Hi all,

I have been trying to fix a VERY intermittent code 43...ESC system. So far I have replaced the knock sensor and replaced the connector with a new one as well. The car drives and idles fine most of the time...very rarely I will get the idle bouncing between 550 & 775 RPM but it quickly regains and then runs and idles fine at arfound 530 to 550 hot and in drive. I read that code 43 could be knock sensor, wiring, ESC module by the blower fan or ECM. Some research suggest that a good test would be to remove connector from knock sensor and see if the ECM is sending 5 volts to the knock sensor with key on. I tried this test on my 1985 but was not getting 5 volts at the connector, Can anyone confirm that a 1985 c4 would also have this 5 volt reference with key on engine off at the knock sensor connector. Oddly the check engine light only cam on for about 2 seconds and then went off but I was able to pull the 43 code by jumping a & b at the adl connector. If the car is supposed to have the 5 volts at the knock sensor...I would think the check engine light would remain on and there would be more drivability issues? Is it possible that the ESC module can be bad and is preventing the 5 volts going to the knock sensor? Can someone please confirm how code 43 impacts the 1985 L98 engine.

Thanks so much for your time

 

5v at the KS connector while key on engine off is a check that is done on vehicles without a ESC module i.e. '90-'91 and maybe '92-'93??? You should consider getting a FSM for your '85 C4 or this might help... https://www.corvetteforum.com/forums...post1600002941

 

Hi Stew,

Thanks for the advice. I was researching online and there appears to be many that think the 5 volt reference signal applied to the 85. After trying that test on mine and not finding 5 volts. I dug a little deeper and I believe you are 100% correct....my car with an spark module should receive a AC signal from the KS and then send a signal to the ECM to retard timing when knock occurs. Thanks again for your advice...glad you guys are so knowledgeable

 

The knock sensors on these cars (until 1996 when they changed completely) have a resistor inside them that tells the car that it's in place. If this resistor is damaged, it will throw Code 43. The resistor is far more delicate than the sensor itself, so it's very common for the code to be thrown when the sensor itself is still functioning normally. When you replaced the sensor, did you torque it to spec (15 lbft) or just crank on it?

 

Hi guys...thanks again for your advice.
Today I loosened the knock sensor and used a torque wrench to tighten it to 15 ft/pds. While there I also checked for battery voltage at the spark control harness with the key on and made sure there was continuity on the wire coming from the knock sensor (E)....I also ordered a new spark control module just to be safe. Since the issue is very intermittent...I figured it couldnt hurt. I also double checked my base timing with computer connector disconnected at its exactly 6 BTDC. Can someone let me know if advancing the timing higher will shorten the cranking time when starting the engine.

Thanks again for all the suggestions

 

Unfortunately an '85 has a unique ECM, so you can't do what the 86-88 guys do and update to the '89 code that removes the cold start injector and optimizes the timing tables to improve startup. You can still manually copy over the timing tables from an '89 onto the program for your '85 and burn a new chip and see if that helps though, or you can go to the trouble of swapping ECMs to the newer 86-89 style. In case you're curious about that, here's really awesome info: https://www.eecis.udel.edu/~davis/z28/ecm_swap/

 

I recently finished resealing the upper half of the engine and put in all new injectors, starter, fuel pump and regulator, catalytic converter, ignition module, plugs, wires, distributor, rotor, both battery cables, MAF, IAC, TPS.etc
I have replaced all vacuum lines and verified no vacuum leaks...and when warm....the car idles at 550 -570 so I am happy with the way its been working. I just wanted to ask if there is something I can do to shorten the crank time when starting the car. Right now it takes a few seconds of crank time. I thought I can shorten that by adjusting the base timing a bit. Not sure if there is anything I have overlooked

 

Again, the issue is the actual code on the ECM. That's just how an 85 runs. If you want your 85 to start like an 89, you'd have to actually modify the code on your PROM to match the 89. You'll never get it all the way there because again, totally different computer, but you can at least use the ignition tables from the 89 and see if that helps.

Or, as I mentioned, you can swap to the same ECM the 89 uses and use its code.

 

Now I get it....I knew the later models did away with the cold start injector and ignition curves. I didnt know the 85 had a somewhat longer crank time due to the software. Thanks again for your advice

 

Make sure the fuel relay is working as you'll get no fuel pressure until the oil pressure builds up and the oil pressure switch takes over for the failed relay.

