4.3 vortec 4x4 engine / fuel issue

99 S-10, 170,000 miles

New mechanic here guys so be gentle. I am getting two P0-300 codes from my truck. "Multiple / Random cynlinder misfire detected". Starts cold, then misses BAD as it warms up. It tries to stall, corrects itself for a few seconds and repeats. However, this truck runs smooth as a babies arse with even the slightest pressure on the gas pedal. Just misses at an idle!!! And, there is no hesitation in reving at first depression of gas pedal.

I changed the pickup coil, plugs, wires, o2 sensors. It seems like a fuel delivery issue but doesnt make sense that it only happens when it is warmed up. It smells real rich when first started, but no smoke. I was thinking maybe a throttle sensor or something fuel related.

Thoughts? Oh, and after it stalls, I have to floor it to get it to start up again. Almost like it is flooded. Also, what is meant by "computer goes through a complete drive cycle" to clear a code (I use a handheld and "erase" codes when done).

How can I determine what cylinders are the problem? I tried to unplug wires and arch the spark on each cylinder, but it is a random thing.

Do you have a haynes manual for your truck?

yes. But it doesnt tell me anything other than what the code is, which is also displayed on my "code scanner"

I had similar problems, and it was actually a battery terminal coming loose. I would have to slightly apply the gas to keep the engine from stalling out or idling at 300 rpms. Especially if you have aftermarket terminals, I would check them and your battery cables first.

Ignorance on my part, and trying to learn this stuff so...

How would loose battery terminals cause cylinder misfires? I thought the engine ran off the alternator.

Again, I managed to change my old 92 4x4 4.3L Blazer motor by myself, but this is CRAZY!!

Sounds like a fuel problem to me if it clears up when you touch the gas.

Changed the fuel filter lately?

Random Cylinder Misfire means one of the sensors, like crank or knock sensor, detected a hickup. This hickup can be caused by a lot of different things. Change your fuel filter first, always a good thing to do on a high milage vehicle. Take some fuel injector cleaner, Techron stuff (guys on Tahoe section have this problem all the time and say Techron is only way to clear it up), put it in the tank and fill with only 12 gal of gas. Let us know how this works and we can go from there. Good luck.

P0300 is usually related to a fuel or spark problem 95% of the time.

A good fuel injection cleaning will usually fix it up or a new fuel filter or a fuel pressure regulator.

Seeing you have pretty well replaced the whole ignition system, I would be leaning towards the fuel.

Too bad you installed all those new parts when most of them were probably in good shape.

Hi. Ok, filter changed. PVC valve changed. Not helping!

Is there a way to check any further issues before I have to take it to a dealer and have them charge me $100 for diagnosis?

I also put in a bottle of cleaner techtron and ran it at a higher idle (2500 rpm's) for about 10 minutes. Does it normally take longer, if so how long? I can drive it around if it speeds things up a bit.

Would the injectors be diagnosed by a fuel pressure check? I can get ahold of a pressure gauge for fuel systems. If I used cheaper plugs, could it be that they are not burning as hot or causing this also?

Possibly the IAC (Idle Air Control)? Have you cleaned the throttle body and IAC passage?

Possibly the IAC (Idle Air Control)? Have you cleaned the throttle body and IAC passage?

No, an IAC will not cause a P0300.

I would check fuel pressure.

Thanks! I will get the guage and check tomorrow, oh tomorrow is today, and let ya know the readings. Is correct (or adequate) pressure 60-65?

Thanks! I will get the guage and check tomorrow, oh tomorrow is today, and let ya know the readings. Is correct (or adequate) pressure 60-65?

61-66psi with the key at the first position.

Do not start it and then put the key back to that position, go from off to the on position for the proper fuel pump priming sequence.

Hi - Finally got around to checking pressure. Without starting the truck and with the key in the ON position pressure was 55. After starting it, pressure was 59. I hit the "release" button a few times and it never went above 59.

Is the best place to start the fuel pressure regulator?

