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6.5 Poor fuel mileage and loss of power

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martym42

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I have a 1994 GMC with a 6.5 turbo with a 4:10 rear end, in the past I have always gotten about 18 mpg empty and about 15 - 16 mpg towing presently I am getting 13 mpg empty and 10 mpg towing.
the engine starts well, runs smoothly, burns no oil, runs excellent except for the loss in fuel mileage and loss of power.

I dont know what would cause this problem ? is there an issue with the engine or could it be the new California diesel any suggestions would be appreciated.

The vehicle is not used much has about 113000 miles, used mostly for towing goose neck or dump trailer

Thanks
martym42
 
Welcome aboard, Marty! This here is the best 6.5 forum on the 'net, and you're gonna like it here!

Couple things first ... fill in your signature so we can all see what you're driving, what (if any) modifications you have made, and so we can give you suggestions that fit YOUR truck, not just a generic truck. The more accurate you are, the better we can see, hear, and understand your ride!

As for your question:

A] The new diesel surely doesn't help your mileage. Are you using any additives? Most of us add a quart of the cheapest SAE30 we can find to every full tank, just to help keep our injection pumps lubricated and to up the BTU capacity of the fuel. The extra mileage and power pays for the oil. Lots of others (myself included) use a c_ocktail of 8oz Powerservice Silver and 8 oz of 2-stroke about every 3-4 tanks, or every tank when I'm pulling heavy. The increase in Cetane number helps with the power and mileage.

B] You might want to start thinking about changing your injectors. They're only rated for 100,000 miles, and when they start to crap out, bad things happen... mileage and power go down sharply, and bad injectors can spray fuel streams (rather than mists) into your cylinders which can cause burn-throughs and other issues. Worn injectors can also 'leak-down' when the truck is shut off, letting raw fuel into the cylinders. This washes off the oil on the cylinder walls, and can cause the rings to grab the wall in the first few rotations upon starting - bad stuff to have happen.

Your injectors are over the change interval; if your injectors are showing the signs of age (mileage down, power down, white smoke on startup, etc), you would be wise to invest a little time and parts in your truck.

I'm sure others will chime in here with some advice; they're all good guys.

Again, welcome to the family!

Jim
 
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similar truck, mine went south when turbo failed to operate correctly, I would start with fuel supply(lp, filter), verify turbo operation- these are 2 pretty easy things to get to quickly.

X2 on injectors, I had a burn through on an old vw diesel truck. not a good thing, but kills plenty of mosquitoes:D.
 
Good points, Doc - I shoulda mentioned them.

Marty, there is a Diagnostic Checklist in my signature that will help to let us 'see and hear' your truck... print it off, go through it carefully and ask for help with tests if you need it. Then copy the checklist into a new post, ass in your findings after each question (use a different color) and that will give us some valuable infomation about what is working and what isn't.

For everybody's benefit - going through that checklist every so often is just a good way to make sure your truck is getting the regular maintenance and attention it needs.
 
You might also want to check the timing chain. It is also rated for 100K miles and it also affects mileage and performance.
 
JiFaire said:
For everybody's benefit - going through that checklist every so often is just a good way to make sure your truck is getting the regular maintenance and attention it needs.
Click to expand...

Excellent advice and I'll do just that when I replace my LP next weekend.:thumbsup:

BTB, what were you really trying to say when you wrote, "ass in your findings after each question...."):D
 
One thing the guy's for got is did you add new tires recently? More specifically did you increase the tire size from stock? If your tires are bigger than stock it would make up the difference you are seeing in fuel economy. My burb gets the same as you are getting now but w/ larger tires the mph is off by 2-4 mph depending on how much bigger than stock you go.
 
When did we start adding a quart of sae30 at each fillup? Did I miss a memo? I'm guessing because of the crap fuel we're getting now. Any downside to pumping the oil in? 1 qt to a full tank?
 
Paveltolz said:
Excellent advice and I'll do just that when I replace my LP next weekend.:thumbsup:

BTB, what were you really trying to say when you wrote, "ass in your findings after each question...."):D
Click to expand...

