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Steve, Auto Service Technician
Category: Car
Satisfied Customers: 5494
Experience:  25+ yrs experience as a professional working technician; ASE L1 master technician
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98 Chevy Tahoe: Check engine light..egr..rotor..vacume leak..manifold

Customer Question

98 Chevy Tahoe, Check engine light on, Codes P0300 and P0420 present.Changed all 4 O2 sensors, Temp sensor, all plugs and wires, fuel injectors, checked egr (it's ok) changed rotor and cap. Checked for vacume leak in manifold, none present. Replaced fuel filter, checked fuel pressure it's about 60 psi and holds pressure. Right side plugs are fouled changed them several times. Spewing black exhaust. Engine running very rough. Any help to answer this headache of a problem would be of great help. I have found the same symptoms posted on other sites but no answers. PLEASE HELP ! 5.7liter 350 vortec engine
Submitted: 11 years ago.
Category: Car
Expert:  Steve replied 11 years ago.

If you are sure you want to pay $100 for diagnostic help, Id be more than happy to walk you through diagnosis if you want to prepay the $100 deposit... my experience though is that such large amounts are rarely paid after spending a couple hrs helping customers with the problem...

The concern is that for the same or possibly less money, you could take your vehicle in to a technician and have the problem diagnosed professionally and actually save money and headaches! :)

Customer: replied 11 years ago.
Reply to Steve7654's Post: I will pay if you are certin you can get me through this. I have already paid over $1500.00 in parts. Please don't tell me something like the cat is clogged on one side being that it is a two cat vehicle. This problem started getting worse after all the sensors were replaced etc. I am at my wits end. I have had assistance from technicians as yourself without any luck. I am desperate at this point.
Expert:  Steve replied 11 years ago.
what kind of test equipment do you have available, and have you had a technician look at the vehicle yet?
Customer: replied 11 years ago.
Reply to Steve7654's Post: So far the only machine we hooked it up has been a scanner. The code P0300 is very generic in nature. The problem is very real however.
Expert:  Steve replied 11 years ago.

Lets start with the code definitions; the P0300 indicates a multiple cylinder misfire, which can have many possible causes. The P0420 indicates a caT failure (low efficiency). A low cat efficiency failure can only be due to a defecticve CAT; it indicates that teh secondary O2 sensor signal is too active (closely following teh pre-cat O2 signal in switching frequency). The only thing that can set teh P0420 is a failing CAT, likely caused by running the engine with a misfire condition. When an engine is moisfiring or running rich (black exhaust smoke) the unburned fuel goes down the exhaust stream and oxidizes in teh CAT, causing internal damage. So, teh vehicle is going to need at least one CAT, probably 2 after repair to make the light go off and get rid of the P0420 code.

You dont want to even think about CAT replacement untill teh rich running/ misfire condition is corrected, or the new CATs will be destroyed very quickly.

What type of scanner are you using to access the codes and datastream? In particular, can you access the freeze frame data for the P0300 misfire? Also, does your scanner have misfire graphic/ cylinder balance testing capability?

One critical measurement we will need before starting will be fuel pressure measurements, both static and running, and engine vacuum measurements, both at idle and at speed.

Expert:  Steve replied 11 years ago.

Before considering any possible control system causes for the rich misfire condition, the mechanical portion of the fuel system has to be verified to be in good working order. No control system in tha world can do it's job if the mechanical things it is trying to control have problems. Also, we are going to need compression test readings early in the process here.

Here is the procedure for fuel system checking:

Expert:  Steve replied 11 years ago.
Expert:  Steve replied 11 years ago.
Expert:  Steve replied 11 years ago.
Expert:  Steve replied 11 years ago.
Expert:  Steve replied 11 years ago.
Expert:  Steve replied 11 years ago.
Circuit Description

When you turn ON the ignition switch, the Vehicle Control Module (VCM) turns ON the in-tank fuel pump. The pump remains ON as long as the engine is cranking or running and the VCM is receiving reference pulses. If there are no reference pulses, the VCM shuts the fuel pump OFF within 2 seconds after you turn the ON the ignition or if the engine stops.

An electric fuel pump pumps the fuel through an in-line filter to the Central SFI unit. The pump is attached to the fuel level meter assembly inside of the fuel tank. The pump is designed in order to provide fuel at a pressure above the regulated pressure needed by the injectors. The pressure regulator keeps the fuel available to the injector at a regulated pressure. Unused fuel is returned to the fuel tank by a separate line.

Diagnostic Aids

Tools Required

  • J 34730-E Fuel Pressure Gauge.
  • J 34730-1A Fuel Pressure Gauge with J 34730-250 Fuel Pressure Adapter Kit.
  • d 42873-1 3/8 Fuel Feed Pipe Shutoff Valve and the J 42873-2 5/16 Fuel Return Pipe Shutoff Valve.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. Wrap a shop towel around the fuel pressure connection to absorb any small amount of fuel leakage that may occur when installing the fuel pressure gauge. Turn ON the ignition leaving the engine OFF, the fuel pressure should be 415-455 kPa (60-66 psi). This pressure is controlled by a spring pressure within the regulator assembly.

