Sudden drop in gas mileage

[SDSeville]

Re: Sudden drop in gas mileage

Thought I'd try the simple things forst so figured I'd clean the MAP sensor. Turns out that this engine doesn't have one. But I did notice quite a bit of dirt and oil around the throttle body gasket. Removed that, cleaned it, and replaced the gasket. Plugs are pretty rusty so think I'll wait for the weekend to tackle that chore. I'll squirt break free on them every day and keep my fingers crossed. Ther are some pretty scary stories out there about spark plugs breaking off and the work it takes to get them out.

As far as the O2 sensor, think I'll do the obvious stuff first and then get hooked up to a scanner to see what it shows for codes before I replace too many more parts.

On the positive side, this truck reminds me of the early days when you could actually sit on the fender well to work on an engine!

Good idea on the break free. I have a couple "wish I would have used it" stories -- Luckily not on my truck.

 

[bruceb58]

Re: Sudden drop in gas mileage

Thought I'd try the simple things forst so figured I'd clean the MAP sensor. Turns out that this engine doesn't have one.

Rockauto.com says you have one but I doubt you will be able to clean it.

Maybe you meant MAF.

 

[PiratePast40]

Re: Sudden drop in gas mileage

^^^ yes, you're right. I was thinking of the MAF since it's in the air flow and can get dirty. It's possible to clean those and that may clear up a problem. But I've found out they didn't use one on this application so it became a mute point. I've also found the MAP sensor and you're right again. It's doubtful it could be cleaned and it's not in the airflow anyway so it's not subject to the same contamination as a MAF would be.

Thanks for helping me use the right terms and look at the correct components.

 

[RogersJetboat454]

Re: Sudden drop in gas mileage

Mitchell on demand is showing a MAF for the 4.9L and Windsor engines, but nothing for the 460. I'm guessing this is a speed density system.

If you have a buddy with a decent scan tool, have him plug it in and check fuel trims, O2 sensor voltages (graphed), intake air temp, MAP voltages, and confirm that the engine is getting up to at least 190?F (enabling closed loop). Would also be good to confirm you don't have any vacuum leaks.

BTW, FWIW I haven't read an owners manual lately, but they usta say 30K for copper plugs, 60K for platinum, and 90K for iridium. Good idea to replace the plugs and anything else needed in the secondary ignition, disconnect the battery for a few minutes then see where you're fuel mileage is after that.

 

[PiratePast40]

Re: Sudden drop in gas mileage

Thanks for the tips. I'll start asking around for a scanner and how to use it. Looks like 96' was the first year they started with the OBD scanner capabilities for this engine.

Got the plugs out without any problems. The Break Free worked great and didn't have any problems at all with the plugs being rusted in the head. Of course, there's plenty of interference to still make some of them a pain to access. The electrodes on the plugs were worn down to little nubs. They were just barely visable over the insulators so it was definately time for them to be replaced. Took a quick test drive and it seems to run better. Acceleration is nice and crisp, at least as much as you can expect from a 7000 lb vehicle.

 

[Big Bubba]

Re: Sudden drop in gas mileage

PiratePast40,
Yeap, you said earlier that you can sit on the fender's of your truck like on the older vehicles back in the day. Yeap, when you said that I just traded my 1999, Chevrolet Silverado, 1500, 4X4, LS model for a 2000 Chevrolet, LS model, 3500 dually-4 door crew cab, with Vortec 7.4liter, 454 engine. Becuase i just bought a 2000 Dutchmen, 30 foot fifth wheel with super slide that both is comepletely paid for and have titles for. Anyway, I do like the fact that you have tons of room under the hood and mine is the same. Yeap, I had to replace the serpentine belt on mine becuase of that annoying squeeling, that drove me absolutely crazy. I am kinda a perfectioness and have to have things almost working darn perfect or I am not happy. Yeap, I loved the fact you had room to get at stuff inside the engine compartment and that is definitely what I have on my 2000 Chevrolet Dually, 3500 crew cab. Yeap, I will be replacing my plug wires and spark plugs as well. Mine seems to have a very slight miss every once in a while at idle but under load it seems ok.

 

[ezbtr]

Re: Sudden drop in gas mileage

change the dist. cap and rotor as well