I really have no damn idea... senior moment, I guess. I think it should be 'add in your findings'

LOL. My bad.
 
Detroit Dan said:
When did we start adding a quart of sae30 at each fillup? Did I miss a memo? I'm guessing because of the crap fuel we're getting now. Any downside to pumping the oil in? 1 qt to a full tank?
Click to expand...

You gotta read your memos, Dan! :D

The ULSD is crappy for lubrication in the DS4s... so most of us were adding the old Powerservice Silver and 2-stroke, or Stanadyne additive. Then Bill Heath mentioned to Tim and I that he uses cheap (no additives) SAE30, the cheapest stuff he can get.

I've been using it ever since (except when I pull heavy, wanting a bit higher Cetane count) with no problems.

I thought you knew or i woulda sent you a carrier pigeon.
 
turbovanman said:
How do you check for bad injectors on the truck?
Click to expand...

The only way to be REALLY sure is to pull them out and test them with a pop tester... you look for release pressure and spray pattern.

Indicators of deteriorating injectors can include decreasing fuel mileage, power loss, overheating, white smoke on startup, cylinder imbalance, knock/miss, and a few others.

But there's only one way to be really sure.

As far as the 100,000 mile barrier, it ain't like they hit 100K and just die... some go before that, some later, some last virtually forever, to hear some people speak. But it's like timing belts... the consequences of failure are pretty bad. If your injectors are 'leaking down' pressure when not running, that fuel washes the oil off the cylinder walls, and can cause the rings to 'grab a wall' on startup ... that would be a bad thing. A direct squirt of diesel (rather than a fine mist) could create a hot spot in a piston, and can result in burn-through. This is also a bad thing.

Or nothing might happen. It's one of those insurance things... you buy insurance, hoping you'll never need it, right? Same reason we change injectors at 100,000 miles. If yours are starting to wear out, then the increase in fuel mileage will help to pay for them in pretty short order.
 
JiFaire said:
The only way to be REALLY sure is to pull them out and test them with a pop tester... you look for release pressure and spray pattern.

Indicators of deteriorating injectors can include decreasing fuel mileage, power loss, overheating, white smoke on startup, cylinder imbalance, knock/miss, and a few others.

But there's only one way to be really sure.

As far as the 100,000 mile barrier, it ain't like they hit 100K and just die... some go before that, some later, some last virtually forever, to hear some people speak. But it's like timing belts... the consequences of failure are pretty bad. If your injectors are 'leaking down' pressure when not running, that fuel washes the oil off the cylinder walls, and can cause the rings to 'grab a wall' on startup ... that would be a bad thing. A direct squirt of diesel (rather than a fine mist) could create a hot spot in a piston, and can result in burn-through. This is also a bad thing.

Or nothing might happen. It's one of those insurance things... you buy insurance, hoping you'll never need it, right? Same reason we change injectors at 100,000 miles. If yours are starting to wear out, then the increase in fuel mileage will help to pay for them in pretty short order.
Click to expand...


Just curious, a friends truck starts rough, like its not firing on 2 cylinders, lots of white smoke. Has fairly new injectors and glow plugs. He can't test the last 2 on the turbo side, lol, so those are probably the ones that aren't working. :sad:
 
That could also be crappy glowplugs... same symptom, only takes one bad crimp to do it.

That's why you can't tell bad injectors only from the on-the-truck symptoms..
 
94-95 GMTD'S scan tech and probably others allow you to shut down individual cylinders-poor performing injector might not be caught but a nonfunctioning would---correct?? like pulling plug wire for dead plug on a gasser.
 
A non-functioning injector (and usually even a poor one) can also be seen by cylinder imbalance with a techII or GMTDscan, as you say. Usually, if you have a non-functioning injector, you'll have a big-time miss that alerts you, along with the smoke. Just before your engine craters. :D

A poor injector, or one that is wearing out, or developing a bad pattern, is harder to find. Those are the ones we need to catch - before they go out - and it's really difficult and uncertain.

Like I said, changing at 100K isn't a bad idea, just for preventative maintenance.
 
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