2000 blazer hesitates

It has 217,000 miles on It.... Vortec 4.3 V6 ... Engine stumbles when you accelerate. Check engine lite on. I had Auto Zone to a code scan - I got back P0300 & P0420...So I repleaced Ignition Cap & Rotor, Wires & Plugs (coil was replaced about 1 1/2 years ago.....) Still hesitates & and Engine light is on,.....What gives?

Pull off the air intake and open the throttle valve by hand, take a look inside and if it's got lots of carbon buildup in there, start with an induction flush. At work I use a kit from BG products, works great. Unfortunately it requires a special tool, which means you need to take it to a shop for this. Carbon on the valves can cause both codes... it absorbs gas when you accelerate and makes it run lean (random misfire code), and then on decel it keeps releasing that gas making it run rich and plugging the cat (cat inefficiency code). It certainly isn't a guaranteed fix, but chances are it needs it anyway. Plus on a very dirty engine it can get you up to a 15% increase in gas mileage (or, rather, undo the 15% drop you got over time from the buildup). The good news is that there's about a 75% chance that the flush will burn off the carbon in the cat and get it at least to where it doesn't set the code anymore.

If AZ did not clear the codes after the repair, it will take a while for the SES light to go off on its own, provided it is repaired. Have you checked fuel pressure and leakdown?

If it's still hesitating, which I'm guessing goes along with the P0300, the problem isn't fixed, but as old master said, the codes should be cleared any time a repair is done. That way you can verify what, if anything, you fixed. The p0420 usually takes longer to reset.

I had very similar issues before I cleaned my MAF sensor. I would suggest giving that a cleaning.

Pull off the air intake and open the throttle valve by hand, take a look inside and if it's got lots of carbon buildup in there, start with an induction flush. At work I use a kit from BG products, works great. Unfortunately it requires a special tool, which means you need to take it to a shop for this. Carbon on the valves can cause both codes... it absorbs gas when you accelerate and makes it run lean (random misfire code), and then on decel it keeps releasing that gas making it run rich and plugging the cat (cat inefficiency code). It certainly isn't a guaranteed fix, but chances are it needs it anyway. Plus on a very dirty engine it can get you up to a 15% increase in gas mileage (or, rather, undo the 15% drop you got over time from the buildup). The good news is that there's about a 75% chance that the flush will burn off the carbon in the cat and get it at least to where it doesn't set the code anymore.


I cleaned the throttle body with carb cleaner and sprayed the rest of the can in the throttle body while it ran....then I put 2 bottles of STP super concentrate with a tank of 92 octane gas and let it run out....still runs bad and gas milage sucks......

I had very similar issues before I cleaned my MAF sensor. I would suggest giving that a cleaning.


I replaced that about 2 years ago.....

I replaced the front c02 sensor about 2 years ago, but not back one....ill do the fuel pressure test this weekend......

If AZ did not clear the codes after the repair, it will take a while for the SES light to go off on its own, provided it is repaired. Have you checked fuel pressure and leakdown?


I took these fuel line pressures

59- Ignition ON

56- Car Running ( i heard it supposed to 3 to 10 lower then ignition ON)

It slowly dropped after about 3 minutes with ignition off

Then I disconnected the Battery for 5 minutes to clear the codes...

Engine light is now off......

I took these fuel line pressures

59- Ignition ON

56- Car Running ( i heard it supposed to 3 to 10 lower then ignition ON)

It slowly dropped after about 3 minutes with ignition off

Then I disconnected the Battery for 5 minutes to clear the codes...

Engine light is now off......

The GM "dealer issue" shop manual says key on, engine off, pump running, pressure must be 60psi to 66psi and must remain above 55psi for 3 to 5 minutes after the pump shuts off. GM does not publish any "engine running" fuel pressure specs for the CSFI system, there are far too many variables involved. Check pressure and leakdown again and post your results.

The 3 to 10 psi drop with the engine running is for older CMFI engines only.

Disconnecting the battery will turn the SES light off in some cases, but it does not clear the history file, (your codes are still in there), or reset the monitors. On OBDII equipped vehicles, a scan tool must be used to clear and reset the PCM.

The Autozone near me won't clear codes, period. They will read them, but I guess they got their butts kicked by the emissions people, anyway, they told me that their new corporate policy will not allow them to clear codes. I went out and bought a (good) cheap scanner on E-Bay for $60 USD that will take care of erasing codes. You will save that on one trip to the garage.

The GM "dealer issue" shop manual says key on, engine off, pump running, pressure must be 60psi to 66psi and must remain above 55psi for 3 to 5 minutes after the pump shuts off. GM does not publish any "engine running" fuel pressure specs for the CSFI system, there are far too many variables involved. Check pressure and leakdown again and post your results.

The 3 to 10 psi drop with the engine running is for older CMFI engines only.

Disconnecting the battery will turn the SES light off in some cases, but it does not clear the history file, (your codes are still in there), or reset the monitors. On OBDII equipped vehicles, a scan tool must be used to clear and reset the PCM.


P0420 came back.Others gone..........Looks like its the catalytic converter......

Here's the diagnostic flow chart for P0420. There are three pages involved, but only one page of actual testing. Follow the instructions EXACTLY, or you might be replacing the converter needlessly:

http://i561.photobucket.com/albums/ss58/cwhook/P0420page1.jpg

http://i561.photobucket.com/albums/ss58/cwhook/P0420page2.jpg

http://i561.photobucket.com/albums/ss58/cwhook/P0420page3.jpg

Here's the diagnostic flow chart for P0420. There are three pages involved, but only one page of actual testing. Follow the instructions EXACTLY, or you might be replacing the converter needlessly:

http://i561.photobucket.com/albums/ss58/cwhook/P0420page1.jpg

http://i561.photobucket.com/albums/ss58/cwhook/P0420page2.jpg

http://i561.photobucket.com/albums/ss58/cwhook/P0420page3.jpg

Dang bro'... :confused:

I was expectin' to see the whole FSM scanned into that page.

15 pics Pffft. :mad: :crying:

:grinyes: P0420 is not difficult to diagnose. The parameters programmed into the PCM have very close tolerance with little room for error, so everything must be right or it'll set the DTC. The tolerance can be widened but it requires flashing the PCM. There is a TSB that addresses it... just can't remember the number.

:grinyes: P0420 is not difficult to diagnose. The parameters programmed into the PCM have very close tolerance with little room for error, so everything must be right or it'll set the DTC. The tolerance can be widened but it requires flashing the PCM. There is a TSB that addresses it... just can't remember the number.


Will check it out.....