Please help - gas mileage is killing my business

[smartassu]

Bought a 2000 GMC Jimmy SLT in January 07 from CarMax.

Has been in the shop 14 times since I've owned it - I was smart and bought the extended warranty. Vehicle had 55,000 miles originally.

I previously owned a 1990 Jimmy 2dr 4wd - would get me 300-350 miles per tank (18 gallon). I miss that truck!!!!

This Jimmy originally got 4 mpg - no joke! Now I am getting about 6-9 mpg in city and average 15 mpg highway. Speed, nor cruise control don't seem to make a difference.

Here is the list of things that have been repaired in regards to fuel in a year's time:

- Fuel Pressure Regulator - was leaking - replacement helped gas mileage go from 4mpg to 6-15mpg

- 4 Fuel Pumps - yes, 4

- Catalytic Converter replaced

- 3 Mufflers - crazy right?

- A part that is connected to the Air Intake - helped improve lag problems I was having.

- Fuel Filter Replaced

- Changed thermostat as I know sometimes this can cause an engine to run cold and use more gas.

Truck now has 66,000 miles on it. I have tried to add Seafoam to clean out the injectors. I have babied the thing while driving - never seems to matter. I can do 60mph or 85mph and get the same mpg's. The city driving sucks the most gas - I can literally watch the gas gage move on a quick trip to the grocery store (app. 5 miles each way).

I am fairly knowledgeable with cars and mechanics and am not afraid of doing basic things at all. I asked a mechanic if he thought a tune-up would help, he said he didn't think so.

I own a small business and the cost of travel is really putting a ding in my finances. The cost of running this truck will eventually shut me down.

I get on average 200 miles on a full tank of gas.

Can anyone help? Suggestions?

[534BC]

A couple of questions while I think about this and take shots in the dark.

total Vehicle weight
AVERAGE SPEED
average trip time
Does convertor lock up (or rpm at 60 mph)
does it shift into 4th gear (or rpm at 60 mph)
does it run good
does it run on all cylinders
does it have the power it should?
does it smoke any?
any other known problems?
is it in 2wd.
is odemeter correct?

Ok, that's a start perhaps. Both of your mpg are quite low for just a stock vehicle and are probably the same cuase doing the low city 6-9 and the low hi-way 15. The city mpg is really in worse shape than the hi-way so maybe we should concentrate on diagnosing that (short trips, average speed, frequent stops, driving style, idling, ect.) and if the city mpg gets fixed the hi-way will probably be fixed automatically.

Another verification on numbers.

200 miles per tank = ??? gallons.
if it's about 15 gallons then that's 13 mpg, right?

if it's 18 gallons then it is 11 mpg. Either way it sucks.

[MT-2500]

A lot of things can affect gas mileage.
Driver and the way it is drove and maintained is one of the main ones.
But the lack of information on the tune up end it is hard to say.
I would go for a good tune up with Ac delco parts.
And find a good repair shop that knows what they are doing.
To many fuel pumps and cats on yours.
It has a engine running problem.

Or just pitch it back and get another one.
Have it checked out and Drive it and check gas mileage before you by it.

Are you using it for transportation or delievery stuff.
What is the actual gas mileage?
MT

[Chris Stewart]

Tire pressure...empty...33psi front & 31psi rear.
Loaded...35psi front & 33psi rear

[bozr]

Well, not exactly the most common tool but if by chance you know someone who has an infrared pyrometer, I have used mine to diagnose leaky injectors.

By taking temperature readings (at idle) directly at the exhaust port, a bad injector will put that cylinder about 75 degrees cooler than the good injectors. Pulling the plug and finding it wet can confirm it. If all the cylinders are close to the same temperature then the problem would be fuel management.

On a similar note, a few weeks ago I found out my local NAPA does ultrasonic cleaning for something like 9 bucks an injector.

[smartassu]

Thanks for all the replies!

I will attempt to answer all questions in one post here.

As far as weight goes, I carry mostly hand tools, telephone wiring, and phone jacks - I would say no more than 150lbs.

My average speed would be 30-40mph city and 65-70mph highway. I used to live in Chicago and there was a lot of stop and go traffic there and I initially thought that was the problem, but now I live in a small town in Wisconsin with a lot less stop and go, and it is no better.

I watched my rpm's last night while driving on the highway - I was doing about 65-70mph and they stayed pretty consistant at 2200-2400rpm.

I would say it feels like it shifts into 4th gear. Sometimes though I would have to say that it shifts a little hard at times, or seems to drag somewhat.

