1999 blazer running bad when cold

[chevy05]

I got a 99 blazer and when you start it while it is cold and you try to drive up a incline which is what my driveway is. It pops and bogs and sounds like it is backfiring threw the air box and will not drive. If I turn it around and drive it down my driveway which is about 1800 feet long by the time I get to the bottom it will run fine when you take off down there. You can go driving and it runs great and drives great. Any advice

[Rick Norwood]

Please Post back with engine size and also 4wd or 2WD

[old_master]

Is the SES light on? Any codes in memory?

[chevy05]

4.3 4wd and no check engine codes at all I have scanned to double check.

[old_master]

I would suggest checking fuel pressue and leakdown first. If that checks OK, it could be a problem with condensation building up inside the distributor cap, (very common). If that's the case, replace the distributor cap and rotor with AC Delco parts. Put a dab of silicone dielectric grease on each terminal, inside and out, of the cap.

[chevy05]

Okay I will check the fuel pressure tomorrow but I didn't figure that would even be a place to start since once you let it warm up for 5 minutes or drive it for a 1/4 mile it will run perfect from then on. I am really thinking the condensation in the cap could be a problem.

[old_master]

These engines rely heavily on correct fuel pressure and volume, if it's low, or the fuel filter is restricted, they don't perform well. If there's a problem with fuel delivery, it usually does not set a DTC or turn the SES light on. The condensation issue may or may not set a DTC, depending on how long it misfires.

[chevy05]

What would cause the fuel pressure to become correct after it runs for a couple minutes and warms up to make it run good then.

[old_master]

It's probably not the fuel pressure that's changing, engine temperature is.

Before fuel can be burned efficiently, it first must be atomized. This is done when the fuel exits the injector as a pressurized mist. Next it must be vaporized. This is done with the heat in the intake port, on the back side of the intake valve. Then it must be compressed, then light the fire!

When the engine is cold, the vaporization process is almost nonexistant so it relys on good fuel atomization and a richer fuel mixture. If fuel pressure is low, the fuel is not atomized as well and performance suffers. As the engine warms up, and the fuel is vaporized, the mixture can be leaned out and the engine will run fairly well, even with slightly lower fuel pressure.

If fuel pressure drops below 60psi it will cause driveability issues warm and cold. Below 55psi it will require a shot of carb cleaner to start. Below about 43psi, the engine will not run because the poppet valves won't even open to squirt fuel.

[TCGrove]

I've got a 99 Blazer 4wd with the 4.3L. I've had every problem known to man including yours. I did all of the following: New plugs, wires, distributor, fuel filter, cleaned the MAF sensor...but honestly I think changing the O2 sensors did more for this problem. I makes sense given the sensors have some warm up time. I eventually gutted the cat converter but it started throwing a code. So I got the little extender that pulls the sensor behind the converter out of the gas stream. It runs great and no more hesitation. I swear I thought I was going to get rear ended sometimes pulling out in traffic. Eventually I also replaced the entire distributor while fixing the intake manifold gasket at 155,000 miles. The little gear on the end was shot. The fuel pump totally went out when it bit it. I hope this helps. Look at the O2 sensors.......

[old_master]

O2 sensors are a possibility for causing a misfire on a warm engine, but not on a cold start. The sensors take about 2 to 3 minutes to warm up, during that time the ECM ignores the O2 sensors. When an O2 sensor gets old, (100,000 miles or more) it becomes "lazy" causing the ECM to increase injector pulse width which richens the fuel mixture = poor fuel mileage.