sudden drop off in fuel economy

[Hammer]

2003 4.3L V6 W
5sp manual-3.42 gear
50,000 Miles
Reg cab Xtreme

after i've hit the 40,000 mile mark my mileage has been dropping pretty heavy. i used to get 370 to a tank. i'm at 50,000 now and I am doing all i can to get 330 out of it, 320 usually.

i have seafoamed the intake, seafoamed the gas, seafoamed the oil.
i have replaced fuel filter about 6,000 miles ago
i have changed oil every 3,000 miles
i have an exhaust rattle.... have for about 20,000 don't think it is cat related
i have repacked wheel bearings at 36,000
i have greased it all every oil change
i have kept all fluids perfect

symptoms, in the morning or after about 12-14 hours, it will not fire immediately like it used to, i let the ignition stay on for 10 seconds or so (checked coil didn't see any arching)

idles great, has great power (especially after seafoaming the fuel lines and replacing fuel filter)

I HAVE NOT CLEANED-REOILED my K&N air filter since i put it in at 20,000 or so, it has about 30,000 on it... maybe the issue?

could it be O2 sensors, not egr, i don't have it -2003, or the cat?

thanks,

[Sparky2263]

I'm guessing if you put it on a scanner, you'd see cam offset (sync) more than 5 degs off. Just watch cam offset parameter and adjust distributor 'til it hits zero. While you've got it on the scanner I'd do a crank re-learn just for ****s and giggles.

Other possibility is a good old fasioned high pressure (yes, 100-140 psi, OMG!) injection cleaning. I'll find the bulletin that addresses it.

[Sparky2263]

This bulletin is being revised to update the procedure, models and model years. Please discard Corporate Bulletin Number 03-06-04-030 (Section 06 - Engine).

Condition

Some owners may comment on driveability symptoms of long or hard starts, chuggle, rough idle and light or intermittent misfire. The malfunction indicator lamp (MIL) may also illuminate.

Cause

Due to various factors, the fuel injectors may become restricted. Extensive testing has demonstrated that fuel related issues are the cause of clogged injectors. At this point, no specific fuel, fuel constituent, or engine condition has been identified as causing the restriction. The restriction causes the engine to operate at a lean air fuel ratio. This may either trigger the MIL to illuminate or the engine to develop various driveability symptoms.

Correction

Fuel injector restrictions, deposits, can be cleaned on the vehicle using the following procedure. Under NO circumstances should this procedure be modified, changed or shortened.

Note: The recommended mixture amount of GM Top-Engine Cleaner has been increased from 5% used in prior procedures to 10%. This increase was implemented to improve the effectiveness of the injector cleaning procedure. Follow the cleaning procedure carefully to minimize the risk of fuel system damage. Testing has demonstrated that exceeding the 10% cleaning solution concentration does not improve the effectiveness of this procedure. GM Top-Engine Cleaner is the only injector cleaning agent approved for use with General Motors fuel system components. Other injector cleaners may cause damage to plastics, plated metals or bearings. General Motors has completed extensive laboratory testing of GM Top-Engine Cleaner, and can assure its compatibility with General Motors fuel system components, as long as the cleaning procedure is followed correctly.

Important: This bulletin recommends the use of pre-measured containers of GM Top-Engine Cleaner, P/N 12346535, - U.S. USAGE ONLY. Pre-measured containers eliminate the need to measure the amount of GM Top-Engine Cleaner being added to the J 35800-A, injector cleaning tool. In Canada, use P/N 992872. It will be necessary to measure and dispense Top Engine Cleaner into the J 35800-A, injector cleaning tool.

Injector Balance Test

The following tools, or their equivalent, are required:

^ J 35800-A Fuel Injector Cleaner
^ J 37287 Fuel Line Shut-off Adapter
^ J 42964 Fuel Line Shut-off Adapter
^ J 42873 Fuel Line Shut-off Adapter
^ Four pre-measured GM Top-Engine Cleaner Containers, 24 ml (0.812 oz) each, P/N 12346535, U.S. USAGE ONLY. In Canada, use P/N 992872, 443.6 ml (15 oz) bottle. It will be necessary to measure and dispense 48 ml (1.6 oz) of Top Engine-Cleaner into the J 35800-A, injector cleaning tool twice during this procedure. See Injector Cleaning Procedure below for further information.

Important: Analysis of returned Multec II injectors indicate that terminal fretting corrosion may also be responsible for the conditions described in this bulletin. Since fretting corrosion (microscopic in nature) is nearly impossible to detect in the field, the following steps should be taken to ensure that fretting corrosion is not an issue before performing any additional diagnostics.

Fretting is a term used to describe a tiny rubbing motion that occurs between two surfaces. Fretting corrosion refers to a build up of insulating, oxidized debris that forms on electrical connections due to a fretting action at the contact points. Injector vibration, engine vibration, and electrical harness movement are contributors to the fretting motion.

