Wazzugar

I'm having a strange problem with my 86 4+3. I'm running the 89 APYP bin with CSI disconnected. The engine will start after the first crank and run fine. If I shut down the engine and try to restart, it will only crank. No amount of throttle input will get it to start and no codes are thrown. I have the chip in a Moates adapter. If I remove the chip and reinstall it, it will start immediately with a large RPM flare. It does not seem to be temperature dependent, it will occur on a dead cold engine, or a hot engine. However, If I let the car sit for several hours it will fire. Also, if I remove the adapter and put the stock chip in place, it will start reliably. Any ideas?

tequilaboy

Could be that the ecm is not seeing enough distributor reference pulses on subsequent restart attempts.

89 ($6E) bins wait to see 8 distributor reference pulses before activating the injectors, so they behave differently during cranking than earlier $32 and $32B bins.

No idea why it works the first time however, but fails to start afterward. Is the fuel pump priming?

Why would you want to run APYP on a 4+3 car??? I would choose an 87-88 $32B bin for this application.

I understand that 86 4+3's in particular are problematic due to the lack of a 1st gear switch and different shifter switch wiring compared to the later cars.

The 86 O/D on/off switch is connected to pin C8. The same pin is used for the first gear switch in 87-88 cars.

That being said, I would still use an 87-88 $32B bin with the O/D on/off switch repinned at the ecm to the 87-88 pin C7. I think I could live without the 1st gear switch and call it a 4+4....

Now there is another alternative....I did hack in the 86 first gear detection logic into an 88 4+3 bin years ago for use in 86 cars. The switch input was also changed in the programming to avoid the repin operation mentioned above. A few users on this forum have used my hybrid 86/88 bin with good results, but I've received very limited feedback on it over the years, so ymmv.

https://www.corvetteforum.com/forums...-86-4-3-a.html

Wazzugar

Yes, the fuel pump is priming for a few seconds as it should. I have the OD switch going direct to the overdrive relay. I thought the car was supposed to have a momentary switch but it has a toggle, so it just works as 4+4 and I didn't ask too many questions. I picked the $6E code since the hack was well documented and I could get rid of the cold start solenoid. I have started by tracing through the code to compare timing between $32 and $6E. I am also thinking for the very first crank the FMD chip may give extra fueling. I have a mild cam which may contribute and the fuel injectors are different between 86 and 89. If it was fueling it would have to be lean since clear flood doesn't help.

Wazzugar

You were spot on about the 8 reference pulses tequilaboy. I forgot to put into my tuning notes that I modified the crank fuel multiplier table (months ago). I added more fuel in the first few reference pulses to light the engine off faster. I think I was adding too much fuel before the engine could get up to speed. I also had zeroes at the end which I think means it would never give any fuel after 24 pulses....not sure what I was thinking there. Engine would start in this condition if it sat long enough. Here's a copy of the table. I went back to stock multipliers and the engine starts reliably now.

Reference Pulse Multiplier (stock) Multiplier (mod)
1 0 0
2 0 0.5
3 0 0.5
4 0 0.5
5 0 0.5
6 0 0.5
7 0 0.25
8 0 0.25
9 0.5 0.25
10 0.5 0.25
11 0.25 0.25
12 0.25 0.25
13 0.25 0.25
14 0.25 0.25
15 0.25 0.25
16 0.25 0.2
17 0.2 0.2
18 0.2 0.2
19 0.2 0.2
20 0.2 0
21 0.2 0
22 0.2 0
23 0.2 0
24 0.2 0

tequilaboy

Glad you found the problem and thanks for the follow up.

Now go out there and fix that O/D switch set-up. Manual control is so primitive.