading from the sensor. The reference is still connected so I assume it still works as a single wire. I made the heated before I moved to OL tune. I also have an AEM WB sensor with a gauge. the WB is also connected to the ECM but not functions as a O2 do. It is just for logging and tuning (With Dynamic EFI, you can use "auto learn" feature using a WB).
I'll update when I'll have a drive with the O2 ground is disconnected.

PS: As I mention I run headers. Because of them, a single wire is getting cold while cruising or idle.

 

Thanks for educating me about the various EFI systems out there!

When installing the Holley Sniper EFI system I added a O2 to my C3's 427 with long tube headers and the Chambered Exhaust. Holley's Support people warned me about the sensor net getting a good mix of all the four cylinders in time by installing it too close to the Headers or the joint. I simply drilled the O2 hole in the pipe under the passengers seat and it gets a good mix and is heated. I am not using any kind of X pipe or crossover, straight dual exhaust. I have the AEM Digital Air/Fuel Ratio gauge and used it to help setting up Carburetors on my C3 as it was the best way to see where it was. I have an idea about using it on the other O2 sensor location on the drivers side.

For a normal unheated Oxygen sensor to start working it has to be hundreds of degrees (F) and they operate at 600* (F) in normal condition. Normally the exhaust should heat them up properly after a couple minutes. What most people don't know is that the unheated O2's have a habit of going back into Open loop after sitting at a long light. I understand why they are desirable as a simple upgrade to most earlier C4's IF you worry about emissions.

The material inside the Oxygen sensor (Zirconium ?) slowly starts producing an oscillating signal as it heats up and it might be able to do all its signaling thru the 1 wire. I don't think it has a reference voltage used, it basically makes the electronic signal after getting Hot. They would have a good ground through the exhaust system. The CTS has a 5 Volt reference Voltage to ensure it gets "accurate" results.

Good Luck!

 

An update. I disconnected the ground only of the 3 wires heated O2. I don't know why but the ECM still got a reference from the sensor. I watched the WB gauge and the O2 sensor reading and they are quite the same. when I rich I got > 700 mv and >13.1 AFR. I don't know f the heated still got the ground maybe from the header itself? I don't really know if the heated still worked while I was driving. the hesitation of the engine still occurs after ~17 mins /190F. I don't think It happens only when I lean. It also happens when I put some throttle and go 13:1 AFR. Maybe its better just disconnect the connector itself...

EDIT: I disconnected the connector itself, means no reference to the ECM and to power to the heated O2 sensor. same results

 

An update: I replaced the 20 years Jacob's coil with a new MSD Blaster 3. Took a drive of 40 miles without stopping. 15 first miles were city drive. smooth driving no issues.
I couldn't find specs for the Jacobs coil by it seems way out of range.