  1. The fuel pressure that continues to fall is caused by one of the following items:

    • The in-tank modular fuel sender is not holding pressure.
    • The fuel pressure regulator valve is leaking.
    • A Central SFI injector is leaking.
    • Fuel line is leaking.

  1. When the engine is idling, the manifold pressure is low (high vacuum) and is applied to the fuel pressure regulator diaphragm. This will offset the spring and result in a lower fuel pressure. This idle pressure will vary somewhat depending on the barometric pressure; however, the pressure at idle should be less indicating the pressure regulator control.

  1. If the fuel pressure is less than 415 kPa (60 psi), it falls into the following 3 areas:

    • The pressure is regulated but less than 415 kPa (60 psi).The amount of fuel reaching the injector is sufficient, but the pressure is too low. The system will run lean, hard starting cold, no start, overall poor performance, and may set a DTC. Refer to Engine Cranks but Does Not Run. See: Computers and Control Systems\Testing and Inspection\Procedures\Diagnosis By Symptom\Engine Cranks, But Will Not Run
    • A restricted flow causing pressure drop-Normally, a vehicle with a fuel pressure of less than 300 kPa (44 psi) at idle is inoperable. However, it the pressure drop occurs only while driving, the engine will normally surge then stop running as the pressure begins to drop rapidly. This is most likely caused by a restricted fuel line or plugged filter.
    • A leaking or contaminated pressure regulator valve or seat interface may not allow the regulated pressure to be achieved.

Notice: Do not allow the fuel pressure to exceed 517 kPa (75 psi). Excessive pressure may damage the fuel pressure regulator.

  1. Restricting the fuel return line allows the fuel pressure to build above regulated pressure. With the battery voltage applied to the fuel pump test terminal, the pressure should rise above 455 kPa (66 psi), as the valve in the return line is partially closed.

Expert:  Steve replied 11 years ago.


If you want to take some time tomorrow and gather:

(1) Compression test readings

(2) Fuel pressure test readings (both static and dynamic)

(3) Engine vacuum measurements (both at idle and at speed)

(4) Check for freeze frame and misfire data stored when the P0300 code set

We will ahve a good basis to decide what direction to take in finding the cause of the P0300 misfire and rich running condition you are experiencing.

I'll check back tomorrow night to see how you made out...

Expert:  Steve replied 11 years ago.
BTW: I do realize you stated that system pressure is about 60psi; you dont state under what conditions the measurement was taken. If teh pressure is climbing out of spec, or dropping below spec when the problem is occurring, that would indicate problems that need to be addressed before contuinuing. The fuel pressure spec on this engine with throttle open is 66psi, so a 60psi reading under these conditions would be 10% below spec/ abnormal.
Expert:  Steve replied 11 years ago.
Also, while youa re doing your testing, jot down the fuel trims (block learn/ integrator counts) for each bank when the engine is warm...
Customer: replied 11 years ago.
Reply to Steve7654's Post: Steve I was hoping for a definitive fix. Your assistance here is over the top. I'm afraid I am not qualified nor do I have any more time to put into the problem. This looks time consuming. I will leave it to the professionals. I'm going to the Chevy dealer in the am. Thanks for your help but I have to decline.
Expert:  Steve replied 11 years ago.

Im afraid I cant help yo pinpoint the problem without doing testing to locate the source... anything else is just pure guesswork and will result in needless replacement of many good parts costing hundreds of dollars unnecessarily.

The description of the symptoms you have provided points to the likelihood of a base mechanical problem creating a low engine vacuum condition, such as a burned valve, low compression on one or more cylinders, worn camshaft lobe, or other internal fault...

Good luck, Id be interested in knowing what they find!

Customer: replied 11 years ago.
Ok, Thanks. I'll keep you posted. I may find something out tomorrow or by Monday. Being that I'm at work now and will be on vacation all of next week. It may be jun 27 before I get back to you. Thanks for your concern, Tom.
Customer: replied 11 years ago.
On Thu, 30 Jun 2005 22:26:39 -0800,[email protected] wrote:
> Hello Steve. I have a resolution to my problem. What a nightmare. Thanks
> for your help.
> Tom.
> Fianlly back on the road. After all I have done and spent on this Tahoe.
> The Dealer found a GM Bulletin that states when replacing the Fuel
> injectors you must replace the whole unit, (spider injector). Do not
> replace the injectors indivually at $88.00 a piece X 8 like I did. There
> is room for error when installing them. With the kit they are factory
> installed. There will not be a problem. I am very happy to say the least.
> I have spent $1,300 at the dealer with diagnosis and repair. well worth
> it. I spent $1,100 on my own diagnosing and repair for a total of $2,300
> for a check engine light and failure to inspect in the state of NJ. That
> is the most expensive inspection I have ever had to endure. I hope I never
> have to go through this again. I also hope that in the future that if
> someone has the same problem they take it the dealer first or read this
> note to assist them. It was a major headache for quite some time. At least
> I now have a truck that runs great with all new parts. It should last a
> few more years. Thanks for all the help and concern.
> One thing that pointed the Chevy Mechanic to research bulletins was he
> stated the fuel pressure in 7 of the cylinders was 8 PSI while in cylinder
> #1 it was 20 psi. MAke sure you have an A rated GM Mechanic when you have
> service. It pays. And so will you.

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