New Plugs - Worse Gas Mileage ???

[PewterPower]

I have a 2003 H2 Lux with 84000 miles. I changed my plugs from the Factory Plugs to the Bosch Platinum Fusion plugs (4509). My gas mileage went from about 9.8 avg to 7.5...... Why would this happen ? I did not change the plugs for any other reason than I thought it was time (past time). I have an Airraid filter in the stock air box.

Any ideas ?

[H2 Doberman]

I'm no mechanic but it may take the computer a few miles to adjust to the change.

[IRA51]

Bosch plugs blow,use NGK or AC Delco. and always change the spark plug wires at EVERY plug change . also change the plugs with a cold engine never remove a plug from a hot or warm engine with aluminum heads and also use anti seize compound on the plugs and dielitric grease in the wires and any eletric conection under the hood . you may have f*cked up a plug wire when you removed it .check the summit or jegs catologe for some MSD 8.5 mm wires they do a nice job and have a very low resistance and are priced less than what the dealer has .

[mdoyle]

Ditto on Bosch bringing the "suck" when it comes to plugs. I had a similar experience in my old Tahoe with them and went back to AC Delco.

Same heat range, ect- Bosch performed poorly.

[Carbine]

Ira51 Quoted:

always change the spark plug wires at EVERY plug change

Why? Test them with a multimeter and save yourself the money.

[QUOTE.check the summit or jegs catologe for some MSD 8.5 mm wires they do a nice job and have a very low resistance and are priced less than what the dealer has [/quote]
Again Why? I am not trying to comdemn or play devil's advocate but if the wires are designed to carry the load in the first place go ahead and check them with a multimeter. If you suspect weak glow plugs and blame them on the wires stop snap diagnosing and verify power Volts direct current and Amps with a multimeter during the glow plug cycle at cold start. Check resistance at the glow plug and check each of them to see if they vary. They might vary from manufacturer to manufacturer, bad lots ect... Watch for load/current drop at the volt guage and that will give you a general idea if in fact if the glow system is working.

[IRA51]

GLOW PLUGS ? You must be drinking too much Diesel . you should not expect the plug wires to last more than 3 or 4 years .plugs will last 70000 to 100000 miles and you should change the wires at each plug change .as they get older they get dryed out and are not as good wet as new ones and the resistance goes up putting strain on your coils as well. In Arizona the wires get dry and brittle in 2 or 3 years the high heat takes it's toll . if you want optimom preformance and fule milage and prefer not being stranded due to wet wires that cause a miss or won't even start change them at EVERY plug change. also you have a 50/50 chance that you will damage them changeing the plugs .they get stuck and you pull harder and you know what happens.spend mega bucks on a Hummer and cheap out on reuseing old wires I don't think so . Glow plugs indeed ,you must be on something bro. you don't know $hit from shineola. so when you shower you don't change your underwear too.

[jmsspratlin]

Originally Posted by Carbine

Ira51 Quoted:
Why? Test them with a multimeter and save yourself the money.


Again Why? I am not trying to comdemn or play devil's advocate but if the wires are designed to carry the load in the first place go ahead and check them with a multimeter. If you suspect weak glow plugs and blame them on the wires stop snap diagnosing and verify power Volts direct current and Amps with a multimeter during the glow plug cycle at cold start. Check resistance at the glow plug and check each of them to see if they vary. They might vary from manufacturer to manufacturer, bad lots ect... Watch for load/current drop at the volt guage and that will give you a general idea if in fact if the glow system is working.

Glow plugs?....is your Hummer a diesel?