Engine/Transmission (1998.5 - 2002) MPG Decreasing

[RMHanes]

Have a 1998.5 with about 132K miles and recently noticed that the mpg has slowly been going down. Has been relatively steady between 20 and 22. The past few fill ups it has been going down and now is around 18. Have the 5 speed manual. Checked air pressure and fluids and air filter, all were good or adjusted. Remember reading that this could be a sympton that the fuel injection pump needs an overhaul or to be replaced. Wondering if it could also be a defective fuel injector or two. How can the injectors be checked without removal from engine? Once the weather warms up a bit will check the fuel pressure that the lift pump is putting out. What can be checked to determine if it is the fuel injection pump? Thinking that either of these are a bit pricey so do not want to "easter egg" this.

 

[JKosten]

Have you checked the lift pump? There is a screen in it that can clog and the pump itself is prone to fail.

Have you noticed a decline in power also?

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[GaryOwen]

May be due to winter diesel blend.

 

[Sticks]

Yeah, it's winter. Between the winter blend, and the warmup time, your mileage will drop.

Come late spring if your mileage has not returned to normal, then bring it back up.

 

[mwilson]

May be due to winter diesel blend.

I agree, my moneys on the fuel.

Mike.

 

[MLee]

It has been rumored that a change of the fuel filter will help. Do you think?

 

[Wyomingcowboy]

I have complained of this problem myself and changed both fuel filters. (OEM and AirDog) On my last fill up I was about 15.5 - which is about normal for this winter but very sucky overall. Worst part is my 46 mile commute and disabled car at the moment.

 

[RMHanes]

Thank you all for the suggestions. Did initially think of the fuel, winter blend as mentioned, but this was happening in summer and has continued since then. About three years ago the OE lift pump failed while it was being inspected and was just about to install a FASS arrangement. Since did not have the time or money at the time the dealer installed the revised lift pump where it is now in the tank. Still looking at changing to the FASS pump but think that will lift the bed instead of dropping the fuel tank. Going to do this myself with no help is the main reason. Just hope that the bolts for the bed are not too "rusted" in place. Any suggestions on which FASS arrangement to go with? Any one have experience with removing this lift pump from the tank? Know that there is a o-ring that needs to be replaced and that it might come out of the groove leading to a fuel leak when filling.

 

[Sticks]

Put the OEM Carter lift pump back on (mounted at the OEM point on the engine block) and leave the in-tank pump there and working. You will have to remove the aluminum block the dealer put in where the Carter goes (3 bolts) and splice in the power wire to the Carter off the in-tank pump, but your fuel pressure should jump to at least 20psi.

I've done dozens of the two pumps on customer vehicles, and have had no problems with it.

 

[PC12Driver]

Think like a diesel, not a gasser. We throttle by controlling fuel flow -

Restricted fuel supply = reduced power
Reduced power = higher MPG

If your fuel supply was being restricted by a fuel filter or a failing pump, you'd have other indications before you'd see a change in the MPGs (primarily low power or sagging under load). Lower MPGs mean extra fuel is going in when/where it shouldn't. Possibly a bad injector (dribbling extra fuel), maybe the IAT (thinking it's warmer than it actually is and retarding the timing), or possibly the injection pump is on its way out (hard to believe the lift pump just happened to fail when it was being inspected).

A laser thermometer on the exhaust manifold may be able to tell you if you have a cylinder that is hotter/colder than the rest. The IAT is pretty easy to replace and relatively inexpensive. Then I'd move to testing the injectors, followed by replacing the VP44.

Assuming there isn't anything binding in the driveline...

 

[BMelhuish]

Have you checked for codes? If so, what were they?

 

[RMHanes]

Thought knew what everything was but am at a loss as to what IAT means? Will get a laser thermometer and try that also.

 

[PC12Driver]

IAT = Intake Air Temperature

It directly effects timing advance. Colder temps get more advance (more power, lower egts, better mpgs) and warmer temps get the opposite.

 

[RMHanes]

Well, got the temperatures checked and all were about the same. Tried to find the IAT sensor but the service manual was not a help. Where is this sensor located? Looked around the intake manifold and did not see it.

 

[Sticks]

Between cylinders 5 and 6 in the cylinder head under the intake plate, left hand side of the engine head close to the block.

