2002 Blazer stumbles slightly at speed

[reefgeorge]

151K miles. Feels like small interuptions in power intermittently at speed like spark or fuel problems. No codes. Did plugs, wires, distr, rotor, fuel filter thinking that was the reason. No change. The fuel pump died. Replaced that, no change. Fuel pressures look good at the curb. Fuel mileage is nominal. The truck idles rougher. I do a lot of highway driving and breakdowns are a real pain so I am trying to catch this while the truck is still running but of course I have no identified failure (yet). I plan on another fuel filter change (because of the new fuel pump), check vacuum, check again for soft codes.

What are the most likely causes for this vehicle/year/mileage?

Thanks,

George

[MT-2500]

Originally Posted by reefgeorge

151K miles. Feels like small interuptions in power intermittently at speed like spark or fuel problems. No codes. Did plugs, wires, distr, rotor, fuel filter thinking that was the reason. No change. The fuel pump died. Replaced that, no change. Fuel pressures look good at the curb. Fuel mileage is nominal. The truck idles rougher. I do a lot of highway driving and breakdowns are a real pain so I am trying to catch this while the truck is still running but of course I have no identified failure (yet). I plan on another fuel filter change (because of the new fuel pump), check vacuum, check again for soft codes.

What are the most likely causes for this vehicle/year/mileage?

Thanks,

George

151K coulkd be many things.
Did yoy use a delphi fuel pump for replacement?
Have you checked for good fuel pressure?

Hoe is secendary ign?
How many miles on plugs and wires and cap?
Fuel and air filter?
Good Ac delco plugs?

[reefgeorge]

Fuel pump is Delphi. Pressures (all at stand still) after the replacement were:

61 psi at 2 second prime
56 psi after 5 min leak down
56 psi after start

Distr/rotor/wires/plugs all new and all AC Delco. Just got the special tool for the PITA #3 plug and will do that last one this weekend.

Fuel filter is new

Air filter is 4k miles

I'll get a spark tester on the ignition wires and see how much energy I have.

What are the first things to go - ignition switch, MAF sensor, ignition module, coil, injectors, crank position sensor, etc.?

Thanks,

George

[MT-2500]

Originally Posted by reefgeorge

Fuel pump is Delphi. Pressures (all at stand still) after the replacement were:

61 psi at 2 second prime
56 psi after 5 min leak down
56 psi after start

Distr/rotor/wires/plugs all new and all AC Delco. Just got the special tool for the PITA #3 plug and will do that last one this weekend.

Fuel filter is new

Air filter is 4k miles

I'll get a spark tester on the ignition wires and see how much energy I have.

What are the first things to go - ignition switch, MAF sensor, ignition module, coil, injectors, crank position sensor, etc.?

Thanks,

George

To many things to chose from.
Proper testing first before throwing parts at it.
I would get it on a good engine capable scanner and check for codes and all sensor readings for up to par.
Watch and check for missfires.
Post back any codes.
Fuel pressure may be low or fading out after on road running or spark getting weak someplace.

The only true test on a fuel pump is to check the direct fuel pressure from fuel pump.
A quick test is to block off the return line and see if fuel pressure comes up to 75-85 lbs.
But do not run the pump at full pressure very long.
Also when testing fuel pump and pressure you need to tape a gauge to outside windshield or outside mirror and drive it on the road for 20 -30 minutes
until the pump gets has run a while to check for a pump fading out after hot.

Hard to start cold or hot and fuel pressure testing guide lines.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
If pump has full pressure with return line blocked and low pressure without it blocked most usually the fuel pressure regulator is leaking or not holding pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak in system.
Post back fuel pressure readings.



And run a full fuel pressure test

[reefgeorge]

Will do on the running check with a scanner.

WILL DO ON THE RUNNING CHECK OF FUEL PRESSURE. wHERE DO YOU ROUTE THE TESTER HOSE, UNDER THE REAR SIDE OF THE HOOD THROUGH THE SEAL?

Are there any injector cleaner fuids for the gas tank that are OK to use?

What kind of shape are these injectors likely in at this mileage?

Thanks,

George

[sreed25570]

have seen small intermittent dropouts in the TPS causing a "surge" feeling test if able voltage should steadily raise and fall any dead spots replace TPS

[MT-2500]

Originally Posted by reefgeorge

Will do on the running check with a scanner.

WILL DO ON THE RUNNING CHECK OF FUEL PRESSURE. wHERE DO YOU ROUTE THE TESTER HOSE, UNDER THE REAR SIDE OF THE HOOD THROUGH THE SEAL?

Are there any injector cleaner fuids for the gas tank that are OK to use?

What kind of shape are these injectors likely in at this mileage?

Thanks,

George

I usually go out rear of hood.
Sometimes at rear corner and tape gauge to windshield or rear view mirrow.
Any place to not pinch tester fuel line.

Injectors are usully still good if good clean fuel is used.
In a engine capable scanner there is a built injector balance test for testing injectors.

[blazes9395]

I agree that proper testing is always a good idea and this case is no exception. That said though, have the injectors ever been cleaned, and/or replaced? It could be the case where a injector is getting dirty/worn giving you an intermitant miss/stumble.

[b1lk1]

There is no injection cleaner that can be put in the gas tank that will do anything useful. They are all garbage. THe only way to have injectors cleaned is by professional cleaning services out of the engine.