Chasing a misfire (mostly P0300)

[Leeann_93]

I am getting closer and closer to pointing at the fuel injectors. Just to clarify, the 98 does have the poppet valves where they may stick open or closed. Right?

Right. You can, however, upgrade to the MFI spider.

[oldtymehockey]

Thanks LeeAnn. I happened upon a 98 Blazer today which had been completely wrecked. I took the upper manifold off to see the injectors. They were dirty but there were no signs of puddles of fuel which would indicate leaking. I grabbed the spider assembly. Paid $16. I would like to give them a try and if it works I am getting off cheap. Else, I am not out much money.

I also grabbed the computer just in case.

Wish me luck!

[Leeann_93]

That should help - at least you'll have a replacement spider if it is bad. And having another computer for testing is always a good thing.

[oldtymehockey]

Well, the Grrrrrr continues...

I put the new spider injector assembly in. I actually put it in about three times. When I got it all together with the new (used) injector it started and idled very smooth. When I throttled up in park there was a very slight miss. At that point I was thinking I need to run some gas through it and if there were a plug or something that was fouled it would burn off.

I got on the interstate and the misfire was there just as bad as before.

First and second are fairly decent. Just a little hesitation. When I get to highway speed and let off the throttle the SES light starts flashing indicating a current misfire. When I try to get into the passing gear or even get it to downshift to 3rd it will go but there is no power and I hear a rattling sound from the top of the engine. The tech at the local Chev dealer said it is most likely exhaust noise. I also hear it backfire from the top. Like when it had trouble starting and a puff of smoke came from the throttle body.

Thanks for all your help so far, all. I have been working on cars and motorcycles since I was sixteen. I have always been just a notch above a hack. By asking many questions I have for the most part avoided royally screwing things up. So I really appreciate a place like this to share ideas and get help.

[oldtymehockey]

Holy smokes! The chase seems to have finally come to an end. It went like this...

First I changed the cap and rotor. The misfire continued and eventually got worse. That is what led me to this thread looking for help. When it got bad in December I started changing out other parts. In the process of changing out other parts and time the cap and rotor I replaced in August went bad. I replaced the cap and rotor (aftermarket) with AC Delco. Guess what? Runs great. The SES light even shut itself off!

The lesson: Take the advice given here about only using AC Delco ignition parts and don't learn the hard way like I did.

[chevymaher]

Holy smokes! The chase seems to have finally come to an end. It went like this...

First I changed the cap and rotor. The misfire continued and eventually got worse. That is what led me to this thread looking for help. When it got bad in December I started changing out other parts. In the process of changing out other parts and time the cap and rotor I replaced in August went bad. I replaced the cap and rotor (aftermarket) with AC Delco. Guess what? Runs great. The SES light even shut itself off!

The lesson: Take the advice given here about only using AC Delco ignition parts and don't learn the hard way like I did.

Congrats it always good to hear when torture ends.

[oldtymehockey]

The torture is almost over. Hopefully. It turns out the light did come back on yesterday. P0300 again. I have AC Delco wires on the way to replace the BWD I installed in January. The all ignition parts will be AC Delco.

She does run 1000x better with the AC Delco cap and rotor though. Hopefully the wires bring it all together.

[redfury]

I am getting closer and closer to pointing at the fuel injectors. Just to clarify, the 98 does have the poppet valves where they may stick open or closed. Right?

Right. You can, however, upgrade to the MFI spider.

I always like the idea of an UPgrade, what is the difference and what exactly is the upgrade to? I can only assume it's a part from a newer vehicle that will work with the older vortec CPFI?

[redfury]

The torture is almost over. Hopefully. It turns out the light did come back on yesterday. P0300 again. I have AC Delco wires on the way to replace the BWD I installed in January. The all ignition parts will be AC Delco.

She does run 1000x better with the AC Delco cap and rotor though. Hopefully the wires bring it all together.

I've reread through the entire post again. A random misfire with no specifics to any particular cylinder didn't point me to the fuel injection system having problems. IF you had a problem, you'd end up with a P030x code ( where the "X" would be a cylinder number ) the P0300 is exactly that..RANDOM.

So, if it's a RANDOM code, then you have to concentrate on the things that would CAUSE a random problem and NOT a specific problem. If you had one bad sparkplug/wire...you'd get a specific code, same with a sticky poppet valve. If the fuel pressure regulator was bad, you could get randomness from that, or if the distributor wasn't doing it's job, you'd get randomness from that.

If the old cap went bad already, I would suspect a bad distributor being the cause for your problems. Even a crappy cap should last longer than that if the distributor was working well. You may just simply have a worn out distributor shaft that is wobbling around causing a random misfire. When you put stress on the motor, that stress is put on the distributor shaft also ( torque is applied to all the moving components of the engine ) which is going to throw it off even more. Chances are that you may damage the new cap. Might be worth taking a look at it and comparing it to the old one you replace it with.

