1997 S10 4.3 with a P0300 Random misfire Problem.

Hello, I have a 1997 S10 with the 4.3L V6 that is throwing a code P0300 random misfire. It starts up and idles but when you give it gas it starts missing and backfiring. I have checked and changed the timing chain and gears, ran a compression ck with 150# on each cylinder. I installed a new distributor, ignition module, crank sensor and still no change. I also had another PCM and that did no good. I checked the firing of injectors and all fire and spray fuel. I did a fuel pressure test and have 60lbs steady when key is turned off. HELP!!! I bought a code reader that has freeze frame data and also live feed. Trim values are Short trim 1 (%) -7.8, Long trim 1 (%) 0.0, Short trim 2 (%) -8.6 and Long Trim 2 (%) 0.0. I can give more freeze frame info if needed. Help!

Do you have the ability to read the misfire history? Even though it is a multi misfire you will see which cylinders have a high misfire when you look at history. If you can not look at history I would find someone with a scanner that can get the cylinder misfire history so you know which cylinders are causing the p0300. I would then do a running compression test on the cylinders with high misfire count. You may have a lower intake gasket leaking and not pulling enough fuel when you come off idle. I have a few of these where compression was ok when cold but once it warmed up compression was lower due to a valve sticking in the guide. I all ways do a running compression test I have a pressure transducer for my scope that I use for this but can be done with a conventional compression tester i would remove the schrader valve so you can see the positive and negative pressure it is usually about half of the static compression if you have 150# you should see around 70-75 # running. This test can also indicate a plugged cat the running compression should be higher if you have a plugged cat.
Hope this helps

Thank you Ephratah, Would doing a vacuum test reveal if I had a intake leak. I have installed new gaskets in the plenum and also have no water leaks. I have 16 lbs of vacuum at idle. I will try and get a Misfire history. I have changed all the spark plugs and wires. Each plug is firing good. All injectors are firing. I have also disconnected the exhaust before the Cat to see if it was clogged and is not. This one has me stumped. I have been doing this for over 35 years and never run into one like this. It is eating my lunch to say the least!

Also what type of scope do you use. That is one tool I need to get back into my arsenal. My shop burnt and all tools were lost over 35 years worth of Motorcycle and Automotive tools and equipment lost. My shop was 40 X 200 feet. It was full!

Take a look at 02 sensor readings with higher rpm if you have a weak fuel pump it can deliver 60 psi at a low flow rate but when I higher flow rate is required it won't keep up and your 02 sensors will go full lean and your fuel trims will max . I like look for this on a road test but you should be able to spot it in the shop as well .

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I have a vantage pro that I use as a scope
How have you checked the injectors for flow? Do you have the ability to check the pressure drop on the injectors? snap on has a injector drop test built into the scanner. Prior to that I made up a harness for testing the injectors and used a OTC injector tester I cut the leads to the original plug and wire harness directly for testing these I also have a snap on tester for this purpose.You hook a fuel gauge up to the fuel rail charge it with pressure and watch the amount of pressure drop on each injector. These 4.3 are know to have sticky injector popets the fuel pressure has to over come the spring pressure in the injector and force it open there is a up grade which does away with the popet design and puts the injector on the end of the line where the OEM design has injector where the main plug is. Did you install a new distributor or used one? I know to check the shaft bushings for wear you have to lift up on the rotor . when it is running the gear forces the shaft up against the thrust washer due to the design of the gear on end of shaft.

I would agree a fuel flow contriction is a likely candidate
Also have you checkout the TPS? both in live data and with meter?

Thanks but when I rev it up to 1500 rpm's the fuel pressure is still at around 57 58 lbs. It will not rev over 2000 rpms and misses and backfires so much I do not want to try and push it even up to that. It is very hard to start and I ended up burning up the old starter. I went and bought a new one and put it on and it ended up breaking the teeth off of the Bendix. I am running into I did another vacuum test and now it drops down vacuum the more I rev it. By this test I am thinking it might have a bad cat that might have an intermitted blockage. What do you think?

What do the 02 sensors say when you do that

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I will have to read it when I get the engine going again. I had to buy another new starter and will remove cat and see if it will crank easier then.

You may want to check the fuel pressure regulator they are known to leak and cause a hard start and a rich condition it is located in the upper plenum. if you can get the plenum off hook up a hand vac pump to the vac. nipple on it and see it it will hold vac

I'm pretty sure you said it has a new cap and rotor, but I have seen these caps be bad right out of the box!
Here is a good test to check for cross-firing inside of the cap.
Engine idling, perform the RPM drop test at the distributor cap using a test light. Spark should jump to your light on every cap electrode. The spark should be capable of jumping just past the end of the plastic distributor housing electrodes (where the plug wires connect). Internal cross-firing will lead to no spark from one or more cylinders unless you put your test light way inside of the cap.

I replace it with a new one when I changed the plenum gaskets.

Scanner Danner, Thank you for your help, I changed the whole distributor with a new cap and rotor. I also had another new cap in a box I tried just in case this was the case. I also checked and made sure the plugs were firing on each cylinder and they are. I will be getting the new starter installed and then run a test without the catyalic convertor hooked up. I did a vacuum test and when I rev it up it falls down lower and lower as it is reved up now. I did this once before but the vauum did not fall like this. I believe now it may be the cat. We shall see. Thank you!

what is your cam retard reading?? it should be close to 0 the pid only updates past 1000 rpm. that could explain the hard starting... is it kicking back or just an extended crank no start??