Something "A-Miss" - MPG Battle!

[Mmusicman]

I have been slowly battling poor MPG's for quite some time now... trying different fixes along the way. The van seems to run ok otherwise. MAF sensor has been swapped, distributor swapped, Catalytic ruled out, O2 sensors working (although one may be lazy), vacuum leaks fixed, etc, etc. Gas mileage avg on highway continues at 10-11mpg.

This weekend I finally tried putting a can of Sea Foam in tank and have finally used up that tank of gas, but no immediate noticeable difference. Then yesterday morning, I noticed a lot of sputtering and surging while sitting at traffic lights. This is something this engine has never done before.

I got on the highway and ALL OF THE SUDDEN I'm getting 15mpg! (I have the overhead console which I've found quite accurate). Suddenly, I only have to barely push the gas pedal to maintain speed, whereas before I was pushing almost 3/4 way to floor, and having to hit passing gear on slightest incline, or to accelerate... or sometimes just to maintain high speed.

Suddenly I had excellent power, and excellent gas mileage. After getting off highway, the idle at lights was completely smooth as could be. Typically before this, I was seeing a slight vibration in mirrors when stopped.

Well sadly, the old condition returned on the return trip back home.

So either the Sea Foam started to solve my trouble, or it's another intermittent issue. I will put in another can of Sea-Foam, and finally change the plugs too, even though the couple I looked at looked ok.

It would be my guess now that I have a cylinder not firing for some reason. If it's an injector issue, I have no experience with that. My saga continues. I will post the solution that fixed my trouble when I have it.

[Jasen]

If it's an injector issue, I have no experience with that.

yet

[Sailing_Faith]

I don't have any (good) suggestions, but I would say if you can get back to 15 mpg on that great big ole van turning those big meats you will be doing well...

I still can not get over your good fortune at that deal!

Good luck man!

[crusader1xxx]

I read your post from July about this and I am just going to throw this out there...

In the other post you kept referring to another 2000 Van and was comparing the two. I don't know about you but comparing gas mileage between two vehicles even the same year is next to impossible. there are so many variables to consider from something as simple as a few lbs pressure in the tires to the type of gas you have in each vehicle. the only real way would be to run each on a dyno where the conditions can be kept the exact same.

Second..you say you are getting you mileage off the computer in the head unit. The only way to really measure your mileage is how much fuel you put in vs how many miles you get. The best way is to fill you tank (if you know how many gals it holds) with out topping it completely off and run it bone dry. See how many miles you went vs how much fuel you used. That would be how much you get per tank. or you can do half a tank etc...If it were me I would ignore the overhead and do the calculation yourself...

As for this thread, my guess is that the sea foam temporarily clean up the injector unit but only for a brief time. Pull the EGR and clean it out, then block it. You will find it makes a huge difference in both performance and mileage. I did mine and it made a noticeable difference in the way it ran, and I too thought mine was fine. The problem is it puts gunk into your intake and over time just messes everything up.

[Coaster]

it sure sounds like EGR not seating right

[Sailing_Faith]

Crusader,

No need to run the tank dry. I fill up every time, and track miles driven every time. Miles / gallons gets the number.

[SilverBullet1997]

start tracking your miles with Fuelly.com

[Mmusicman]

...Second..you say you are getting you mileage off the computer in the head unit. The only way to really measure your mileage is how much fuel you put in vs how many miles you get...

Actually, I have found the overhead gauge to be quite accurate! I double-check the mileage divided by gallons used, and it's always right on with the gauge. It also show me instant readings, which is where the average comes from... and gives me a very good indication of what's going on.

Regardless... I did temporarily start getting good gas mileage (like it used to) and had much more noticeable power.

Last night I changed the spark plugs, but haven't had it on the highway yet to see if they were they were the problem. I don't think so... because I still have the slight vibration in the mirrors that briefly went away when the problem went away. I go on a highway trip tomorrow.

I have added another can of Sea Foam... to see if it can pick up where the other may have left off?

I have never received any codes, but there is definitely a problem. I'm going to look into doing the EGR valve mod next...

