2001 Blazer rough idle, p0410, and p0300 codes




Hooked it up to a reader and it was giving me a p0300 misfire code but when i looked farther into it cylinder 2 was the only one showing misfires with a past misfire in cylinder 1. It idles really rough when started. I tested to see if it was a brake booster vacuum leak by pressing the brake and seeing if it started idling better and nothing changed. If i rev the RPMS up it seems to idle a bit smoother. Replaced wires, plugs, cap and rotor and nothing changed same misfire code. Sprayed ether around intake and vacuum lines around there and there was no change in rpms. Check engine light flashes when i go over 30mph but when i slow back down below 30mph it stops flashing. Please help i have no clue what it could be or if I checked for a vacuum leak in the correct spots.

Occasionally the brake light and ABS light come on but then when I turn the vehicle off (like to get gas) and turn it back on they go away. I don't know if this has anything to do with it or not.

 

Welcome to the forum!


P0300 is almost always caused by a fault in the secondary ignition system, (spark plugs, wires, cap, rotor, and ignition coil). These engines are very fussy when it comes to tune up parts. Spark plugs, (41-993) and the cap & rotor, really need to be AC Delco. The aftermarket stuff just doesn't cut it. Other possibilities for P0300 include: The original injector assembly. It can been upgraded on your engine, and it is plug and play. Pretty easy to check if the injectors are causing a problem, but it would require a bi-directional scan tool to perform an injector balance test. Distributor bushings like to wear out too, which is another easy test, but requires a scope. The camshaft position sensor must be adjusted correctly, but that also requires a capable scan tool to check and adjust it. Low fuel pressure, or excessive leakdown, can also cause misfires, here's a link that explains how to test it. Post all 4 readings and we can analyze them. https://blazerforum.com/forum/2nd-ge...eakdown-88305/

 

A guy I know put his computer on the blazer today and the only code was the P0300 code. It still ran really rough and the check engine light flashed when I reached 30mph. The computers showed almost 2000 misfires (cylinder 2) in the 5 min drive we took and only 1 misfire in cylinder 1. Would this be safe to say that I need to replace the spider harness to fix this?

 

Nope, that would be a guess. The problem needs to be diagnosed before replacing anything. P0300 means that more than one cylinder misfired, and your test confirmed that, but it did not diagnose the problem. The cause of the misfire is something that is specific to cylinder #2, which could be any number of things. Fuel trims will tell if it's a lean, or a rich, misfire. That will eliminate several possibilities as being the problem, but it will not pinpoint the problem. Have your friend check, and write down, long term fuel trim and short term fuel trim for bank 1 and bank 2 at idle, and at 2K rpm, then post all 8 numbers. That will point us in the right direction for diagnosing the cause.

 

I wrote them down earlier when we did it but the motor was only a little bit above idle. Long term FT B1-3.9%
Long term FT B2-25%
Short Term FT B1-13.3%
Short Term FT B2-27.3%

 

That's a good start. Negative fuel trims mean the fuel mixture is too rich, and the PCM is subtracting fuel trying to get the mixture correct. Now we know it is a rich misfire, on both banks. Either too much fuel is going in, possibly from a leaky injector, or the correct amount of fuel is going in, but it's not being burnt due to an ignition, or other problem. On bank 2, where cylinder #2 is, both trims are indicating richer than bank 1. Which makes perfect sense being that cylinder #2 was misfiring more than cylinder #1, which is on bank 1.


Time to check fuel delivery. This link will help you. Post all 4 readings: https://blazerforum.com/forum/2nd-ge...eakdown-88305/