99 Blazer Engine Hesitation

[Pinokeeoh]

I have 99 Blazer that when driving at pretty much any speed I feel a quick (split-second) hesitation. Doesn't seem to matter what weather conditions, terrain or temp I'm driving at. Any suggestions???

[GMMerlin]

Does this happen just once while you are driving or happens constantly?
Could be a secondary ignition misfire or a sensor internal fault.

[johnblazer]

I have this same problem on a 2001 Bazer. Bought it used with just 38k. Didn't notice this until taking a long hwy drive.

It is almost an imperceptable hesitation, and only noticable at highway speeds, but very annoying and worrisome. I first discovered that it only happens when the airconditioning is on, but then discovered it happens when front defroster is blowing as well. Doesn't seem to happen when just the blower/vent was running. I haven't checked yet on heater. It happens with cruise control on or off.

It does not happen when the air/defrost etc is off. Anybody have any idea what this is?

[kernwaffe]

Many people don't think that the A/C is useful during winter, so many people get mad when you tell them that defrost uses the A/C- It uses it to remove the moisture in the vehicle (window fog), same as it does in summer (humidity), but doesn't run as long in winter due to cold temperatures. I also have an '01 blazer 2dr 4wd, which sets a code for secondary air injection pump (which I guarantee yours sets off). I do not understand what it is (lol), but I know the CEL flashes at speeds of 70mph+ w/ A/C (defroest) on. I have another code that keeps the check engine light on, so i only see it flash under your similar conditions, but my other code is a self check for the flapper valve for fueling the vehicle, so I can never get the tank 100% full; which BTW is caused by filling up while the engine stays running. I am listing this here, because I have hesitation with mine, which I have not scanned yet, but potentially might be the injectors (california there is a 200,000mile/10yr recall on those).

[kernwaffe]

Straight from Alldata- wish I had Mitchell On Demand!
Bulletin No.: 04-06-04-015A
Condition
Some customers may comment on the SES/MIL light being illuminated. Upon investigation, the technician may find a DTC P0410 indicating that there is a concern in the secondary air injection system.
Cause
Water may have collected in the AIR pump or the vent solenoid and frozen or corroded the pump.
Correction
Replace the AIR pump and install a new inlet hose and solenoid tube assembly using the following procedure:
Raise the vehicle on a hoist.
Remove the shield covering the AIR pump.
Remove the hose and vacuum lines from the pump and solenoid.
Remove the AIR pump inlet hose. Do not re-use the hose.
Disconnect the electrical connectors from the pump and solenoid.
Remove the pump mounting bolts and pump.
Transfer the isolators from the old pump to the new pump.
Install the new AIR pump and bolts to the vehicle.
Tighten
Tighten the bolts to 17 N.m (12.5 lb ft).
Route the new hose assembly up between the engine and fender. The hose should come up near the area between the battery and the coolant bottle.
Connect the new inlet hose to the pump inlet.
Connect the old outlet hose to the pump outlet.
Connect the electrical connector to the pump.
Connect the electrical connector for the solenoid to the connector on the new hose assembly.
Connect the hose with the white nipple to the vacuum source hose.
Connect the remaining hose to the shut off valve.
Install the shield covering the AIR pump.
Lower the vehicle.
Remove the coolant reservoir nut nearest the battery.
Route the new hose assembly between the battery and the coolant reservoir with the solenoid on top.
Install the solenoid bracket onto the coolant reservoir stud and reinstall the nut back into the vehicle.
Use a wire tie to keep the new hose assembly away from the engine. On four cylinder engines, tie to the AIR outlet hose. On six cylinder engines, tie to an available hole in the radiator fan shroud.