[Mmusicman]

...I still can not get over your good fortune at that deal!...

Thanks! I get thumbs up all the time... and really enjoy the ride and the view!

[Sailing_Faith]

Have you thought about a tin can block off plate? I have found GM with their silly electronic EGR valves can create a multitude of problems from a faulty EGR.

I know another poster mentioned it, but hearing about your "shaking mirrors" makes me think that may well be it. If you are experiencing a slightly stuck open EGR, the o2 will read low and the computer will dump fuel to make it up.... That would sure knock your mileage down!

I was working on a 3.8 that had been to 2 different dealerships in Michigan. They had changed parts all over that car. I made an EGR block off plate out of an aluminum can And the supposed "transmission problems" went away. Mileage went up immediately (also as registered by the onboard computer)...

Take your favorite soda (or beer if you drink beer in the can like the savages) cut the top And bottom off. Take the resulting sheet of aluminum and fold it in half.

Take the valve off, trace the shape of the EGR gasket on the aluminum and cut it out with sicisors. A hole punch works well to make a place for the bolts to poke through, put the gasket back on, slide the plate over the bolts and mount the EGR back on top.

If your EGR is sticking, you will get the smooth idle and mpg back.. Of course you will also see a 404 code soon, but good for troubleshooting anyway... And it is free!

Good luck!

[crusader1xxx]

Sorry, I thought you had only ever used the overhead unit so I thought maybe the unit was out.(calculating your mileage)

I hope you find the problem, like I said in my post and others as well have said it, block off that EGR valve and you van will run like a different beast in a better way. Mine sure did. I used an old piece of flat sheet metal to make mine. i just traced the EGR outline onto the metal and cut it with a jig saw. I wanted something a little thicker then a can. i sandwiched it between two gaskets and she has been running great ever since, about 2 months now.

Good luck.

[krewchief]

Here's my take,for what it's worth.... First, misfires. Misfires always will count up in the misfire counters in the PCM - so with a real scan tool you can look at both 'live' data which will show current misfires, and 'history' misfires which count up and accumulate over time. Ignition misfires are the most prominent, and will always increase the misfire counter. Typically when ANY late model GM product (96=) logs more than 300 misfires on any single cylinder it will turn on the SES light and store a code. This would be your first place to continue with your diag. The earlier generation vortec engines were notorious for the crab-style injector going bad, and plugging up causing no starts and drivability problems. We have seen quite a few bad injectors in the later model ones. The key here really is that you need to befriend a good mechanic or buy an good, expensive scan tool - I have over $12K invested in my scan tool alone! When driving, you can look at your fuel trims and start to determine what the problem is. By graphing the 02 sensors and looking at fuel trim, you can see if your engine is running lean - this could be from bad 02 sensors, bad fuel pump, restricted fuel filter, dirty MAF, bad fuel pressure regulator, leaking intake gaskets (unmetered air), etc.

Ignition misfires can always be felt in the seat of your pants, like a kick - when the spark that's supposed to go to a particular cylinder goes somewhere else, you can always feel that. Fuel misfires can be a lot more tricky, because with a restricted injector or plugged fuel filter, fuel is still getting into the cylinder and it's firing, just with not enough power. Mechanics typically allow +/- 10% fuel trims as normal. I suspect if you have a way to watch the fuel trims when you get out and drive your van and have to go 3/4 throttle, you'll see you fuel trims at +25%, which is just about where the computer strategy gives up and turns on the SES.

Also, while seafoam is a very good product (I have 5 cans in my toolbox and I use it), it's no magic fix in a can. I have to believe it was just coincidence that your van ran better with it in the tank. I use it primarily to decarbonize intake manifolds and EGR passages. Hope this helps, and go ahead and write me back if I can help you work through this!

[Mmusicman]

I appreciate all the great suggestions and advice! I will look into doing the EGR block plate this weekend. The new plugs did not rectify the problem.

@krewchief: I have watched the fuel trims while driving, and (I assume) they seem to read high. The O2 sensor seems to have issues (either reads real high or low) unlike the bank 1/1 and bank 2/1 sensors which sweep up and down normally. But I suspect the pegged readings of the final o2 may be due to other issues? I have a new one... just haven't had a chance to change it yet.