 

Interesting, I didn't know it worked that way on the older cars. There's a similar function in the newer C4s but it's the CCM that gets overridden.

So yes, absolutely. If the oil pressure switch is what's engaging the fuel pump, that'll certainly take a while.

 

Are you sure on that? On my 91, it looks like a backup circuit and not the only one supplying to the pump. I know people have called it a lot of things in the past, including a safety switch but the circuit seems to be little more than a backup for the main system.

 

Hi Stew/all..yes I have already replaced the fuel pump relay and verified that the oil pressure switch closes with engine RPM over 400. The car does start and run well. I was interested in getting it to start a little sooner but it may just be the way these l98 engines are. or maybe im just a little OCD..lol

Thanks again for all the advice

 

If everything is good, you may have to delve into the chip and change things. Is it worth that to you to figure this out, buy the equipment, etc, etc?

 

I can't speak for his car, and I can't speak for yours. But on my '94 and '95, the oil pressure switch is tied into both the "CHECK GAUGES" light and the fuel pump. You can actually confirm that the oil pressure switch has direct control over the fuel pump by plugging in a laptop and sending the serial data command to the CCM telling it to disable the fuel pump. When you do that, everything works...but it takes a while to crank. The moment the "check gauges" light goes out (because there's oil pressure) the fuel pump kicks in and the engine starts.

It's not a safety switch as people incorrectly assume (it has no ability to shut off the fuel pump in the event oil pressure drops to zero unless it's the only thing powering the fuel pump at that time), but it is most certainly tied into the fuel pump circuit.

As always, check your year's wiring diagram in the FSM for reference.

 

No I'm not sure just quoting what others have said here. I think on one thread I said "that schematic has me confused and never understood how it works."

May I ask how you start your engine? I've seen people from off, go straight to crank and never give the ECM a chance to prime the pump for 2 seconds. Not going to say always but when I remember to, I normally key on until I hear the relay kick off then crank. Seems to make a difference from cranking for a few seconds to starting almost instantly.

As aklim mentioned^^^getting into PROM tuning may be the way to shorten it. On my '90 I've noticed the same and noted that the Bosch IIIs might be a major contributor. After changing the battery offsets the crank time seem to be worse. I had to increase the crank multiplier and it's better now. Of course this is all done in the PROM.

 

Hi Stew/all,

I typically turn key to on position..allow the puel pump to prime and then crank. I guess its just the fact that the older technology/software will not be as exact as the newer. Its certainly not worth changing out ecm and/or proms just to shave a second or two off he cranking time. Im sure that would just open up a new world of hurt with all the other impacted systems. I should just leave well enough alone I guess. I know these vintage engines are also known for their exhaust smell even when everything is working fine. I tried getting rid of my exhaust smell by replacing pretty much all ignition parts, new injectors, FPR ,resealed the upper half of engine, replaced all rubber hoses, new O2 sensor, new catalytic converter, etc. the The car has 107k miles on the clock but I had rebuilt the engine nearly 20 years ago when the car had 93k miles with new pistons and edelbrock aluminum heads.and nearly stock compucam. I believe compression stayed pretty much the same.
Thanks all for the great suggestions

 

From what I see, it is a backup circuit just in case your pump circuit fails and there is oil pressure and not the other way around where it shuts off if the pressure falls below a certain threshold.

In one fluid motion, I turn it to crank and when it catches, I release. Try this. Hook up a fuel pressure gauge and watch it hit top pressure and see how fast it is. The rest of the time is unnecessary assuming your fuel system is up to par. Considering the speed electricity travels, the current gets to the pump faster than you can turn it from "Run" to "Crank". I see the gauge instantly shoot up to 43 or whatever it is set to and it keeps priming for the rest of the 2 seconds.

I don't follow. What battery offsets and how does it affect the crank time? If there is enough power going to the starter, it should not change but I am not sure what offsets you refer to.

 

Approaching 4 months of owning my C4, my initial thoughts on the crank time to until it catches seemed a little long. Perhaps not being stock i.e. bigger cam, SR intake changes the starting fuel requirements???

https://www.thirdgen.org/forums/diy-...155-710-a.html