Hi there. I searched through the P0300 stuff, but didnt find anything that has helped me figure this one out.

Its a 2001 4.3 blazer, 2wd/auto. I put a new motor in this truck in June because the owner overheated the old motor repeatedly until it lost all compression. Plugs, wires, cap, and rotor were also replaced at that time. It has run fine until recently.

The front cylinder on each bank (cyls 1&2) are completely dead. I can watch the misfire counters on the scanner and they misfire 100% of the time. The other 4 cyls show no misfire counts at idle.

I checked fuel pressure: 62 when priming, 60 when the pump shuts off, ~52-55 while running.

I checked injector flow balance through the scan tool. Cyl 1&2 injectors drop the same psi as the other four. Turning injectors 1&2 off while running shows no rpm change at all.

I checked spark. I get spark through those plugs. There is no indication of cross-fire inside the distributor cap or on the plug wires.

I pulled the plugs. Both are dry. This is where i started to get confused.

I checked compression. The results were 150 and 130, definitely not low enough to completely kill the cylinder.

I checked EGR and EVAP function through the scanner. Both are working and show a major difference between on and off.

I took off the plenum and manually actuated injectors 1&2 with jumpers, both spray. I also took out one of the other tubes from a cylinder that works and laid them next to each other to compare the spray pattern while cranking and being fired by the computer through the harness. They appear identical.

I also ohmed the injectors and both showed in spec, 13ohm.



So here is the problem. I have 2 dead cylinders that have compression and spark, and while the injector tubes ARE shooting what appears to be the same amount of fuel as the other cylinders (which work perfectly, 0 misfire counts), the plugs appear dry/normal. I would think that a dead cylinder thats getting fuel would have black, wet, fouled plugs. But if the injectors are spraying fuel into the runner, and the cylinder is not firing, and the plugs are dry.... where is it going?

The only thing i can think of now is possible valve actuation problems, but if the intake valves werent opening at all, wouldnt i get bad compression readings and valvetrain noise? And if they were opening AT ALL, wouldnt i have wet, fouled plugs?

I'm lost on this one and the customer really needs this truck back, please help!!

 

Update:

I ruled out any valve issues. I pressurized the cylinders and they seal well when the valves are closed, and definitely blow air through the valves when they are open.

I left an intake valve open and a plug out and jumpered the injector and could clearly see the fuel spraying into the cylinder... and im still wondering how i can have dry plugs on these dead cylinders.

I checked the voltage supply to the injectors while running, no problems.

I checked to make sure the injectors were being switched by watching the control side (switched ground) with a meter set to frequency. I imagine there is no noid light for this app and i dont have a scope so that is the best i can do.

I grounded the injector full open with the engine running, and it didnt change anything. I guess i didnt expect it to since i already know i can turn off those injectors without RPM drop.

I switched plugs between dead and good cylinders with no change.

I pulled spark plugs from adjacent cylinders 1&3, 3 being a good cylinder, and watched the spark side by side.. they appear to be the same. I dont see how this can be an ignition issue.

I am leaning towards this being a control issue, something the computer is doing either based off bad input data, or just because it is messed up.

Tomorrow i am going to run a timing light off each wire and see if cyl 1&2 are firing WAY out of time. It would have to be tremendously off for the cylinders to not fire at all, but i am grasping now. Anything i can eliminate seems better than standing around staring at it.

 

Well, i did some more tests.

I did a running compression test. I had done a static compression test before but since i was already in there i did it again.

All cylinders had 150-155 cranking psi.
All the running cylinders had 100-105 psi running compression at idle.
Both the dead cylinders had 65-70 psi running compression at idle.
All cylinders had the same 120 psi under the snap throttle test.

I have had other motors work just fine on 65 psi running compression. So that info told me SOMETHING was different on those two cylinders, but it also told me they werent restricted in any way, so what i really learned from that test was NOTHING.

I started to compare ignition timing on individual cylinders with my timing light, but it died after only 3 cyls, which gave me inconclusive info.

In desperation i called my friends back fishing for ideas.

One mentioned that i had checked spark with the plug in open air, but not in the cylinder.... Ok, true enough. The conditions in the cylinder (rich, lean, etc) do affect the resistance across the plug gap, but it didnt seem to me that a strong blue spark in open air would completely fizzle in the cylinder, especially when its coming from the same source that is adequate for all other cylinders which are firing.

Nonetheless, i pulled the wires at the cap this time to watch spark there, with the plug firing in the cylinder. Lo and behold, no spark coming out of either terminal 1 or 2, and strong spark from all others.

Ok, so now i know that i get spark at the plug in open air, but not in the cylinder?

I took the cap off to measure resistance across the cap terminals. As many of you have probably noticed, the terminals for some cylinders run all the way across the top of the cap, so there ARE parts of the electrical connection that you can't just eyeball. The resistance for both terminals was 0.1-2 ohms. No problem.



The terminals for cyl 1&2 happen to run alongside each other on the cap. When i first put the scanner on this truck and found out those cylinders were misfiring, i looked at that distributor cap and was immediately suspicious. I popped it off the distributor, leaving all wires attached, and flipped it up to look at it as i leaned over the front of the truck. I didnt see anything from that distance of 1 foot, which as it turns out, was about 4 times farther away than i needed to be when looking at it.

Fast forward 3 days and ive run every test known to man (some of which i haven't mentioned here) and eventually come back around to checking things id already checked, like the distributor cap.

I took it off and got my face right up next to it, this time.






And there it was. The damn distributor cap i just put on 6 months ago. Whenever Cyl 1 or 2 would fire, it would fire into the cylinder with less resistance (i.e. the WRONG one). I was right about a few things: It had spark, compression, and fuel, and it seemed like the cylinders were just firing at the wrong time. However, i didnt think it was the cap until i saw that.

I put on a new cap (lifetime warranty this time..) and all was well.

So, of course i am not happy it took me 3 days to figure this out. I like to think of myself as a pretty good tech.

What really bugs is me that some windbag could have looked at this thing and said 'Drrp Drrp, just needs a Tune Up ' and thrown parts at it, and they PROBABLY would have fixed it.

Oh well. My fiancee told me yesterday that me not throwing someone else's money at it is what separates me from so many others in the business, and even though it doesnt make me feel better about 3 days on a distributor cap, i guess that's true.

 

wow, that sucks ***! I kinda went through the same crap with mine, what brand of cap was on before?