I'm not sure where you see misfires... maybe my cheap OBD2 reader doesn't see them? They way you describe it doesn't seem to be my issue. I'm still leaning toward a possible injector issue, but I'll do the EGR block next.

[Leeann_93]

My AutoXray TechScan AX7000 can read misfires; my other scan tools cannot.

You do have to step up a little in the $$ to get a tool that will see misfires. See if you can borrow one from a friend or find a mechanic who has a tool with that capability.

Seafoam is fine for decarbonizing, but I prefer Chevron Techron in the fuel tank.

[Mmusicman]

I've had others suggest Techron too... I'm going to give that a try. Thanks for the info on the reader. Mine only reads most of the basics, unless the limitation is in the software... I'm not sure. I haven't found another free software.. I'm using OBD 2007 Lite that came with reader. No gauges which suck... just text readings.

UPDATE: I put the new O2 sensor in. Apparently I only have 3... one at each header, and one AFTER the catalytic converter. Before I hooked up the new... I was curious to see what would happen if I left it disconnected. The OBD reader showed it going into a "default" level of 0.45 and stayed there steady. Ironically, my mileage was not affected much.. got about 12mpg.

After hooking up the new O2 sensor, I saw it functioning as it should.. swinging high and low. My mileage did seem marginally better... about 13mpg now.

I believe it is possible if my issues are injector related, the faulty injector may be working somewhat, and intermittently, since I saw occasional 14mpg, then back down to 12... on same flat highway.

I haven't done the EGR block-off yet... should get to it hopefully sometime this week...

[redfury]

What are your long term fuel trim readings? Do they change with an increase in RPM. If they are high at idle, and then decrease as you increase RPM, then you likely have a vacuum leak somewhere. It's not always the easiest thing to understand. I've been "battling" my high fuel trim readings, trying to determine what is causing them. I have no misfires, good fuel pressure, but had lean bank codes for both banks. I get an average of 15.5mpg out of my V8 van. The lean bank codes have gone away on their own for some reason. I suspect possibly an underperforming fuel pump since I started having the lean bank codes shortly after the pump started to make that electric whirr they are known to make.

Ultimately, as long as your O2 sensors are switching above and below .450mv and doing it relatively quickly, then you can forget that side of the system for being an issue. It's going to be a matter of fuel delivery, making sure your CMP retard is within range and that you have a MAF sensor that is clean and creating a good pattern on a scope. I'd love to own a PICO scope for this reason alone. It would help diagnose many things, including the health of that noisy fuel pump.

My EGR is disabled in the computer, but I haven't put a block off plate in mine, which if the EGR is letting a tiny bit of air past the pintle, it could be the source of my problems. I think I'll be giving the soda can fix a try, since it costs darn near nothing.

[WinnieVan]

3M -- I know this is VERY OLD -- but did you solve this problem?

Mine sounds very very simlar.

[Mmusicman]

UPDATE! (4 YEARS LATER):

Well, let me answer your question.. and post an update almost 4 years later!

It turns out that there really wasn't anything specifically wrong! My engine was fine, nothing was wrong with it.
Turns out it was all nothing more than not having enough power attempting to overcome unknown increased load resistances, or knowing the conditions around me while taking my mpg readings. This (at the time) was new to me.. and I didn't really know yet certain things that could be affecting me.

I have since learned that my erratic mpg's were (and still are) mostly due to driving conditions...
* head winds (the biggest factor for me)
* road type
* travel speeds
* driving style
* and overall conditions

Over the years I have attempted and tried numerous things, and have done numerous fixes:
* Eliminate EGR valve (changed nothing)
* Change 02 sensors
* Seafoam for injectors
* Tune-Up (plugs, wires, cap rotor)
* New distributor
* New MAF sensor
* Mods to IAT sensor
Not one single thing really changed anything (for me) with regard to mpgs!
I have watched and studied OBD readings, and found nothing to far out of normal.
I've still never seen any thrown codes either.

