2011Corvaroguy

Hello
I just recently bought a 1986 Corvette with only 3800 miles. What I KNOW: Car starts immediately, runs, however right around 170 degrees it starts running rough - the check engine light flashes on and off - and it will then completely stall. It will start - awhile later after cooling down. Had a rebuilt ECU unit installed. Had a reprogrammed chip installed. A new MAF. A new Ignition control module. I drove it some 90 miles bringing it home - it would stall out - BUT immediately restart. The car was kept indoors - driven only about 100 miles a year owner was a collector. I can start it up in the garage - wait until it gets up to around 170+ degrees - runs rough - Check engine light flashes - stalls completely. I bought the little GM OB1 code reader and it continues to say bad ECU code 51 IF I am interpreting the flash counts correctly? Before running to the catalogue to buy this, appreciate any suggestions. The GM OB1 is quite primitive in displaying problems or being bidirectional to reboot. I've read about "closed loops". I have disconnected - reconnected the battery. Hired an expensive "we come to your home mechanic" - became apparent this type of problem way beyond his experience. He suggested to run the AC to turn the fans on. No difference. When he replaced the ignition control module, he put a brand new distributor cap on and a new rotor. We pulled several plugs and they are SUPER clean. We disconnected the battery - let it sit - then reconnected hoping this might remove any other stored codes. ?? Could this possibly BE a fuel pump going bad? The only other piece I have not yet looked at is the Idle Control Valve. Hope someone may know of the solution!

Topduarte

Fuel injectors and/or fuel pressure regulator acting up when it gets warm. That is my guess.

vader86

It is definitely a closed loop issue. The car restarts because it always resets to Open Loop operation until an internal timer is crossed, then if it sees proper engine temps and readouts from sensors, it will go back to Closed loop, the timer gets shorter the hotter the Coolant is reading. Usually the longest timer is at the 150-160F range and the next one up is about 170F, but I'm going from memory. If the engine is hot it will almost immediately go to closed loop.

Open loop mainly uses MAF, CTS, TPS voltage, plus the timing/fuel tables in Chip memory. I forget at the moment the parameters for the CSI circuit.

Once it goes closed it will read all sensors and another table in memory. O2 sensor failure wouldn't cause your issues to be this bad. An OBD1 cable and laptop are what I'd hook up and I'd look at my TPS voltages, IAC counts, and then whats going on with the fuel when it starts doing it. A bad vac leak could do it. Since you've replaced the chip and ECM once though, the scan read out would probably look off from the start, and one of the two or both are possible culprits. If you have not looked at the fueling system, that place is next. Hook up a FP sensor to the rail and tell us what pressures you're running at idle on startup, and after shutdown. You may also want to check injectors with noid lights.

383vett

If he drove 100 miles a year, the fuel must be pretty musty. Siphon, refill with fresh gas and retry.

MatthewMiller

I agree with vader86: it's a closed-loop-only sensor that has gone bad. I wouldn't necessarily assume it is not the O2 sensor just because it stalls outright. Those things often fail slowly, so we notice a problem until they fail outright. However, if a 1986 car has been sitting for a long time and/or was run with old fuel, it's entirely possible the O2 sensor is really bad or DOA. It's also possible a mouse chewed through the signal wire from the sensor to the ECU and it isn't seeing any signal from the sensor at all. On a car this old and unused, these are both real possibilities. The good news is that they won't be hard to fix once you know what the issue is. I don't know what - if any - other sensors are only read in closed-loop mode by the 1986 ECU, but if you can get the list of them, then I would focus on them. Vader's recommendation to data-log the car is good: you need to initially focus on the readings from the closed-loop-only sensors, noting what they read after the car starts to falter.

tequilaboy

Code 51 is either a bad checksum or incorrect mask id in the bin. Suspect chip programming was botched or a bad connection between eprom, memcal and ecm. Doesn't make sense that this would occur at 170 °F.

MatthewMiller

Maybe the two are present but not related? IME, for example, O2 sensor failures don't always trigger a DTC at all.

zbo2

My sons 94 camaro did that ... bad knock sensor no codes... took a while to figure that one out.. replaced the only one I saw 2 times till i realized there were 2 of them.... replacing the other one did the trick.