P0300 Codes Got Me

[MT-2500]

Some place in the snapon scanner there is a injector balance test you can do with engine off and with a fuel pressure gauge hooked up to watch how much fuel pressure dropem with injector on a certain amount of time.

On The 02 sensor wiring tsb I posted I had only one blazer that done it.
It had a dead miss on one cylinder.
That I finaly got out by moving the wiring around and switching and unpluging and pluging back up 02 sensors.
If you can find the wiring plug in on the 02 sensor wiring harness you might try unpluging it and see if miss goes away.
Not sure that is your problem but I had one that done it big time.
Drove me ragged trying to find it.
If you want the TSB on it e mail me and I will pdf it to you.
Good luck.
MT

[MT-2500]

Some place in the snapon scanner there is a injector balance test you can do with engine off and with a fuel pressure gauge hooked up to watch how much fuel pressure dropem with injector on a certain amount of time.

On The 02 sensor wiring tsb I posted I had only one blazer that done it.
It had a dead miss on one cylinder.
That I finaly got out by moving the wiring around and switching and unpluging and pluging back up 02 sensors.
If you can find the wiring plug in on the 02 sensor wiring harness you might try unpluging it and see if miss goes away.
Not sure that is your problem but I had one that done it big time.
Drove me ragged trying to find it.
If you want the TSB on it e mail me and I will pdf it to you.
Good luck.
MT

[Gabe25]

Thanks MT, Got the serive Bulletin. I tried the fuel balance test with the pressure gauge, but had to stop. My pressure gauge decided to spring a leak. It was old so I'll pick up a new one today and continue. Anyway, Tried spraying carb cleaner around the upper Plenum looking for possible cracks. Still nothing. I drained the gas and replaced it with fresh gas and bleed the fuel lines, and still nothing. This blazer is my sons and I was thinking mybe he bought some bad gas. It was just a thought. Anyway, I'll continue again tonight after work and hope to get her running by this weekend. Will let you know. Thanks again.

[Gabe25]

OK completed the fuel injector balance test with my new fuel pressure gauge and everything tested OK on all the injectors. Now, I remember before reading somewhere that someone was having trouble with there idle and it turned out to be a ground strap from the firewall to the engine. Anybody heard anything like that. This trouble has really got me thinking.

[Gabe25]

OK, the saga continues. So far I’ve replaced the following:
1. Lower Plenum Gasket Set (Leaking Water)
2. Spark Plugs ( Burning OK)
3. Cap (Little Worn)
4. Rotor (Small Carbon)
5. Wires (Uneven Resistance)
6. Cam Positioning Sensor
7. Coil (Weak Reading)
8. Injector Fuel Regulator (Leaking)

Testing performed:
1. Fuel Pressure.(62psi)
2. Leak Down 20 mins. (59psi)
3. Fuel injector Balance test. (All passed)
4. Fuel Pulse test. (All passed)
5. Cylinder Compression Test (All even)
6. Blue spark at end of plug wire
7. Triple check #1 is TDC
8. Distributor set on #6, Firing order #1.

Trouble is still there. P0300 Code. Multi Cylinder Missfire. Cylinders 4 & 5. I had one more thing to check. O2 Sensor, which I’ll be testing tomorrow. Does any body out there have any ideas. I’m kind of running out of them.

[MT-2500]

Gabe
Just a few things I casn think of to recheck or check.
Did you set the camshaft retard setting on the scanner?
Does scanner show lean fuel trims?
Can you feel a dead miss on the road?
Does pulling no 4 or 5 plug wire affect it any?
Have you double checked the injector plug in?
Liked locked down good and tight?
Is it getting injector pulse on 4 and 5 cylinders?
Does unpluging mass air flow sensor make any difference?
Good luck
And hang in there.
MT

[Gabe25]

MT, Thanks for the ideas. Here is the answer:

Did you set the camshaft retard setting on the scanner?
Yes
Does scanner show lean fuel trims?
Yes
Can you feel a dead miss on the road?
Yes
Does pulling no 4 or 5 plug wire affect it any?
Yes
Have you double checked the injector plug in?
Yes
Liked locked down good and tight?
Yes
Is it getting injector pulse on 4 and 5 cylinders?
Yes
Does unpluging mass air flow sensor make any difference?
No: When pulled. Only Error code for Mass Flow comes up. Reinstall and error clears

[Gabe25]

MT, Do you know what the voltage reading should be on the injector pulse? I cleaned the MAF screen wires. They were full of carbon, but no change on misfire. Still searching.

[16th hippy]

one side is constant power(battery voltage). not sure if it grounds when vehicle is off/not running.

the other is the ground controlled by PCM/ECM to fire it.

[MT-2500]

Originally Posted by Gabe25

MT, Do you know what the voltage reading should be on the injector pulse? I cleaned the MAF screen wires. They were full of carbon, but no change on misfire. Still searching.

As 16th said.
One side is 12 volt positive key on.

They trigger off of a ground side pluse.
The PCM controls the ground pulses the ground signal width engine running or cranking.
The pulse width sometimes can be read on scanner.
You will need a lab scope to measure the pulse width.
MT

[Gabe25]

The Final Chapter comes to an end. This has to be one for the books. After going through all of this testing. It comes down to fuel injector 4 & 5 tubes are pluged. Gas was being restricted. I was going to attempt to open them up and clean them out, while removing it from the lower Plenum. Injector tube number one broke. With that, I just said the hell with and picked up a new CPI Unit. Slamed it in and my son is smiling.
MT & 16th. In Hawaii we say Much Mahalo for everything. Which means Thank You Very Much for all of your help. Have a great Weekend. I know I will. Off to the Beach.

[MT-2500]

You are welcome and thanks for posting back how it went.
Glad all is well and fixed.
Enjoy the beach.
We may get some snow today here in Mo. USA.
Good luck
MT