 

[Vaughn MacKenzie]

Actually it's beyond the intake plate, at cylinder 6 right beside the injector fuel return line. Item 3 in the pic. If I recall correctly it's the same size as a smaller spark plug socket. Pull it out and blast it with some brake or carb cleaner and you should be good to go. Out of curiosity do you have an exhaust brake that you use regularly? If so the IAT should be pulled and cleaned every 30K miles or so, since exhaust gases get blown back into the intake and create an oily film over everything.

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[SnoKing]

Vaughn, thanks for the reminder. I cleaned mine yesterday. It was not bad given that I use the exhaust brake all the time. Chris

 

[RMHanes]

Vaughn, Thanks for the picture. Will look for it tomorrow and might spray some lubricant on it just in case it is rusted in place. Do not want to snap it off and then will have a bigger problem to deal with. From the picture and what can remember of the engine layout, a mirror would be a great help also. It has got to be right near the firewall also so not a lot of room to work in but will manage as best as can. Have a good set of deep sockets to get over the sensor also. Might make sure that use 6 pt vs. 12 pt so do not round it off. If can get a wrench in there will do that instead but leverage might work better with a ratchet. Do not have an exhaust brake, just about 133K miles of travel.

 

[CM_Guest]

This may sound crazy, but how are you driving it? And what transmission do you have? I just bought an '02 a few weeks ago. It's the first 24-valve I've ever really been around and the first "electronic" engine I've actually owned. I've put injectors in newer trucks and done head gaskets and things like that but I've never owned one or driven one long-term. I got the factory service manual on CD-ROM for mine and started reading. And from what I've read, and putting it in context with what I've observed, it MIGHT BE possible for a 24-valve truck to sort of get "lazy" during the winter if it has an automatic transmission and you don't run it hard from time to time. I learned that the 46RE and 47RE transmissions are "smart".

They "learn" driving styles and "adjust" their programming in response to how they're driven. And the temperature has a big effect on it. Because there are also different "modes" the engine and transmission use depending on ambient air temp, coolant temp, etc. And the main "calibration" the transmission does is governor pressure. An electronic solenoids controls oil flow to produce a certain line pressure (which operates the transmission elements for applying and shifting gears) and I think it basically uses pressure/vs engine speed and load to plot a map of where the governor pressure is under various conditions. What that means is to get a "good" map or calibration, you need to be running the engine up to wide open throttle (WOT) during several different situations of speed and load frequently and year-round for the transmission to keep a good map of governor pressure and a good baseline to calibrate from. AND it needs to be above 50 degrees outside for it to calibrate.

So what COULD be happening is that your truck is running on "old data" so to speak and as it's getting warmer outside but maybe not warm enough to recalibrate and without some good hard acceleration runs to map governor pressure. Your engine and transmission may be just getting more and more "lazy" because the transmission has "learned" all it can with what it has. And maybe it has learned NOTHING since it was maybe a LOT colder or a LOT warmer than it is now. Usually warmer is better for fuel economy because the engine oil flows more easily. But your transmission oil will too. Which means the pump is going to produce LESS line pressure at a given speed. Less line pressure means later and slower converter lockup and shifts. So the PCM needs to see some WOT governor pressure at warmer air and oil temps and compare it to possible FASTER engine acceleration and come up with new data to operate the transmission with. And the more you baby it because the fuel mileage is dropping, the more the mileage is GOING to drop. And I learned/theorized all of this AFTER noticing the exact OPPOSITE on my '02. The day I bought it was cold as hell. And it was a "new" truck so I wasn't getting on it much, and it wouldn't have made an immediate difference if I did. But when I did get on it, all I could do is compare it to my '94 and tell myself I bought a DOG. I pretty much babied it for a couple of weeks because it ACTED like a baby. AND because the mileage was NOT GOOD. The day I left the dealership with it I was cruising on a state highway with some decent but not HUGE hills here and there with flatter spots between and I was seeing 16 mpg at BEST doing 60-65. I was not impressed and figured it had to have some fuel system issues and that I'd be lucky to get 12 on the interstate. Well, I drove it home and in the process came up 3000+ feet in altitude and then it REALLY seemed weak and doggy. It was still cold and I was still babying it but mostly because I was embarrassed how slow it was.

