1996 Yukon Code PO 300 Random Missfire??

[Will]

New Member here; I picked up a 1996 5.7, 4X4 Yukon with 109K miles on it over the weekend from the original owner; the Service Engine Soon light was on so I had a freind pull the code and found "PO 300".
The previous owner changed the following items a few weeks back:
Cap, Rotor, Plugs and wires (I double checked the wires etc.)
Other than the Random Missfire the alternator showed to be "back feeding" I guess it may have a bad diode. But any other ideas what else to check?
The IAC Valve, Mass Meter, O2 sensors etc. all checked out ok.

Edit: the previous owner also had the intake changed a year ago.

Thanks for any help

Will

 

[Dr. Overkill]

I had a 0303 code set that pinpointed the #3 cylinder, which I traced to a bad cap on my brand new distributor. Your code isn't cylinder specific, so that probably rules out a bad plug or bad wire. I'd look into the distributor cap, coil, coil wire, and of course, look for corrosion on terminals in the coil, cap, and wires.

After the #3 misfire, a couple months later it developed another misfire that never set a code. I found substantial corrosion (white powdery stuff) in the terminal on the coil. Cleaning it helped, but I replaced the coil soon after anyway as it had nearly 140k miles on it.

_dennis

 

[Will]

I had a 0303 code set that pinpointed the #3 cylinder, which I traced to a bad cap on my brand new distributor. Your code isn't cylinder specific, so that probably rules out a bad plug or bad wire. I'd look into the distributor cap, coil, coil wire, and of course, look for corrosion on terminals in the coil, cap, and wires.

After the #3 misfire, a couple months later it developed another misfire that never set a code. I found substantial corrosion (white powdery stuff) in the terminal on the coil. Cleaning it helped, but I replaced the coil soon after anyway as it had nearly 140k miles on it.

_dennis

Thanks

I checked the Cap and Rotor and all looked great.
I havent checked the coil yet but I will tonight.
Im trying to determine if it is ignition or fuel.

Will

 

[Dr. Overkill]

You can have problems that aren't visually apparent, too. My bad cap was in the circuit that runs across the cap from the inside terminal to the outside terminal. It was invisible. In the process of trying to rule out a bad plug wire, we stuffed a piece of large gauge generic wire in the cap's terminal, and then touched the other end to ground, and started the engine. The spark coming from that cylinder (#3) was much weaker than when we did the same with the cylinder adjacent to it on the cap (probably #5 or 7). We did a similar test with the plug wire and got the same result, so the plug wire was fine. The problem was isolated to the cap. It was a brand new cap that had developed a problem in that one circuit.

The old style caps with the terminals on top didn't run the risk of this problem. I'm not sure why GM changed the design--it's not like there's a lack of space for a traditional distributor cap and wires back there on the Vortec.

_dennis

 

[Will]

Thanks

Edit - Since your problem was on one cylinder i understand how you ruled out other items; but since mine is a multiple cyl. missfire; how do i know the problem is in the cap and not the coil? Sorry if that is a stupid question

Thanks again

Will

 

[Dr. Overkill]

Thanks

Edit - Since your problem was on one cylinder i understand how you ruled out other items; but since mine is a multiple cyl. missfire; how do i know the problem is in the cap and not the coil? Sorry if that is a stupid question

Thanks again

Will

As they say, the only stupid questions are the ones you don't ask :yesnod:

There is a bench test you can do on the coil in which you test the resistance between 3 different pairs of contacts. Check your service manual for them. Note, you probably need a good test meter. My dad and I have cheap ones (Sperry) and we both got readings that should have been out of range on my old coil. The screwy thing was that we got the same out of range numbers on the new coil I purchased... Go figure. But it's a place to start.

It might also be a bad coil wire. Remove it and check it out for corrosion, and put the meter on either end and wiggle it to see if there are any obvious breaks that are intermittent with engine vibration.

Also look down in the hole in the coil to see if there's any corrosion like I had. Check inside the cap for any cracks, any carbon tracers (lines), corrosion on the terminals and rotor contacts. Check the spring tension on the rotor center terminal (I can't remember if the center terminal in the cap is sprung--don't think so but check it anyway) to make sure they're making good contact.

If you don't find anything with those checks, I'm not sure what to do short of replacing parts starting with the cheapest or the oldest and most likely in need of replacement. Coil is about $35-60 depending on what brand you get. Cap is around $50, I think. A coil wire could probably be had relatively cheap, but you have to find a store that sells individual wires (I found one, but it wasn't a mega-chain...CarQuest I think).

Or if you know anyone nearby with a similar year Vortec who's willing to let you throw his coil wire on for a test, then dizzy cap, then coil... I'm not sure if these parts cross reference between the pickups and Tahoes but I would think so.

_dennis

 

[Will]

Thanks for the help

Will

 

[tofargone]

Had the same issue on my 98. I started pulling wires and found that a few of them had a tiny bit of corosion that made the connection to the distributer flutter especially when the motor was really working. Cleaned them all up made sure they were snug, reset the code and no problems since. I love it when the solution is free.

 

[Will]

Well found the problem; leaking fuel pressure regulator.

 

[Dr. Overkill]

Wow, I was completely off the mark on that one! Glad you figured it out. Isn't the regulator inside the plenum? How'd you track it down?

_dennis

 

[Will]

Wow, I was completely off the mark on that one! Glad you figured it out. Isn't the regulator inside the plenum? How'd you track it down?

_dennis

Ya the intake has to be removed. My brother called me and reminded me that his scanner (Solus from Snap On) had flagged a possible leaking fuel pressure regulator. So sure enough that is what it was.

 

[GMCYukalade]

Also - remember that these Vortecs like to wear out their dizzy gear.. - so thats another thing to keep in mind down the road if it starts missin a lil here and there and you can't find it

 

[Will]

Also - remember that these Vortecs like to wear out their dizzy gear.. - so thats another thing to keep in mind down the road if it starts missin a lil here and there and you can't find it

Thanks; will do.

 

[Dr. Overkill]

Also - remember that these Vortecs like to wear out their dizzy gear.. - so thats another thing to keep in mind down the road if it starts missin a lil here and there and you can't find it

Wow, I'd never heard that. I guess I shouldn't be quite so bitter about the base of my dizzy breaking (where the cap screws to it in the front). I had to replace the entire dizzy, so now I won't have to sweat the gear for a good long time.

(I was bitter over GM using plastic for the base in the dizzy--plastic? seriously? Be careful to never cross-thread or over-tighten those two dizzy cap screws, gang...)

_dennis