98 Sierra 5.7 Misfires

pbellone · May 17, 2009

I have a tough one. I have research many forums for answers and now I will test the techs here.

1998 GMC K1500 5.7l Truck-Back in Feb I set a P0300/5 codes. Truck ran OK so I cleared the code and it seemed fine for a month. It set again. I ran a bottle of top end cleaner down the intake and left it soak. Started it up and burned it off. Changed the cap, rotor, and plugs. Plug 5 was caked up a little. Couple of hundred miles the next week, back again and a distinct rough accelleration. Mileage dropped and lousy feel. Ran reader looking for fuel trims compensating for something and other bad things. Except for the misses, all appeared ok. Replaced all wires, plugs, PVC, cap, and rotor again. Plug 3 & 5 were caked up with gray crap with only 400 miles since new. Still runs a little rough and I suspect it will set the misfire code again. I am being advised I will need to do the dreaded intake manifold gasket repair. I don't mind doing it as long as it has a problem. The fuel injection setup is a mystery to me along with testing and repair.

Any suggestions here besides reading dry TSBs or letting a deal change parts until it runs good? This truck has been well cared for over 105,000 miles. The only item I introduced prior to the problem (3 months) was Lucas fuel additive. Ten ounces per 26 gallons of fuel.

IDEAS???

bill190 · May 17, 2009

For the misfires, try reading this...

(He has a neat propane torch vacuum leak test suggestion.)

http://www.random-misfire.com

As to learning about fuel injection, testing, and repair...

The key is having the "right instructions", the "proper tools", and learning how to use those tools.

The "instructions" are in the GM Factory Service Manual Set (which includes 4 separate books for my vehicle). They are...

Service Manual (How to fix just about anything on the vehicle.)

Fuel and Emissions Service Manual (All the engine computer and fuel injection stuff)

Unit Repair Manual (Tear down of specific components.)

Electrical Diagrams (All the wiring.)

You can order a service manual set from your dealer.

Then the key for the fuel injection stuff is the "boss" who "issues the "orders"!

(This would be the engine computer.) Let's call him the "Sergeant".

The Sergeant (computer) bases his decisions on when to "fire" (spark) or "resupply the troops" (fuel) from various "look-outs" (sensors) around the engine!

And that is all there is to it!

Think of the computer as a "boss" what has to make decisions. Actually they even call it "Computer Command Control" in my fuel and emissions book!

The boss (computer) wants to make the RIGHT decisions, so he gathers all the information he can from the various sensors...

How far is the throttle open?

What is the engine temperature?

What is the air temperature?

What is the air pressure?

What does the exhaust look like?

What speed is the vehicle going?

Etc.

Then based on that and other information, the computer decides how much fuel to send to the engine and decides when to fire each cylinder.

The problem occurs when a sensor is not doing its job correctly. Maybe the engine temperature sensor is telling the computer the engine is cold when actually the engine is hot. Or maybe the throttle sensor is telling the computer the throttle is slightly open when it is actually fully closed.

Then the computer can't make the right decisions with this wrong information!

The KEY to checking each sensor is learning how to measure voltage with a volt meter and how to measure "resistance" with an ohm meter. These two electrical meters are usually combined into one meter called a multimeter.

Many factory service manuals have basic instructions on how to use test equipment like volt and ohm meters. Also there is tons of information on the internet about this.

So if you can learn to use a multimeter, can get the factory service manual set, and can also get the other service tools needed for each test like a fuel pressure gauge, then all you need to do is follow the instructions in the book.

The factory books take a bit of getting used to. Learning how to use them. But start with simple stuff at first. Then you will be doing more advanced things in no time at all.

superjim · May 17, 2009

My guess at this point...intake gaskets.

You are fouling plugs 3 and 5.

As you know already, they are side by side.

There has to be some common element doing that.

Thus my guess for intake gaskets.

pbellone · May 17, 2009

Bill,

Thanks for the detailed reply. My next step is to do a fuel injector cleaning. I have to borrow/get the tool and I already have the GM top end cleaner. I may be forstalling the inevitable intake manifold gasket change. This will be the 3rd one in my family of GM vehicles. Maybe the easiest of the 3 also. Never had this trouble with my '76 or '84 350's.

Pete

pbellone · May 17, 2009

As I told Bill, thanks. I am feeling you are right but I want to try other things before doing the gasket and still have the problem due to clogged injectors or some other simpler repair.

Pete

My guess at this point...intake gaskets.

You are fouling plugs 3 and 5.

As you know already, they are side by side.

There has to be some common element doing that.

Thus my guess for intake gaskets.

superjim · May 17, 2009

As I told Bill, thanks. I am feeling you are right but I want to try other things before doing the gasket and still have the problem due to clogged injectors or some other simpler repair.

Pete

My guess at this point...intake gaskets.

You are fouling plugs 3 and 5.

As you know already, they are side by side.

There has to be some common element doing that.

Thus my guess for intake gaskets.