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Go Back   Dodge Cummins Diesel Forum > 2nd Gen. Dodge Cummins 98.5-02 24V Forums > 98.5-02 Powertrain
98.5-02 Powertrain Discussion of components that are directly involved in the power production and all that is needed to get and keep the truck moving . Engine , Transmission Ect..NO ADVERTISING .

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Old 11-03-2012, 03:28 AM   #1 (permalink)
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Bad mileage .

I'm sure this has been beat to death but I don't know piss about these motors so gimme a break. 98.5 5 speed 3.73 s gettin 10 mpg no matter what I'm doing, towing, highway, town, babied, hot rodded, has absoloutely 0 smoke even luggin er out. Used to get 18 ish then just within the last year it fell off. Lift pump has 12 psi, did an overhead a few weeks ago and it didn't help other than quieting the idle. They were .035 out. Any ideas ?

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Old 11-03-2012, 08:31 AM   #2 (permalink)
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Fuel pressure is low. You don't want anything below 14 PSI at any time.

Error codes?

What size tires are you running?

How fast are you driving?
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Old 11-03-2012, 11:12 AM   #3 (permalink)
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285/75s it don't matter if it is it or 85 still het 360 is miles on the tank. Are you sayin the lift pump as another idea or is that for sure that could cause an an 8 mpg drop?
Also what lift pump would be one to look at ? From what I've looked at so far I've seen a few guys mention dodge lift pumps aren't too good?
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Old 11-03-2012, 11:14 AM   #4 (permalink)
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Every once in a while the truck cuts out like the connection on the pcm is loose, it will cut out then a check engine light will cone on for a few min, go off and when I get a code reader on it there are no codes.... Odd right? But I think that's a separate issue

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Old 11-03-2012, 02:23 PM   #5 (permalink)
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littleredstroker........Replace your stock lift pump with a Airdog, Fass. There are others too. Just throw that stock liftpump in the trash if you don't want to pay $800-1500 for a new VP. Get atleast a mechanical fuel pressure gauge that sets atleast 20 psi at idle. But not much over 20 psi. A 100 gpm pump is fine. Is the fuel filter clean? Any hard starts?
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Old 11-03-2012, 02:31 PM   #6 (permalink)
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Quote:
Originally Posted by littleredstroker View Post
285/75s it don't matter if it is it or 85 still het 360 is miles on the tank. Are you sayin the lift pump as another idea or is that for sure that could cause an an 8 mpg drop?
Also what lift pump would be one to look at ? From what I've looked at so far I've seen a few guys mention dodge lift pumps aren't too good?
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(Shaking head...)

First off remember for every 4 pound of tire you add to the truck your required 1 HP more to keep the truck rolling. You'd be surprised how much weight those 285's added to your truck. Racing slicks are roughly 20-25 pounds... Ultimate for rolling resistance (near zero) and nearly nothing for rotational mass.

Here are my 235's with 21-22 MPG...


Then second off... 55MPH is the best speed. When you jump to 65 MPH your wind drag doubles, then at 75 MPH its 4 times as much. so at 80 MPH it roughly 6 times as much drag as at 55 MPH. Slow down going pass 2K on the tach does no good period.

Fuel pressure... should be between 14-20 PSI as a norm. Anything below that your beat you pump to death.
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Old 11-03-2012, 03:05 PM   #7 (permalink)
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... I had the same tires and drive the same way when it got 18????? So not sure why you are " shakin Ur head"??

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Old 11-03-2012, 03:39 PM   #8 (permalink)
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Well then the only thing I can think of is that your low fuel pressure ate the injection pump.

Why I'm shaking my head is because a lot of times (not this time) people add larger tires and shortly after-ward start complaining of poor mileage. So sorry for the comment.

Also remember you can't use a percentage offset to calculate odometer mileage either. Like some friends been doing study work on this and found out that the offset varies with speed so if you drive slow you need less offset but if you drive faster you need more. So the idea of percent offset doesn't work and the only way to know for sure is to use a GPS to check actual miles.
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Old 11-03-2012, 04:33 PM   #9 (permalink)
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I see what you mean there. How spedo is 3 mph fast at 30 5 at 45 and 10 at 80, right? ( just throwin out #s) how do I test the injection pump

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Old 11-03-2012, 04:45 PM   #10 (permalink)
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I see what you mean there. How spedo is 3 mph fast at 30 5 at 45 and 10 at 80, right? ( just throwin out #s) how do I test the injection pump

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As for the VP44 the only way for you to test it is to replace with a known good pump. The only way to test your current pump is on a very very expensive test bench.

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Old 11-03-2012, 08:02 PM   #11 (permalink)
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Oh my.... So would a stock injection pump keep up with a set of 90 hp inj and a tune? Or.... What would be an upgrade?

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Old 11-04-2012, 12:35 PM   #12 (permalink)
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Oh my.... So would a stock injection pump keep up with a set of 90 hp inj and a tune? Or.... What would be an upgrade?

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I agree with Moparman. Bad VP. As for a upgrade VP. A SO pump is the better of the two ( HO vs SO). that will hold your 90 injectors fine. IF YOU have a good L/P to feed it properly. Goes back to good fuel pressure. Minimum of 15 psi most people shoot for 20 psi at idle.
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