Hi all
Here a little problem I have with a 2000 Blazer 2 dr 4x4. The vehicle suffers from an exceptionally bad misfire starting at about 1400 rpm which makes it undriveable. However if the MAP sensor is disconnected it runs perfectly well. I have replaced wires, cap, plugs, rotor, TPS, MAP and the intake manifold gaskets. When I bought the vehicle I had to replace the CAT as someone had bored 2 holes in it, and the injectors.

Has anyone had, or heard of this problem as I am stumped by it. The only DTC thrown up when the MAP is connected is the ever so useful P0300 random multiple misfire.

Hoping someone can be of help

 

fuel filter plugged????

 

Thanks for the suggestion but not to my knowledge as it was replaced along with the CAT etc. If it was plugged up I would assume the problem would be there permanently. I`m beginning to wonder if its possible to have the rotor in the wrong position in relation to the cam sensor i.e 180 degrees out.

 

hmmm you know you can check your rotor. a few years back a guy came in with a chevy van with the same distributor set up as a s10 and he somehow ended up missing the holes on the distributor with the rotor screws so it was a little bit off. it idled fine but when you tired to accelerate it would run like crap. should take but a minute, also while your in there check the cap and rotor for cracks, and check the up and down motion of the distributor bearing make sure its not bad and the rotor isnt grinding into the cap

 

Thanks to you abig84. I did go rummaging thru the ignition system as you suggested today. I reset the base timing on the distributor, and it now appears to run above 1400 rpm with no discernable misfire. However I will have to give it a few runs just to check. Just by any chance do you have any idea what the MAP sensor and also the advance/retard should read at idling. I use an ACTRON CP9150 scantool.

 

Base timing is not adjustable on this engine. What exactly did you do?

Your CP 9150 is an entry level scan tool, it will only access limited basic data. Ignition timing at idle should show ~17 degrees on your scan tool. It is not adjustable. What the scan tool is showing you is how many degrees of advance that the PCM is applying. If the engine runs better with the MAP disconnected, fuel trims should show positive when the MAP is connected, indicating a lean condition. Have you checked fuel pressure and leakdown?

 

Thanks for the info Captain

The injectors should be ok as they are less than a year old and the fuel pump has been replaced too as it crapped out on me 6 mths ago. Whoever had it before me put the wrong pump in, but worth a check. I have no idea what I did wrong, BUT, the last time I looked at the advance/retard on my scan tool, it was reading +20 not -17, so just maybe I managed to somehow get the dist a tooth out. I will check what you suggested and also the advance/retard later today.

 

If the PCM is operating in cell 17, (which it should be), in closed loop, operating temp, at idle, ignition timing advance should be +21 degrees. It can, and does, move around a bit. If the distributor is off one tooth, (or more) P1345 will set and turn the SES light on.

P0300 is almost always caused by a fault in secondary ignition: plugs, wires, cap, rotor, ignition coil. It can also be set by low fuel pressure, worn distributor gear, worn distributor bushings. If you post LTFT & STFT at idle and at 2K rpm, it will help in determining if the misfire is being caused by ignition or fuel. First step is check fuel pressure and leakdown. Just because a part is new, is not a guarantee that the part is good. Key on, engine off, fuel pump running, pressure needs to be 60psi to 66psi. It must remain above 55psi for at least 10 minutes after the pump shuts off. If it passes both parts of the test, fuel delivery is OK, and we move on to something else.

 

OK fuel pressure test ok 60 psi key on engine off, fell to 56 psi after 11 minutes.

Here is the info from the scanner

At idle with the MAP disconnected, IGN 18, LT FTRM1 -2.3, ST FTRM1 0, LT2 -2.3, ST2 0, Fuel SYS 1 and 2 open.

At 2K MAP disconnected IGN 34.5, LT1 1.6, ST1 -5.5, LT2 -1.6, ST2 -7, Fuel sys 1 and 2 CLSD.

At idle MAP connected, IGN 16, LT1 -0.8, ST1 1.6, LT2 -2.3, ST2 3.1 Fuel sys 1 and 2 CLSD

At 2K MAP connected, IGN 35, LT1 -2.3, ST1 -1.6, LT2 -3.1, ST2 -1.6 Fuel sys 1 and 2 CLSD

 

The bottom set of trims, 2k rpm, indicate slightly rich. Which means either #1 it's not getting enough air, (not likely because the other trims are fine), #2 too much fuel is being introduced, (not likely because the other trims are fine) or #3 the fuel is not being burned, (ignition problem).

You mentioned it was undriveable around 1400 RPM; double check the wiring on the distributor cap, pay close attention to cyls 1 and 3, easy to transpose them :



Also:
Did you install an AC Delco cap & rotor?
What brand and part # were the spark plugs that you installed?
Do you have access to a scan tool that is able to view camshaft retard?