Yet another PO300 question

**J-wall** · 07-23-2009, 04:40 PM

First, I would like to thank this forum and its members for helping me find the confidence to tackle all of the issues listed below to this point on my own. This place is a great resource and I thank you for taking the time to read my "story" and I apologize for its length. The problem centers around the dreaded PO300 engine code and I have read many of the posts with people experiencing that code, but my experience has been a bit different.

I own a 2004 Tahoe that I got used with 75,000 on the clock and it now has 80,000. (This is my first GM, and my first V8, and I love it to death.) The engine light came on as I drove it home when I bought it (2 hrs on interstate) and after my wife read what the engine light could mean in the manual, we were not that worried, checked the gas cap and set off again. The light went off after a while and as long as we did not get on the freeway, it did not return.

Then I found this forum and took it to Auto Zone and got the PO300, but no specific cylinder. So, I proceeded to replace plugs, wires, air filter, and fuel filter to no avail. I was told that because it was not giving a specific cylinder # that it was a fuel deliver problem.. I rented a fuel pressure gauge and sure enough, the pressure was low and the needle bounced continually, so I replaced the fuel pump. The pressure is still low (45-48 psi), but the needle calmed down and will remain steady.

After all that, the light still came on on the interstate so I took it to a garage and their code reader said cylinders 1 & 3 were the culprits, so last Saturday morning, I put on the proper 2 new coils. I took a trip on interstate yesterday and the dang light came on again. The same garage today told me that it was now cylinders 5 & 8 that are misfiring and I needed to change those coils. I stopped at the GM dealership and talked with them there and was told that the coils are a rare problem and it is most likely deeper, which at this point sounds reasonable. I have an appointment Monday for a full diagnostic, which I wish now i had done many dollars and hours ago.

I was wondering if anyone had any ideas as to what to expect or anticipate, I would greatly appreciate your help.

Thanks for reading,

Josh

**bill190** · 07-23-2009, 08:19 PM

There is more to a fuel system than the pump. There is also a fuel pressure regulator! (And a lot more than that...)

Here is a fairly complete fuel system service...
http://www.asashop.org/autoinc/june98/techtotech.htm

Then for PO300, there is a web site just for that...
http://www.random-misfire.com

Then with newer vehicles, due to the complexity of many systems and the numerous components involved, there is a *new* way to troubleshoot them and that is to "TEST" components before replacing...

And for this you need the instructions/specifications. Or a GM Factory Service Manual set for your year/model vehicle, which you can order from helminc.com or a GM dealer. A '99 set I have has about 6000 pages of information in the 4 book set.

So get a Factory Service Manual set, learn to use a multimeter (used a lot for testing), get a fuel pressure gauge, an exhaust pressure gauge, and you will pretty much be in business for the "next time". You will then be able to test things instead of guessing and replacing.

This takes a bit of study and reading, but a Factory Service Manual set (for your vehicle) is a "school" on just your vehicle. It explains how all the different gizmos work. Quite interesting! And it sounds like you are interested in learning about this stuff.

Then the other thing is learning how to use a multimeter. For vehicle testing, you mostly use the volt meter function to measure DC voltage. And you would also use the ohm meter function to measure electrical resistance or ohms.

Here is information on how to use a multimeter. Just keep reading until you understand it.
http://www.google.com/search?hl=en&s...imeter&aqi=g10

**bill190** · 07-23-2009, 08:29 PM

P.S. It is good to have a long post. The more information, the better!

I've been repairing things most of my life (non-automotive) and I like the customers who communicate the entire story the best. From these people I get the *complete* picture. They also tend to get their products fixed better because the technicians understand the problem better.

So it's a good thing!

**Jimmiee** · 07-24-2009, 08:45 AM

Bill,

Your long posts are very informative! Keep them coming.

**J-wall** · 07-24-2009, 08:59 AM

Thanks for your responses so far, I appreciate them. I will keep the details coming as the situation develops.

I would love to purchase a personal "school" for my Tahoe, but my wife and I have a baby on the way so we will see. I am also new at going at these things this deeply, so it is a bit of an ocean through a straw feeling, but I think I like it.

**EvilNetwork** · 07-24-2009, 11:05 AM

Originally Posted by bill190

Then the other thing is learning how to use a multimeter. For vehicle testing, you mostly use the volt meter function to measure DC voltage. And you would also use the ohm meter function to measure electrical resistance or ohms.

Here is information on how to use a multimeter. Just keep reading until you understand it.
http://www.google.com/search?hl=en&s...imeter&aqi=g10

My Fluke meter is the most important tool in my box. Good advice to be giving!

--Joe

**J-wall** · 07-27-2009, 03:15 PM

Well, I took it to the dealership today and their conclusion was that the intake gasket was leaking causing a loss of vacuum and hence the misfires. They said that the engine would surge when they sprayed throttle body cleaner around the intake, and that seems plausible to me. They also recommended cleaning the fuel injectors and they said they could do it with a pressurized cannister of cleaner that would bypass the fuel system.

My question now is how hard is it to change that gasket? And, what is the pressurized cannister they refer to for cleaning the injectors?

Thanks,
Josh

**Jimmiee** · 07-27-2009, 04:14 PM

Originally Posted by J-wall

Well, I took it to the dealership today and their conclusion was that the intake gasket was leaking causing a loss of vacuum and hence the misfires. They said that the engine would surge when they sprayed throttle body cleaner around the intake, and that seems plausible to me. They also recommended cleaning the fuel injectors and they said they could do it with a pressurized cannister of cleaner that would bypass the fuel system.

My question now is how hard is it to change that gasket? And, what is the pressurized cannister they refer to for cleaning the injectors?

Thanks,
Josh

The intake is just bolts and nuts. You should be able to do it with a little help from your friends.

The pressurized cleaning is a can of cleaner that is added through the pressure test port. It bypasses the fuel system and goes right into the the injectors. This method works very well and you will see immediate results.

**J-wall** · 07-27-2009, 05:42 PM

Is the pressurized cannister something that only dealers can use to clean the injectors, or can I get some myself and if so, where?

Thanks to all!

**Jimmiee** · 07-27-2009, 07:41 PM

Originally Posted by J-wall

Is the pressurized cannister something that only dealers can use to clean the injectors, or can I get some myself and if so, where?

Thanks to all!

Once you have the tool you can buy the product and do it yourself. The tool is a canister that you fill with cleaner and it's pressurized with shop air. It adapts to the schrader valve and is injected into the system. Wynns, BG, Justice Brothers and Snap On make them among others. You can use any product you wish in the process. The above companies make the chemical and so does many other companies.
Here's a link to the BG tool which is a lot less expensive than Snap On.

Click Here