The mirror shake (vibration) was (and still is) simply bad motor-mounts.

But I also learned that because I have a seriously lifted van PLUS very over-sized tires (31.5")... I seriously struggle with not having enough power to overcome the slightest headwind! If affects this van SO much more than my previous completely stock 2000 cargo van. With even a slight head wind, I am into the throttle almost as far as you can go (on the highway), just before it downshifts. With enough head-wind, I literally can not even maintain certain highway cruising speeds in 4th. This is exacerbated by the use of the AC. If I downshift to 3rd, my power and speed increase immediately, and my fuel economy drops further.

I did not know all this when I first posted this. I started to figure this out when I had thrown some Seafoam into the tank and "coincidentally" got almost 16mpg going north to a job one day, only to see about 11mpg on the southbound return trip home. I soon figured out I had a tailwind going north, and a headwind going south, and the Seafoam hadn't really done much of anything for mpgs.

In addition to all this, I've also come to realize poor head-wind resistance mpg's are further exacerbated by my driving speeds. For example.. a 10mph direct headwind might cause me to literally top off speed at 70-75mph on the highway (I absolutely can NOT go any faster in 4th gear). If I try to do 75 or 80mph, the van downshifts, and my mpgs drop off very significantly. Typically to overcome the lack of torque (in a strong headwind) and inability to maintain highway speeds, I have to manually downshift to 3rd and stay in that gear. My mileage will usually and typically drop to around 10mpg (or even less) in these conditions.

Yet with a moderate tailwind, everything changes! I literally saw 18mpg (going on a trip) doing about 80 just last week! (yes people drive fast on the highways in South Florida). I'm barely pushing the throttle and it's very easy to maintain top highway speeds.

Bottom line: my engine is running perfectly as expected. (200K+ miles)

This stock 4.3 engine simply does not have enough hp/torque (to the rear wheels at my lower than normal rpms), to overcome the increased resistances with over-sized tires and stock 3.42 gear, to overcome the excessive wind resistances forced at a lifted vehicle. It's just more than the stock 4.3 and this modified vehicle was meant to handle. Naturally, the more I push the throttle, the worse my mileage gets.. as you would expect! Light throttle and things are GREAT!

I believe if I corrected the differential gear ratio (for the taller tires).. that might help some. But I'd probably still suffer from head winds... maybe just not as much.

My engine overall, runs fine.
* If I drive easy (65mph), I average about 15mpgs. (side wind)
* If I drive slower.. I can average 17-18mpgs (wind speeds also have less effect).
* If I drive fast (80mph)... I average about 13mpg. (side wind)
* If I attempt to drive 80 into a headwind, I will typically see 10mpg or less.
* Headwinds/tailwinds change these number dramatically, either up or down - or + up to 2-3mpgs.
* I get better mileage in the mountains than on flat roads.
* I believe driving at most efficient speed under ideal conditions that I could exceed 20mpgs.

I know exactly what my mileages are at all times using the overhead console, which I have found to be completely accurate! I always check and compare when I fill up. It gives me instant mileage, as well as average. I have literally started a trip (just 2 weeks ago) getting 17mpg highway (doing about 80)... only to watch that average change down to 14mph (about 2-3 hours later).. still going same direction on the same road, still going the same exact highway speed, driving with the same gas, weight, load, doing everything exactly the same. Turns out wind direction and wind speed had changed from where I started in South Florida, to further up the road in Central Florida where I saw my mileage start to drop significantly.

What I now know is that when people boast or complain about fuel mileage... you have to account for so many other factors as well! How fast do you drive, how heavy are you loaded, what is the wind speed and direction, what is your diving style, what mods have you made to your van, etc, etc.

Certainly a poor running engine will have a definitive affect on fuel economy... but apparently so do all these other things too.

One thing I also know for sure... my V8 van is capable of getting better mileage than my V6 van under anything less than ideal conditions.

[WinnieVan]

Cool Thanks 3M. Appreciate your input (as always)!

[Leeann_93]

Yup, 3.73s would be better than the 3.42s, but you might need 4.11s to see any real improvement.