Seems to run good besides all the issues. Right before the fuel pump starts to go, which may be happening again, it seems to lag. Like I really have to hit the pedal to get it to go. It also seems to slow down faster than normal when I step off the gas pedal and before I even apply the brake.

I believe it is in 2wd - but have contemplated that it may not be just because of the "drag" I seem to feel. Last night I put it in Auto 4wd and then back to 2wd Hi (normal) to see if I felt a difference - didn't really notice much. While in 4wd it did seem a tiny bit more sluggish and tight.

I hope it's running on all 6 cylinders! I have never had the Service Engine light come on - but who knows if that really works for all problems.

Vehicle does not smoke at all, except a little when cold and the smoke is white - no odd color or odor.

Acceleration could be better. As I stated above, it seems I really have to hit the gas pedal to increase speed. I don't do this very often though as I try to save as much gas as possible, but I did attempt it last night.

Odometer seems correct. I watch it closely as I watch my gas gauge go down too - makes me very depressed.

Actual mileage is 66,134 as of March 15th, 2008

I wish I could take this truck back to Carmax. I have attempted to contact the GM many times with zero return phone calls. I put $10,000 cash down when I purchased it and after a year of payments I really can't afford to buy another vehicle.

I will do a tune up on it (myself) as soon as the weather warms up more. I always use the best parts and stick with AC Delco on American made autos.

I check the tire pressure alost daily. They are a good solid 35psi.

I have contemplated having an Injector Cleaning done - I think Merlin Muffler offers this with some new machine or something, but I've wondered if it's just a scam. Any advice? Is that like Napa's ultrasonic cleaning?

Unfortunately I don't know anyone with an infrared pyrometer - but I will be on the lookout.

Like I said before, I had a 1990 GMC Jimmy S-15 right before I bought this one. I carried the same load, had the same exact tires on that one as I do this one and would have to say that I beat that one a little more as far as gassing it - man that thing had power! And I would easily get 300-350mpg. I sincerely regret ever getting rid of that truck - this new Jimmy is causing me some deep depression. I love to drive and it has taken all the fun out of it. I'm almost becoming obsessed with watching the miles go by and the gas dwindle.

Thanks for all previous replies and thanks for all future replies.

[MT-2500]

4 fuel pumps and cat and 3 mufflers sets a red flag on engine running right.
What is going on with the fuel pumps and cats and mufflers?
Are you using the AC-Delco/delphi fuel pumps?
Aftermarket cat or OEM direct fit cat?
And what is blowing the mufflers?

Go for a good AC-delco parts tune up.
And find a good repair shop to check things out on a engine capable scanner.
Also make sure it is going out of 4W drive.
Any add on stuff like Cold air OR K&N STUFF?
After a good tune up and check over.
Fill the tank up and check actual mileage on the highway.
Winter in town sort trips can kill gas mileage.
And fill up ever so many miles does not help figure out gas mileage.
You have to take gallons used into how many mile drove.

[534BC]

Test for going into 4th gear is to drive constant speed and whithout lifting throttle foot simply pull gear selector down to 3rd. You will know if it shifts.

Test for convertor lock-up is to drive constant speed above 45 mph and touch brake pedal without moving throttle foot. If rpm goes up then converotr is locked up like it should.

If it ran on less than 6 cylinders you'd probably know it already and since there's no smoke to speak of then a rich condition may be ruled out. The only thing that comes to mind for low power/sluggish accelleration /poor mpg is worn out rings/engine.

Even though it has low miles maybe it is very worn out for some reason. The drag/slow down is a bit concerning as well.
It seems that it is both in high gear and also the convertor is locking up.

[534BC]

Originally Posted by MT-2500

4 fuel pumps and cat and 3 mufflers sets a red flag on engine running right.
What is going on with the fuel pumps and cats and mufflers?
Are you using the AC-Delco/delphi fuel pumps?
Aftermarket cat or OEM direct fit cat?
And what is blowing the mufflers?

Go for a good AC-delco parts tune up.
And find a good repair shop to check things out on a engine capable scanner.
Also make sure it is going out of 4W drive.
Any add on stuff like Cold air OR K&N STUFF?
After a good tune up and check over.
Fill the tank up and check actual mileage on the highway.
Winter in town sort trips can kill gas mileage.
And fill up ever so many miles does not help figure out gas mileage.
You have to take gallons used into how many mile drove.

I didn't notice mt's post. He's right, fuel pumps and exhaust components.

Retarded timing will cuase bad acceleration, low mpg and be very hard on exhaust components as well. A pyro mentioned above will also give a good indication of timing along with the color of the manifolds.