PO300 scan tool shows cylinder 1 & 2 only
 

1998 blazer with 4.3l W engine, 4wd auto.
Spark to all cylinders. CMP retard at -1 degrees.
Closed loop mode.
High fuel trim readings, first st then lt. Over 25%.
Both B1S1 and B2s1 counts would slip into red area (low counts) on the scan tool.

Wanted to see if there are any suggestions to why cylinder 1 & 2 and where to go next in diagnostics.

Could both o2 sensors been fried at the same time?

Btw I'm using an autoenginuity scan tool. Great tool.

Note: initially two codes. First problem was the distributor tie down was loose. Realigned to get CMP retardation right and tightened it up. Misfire still happening.

So an update. Still could use some help.
Fuel pressure is a steady 57 psi, which has been normal on this vehicle for a long time. No bleed down.
Found and fixed a major vacuum leak.
FT (short plus long) now under 10%
Still experiencing misfire on cylinder 1 & 2.
Noted that knock sensor is occasionally reach 4v reading indicating knock.
B2S1 o2 sensor looks a little lazy.

Still setting the PO300 and it is both front cylinders 1 & 2. What would affect only the front two cylinders????

Thanks
Kevin

How about your secondary ignition parts? Are they AC Delco? From what I have learned from the experts on here, Blazers are very touchy about using AC Delco rotor, cap wires, plugs.

When you say your fuel is steady at 57psi...what is it in the initial 2 seconds when the fuel pump first comes on after you turn the ignition switch to the "ON" postion? Another thing I have learned is that it must be 60-66psi in those first two seconds (key on, engine off).

+1, have you done a tune up?

Fuel pressure key on engine off is 60 psi. Running psi is 57/58. Vacuum is 17 inHg at idle.

I've narrowed this down to a timing problem. Remember this was a badly missing engine with two codes. Found a loose distributor tie down. Aligned using CMP retard and engine was still running poorly and misfire on cyl 1 & 2.

Physically aligneded the crank timing marks and checked the distributor. It is NOT pointing at the six. Adjusting the distributor to point at the six causes CMP retard to be -26 degrees.

How can I determine if the distributor jumped a tooth or if the timing chain jumped a tooth? If timing is off by a negative 26 degrees which direction would I move the distributor if adjusting it by one tooth?

If the cam retard is negative, rotate the distributor counter clockwise. If cam retard is positive then you have to rotate it clockwise.

Okay. Tried adjusting the cam by one tooth to the right. Ended up with positive retard. Distributor cannot rotate enough to set (physical interference) to zero. Pulled and checked the dist gear which looks fine.

Engine runs best when cam retard is -22 to - 26 degrees. Could the timing chain be off?

Key ON, engine OFF, fuel pump running, pressure must be 60psi to 66psi. Your reading of 60psi is the lowest allowable, and if your tester reads high, (calibration issue) you're below minimum spec, which will cause a lean condition. GM does not publish any "engine running" fuel pressure specs for this engine. There are too many variables involved with engine condition, elevation ASL, partially restricted injectors, dirty throttle body, etc etc.

When camshaft retard is off more than ~27 degrees positive or negative, DTC P1345 will set.** This includes if the distributor is installed a tooth off. If cam retard is off more than 2 degrees either way of zero, crossfire inside the cap is likely to occur, and may or may not set a misfire DTC.

If you can not achieve zero degrees camshaft retard when the distributor is installed correctly, the OE distributor hold down bracket must be removed and an old style SBC, (small block Chevy) hold down bracket installed Mr. Gasket 1009 Distributor Hold-Down Clamp Chrome Plated Incl. Bolt/Gasket This will make the distributor fully adjustable.

You can check timing chain slack without any engine disassembly. Rotate the crankshaft clockwise until the timing marks line up. This keeps all slack on the passenger side of the engine. If you go past the marks, do not rotate the crank counterclockwise, rotate one more revolution clockwise. When the marks are lined up, remove the distributor cap. Have an assistant watch the rotor as you slowly rotate the crankshaft counterclockwise. The instant the rotor begins to turn, stop rotating the crankshaft. If the crank moved more than ~12 degrees, replace the timing chain & gear set.

Timing: it's impossible for ignition timing to be incorrect on only two cylinders.

Set cam retard to zero degrees, (gotta snap rpm over 2000 before checking) and make sure the cap is wire correctly:

Attachment 31884





** the distributor drive gear has 13 teeth, each tooth accounts for ~27 degrees of distributor shaft rotation. This is why the PCM is programmed to set P1345 @~27 degrees or more.

Thanks Chuck,

When my son was driving the truck at highway speeds he suddenly experienced vehicle shaking and misfiring. Nursed it to his house and found the distributor cap loose and vacuum tube at back of intake disconnected. Codes PO1345 and PO300.

Retimed the engine by aligninment on crank shaft and distributor and it started and idled with minor misfire. Scan tool said cylinders one and two. CMP retard at -26, -22, -26. Tried it three times. I have the adjustable distributor hold down and dialing in 0 degrees retard causes the engine to run terribly ( snapping rpm over 2k each time).

Tend to believe fuel pressure is okay. I think my gauge reads just a bit low, new fuel pump last summer and fuel pressure same as after install. Similar readings on my other two blazers.

Ill check timing chain slack next, vehicle is an hour away from the garage. My concern is that the timing chain may have jumped one or two teeth. About 190k miles on the truck.

Does the idea of the chain having jumped make any sense?

When P1345 sets, it means camshaft retard is off ~27 degrees or more, (the distributor is off at least one tooth). You may or may not be able to get it back to zero just by rotating the distributor due to physical interferance with the intake manifold. As I mentioned earlier, when camshaft retard is off that far, you will get crossfire in the cap, and P0300 will set in memory, which is exactly what you have.

If/when the timing chain jumps, camshaft timing and camshaft retard will both retard the same amount in degrees. This will cause hard starting, sluggish throttle response & extremely low power under ~3K RPM, (IF you can get it running). Above 3K, you may not even notice a problem. Don't confuse retarded camshaft timing with camshaft retard, they are two totally different things. Camshaft timing is the synchronization of the camshaft with the crankshaft via the timing chain. Camshaft retard aligns the camshaft position sensor with the camshaft, and the rotor segment with the distributor cap terminals.

FYI: Timing chain issues are almost non existant on this engine, especially with only 190K miles.

Check for a vacuum leak along the plenum gasket, probably toward the front, (where cyls 1 & 2 are). There could be a vacuum leak at the lower intake gasket as well, but to affect cyls 1 & 2 there would have to be two leaks, one on each side of the engine. Spark in the cap might also be arcing between 1 & 2 inside the cap, (they're next to each other in the firing order).