Depending on how large the build-up of oxide is will determine how drastic the effect will be on the injector circuit. Worst case is a total loss of continuity in the connection resulting in P0200 (Injector Circuit Fault) and P0300 (Misfire) codes being set. If the oxides create a high resistance circuit, insufficient current will be available to properly open the injector, resulting in unstable fuel delivery and ultimately a P0300 DTC. High resistance in the circuit will not necessarily set a DTC P0200. Keep in mind that a connection with fretting corrosion is unstable. Since movement causes fretting, any vibration may create a condition where the connection varies from an open circuit, to a high resistance connection, to a good connection.

Important: Unless intermittent mis-fire due to fretting corrosion is suspected when servicing 3.1 L and 3.4L engines, skip steps 1 through 4. Steps 1 through 4 may be performed at a future date when the upper intake is removed.

When servicing 2.8L, 3.5L or 4.2L engines (VINs 8, 6, S - RPOs LK5, L52, LL8) skip steps 1 through 4 unless replacing injectors.

1. Disconnect one injector electrical connector.

2. Apply a small amount, about the size of a BB, of dielectric grease, P/N 12377900 (in Canada, P/N 10953529) to the female terminal. Be sure to cover both cavities.

3. Reconnect electrical connector to injector.

4. Repeat steps 1 through 3 for all remaining injectors.

5. Using the Tech 2(R), perform an injector balance test using the procedure in the appropriate Service Manual. Use the Tech 2(R) to select each individual injector. Record the pressure drop of each injector on the repair order.












Important: Technicians must record the results of all injector balance tests and keep them with the repair order. For your convenience, the bulletin has a worksheet shown for recording the test results.












6. Subtract the lowest pressure drop, leanest injector, from the highest pressure drop, richest injector. If there is greater than or equal to a 20 kPa difference between the lowest and highest pressure drop, then the fuel injectors may be the cause of the driveability symptom. See the example tables shown to determine if cleaning is appropriate.

7. If there is less than a 20 kPa difference, between the highest and lowest fuel pressure drop, the fuel injectors are operating properly and further Service Manual diagnostics should be performed on the vehicle. All diagnostic time should be added to the appropriate labor operation for the repair performed and not to injector cleaning.

Although the pressure drop is greater than or equal to 20 kPa, cylinder 2 stands out as being rich when compared to the other injectors. Do not clean the injectors. Replace injector number 2. Charge time to labor operation J5650.

Important: When replacing injectors be sure to add dielectric grease to injector electrical connectors.

The pressure drop is greater than or equal to 20 kPa. Injectors 1 and 3 appear to be lean when compared to injectors 2, 4, 5 and 6. Clean and re-test pressure drop. If cleaning does not restore flow on injectors 1 or 3, replace the detective injector or injectors as necessary. Charge time to labor operation J5650.


Injector Cleaning Procedure

Note: GM Top-Engine Cleaner is the only injector cleaning agent recommended. DO NOT USE OTHER CLEANING AGENTS, AS THEY MAY CONTAIN METHANOL WHICH CAN DAMAGE FUEL SYSTEM COMPONENTS. Under NO circumstances should the GM Top-Engine Cleaner be added to the vehicle fuel tank, as it may damage the fuel pump and other system components.

Do not exceed a 10% cleaning solution concentration. Higher concentrations may damage fuel system components. Testing has demonstrated that exceeding the 10% cleaning solution concentration does not improve the effectiveness of this procedure.

During this procedure, you will need a total of 960 ml (32.4 oz) of cleaning solution (10% Top Engine Cleaner/90% Regular Unleaded Gasoline). That is two tanks of solution for the J 35800-A Fuel Injector Cleaner Tank. Other brands of tools may have a different capacity and would, therefore, require more or less tanks to complete the procedure. You must use all 960 ml (32.4 oz) of solution (10% Top Engine Cleaner/90% Regular Unleaded Gasoline) to insure complete injector cleaning.

Important: Vehicles with less than 100 miles (160 km) on the odometer should not have the injectors cleaned. These vehicles should have any out of specification injectors replaced.

1. For U.S. dealers, empty 2 pre-measured GM Top-Engine Cleaner containers, 24 ml (0.812 oz) each, P/N 12346535, into the J 35800-A, injector cleaning tank. For Canadian Dealers, measure and dispense 48 ml (1.62 oz) of Top-Engine Cleaner, P/N 992872, into the J 35800-A, injector cleaning tank. If you are using any other brand of tank, you will need a total of 96 ml (3.24 oz) of Top-Engine Cleaner mixed with 864 ml (29.16 oz) of regular unleaded gasoline.

2. Fill the injector cleaning tank with regular unleaded gasoline. Be sure to follow all additional instructions provided with the tool.

3. Electrically disable the vehicle fuel pump by removing the fuel pump relay and disconnecting the oil pressure switch connector, if equipped.

4. Turn the ignition to the Off position.

5. Relieve fuel pressure and disconnect the fuel feed and return lines at the fuel rail. Plug the fuel feed and return lines coming off the fuel rail with J 37287, J 42873 or J 42964 as appropriate for the fuel system.