Hi again.
OK, FINALLY found the fuel pressure regulator (who's bright idea was THAT???) and changed it.

It did not help AT ALL. The truck will run great at RPM's above 1800 FOREVER with no MIL codes. Under that, it chugs and misses for about 4 minutes before 2 PO300 codes come back.

Any thoughts?

Did you replace the cap and rotor?

Hi,
Yup. I changed the pickup coil, plugs, wires, o2 sensors, fuel pressure regulator, cap / rotor and ran injector cleaner through it.

None of which has changed the way this truck runs the slightest bit.

I read somewhere about faulty CPI / nutkit and / or central sequential fuel injection.

Any thoughts? Or anyone had experience with these?

You run CSFI not CPI (Same principal but CFI is sequentially injected vs batch fired)... You said you changed the fuel regulator, that's part of the injection unit itself, it's not seperate from the injector, so you replaced the injector itself?

Did you also changed the coil itself or just the ignition module? Have you checked the crank position sensor? Just some things for you to look at... One other thing.... Start your truck in the dark and look in the plugs/cap/coil areas and see if you see any arcing going on.

I know this may sound stupid, but I have encountered this before on other cars. Check or replace the coolant temp sensor. If the sensor is reading wrong it could be telling the computer to dump more fuel than necessary. Usually you wont notice any problem when its warmes up but when its cold it will act like its flooded. try unplugging it and see what happens. JUst a thought.

Actually, the fuel pressure regulator was not a "part" of the injector. It was held in place by a C clip. Remove the clip and pop out the regulator. But, the one odd thing on that was no apparent vacuum line to the regulator when I removed it, so I did not search extensively for one to go on the replacement. I thought I read somewhere that some are "self contained" (or something like that) vacuum's. There are no arcs, already checked.

I tapped out the rivet in the stock ignition module and replaced that. I will jot down the other ideas and let ya know what happens. Crank sensor and coolant temp sensor and post results.

THANKS for being patient guys!
Cynthia

Hi again.
OK, FINALLY found the fuel pressure regulator (who's bright idea was THAT???) and changed it.

It did not help AT ALL. The truck will run great at RPM's above 1800 FOREVER with no MIL codes. Under that, it chugs and misses for about 4 minutes before 2 PO300 codes come back.

Any thoughts?
I had a similar problem with a 94 vortech. Remove the EGR and clean
it. Make sure the valve has the full range of travel without hanging
up. Was there any fuel beneath the plenum on the right side? The computer is changing fuel values because of the lean condition in the plenum.You should also note that the power brakes appear almost none existant at low speeds. Put a vacumn gage on the intake and look for
low readings.You may have more than one problem but eliminate the easiest first.

[pauper_72,just found this site and was just wondering if you ever got the p0300 problem fixed...i am in the middle of the same issue on the kids 98 blazer and i am about to pour a can of gas on it and be done with it. :banghead:

In a way, yes...I found the solution...Some guy just HAD to have my truck, so I sold it for 500.00!!

I know that wont help you, but I do wish you luck. I will NEVER own a used 4.3 again!

Sorry for your troubles but here is to your success in '06!

In a way, yes...I found the solution...Some guy just HAD to have my truck, so I sold it for 500.00!!

I know that wont help you, but I do wish you luck. I will NEVER own a used 4.3 again!

Sorry for your troubles but here is to your success in '06!

Christ, you sold a 99 truck for 500 bux????! If the body alone was good, I would have easily given you double that!

Christ, you sold a 99 truck for 500 bux????! If the body alone was good, I would have easily given you double that!

Last weeked I was having the same issues, although the engine is in an 87 toyota pickup. The problem ended up being that the pickup hose in the gas tank was collapsing on itself. Even with the problem, at idle the fuel pressure was only 55, but when you stepped on the gas the suction from the fuel pump would cause the hose to collapse on itself.

That really really sucks that you gave up on that car. You're not a mechanic and neither am I, but you could've learned more buy solving the problem.