I think the dealership price may be worth the $$ as you seem to be running circles in a parts replacement chase. You can get a new dizzy off ebay cheap enough, but you need to get the CMP retard done to get it spot on. I was getting all sorts of misfires on my van, you could see thousands of them on one cylinder alone. I got the CMP retard set to 0 and they all went away. Of course, I have a new set of problems myself, but I do know that the CMP retard and crank relearn are important aspects to a tune up...unfortunately it isn't just cap, rotor, plugs and wires and you are all set these days. Honestly, I'd really like to change the van to a DIS ignition and do away with the dizzy altogether...I miss my DIS ( distributorless ignition system ).

[Vinnie6]

I echo Redfury. I would really suspect a worn out distributor at this point. I had similar symptoms to yours and a new distributor is what fixed it. Seems to be a common thing in the Astro from what I have seen in this forum.

[Leeann_93]

I am getting closer and closer to pointing at the fuel injectors. Just to clarify, the 98 does have the poppet valves where they may stick open or closed. Right?

Right. You can, however, upgrade to the MFI spider.

I always like the idea of an UPgrade, what is the difference and what exactly is the upgrade to? I can only assume it's a part from a newer vehicle that will work with the older vortec CPFI?

You have CSFI, which is Central Sequential Port Injection. There is one metering body and 6 injectors all in the central unit. Those injectors feed poppet nozzles at the end of each injector hose. Those poppets need a certain amount of pressure to open and they are also quite susceptible to getting dirty and sticking - open and closed. The unit must be replaced if one injector or one poppet valve goes bad. Each poppet is fired in sequence, only when it is firing.

The upgrade is to MFI, or Multi-port Fuel Injection. There is one metering body in the central unit and 6 injectors out on the ends of each injector hose. If an injector goes bad, it can be replaced separately from the rest of the unit. There are no poppet valves.


There is no upgrade available for the CPI, or Central Port Fuel Injection, which is the first type of the spider injection, available in the 92-95 models. Each poppet is fired every time - regardless of which cylinder is firing.

[oldtymehockey]

I echo Redfury. I would really suspect a worn out distributor at this point. I had similar symptoms to yours and a new distributor is what fixed it. Seems to be a common thing in the Astro from what I have seen in this forum.

That will be my next after putting the proper wires in. When we sprayed the wires with water near the plug boots the idle behavior changed. It did this on more than one cylinder. I am learning my lesson with aftermarket parts on this particular vehicle. They don't mix.

I have all my power back with the replacement of the cap and rotor (to AC Delco). The only stumble is now at idle. Basically reversed of the previous behavior.

Progress is being made.

[How-to With Rich]

I had a very similar experience with the Random Misfire, and three shops all missed the fact that the bearing in the distributor had gone bad, and it was allowing the rotor to move around wildly, and make contact with the cap. And YES...the engine ran...poorly, but it ran.

Long story short, one of the shops thought the misfire had to be bad plugs and wires...nope! I had just replace all plugs and wires. They also thought it was the cap and rotor...nope! I replaced those with the plugs and wires. So they figured it was the injectors...and put a very strong injector cleaner in the fuel tank. This cleaner, cleaned alright...it cleaned all of the lubrication in cylinder 1, which allowed the piston skirt to score the cylinder wall. It sounded like one of the lifters had failed, and it was a LOUD (bad lifter-like) banging noise. You know like when a lifter refuses to fill with oil.

By the way, if you live in Oregon, and that LOUD banging noise starts up in your 4.3 as you are going across the Golden Gate Bridge; that SUCKS!!!! Oh yea...the ONLY people that will rent you the truck and trailer you need to tow your Astro/Safari back to Oregon; is U-Haul. And they will be glad to rent you both for about $900 and change.

THEN...when you find out that the engine misfire was caused by a bad distributor (about $350.00 installed), but you wind up having to replace the engine with a remanufactured 4.3 ($4000)...that REALLY sucks.

CONCLUSION: Don't forget to check the distributor shaft!!! I should have caught that issue when the shop was looking at the Random Misfire, in real time on the scanner...why would the Random Misfire be moving from cylinder 1, to 3, to 4, to 5, to 2, back to 3, then to 5, to 6, and on and on? The only reason that has ANY logical outcome is the distributor as it is the only common link to the random misfire moving around like that.

Don't forget to check the distributor shaft!!!

[SPL Astro]

obviously a very old thread, but I had this same P0300 code last week. I've wasted tons of money chasing problems on my old Blazer so I wasn't going to make the same mistake with my Astro. I called my garage I deal with and they couldn't get it in, but they did tell me that this is the worst code to get and could be 15-20 different things, a majority of the time it's the distributor cap. While they couldn't look at it right away they suggested replacing the cap, rotor, plugs and wires. I replaced the cap and rotor and it seemed to help a little, after more questions to some friends on facebook I was told to check the O2 sensors and replace them if necessary. Apparently those are also a cause and they won't throw a code either. Well I just happened to have extras of all 3 sitting on a shelf, so I swapped them out and it runs nice and smooth now. I gave up on the plugs and wires, just no room to get in there.