6. Connect the J 35800-A to the vehicle fuel rail.

7. Pressurize the J 35800-A to 510 kPa (75 psi).

8. Start and idle the engine until it stalls, due to lack of fuel. This should take approximately 15-20 minutes.

9. Turn the ignition to the Off position.

10. Refill the J 35800-A injector cleaning tank per steps 1 and 2.

11. Start and idle the engine until it stalls, due to lack of fuel. This should take approximately 15-20 minutes.

12. Turn the ignition to the Off position.

13. Disconnect J 35800-A from the fuel rail.

14. Reconnect the vehicle fuel pump relay and oil pressure switch connector, if equipped.

15. Remove the J 37287, J 42873 or J 42964 and reconnect the vehicle fuel feed and return lines.

16. Start and idle the vehicle for an additional 2 minutes to ensure residual injector cleaner is flushed from the fuel rail and fuel lines.

17. Repeat Step 5 of the Injector Balance Test and record the fuel pressure drop from each injector.

18. Subtract the lowest fuel pressure drop from the highest fuel pressure drop. If the value is less than 15 kPa, no additional action is required. If the value is greater than or equal to 15 kPa, replace the injector assembly with the lowest fuel pressure drop. Follow injector replacement procedures outlined in the appropriate Service Manual Information for the vehicle. Repeat balance test to ensure that all injectors fall within 15 kPa.

19. Add one ounce (30 ml) of Fuel System Treatment, P/N 12345104 (Canada P/N 89020804), to the vehicle fuel tank for each gallon (3.78 L) of gasoline estimated to be in the fuel tank. Instruct the customer to add the remainder of the bottle of Fuel System Treatment to the vehicle fuel tank at the next fill-up.

20. Advise customer to change brands of fuel and to add GM Fuel System Treatment at every oil change. Regular use of GM Fuel System Treatment should keep the customer from having to repeat the injector cleaning procedure.

21. Road test the vehicle to verify that the customer concern has been corrected.








Parts Information

Parts are currently available from GMSPO.

[Sparky2263]

Bulletin says they increased the % from 5 to 10. I'll tell ya' we did 20%.

Also says 75 psi. I saw 100-140 clean some injectors. Not saying I would personally do that..............

[Hammer]

if it was a fueling problem or a power problem i'd say injectors. i don't believe it's injectors, next time i go to the dealer i'll see if they can run scanner on it quick.

[Hammer]

thanks for the ideas.

[Maximus]

Check the air pressure in your tires and have the alignment checked also.I have seen them both cause a loss in mileage.

[Sparky2263]

Check the air pressure in your tires and have the alignment checked also.I have seen them both cause a loss in mileage.

A big "Amen Brother" to that.

[extreme gmc]

could be o2s failing a llittle early. i believe the service manual calls for them to be replaced at 60xxx...

[Maximus]

could be o2s failing a llittle early. i believe the service manual calls for them to be replaced at 60xxx...I have seen O2's that were bad that didn't get noticed by the ecm on a Silverado.Even when it was hesitating it still didn't catch it.Only after testing them did I find the left bank was going bad.

[extreme gmc]

oic

[Hammer]

hmmmm O2 sensors.... how to test with out paying??? :)

[Hammer]

i know about the air pressure i ran the tires high for too long so i had to drop back down to stock... that could be the reason but i'm not sold on it yet. however my tires have 50,000 on the original goodyears so that meets the warranty

[nomaboy]

i am lucky if i get 200 miles to my tank. it is usually about 170. i also don't see tire pressure changing gas mileage that much....

[Maximus]

i am lucky if i get 200 miles to my tank. it is usually about 170. i also don't see tire pressure changing gas mileage that much....Do some research in to it and you will find out how it can effect mileage.

[Hammer]

um 6 pounds in each tire affected me by about 5-7 miles per gallon. I dropped the pressures from 40 in the front to 34 and 36 in the rear to 32.

but i still think i'm losing out someplace else. this morning i fired it up and it ran like it was 2 cylinders down, quick shut it off restarted it and it ran great. WEIRD.

[Hammer]

thanks for the suggestions guys, i think i figured out that it was tire pressure.

[Maximus]

Glad to help ya out.

[Rocket_97]

um 6 pounds in each tire affected me by about 5-7 miles per gallon. I dropped the pressures from 40 in the front to 34 and 36 in the rear to 32.

but i still think i'm losing out someplace else. this morning i fired it up and it ran like it was 2 cylinders down, quick shut it off restarted it and it ran great. WEIRD.
this may be a stupid question but what is the right amount of pressure for a tire? I always thought it was 32. Does the size of the tire determine how much pressure should be put in it?

[imdawrlus]

is there gas to be saved when you compare a 50 psi tire to a 32 psi tire?? i know the anwser is obviouslly yes, but how much?? after a certain point i cant imagine that much pressure making that much of a difference

[Hammer]

with the goodyear ga's, i ran about 38psi in the front for 40,000 and tire wear was a little more in the middle but not bad, so i lowered it recently and i've gotten the outsides to match back up. i've got 50,000 on mine and about another 10,000 to go still. the trick is not to have the backs to high, i ran the backs at 32-34 and got good wear. I've heard that getting the rated 50,000 out of the GA's is practically unheard of? I too judge by wear like huge does... i run less pressure in winter for traction purposes and more in summer to keep tire heat down.