Poor Fuel Mileage

[Wilycyote]

My mileage has dropped to 16. 5 mpg empty, with combination town and freeway. Had been 18 to 19 mpg. Started right after DC dealer changed the leaking fuel return line. I changed the fuel filter and lost the cat. I bombed this truck last year, and gained a couple mpg towing a 13,400 5th wheel. The only difference removing the cat made was the EGT temp. is 200 degrees cooler and it cools down faster when I get off it. I don't have a heavy right foot. That is why I got over 100,000 miles out of the original auto transmission. I checked the cat by looking at the sun through it and it isn't plugged. I am puzzled so I could use some help. This is my first post, but I check out the site for awhile every night. I appreciate all the help that I have picked up from reading this forum. Keep up the good work. #ad
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1996 ram 4x4 ext cab. BD products, Large injectors,250 torgue plate 4"cat back, Tougue lock. pressure lock, and BD Auto Transmision

 

[fortunate1]

Wily, I don't have a sure-fire answer for you, but perhaps some of the experts will notice the topic and post their thoughts. You don't mention any boost numbers... could they be low enough to affect mpg (loose hose clamps on the intercooler plumbing)? One question that I have for the experts: Can the P-7100 "wear" its way out of spec (excessively retard the timing)? Can this be checked & adjusted by the owner, or must it go to a Cummins or Bosch shop?

I just finished a 1560 mile run through all of Va (500-600 miles of Interstate, the rest mixed) and used 70. 5 gallons - 22 mpg. But I still have the stock injectors, and like you only use a heavy right foot some of the time. I plan to install 215's from a '98 12-valver soon. Wonder what that will do to fuel milage? I'm pretty sure what it will do for POWER!!

 

[fortunate1]

Wily, I don't have a sure-fire answer for you, but perhaps some of the experts will notice the topic and post their thoughts. You don't mention any boost numbers... could they be low enough to affect mpg (loose hose clamps on the intercooler plumbing)? One question that I have for the experts: Can the P-7100 "wear" its way out of spec (excessively retard the timing)? Can this be checked & adjusted by the owner, or must it go to a Cummins or Bosch shop?

I just finished a 1560 mile run through all of Va (500-600 miles of Interstate, the rest mixed) and used 70. 5 gallons - 22 mpg. But I still have the stock injectors, and like you only use a heavy right foot some of the time. I plan to install 215's from a '98 12-valver soon. Wonder what that will do to fuel milage? I'm pretty sure what it will do for POWER!!

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95 2500 SLT, clubcab, 2wd, longbed, a/t, 3. 54, 144K miles, "all the usual refinements"

 

[Andy Redmond]

check the the fuel pressure before and after the filter. You should have at 20-25 psi before the filter and no more than 5psi across the filter. If you have a good lift pump and clean filter you will see little pressure drop. If you don't have at least 20 psi after the filter, I would think the overflow valve is suspect.

Don't rule out an improper filter install or fuel line replacement, is it sucking some air?

Perhaps, it's just Mr. Murphy's coincidence?
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[Wilycyote]

Thanks for the information. My boost is 30 if I am on it hard when I am towing. I haven't experienced any power loss that I can notice. I will start at the beginning and check out the systems you suggest. Thanks I'll give an up date when I get some of the results.

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1996 ram 4x4 ext cab. BD products, Large injectors,250 torgue plate 4"cat back, Tougue lock. pressure lock, and BD Auto Transmision

 

[DSiemens]

Check the overflow valve as wheel. It is located in the upper right corner of the p7100 injection pump. I have heard that this valve can cause all kinds of problems. I believe the pressure before opening has to be 22psi. Make sure that the air system is good, clean filter and such. What boost pressure are you running at. Do you have a power drop or other problems also?

 

[illflem]

Wily,It could the fuel quality,have to wonder if it dropped along with the price of diesel so the oil companies could maintain their high profit margin. It seems like you've checked everything else that would effect your mpgs though you may want to adjust the valves. Heavy right foot will effect fuel economy more than anything else,but then it's no fun!

fortunate1,the timing is something that shouldn't change on well maintained engine,though I've heard of cases where the timing gear has slipped on it's shaft and changed timing. Usually when this happens the timing will change so much that the engine runs poorly,if at all.
If your timing has never been checked have it done,many times it's incorrect from the factory. The tools to do it will cost more than it costs to have it done,not worth it for something that doesn't need to be done very often. Best to take it to diesel mechanic that will adjust the timing to 15. 5°-16°,this seems to be about the best number for power and fuel economy. If you go to Dodge or Cummins they will only adjust it to factory specs,not what's best.

 

[JEmmons]

I had the same problem. The overflow valve needed replaced, only a couple bucks. It did not have the proper pressure. It was down to like 12psi and I think 17 is the bare minimum. This caused a big drop in fuel mileage.

 

[BFaoro]

Yes, check the over flow valve. I dropped big milage and I checked all hoses from the airbox to the intake plenum, everything seemed ok until some one mentioned that the overflow valve could be the culprit. The length is supposed to be . 5 mine was an 8th of an inch shorter so I stretched it out to see if it made a difference. WOW, the milage went back to normal and I had more power. #ad
good luck !! By the way this is only a temp. fix you need to get a new one !

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96 2500 4X4, SC,SLT, 12 valve,lost kitty 5SP 3. 54. line-x liner All stock

[This message has been edited by ROCKYMNT (edited 02-23-2001). ]