But then it started warming up outside and I started driving it harder. And it started running better and SHIFTING better. Before it started improving, it would hit high idle in first, second and third gear and just STAY there for 5 or 6 seconds at LEAST before it would make a very "low performance" shift. I even asked if anyone had any ideas on here. And I got some good advice, but I'm glad I didn't start throwing parts at it. Because I started getting after it a little bit more and it has made a BIG improvement. And the better it runs, the more I'm getting on it. And then it runs better AND the mileage is improving. It's not surprising it was running crappy. It had sat on a dealer lot for at least a few months and no doubt got started, idled around with a little, maybe taken on a short, cold test drive or parts run or something but basically just didn't get WORKED for several months an who knows HOW MANY key cycles. Now it's doing WAY better. I ended up getting around 16 on that trip out here. And the first few nights I drove it home from work I set the cruise at 60 on a flat, straight highway with nothing more than a few gentle curves and a couple of little rises. The first few trips I was getting 20-21 or so. Then it was more like 22-23 and those numbers were the BEST I'd see for any length of time with an occasional 24. Fast-forward to last night. Same road, same speed - a pretty steady 26mpg with dips to 25 and blips to 27 that probably average out to 26.

I do quite a bit of in-town driving where it idles for a few minutes here or there and I drive a lot of back roads home and such so I'm not beating on it by any means. But I do "spirited" driving at times and what I'm seeing is an average around 20 mpg. Which I think is pretty darn good. By the time I start it in the morning, get it out of the uninsulated shed, drive down to the Pump & Pantry for my breakfast burrito and orange juice, it has been more or less idling for a good 10 minutes before I get out of town. And it's still pretty cold for the first 10 miles with a similar situation at night coming home (except then its beer instead of OJ) by the time I stop and get the mail and such. So all in all, I think a 20 mpg average in late winter with no winterfront to really warm it up good, and at least 20-30 minutes of idling per day, is pretty good mileage. I know I have to baby my '94 like crazy and can't get on it at all to average 18.5. Of course it has 4.10s and the '02 has 3.55s, but the '02 is a Quad-Cab and the '94 a regular cab. Figure what, maybe 1000 lbs difference in weight? Maybe a little less? Same tire size on both.

So my recommendation would be to go rod the hell out of it and see if your mileage improves. If you don't already have something blocking your coolers, cover them up GOOD and take it on a good drive and really work it and warm it up. And by warm it up I mean 200 or more if possible. It's possible that it's just needing the cobs blown out of it to get it going again. I have seen 2nd Gen Dodge diesels that were used as ranch trucks at a ranch where they don't do ANYTHING fast and those trucks just puttered around for weeks or MONTHS without getting worked hard and warmed up good and they were JUNK. I looked at the turbo on one and I could hardly spin it with the tip of my finger. That's bad. You know it's bad when they won't even run good enough to BLOW UP, lol. If your truck gets a LOT of idling and/or never gets warmed up good, you CAN get carbon and soot and crap building up on your turbocharger exhaust turbine and in the housing OR in the wastegate to gum things up and cause drag and low boost. That'll bite your mileage hard too. One thing is for certain. We've got medium-duty engines in light-duty trucks and they are OVERCOOLED from the get-go. And there is NOTHING worse on a diesel engine than extended idling in low temps, frequent start-ups and shut-downs, never getting up to FULL operating temp and STAYING there long enough to get the moisture out of the oil and burn the carbon out of the cylinders, head, turbo etc. And "extended idling" can literally be 5 minutes if its 15 degrees. I let my truck idle too much too, but its usually idling BEFORE it's warmed up at all. But even then if you're going to be gone more than a couple of minutes, it's better to shut it off than let it idle because it will lose LESS heat shut off than idling and having heat go out the exhaust, radiator, heater core, etc. I'm better about shutting off this '02 because it warms up more quickly and gets warmer. I think if you go wail on yours some you might see a difference but it might take several DAYS of being on it more to see any improvement. Obviously that's counter to your goal of maximum fuel economy, but then again if it takes more fuel now to burn less later, just say it averages out.

 

[CM_Guest]

As if I didn't write a book already. But I thought of something else. Fuel related like the other guys mentioned. There has been some REAL crappy fuel in my neck of the woods this winter. And we usually don't have that problem being out here where the buffalo roam and NO ONE wants to get stranded or be the reason someone gets stranded. And it's NOT the winter blend fuel that's crap. Because there really ISN'T much winter blend fuel this winter. My mom runs a grocery store with fuel pumps and they've had some problems. And it seems to be water in the fuel more than anything else. You MIGHT want to think about emptying out your tank as much as possible, changing the filter and putting in some good fuel and some GOOD additive at a "shock" level to really get it cleaned out and really run it hard to help that process. Fuel that is "wet" with water is a perfect recipe for corrosion and deposits that can thoroughly gum up a rotary pump with a housing full of crappy fuel all the time.