Splain this to me if you can

[The_Duckslayer]

OK... we have an 03 Envoy and were told that at 100K miles we need to change the plugs. Well I pulled it into the garage changed the oil, changed the spark plugs, and changed the air filter. Prior to my doing all that we were getting 17.5 to 18.5 MPG with it. Since then it dropped to 13.5. I don't get it!? I gapped the plugs at .042 just like the book says. Any thing that I should look for? A 4 MPG change overnight is a bit much. Thanks././Jimbo

[Scott K]

Are you going off of the digital fuel economy display, or using milage with a calculator after fill up?

[Jeremy airjer W]

What kind of plugs did you install?

[The_Duckslayer]

Mileage with the calculator. AC Delco platinum or something like that. I know they were AC Delco Thanks././Jimbo

[Scott K]

Possibly got dirt on the Mass air flow meter when changing the air filter?

[BoxMN]

Hmmm... just a thought... but I have messed up the calculation with my fat fingers before, or maybe somebody tripped the odometer by mistake, making you get worse mileage due to not as many miles?.... I know my wife was done that to me now and then, tripping the trip odometer or adding gas without resetting

The simplest solution is usually it I bet your next tank will back to normal or a bit better with the new plugs.

[Scott K]

Or idle time, letting it warm up? If you have been letting it idle to warm up, most vehicles will use a gallon or more of gas per hour of idle time. So, 15 minutes in the morning, 15 minutes in the evening for a couple days, it adds up fast!

[The_Duckslayer]

Nothing has changed and that is 2 tank fulls that it has dropped. Mama does let it warm up but she has alw2ays done that and I don't think that she does is to the tune of dropping 4 MPG. Looking to check it again probably by the end of the week. However the gauge is dropping quicker than normal still. I just don't get it. There are no engine codes and mama says that it does in fact run better since the plugs were put it. Things that make ya go Hmmm.

Which sensor is the air flow one? I only see one at the back of the intake tunnel and I did not take that out and mess with it when I did the plugs.

[Jeremy airjer W]

No MAF sensor on the straight 6.

Might be worth pulling the vacuum line off the pressure regulator and see if it is leaking. We just had one last month that started on fire because of a leaking regulator. It might even be possible that you didn't get that hose back on the air box above the plugs when you removed it?

[The_Duckslayer]

Checked that last night. There is a small one on the front right of the engine and a PVC valve type hose and they are both connected. They are both connected. It does smell gassy some times though and that should not happen on the fuel injection should it? The one small hose was gas wet when I disconnected it? Is that one on the front the vacum line you are talking about or is that some where else? Does not seem like there would be much vacum there? Thanks for the help././Jimbo

[Jeremy airjer W]

There's your culprit!!!

On the other end of that really small hose is the fuel pressure regulator. Pull the hose off the regulator and see if gas comes out. You may have to have the engine warm, running, cold, not running or any combination of the above. You may also have to wait a little while to see gas. If you see any gas come out of the nipple on the regulator you need to replace it immediately!

[The_Duckslayer]

I'll double check that tonight but I think I remember it being wet. That seemed kind of strange to me now that I think about it as why would you want raw gas inside that air box? I'll let ya know what I find out. Thanks!

[Jeremy airjer W]

What happens is that air box will fill up with fuel if it gets bad enough. One backfire and and she goes down in a blaze of glory (a little bon jovi for ya)!

[deadeye]

Should be about 50 bucks for a new one.

[The_Duckslayer]

Yup that's what it is. Took the tube off and turned the key to the on position and it did not take long to have some gas seeping out. I am assuming, and I really hate to do that, those two spring clips come off of the tubes leading to the valve and that is how it is replaced? Thanks for all the help guys!!!! Take care and N Joy the Hunt././Jimbo

[The_Duckslayer]

Well got the new fuel pressure regulator on last night. DE they are $79.00 at the parts store in Northfield. Actually I put the thing on twice :-( I figured out how to get the old one off easy enuff. I guess next time I should find the fuel pressure releif valve so that I do not get so much fuel all over the engine compartment! I got it off, put the new one on and pushed it out into the driveway to let it sit and air out and the spilled fuel dry up some. After a while I went out and started the thing up and it was leaking fuel from the bottom. I noticed that there was a paper in the box so I though the proverbial "if all else fails read the instructions". Well it turns out that was the receipt but when I pulled it out a small plastic bag fell out with an "O" ring in it. So this time in the driveway at 9 degrees I took it off again and put the "O" ring in. Waited again for a while for some of the fuel to evaporate and started it up again. No leaks this time! Now I just hope that the mileage will go back up again. Thanks for all the help... Knew that I could count on FM to get it right! I owe you guys a brew next time we meet up. take care and N Joy the Hunt././Jimbo

[Jeremy airjer W]

I owe you guys a brew next time we meet up.

I'll see you this weekend!!

Let us now what the mileage your getting is now that you have that replaced. I have not had any customers that keep track of mileage to quiz about the mileage changes so I am kind of curious to see what the difference is.

[The_Duckslayer]

Originally Posted By: The_Duckslayer

I owe you guys a brew next time we meet up.

I'll see you this weekend!!

Sorry airjer, I'll be in West Bend WI this weekend. However will track the mileage there and back and see where we are at now. Will let ya know! Thanks again.

What time do you get to work in the afternoons? Take care and N Joy the Hunt././Jimbo

[Valv]

I am going to "steal" the thread and ask where's the regulator on the V8 5.3 Trailblazer ? I have same issue with bade mileage and want to check all possible causes.

[Jeremy airjer W]

With an external fuel rail it will be on the drivers side of the rail in the middle. You may have to remove the top engine cover to see it/check it.

Its probably fine, you just have to stop driving it like it a Ferrari!

[Valv]

Its probably fine, you just have to stop driving it like it a Ferrari!

Smartypants !!!! I don't drive it, it's the wife's ride. I am a diesel guy.

I'll check it when she gets home. Thank you.

[WingDing2]

I just had to laugh...Airjer diagnosed my problem with my fuel pressure regulator in my truck and when I changed it I got a face full of gas injected under high pressure right in my eyeballs!

Wheres that pressure relief valve? hehe

Airjer- When my oil pressure gauge in my silverado says 80lbs does that means the pressure switch has gone bad? Truck runs fine been driving it like this for a 1000 miles??????

Whats yer call?

[Jeremy airjer W]

I got a face full of gas injected under high pressure right in my eyeballs!

Wheres that pressure relief valve? hehe

[The_Duckslayer]

OK... now that I have put e few hundred miles on the Envoy and checked the gas mileage over the course of 5 tanks I am getting 14 MPG. That is still 4 mpg less than before I put the new plugs in. Do I need to put in some "Super Duper" kind of plug to get that 4 MPG back? Took AC Delco's out and put AC Delco's back in! Can I bring it to a shop and have them hook a computer up to the vehicle and look for something? Just seems real weird! Thanks in advance! Take care and N Joy the Hunt././Jimbo

[Jeremy airjer W]

Do you monitor your mileage with every fill up or due you know that at some point you had 18mpg and after you changed the plugs you noticed your economy was 14mpg. It is winter and fuel economy will differ greatly from the summer! Did you check the air pressure in you tires when you changed the oil?