ok.. so i am stumped and the mechanic at my shop is stumped. 97 blazer 4.3l w vin code 4x4.

symptoms:
hesitation under light acceleration (feels like ignition breaking down, but under load it is fine) bogging down in first gear at first take off each key cycle.

Replaced:
OEM Cap, Rotor, Wires, Plugs, Coil, IAC motor, TPS sensor, TB Gasket, upper plenum gaskets, replaced vaccum lines, fuel pressure 60 psi bleeds down to 55 after about 10 minutes (checked at service port). test driven with cat unplugged, new fuel filter..
(All parts are OEM From dealership, gotta love shop discounts!!!)
im at a loss... thought maybe 02 sensors, but they are switching just fine
(have access to really good scanner, but not a tech 2)

no current or history codes. only pending code is fuel level sensor (i would replace the pump, but it works fine per fuel pressure test)

any ideas? we are freaking frustrated and it is starting to bug me...

 

If the check engine light works, and it's not coming on, and there are no codes stored, the problem is something that the PCM does not monitor, (fuel pressure is not monitored). Checking fuel pressure at the service port shows you regulated pressure, or maximum pump output pressure, whichever is lower. 60psi is borderline low, and if that's all the pump can build, that's a problem. Really need to check pressure and leakdown at the fuel filter outlet. This link explains:
https://blazerforum.com/forum/2nd-ge...eakdown-88305/

 

let the truck sit overnight at the shop and checked fuel pressure at first key on of the day. only came up to 45. cycled it a few times and MOST i could get was 52/53. i know GM doesnt give running specs, but it hovered right around 50 while running... fuel looks very dirty when relieving pressure from the tester also.

got a new pump ordered. will replace it this weekend along with the fuel filter and cleaning the fuel tank.
even if it doesnt fix my hesitation issue it will take care of my fuel gauge that has been inop since i bought the truck!! so money will not be wasted..

 

As mentioned in post #2, checking pressure at the service port shows you the LOWER of regulated pressure, or fuel pump maximum output pressure. Without doing the entire test in the link, you have no idea what the problem really is. If it does turn out to be the pump, AC Delco or Delphi only. Don't waste your time or money on aftermarket stuff.

 

ok. new delco fuel pump. fuel filter, and just replaced the spider injector with the upgrade last night (new a/c delco as well). still misfires under light throttle (up to about 40% then runs fine after). does it whether there is a load on it or not (park, reverse, neutral, drive). going to use scan tool and check live data. possible mass air flow sensor? completely stumped. i would pay to have it professionally diagnosed, but i am afraid of paying for a mis-diagnosis...

 

so pulled codes at lunch. P0300 random misfire and P1381. Did not have description for 1381 in our scanner. pulled it up on alldata and it shows "Misfire Detected No EBCM/VCM Serial data"

any ideas? it has basically no information on it.

ran live data while driving. everything seemed to be normal. only thing off was CMP Retard was steady at -22 and it showed normal range was -20 to 20. everything else was good.

 

Howdy!

Check my thread out, I recently had some similar drivability issues as you (hesitation under acceleration), but different codes. When I first got the codes, there were no noticeable drivability issues, but they progressively worsened over a few weeks time.

https://blazerforum.com/forum/2nd-ge...olution-90048/

Monitor fuel trims for lean or rich condition which might help diagnosis, lots of info here and on web for normal/abnormal values. My research also showed P0300 often occurred with a bad PCV elbow. If nothing else, look it over closely, especially the back facing the firewall using flashlight and mirror or take a pic with flash. If the elbow is suspect, or the fresh air tube and/or PCV valve are easy to pull out of the valve cover grommets, I'd replace everything I mentioned in my linked thread. Cheap fix for me that fixed my problems and now better gas mileage. I also recall reading the IAT can cause some of the issues noticed.

 

Desired spec for CMP retard is zero degrees, with a tolerance of 2 degrees plus or minus. -22 degrees will cause serious crossfire inside the distributor cap, and most definitely set the P0300. It will require removing the hold down bracket from your distributor, and using a small block chevy hold down bracket, or modify your existing bracket, to make the distributor fully adjustable. The P1381 should go away once CMP retard is within spec.

 

thank yall for the help.. BlazinClint.. i have already replaced the pcv valve, elbow, and grommet. new air filter, but it is almost due for an oil change.. i will go ahead and do it a little early.

Captain. I will buy a SBC hold down bracket today and put it on so i can adjust the CMP retard. I will report back with the results monday (dont have access to scan tool on weekends)

 

Cap, with it being that far off would it be possible the timing chain is stretched and jumped a tooth?