im new to this site but use it often for references for my girlfriends blazer, and now im stuck

the code p0300 came one, and on my code reader i read advanced dtc and p1380 was also there, which is random misfire of course

everything is new, im a mechanic so i fix everything on it, new plugs, wires, cap rotor, fuel filter and pcv valve. we drove cross country, and when we got to where we were going the p0300 came on so i changed the dist cap and rotor again, seeing that seemed most likely to cause it, it went off and was good for a while, but i just did the intake gaskets on it and the two codes came on. same symptons as before, hesitant shifting, feels very heavy going between 55-75. Doesnt seem to be misfiring though at all, just when shifting to 2nd and she says it feels really boggy or hesitant on the highway.

i recently did the intake gaskets on it and had the battery unplugged for hours not sure if it reset anything it shouldnt have, but after i did them it seemed like the problem with the misfire was worse than before, but the light came on about 3 days later and turned off after she refueled, but another day passed and it turned on on the highway and was blinking than stopped, which i know means catalyst damaging.

im at a loss, at my shop i found a TSB on aftermarket alarm systems turning the fuel on and off every milisecond for these vehicles, im not sure if thats the case though but may be since everything is new, and im not sure if i could get it reflashed or updated. i also read in the manual that if the key or ignition cylinder are damaged it could cause the car to not start because it has a passivelock feature, not sure if the cylinder could be slightly damaged making the security system want to shut the car down but not enough to completely shut it down.

the parts i have for it are from autozone too, i found that those parts dont work well in blazers, could possibly be that but dont see how they could be that bad of parts.

any other questions please let me know! i hate this code and need the help so please brainstorm!

ps sorry if i confused anybody lol im trying to say everything that i know because i need help, were about to drive back home about 2600 miles in 3 weeks and dont want any problems.
THANKS!!

 

they are going to need more info from you....like what year it is and if you have TBI, CPI or SCFI. Some of these Blazers have issues with fuel pressure causing misfires. Do a search on testing the fuel pressure.

 

Have you tried to clean the egr valve? Not sure if it would cause a random misifre though. You can also disconnect the maf sensor to see if it helps, if it does clean it.
The cap, rotor, and plugs/wires is where I would look first but you already changed them. I have heard of autozone parts working for a month then going to crap, especially the cap and rotor.

 

Yes, they really ARE that bad. P0300 is almost always caused by a fault in secondary ignition. Install AC Delco 41-993 plugs with a gap of .060", AC Delco cap & rotor, (put silicone dielectric grease on each terminal, inside and out) and use Autolite Professional series wires or AC Delco. The P1380 is extremely rare and usually due to a poor connection in the crankshaft position sensor shield circuit or camshaft position sensor circuit. Find the nearest trash can and take care of the ignition stuff, worry about the P1380 if it pops up after you perform the General Motors Driving Cycle

Disconnecting the battery, (for any length of time) will not reset or clear anything, (monitors included) in the OBDII system. It will turn off the SES light only if there are no current faults present.

 

Well its a 99 4x4 zr2, lol sorry about that not sure about tbi or cpi it has the spider lookin injection system, i have yet to hook a fuel pressure gauge to it as well.

I hooked it up to the snap on verus at work and everything looks to be functioning ok and no misfires found, its just a subtle misfire i feel too, which i feel it is the cap and rotor haha but lookin at the tsb's on alldata i found the one with the security system malfunction, saying that it could cause p0300 and p1380 so i feel it might be related to that. If no one else has had that issue i could be wrong though and i can link the tsb here if anyone wants to read it
Even in the owners manual it says that if the key cylinder is messed up it could cause itto not start because of the passlock system, so i was wondering if it could be slightly damaged causing a misfire.
No clue at this point lol so need the help
Edit: also with the verus i reset all the modules and rebooted to see if maybe the security system malfunction, drives better after reset but i know its not fixed gonna see when light turns back on.

 

Your engine has Central Sequential Fuel Injection, (CSFI). Low fuel pressure can also set P0300 so it really should be checked as part of the diagnosis. Key ON, engine Off, fuel pump running: Pressure must be 60psi to 66psi and must remain above 55psi for 10 minutes after the pump shuts off. Your snap on verus should have a menu that shows misfires for each cylinder but the P0300 is random misfires, at least you'll be able to see which ones are misfiring.

While you've got the verus connected, check camshaft retard and adjust it if necessary. Spec, with engine rpm above 2K, is 0 degrees + or - 2 degrees. If it's off less than ~25 degrees, it will cause driveability issues, (misfire, hesitation, stumble, lack of power etc) and NOT set a DTC. If it's off more than ~25 degrees it will set P1345.

Passlock failures can cause no starts, never heard of it causing a misfire though.

 

My blazer started missing and eventually just kept getting worse. I had new plugs, wires, cap and rotor. So I checked the EGR, Coil, MAF and none of them was the problem. Did some more reading online and bought the whole distibutor and put it in. Problem solved havent had a code or miss of any kind since. I guess the gears get worn out over time.
Hopefully this will help someone in the future because I was really getting tired of troubleshooting and I know it could have been quite a few more other things as well causing the problem.

 

I cant get the pics and diagrams to post or I would add those, also.

Circuit Description
This diagnostic DTC is used to determine when the Electronic Brake Control Module (EBCM) has lost the ability to provide rough road data to the Control Module (PCM/VCM) for misfire detection (DTC P0300). When the Control Module detects this condition on the Class 2 serial data circuit, this DTC will be set.

Conditions for Running the DTC

• The vehicle speed is more than 1 mph (2 km/h)
• The engine speed is less than 5800 RPM.
• The engine load is less than 89 percent.

Conditions for Setting the DTC
The VCM determines that the ABS system is unable to detect a rough road condition.

Action Taken When the DTC Sets

• The Control Module stores the DTC in history after the first failure but will not illuminate the Malfunction Indicator Lamp (MIL).
• The Control Module records the operating conditions at the time the diagnostic fails. The Control Module stores the failure information in the scan tools Freeze Frame and/or the Failure Records.

Conditions for Clearing the MIL/DTC

• A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles.
• A warm-up cycle occurs when the coolant temperature has risen 22°C (40°F) from the start-up coolant temperature and the engine coolant temperature exceeds 70°C (160°F) during the same ignition cycle.
• Use the scan tool Clear Information function.

Diagnostic Aids
Check for the following conditions:
An intermittent may be caused by any of the following conditions:

• A poor connection
• Rubbed through wire insulation
• A broken wire inside the insulation

Thoroughly check any circuitry that is suspected of causing the intermittent complaint. Refer to Intermittents and Poor Connections Diagnosis. See: Symptom Related Diagnostic Procedures

Test Description
The numbers below refer to the step numbers on the diagnostic table.

1. This step isolates an open serial data circuit from the EBCM system.

 

Yep.. if I had a 2600 mile drive ahead and still had this problem, I'd replace the distributor.

 

Thanks everyone, actually put a gm cap and rotor on it and its better, no lights or misfires, will do distributor if problem comes back and the injector kit is only 180 where i work so also will try that. Its all good now though, thanks guys!

I did inspect dist when doig intake gaskets looked fine but of course cant tell if clearance is a little off.

Thanks!!