1996 impala backfire on cold start

[j cat]

I am the original owner of this vehicle . nov 1995. since it was new when it was below 20degf out and the vehicle was not used for a few days on first cold start it would spit/backfire slightly on starting.

this year lately even now in the warm weather it will sometimes pop/backfire on first cold start . vehicle has 68,ooomi. no major problems.

runs great, starts quickly. great high rpm performance , no DTC's.

I did notice when fueling up ,that the fuel tank appears to have a good amount of vacuum . In the past you would get not much of that at all.

I have the oem shop manual. any ideas would be great.

[Crime Scene 96]

Well first of all, welcome to the forum.

Secondly, whats those spark plugs looking like?

[j cat]

Originally Posted by Crime Scene 96

Well first of all, welcome to the forum.

Secondly, whats those spark plugs looking like?

spark plugs are the 3rd set installed 2 years ago ac delco . when they were replaced 3,ooomi ago those plugs looked very good with 25,ooomi on them . all looked exactly the same with a light brown /tan color and very clean.. engine runs great at all speeds/loads.

only the cold start pop/backfire , especially if it sits for a few days.

this issue has been with this since it was new. only now it is getting more common and at the warmer air temps.

thanks for your reply.

[Mr pogo]

check fuel pump pressures.
plenty of threads search will find.
if thats ok look at the opti and related.

[j cat]

Originally Posted by Mr pogo

check fuel pump pressures.
plenty of threads search will find.
if thats ok look at the opti and related.

I measured fuel pressure / fuel pressure test. 43psi ign on engine off/39 psi engine on/ 43psi pulled vac line off reg/ ign off pressure held. disabled fuel pump eng started and ran until the pressure hit 15 psi at idle rpm.

pump pressure jumps up to 43psi fast.

checked fuel pump relay . tested good . cleaned switch contacts anyway.

measured resistance of the ECT/IAT both read the same resistance which indicated 70deg F.

removed vapor cannistor. tested control valves and checked vapor line running from cannister to tank. checked that cannistor was taking vapor.
no smell of fuel.

passed all tests no restrictions or leaks.

it is like the engine on the first cold start after a day or more starts up too fast. seems like the crank is about 1/2 a rotation. not enough to get the timing correct.

opti failures are usually at the higher than idle RPM . also you get DTC with this type failure ...at least some missfires especially at upper rpm range..

could be plug wires have high resistance but not open . on a cold start that might cause the engine to pop. I'll have to measure the wires resistance.

[Fast4drSS]

Jcat

Cold start maybe condensation in the the Opti. Check your vents for the Opti.

Mine Opti just freaked out and is send incorrect signals to the PCM causing the motor to backfire.

An interesting test would be to heat the Opti with a hair dryer and see if the problem goes away at startup. The electronics may be slowly failing.

[j cat]

Originally Posted by Fast4drSS

Jcat

Cold start maybe condensation in the the Opti. Check your vents for the Opti.

Mine Opti just freaked out and is send incorrect signals to the PCM causing the motor to backfire.

An interesting test would be to heat the Opti with a hair dryer and see if the problem goes away at startup. The electronics may be slowly failing.

I doubt the pcm is causing this , because this problem was with the vehicle when new. could however be sensor related .


your condensation of the OPTI is very possible as a cause. with age those seals on the opti cap breakdown . I have tested the OPTI venting . could be that the venting check valve restrictor is not venting enough .

I have a new water pump I purchased several years ago because the weep hole would sometimes drip a few drops. it is not leaking now.

when I replace the coolant very soon , I am going to remove the water pump and opti cap to see if the opti cap requires replacing/cleaning and I am sure the cap gasket/rotor as well.

thanks for that tip.