GM Truck Club Forum banner

1 - 16 of 16 Posts

·
Registered
Joined
·
5 Posts
Discussion Starter #1
New guy here with questions. I recently bought my first diesel, a '98 Chevy Suburban 1500. So far I've put on a larger crosspipe, downpipe, 3" straight exhaust, and the PMD has been relocated. I'm getting 12 MPG while empty. What can I do to improve that? What should I be checking? Thanks!
 

·
Registered
Joined
·
7,291 Posts
Welcome to the club. Sorry can't help with your question, but there are several diesel guys on the site and I'm sure when they see your post they can help you.
 

·
Registered
Joined
·
7,979 Posts
Is it a 4X4 or a 2X4 what differential gears do you have? Also you say 12 mpg is that in town or straight highway? Also when was the last time it had the filters changed? Also You have the exhaust all open but have you done anything to get more air in? And taking a closer look at the air filter as well. Diesels are just really big air pumps and you open up one end you need to open the other up as well. And with a suburban doing 70 on the highway with a 6.5L turbo you will only see around 15-18, slowing down to 60-65 will get you much better results since the 6.5L doesnt like to be reved.
 

·
Registered
Joined
·
57 Posts
Is it a 4X4 or a 2X4 what differential gears do you have? Also you say 12 mpg is that in town or straight highway? Also when was the last time it had the filters changed? Also You have the exhaust all open but have you done anything to get more air in? And taking a closer look at the air filter as well. Diesels are just really big air pumps and you open up one end you need to open the other up as well. And with a suburban doing 70 on the highway with a 6.5L turbo you will only see around 15-18, slowing down to 60-65 will get you much better results since the 6.5L doesnt like to be reved.
Good info,I will apply the in and out as I build up my 88.
 

·
Registered
Joined
·
5 Posts
Discussion Starter #5
Is it a 4X4 or a 2X4 what differential gears do you have? Also you say 12 mpg is that in town or straight highway? Also when was the last time it had the filters changed? Also You have the exhaust all open but have you done anything to get more air in? And taking a closer look at the air filter as well. Diesels are just really big air pumps and you open up one end you need to open the other up as well. And with a suburban doing 70 on the highway with a 6.5L turbo you will only see around 15-18, slowing down to 60-65 will get you much better results since the 6.5L doesnt like to be reved.
Hi Mike, thanks for the response. It's a 4X4 with a 3.42 rear. The 12 mpg is mostly in town; maybe 75% city / 25% highway driving. All the filters are new including an aFe Pro Dry air filter. That's the only thing I've done for the intake.
 

·
Registered
Joined
·
7,979 Posts
I would open up the intake more, and 12mpg city is actually pretty good for a 6.5L... as i said above they are good on milage when driven lightly. The best part of a 6.5L is that it will run forever if taken care of.
 

·
Registered
Joined
·
169 Posts
Depending on your mileage, and whether or not you know if the injectors have been changed, worn injectors will cause a loss of power and mileage. Additionally, if your lift pump isn't performing properly you will likely see a drop in mileage and performance.
Your mileage isn't bad if this is your city mileage; though I have seen 4WD Subs get high teens to twenties on the highway.
 

·
Registered
Joined
·
1,514 Posts
6.5 l TD on a 98 1500 4X4 should be more like 16-20 MPG in non towing apl. ! something is not write !?
 

·
Registered
Joined
·
169 Posts
I definitely agree that this being too low, especially being a 1/2 ton. I've seen one ton p/u's getting better around town than this.
Several things can cause this too. Check to make sure you have no blockage in the fuel system; and your lift pump is operating properly. Pressure for this should be no less than 5psi at idle and can be checked at the water drain valve. There is also a fine screen in the filter manager, which is slide on the outside of the fuel heater tube. Also worn injectors will cause low mileage as will a defective injection pump.
If you have checked the level of boost your are building, and are within specs but see a fair amount of black smoke still, I'd suspect injectors as they are often not changed as often as recommended (every 100k).
 

·
Registered
Joined
·
5 Posts
Discussion Starter #10
Thanks guys. The lift pump and injectors will get checked next Monday. Although, considering my last fill up showed 10 mpg, maybe I'll just get them replaced.
The truck has 151,000 miles on it and they may be original parts. It runs great otherwise!
 

·
Registered
Joined
·
169 Posts
Let us know what you find, might be able to help with some more things to look at.

As far as the pump goes, I doubt it's original. GM was clueless about PMD issues and whenever one came in with a problem it often got a new injection pump as the fix-all.
An injection pump should last at least 150k as long as you don't allow water or any other debris in the fuel to get to the pump. Be sure to use a good fuel additive and regularly drain water from the filter housing.
 

·
Registered
Joined
·
5 Posts
Discussion Starter #12
Belated update

Got the new injectors finally. The mileage didn't change much, maybe a single gallon per mile improvement, but it runs smoother and the white cloud at start up is gone, so is the constant diesel fuel smell. So, not entirely what I was hoping for, but I'm still glad it's done.

I've discovered a problem with owning a diesel: Finding a good diesel mechanic! I've asked two places to check the fuel and lift pumps and make sure they are working properly, but all they actually check are the diagnostic codes. Anybody know a good mechanic in the Chicago suburbs?
 

·
Registered
Joined
·
169 Posts
You can check the lift pump yourself very easily. With the engine idling, open the air bleeder on the fuel filter. Fuel should shoot out of it several inches with no air coming out of it first.
Next, you should have a small brass T handled valve by the thermostat on the coolant crossover at the front of the engine. Again, with the engine idling you should be able to open this and drain out water and sediment from the filter housing without the engine stalling. If it stalls, the lift pump is probably not running.
You can also put a pressure gauge on the fitting at the valve too; the pressure from the lift pump should not be less than 5psi, and not over 9psi.

You're going to have a heck of a time finding anyone who'll work on a 6.5L and is competent; they're very few shops that are knowledgeable about them, and the dealerships often won't touch them either, and know almost as little and most shops.
 

·
Registered
Joined
·
5 Posts
Discussion Starter #14 (Edited)
Thanks! I'll check the pump as you described. You are right about the mechanics, the guys I talked to just looked at it and scratched their heads. :no:
 

·
Registered
Joined
·
32 Posts
6.5 l TD on a 98 1500 4X4 should be more like 16-20 MPG in non towing apl. ! something is not write !?
very true i had a 93 gmc extended cab 4x4 1500 6.5 turbo and only thing i did to mine what dual thermostates, and intake. empty i got like 23-26 mpg on hwy... there really isnt a whole lot you can do aftermarket wise for the 6.5L mainly cuz that engine is used more in the miltary then in cilvian vechiles
 

·
Registered
Joined
·
4 Posts
My 82 with a 6.5 and a 400 trans and 4.10 gears and is getting 13.5, around town, it does not like to go much over about 60 mph though.
 
1 - 16 of 16 Posts
Top