Rough Starting w/ back fire

[89_Corvette]

I have an 89 Coupe, which has been one challenge after another for the last few months. I think I am finally making some progress. It runs fine and idles steady at 700 rpm. But the first time it is started after a few days it is difficult to turn over. When it does, it stumbles, back fires and occasionally dies. If I can get it running for even a few minutes, it warms up and runs fine even after it sits for a while. The timing is solid at 6 degree BTDC. Any thoughts would be helpful.

Thanks

[c4c5specialist]

Hi there,

I would be looking for spark scatter.

Either the distributor cap, ignition rotor or wires. Inspect them all to check for any type of carbon tracking, and make sure they are ACDelco.

Allthebest, c4c5

[89_Corvette]

Thanks for the help, all those parts are new, not sure if they are ACDelco, I'll check. What is spark scatter?

[89_Corvette]

Okay, I have some more information. I got a fuel pressure kit from Autozone. When it is cold the fuel rail pressurizes to about 44 psi and it hold pressure well (4 psi drop after 15 min). Once started, it idles rough, a back fire or two and has about 40 psi with 16 lbs of vacuum @700 rpm. Also when I push the accelerator the fuel pressure bounces all over, dipping and spiking.

Once it warms up, a completely different story. The idle smooths out, pressure is rock steady at 48psi, still 16 lbs of vacuum @700 rpm. The fuel pressures remains steady at 48 psi on acceleration.

Any thought as to why it idles rough when cold?

Thanks

[c4c5specialist]

HI there,

Ok, thats interesting.

The fuel pressure fluctuates based on vacuum within the engine, so that fluctuation that you have could go from 30-50, depending on vacuum.

You may just a few injectors that are restricted from deposits. Unfortunately these injectors are KNOWN for problems like this.

I would suggest a bottle of TECHRON 20 oz bottle to your next fuel tank and see if it clears up by the time your finished with the tank.

Allthebest, c4c5

[89_Corvette]

Thanks for the quick response. I didn't mention it, but I replaced the multech injectors with bosch 3s. So the injectors in there are new and only have about 20 miles on them.

John

[c4c5specialist]

HI there,

Ok, that eliminates the injectors .

I would definately be checking for something cold, as if you dont have a vacuum leak, then you will have to start looking at the scan tool data of your engine to determine what the computer is looking to correct.

Allthebest, c4c5

[89_Corvette]

Fuel pressure seems good, 40 - 44 psi

[c4c5specialist]

HI there,
Ok, from this point, I would be looking at the ignition system.

This would specifically entail looking at the distributor cap and rotor.

Look to check for any carbon tracking and see if they are ACDelco.

Allthebest, c4c5