Degrading performance

[Jack in Alaska]

I have a 1998. 5, 3500, 24 valve, 5 speed, 3. 54 gears with 77k miles on it and completely stock.

My fuel mileage used to be high 18 to low 19 mpg running empty and now it is 15 to 16.

My boost max(really getting into it) is 17 and it used to be 20 to 21.

Any suggestions on how to get it back to original without mods & upgrades would really be appreciated. I tightened all the clamps on the intake side.

 

[Runaway]

When was the last time the fuel filter was changed. Mine started to lose mpg, changed fuel filter and it was back to normal. Just a thought.

 

[TURBOs4LIFE]

ONE thing that I found out that you may need to do is to check the accuracy of your boost gauge. Not to answer why you lost fuel mileage but I had the same problem with my boost also. I thought OH crap something wrong with my truck but to find out later it was only a bad gauge. I did not want to have to take it to the dealer for anything! I would rather have wisdom teeth removed.

 

[pwr2tow]

Have your fuel pressure checked. Not enough fuel, no boost. Check for air leaks at intake manifold, map sensor, IAT sensor, plug where you installed boost line, check air filter for being plugged.

Ron

 

[csutton7]

check your intercooler hoses also--blown gasket or loose bolts on the intake manifold, exhaust manifold and turbo---chris

 

[JR2]

My Dad (Jack in Alaska) changed his air and fuel filters day before yesterday and it made no difference. When he is in fourth gear going up his reference hill he can only get the truck to approx 2800 rpm and it will no accelerate at all and that is unloaded. His truck used to hit the limiter at 3200 and accelerate up the hill.

I cleaned the IAT sensor for him (it was filthy) and we checked the intake bolts, the exhaust manifold bolts and turbo bolts. My Dad also tightned all the connections between the turbo and intake horn.

I know he made an appoitment for the truck at the dealer for next week, but does anyone have any other ideas??

My guess is either a bad MAP sensor or a failing lif pump.

 

[MCummings]

Might be your T. P. S.

My truck acted just about what yall are talking about.

No/Low power.

Wouldn't go above XX RPMs. .





MerickNJr

 

[Todd T]

its probably your lift pump. mine lost 70miles off the last tanks average mileage. and I started to notice it would run hard one minute and the seem low on power the next. I also noted the pump would not always cycle before start up, in the last few days.



sure enough I checked the lift pump pressures and it was 7psi at idle. and would drop off to nothing at mid throttle.



it goes in next week for a new pump.

 

[Steve St.Laurent]

AAAAAAAAAAAAAAAAAARGH

Definately check your lift pump pressures - I'd bet money at 100 to 1 odds that it's bad. My truck was running horrible at Thunder in Muncie this weekend - guess what, yep ANOTHER bad lift pump. This one lasted 15,000 miles running great and started to die this weekend on me. It's variable right now, sometimes it's running great other times not. This is lift pump number 4 or 5 (in 74,000 miles) - I've lost count!! There is a new rev lift pump due out soon (Cummins has it listed but can't order it yet - it's on status 80, not available yet) that is supposed to have some improvements (hardened seats). I'm going to be trying out an aftermarket solution on my truck this time along with replacing the lift pump with the brand new one I have sitting on the shelf. I'll post my results in another 20,000 or 30,000 miles to let you know how it works (IF it doesn't die before that).

 

[Todd T]

a bad pump is what kept me from racing this weekend. we checked it sunday morning and sure enough it was going south.



so I was faced with the choice, race and maybe go home on the hook, or watch and wimper, but make it home (370miles) OK.



I did the later and things were fine.

 

[Jack in Alaska]

Poor performance..Again

I have to admit to a "large brain fart" but still have the original problem. .

I received the PS engine indle control for an early fathers day present from JR2. We installed it and it worked great. I also changed the air & fuel filter at the same time. After all that the performance problem got worse not better. Now I was really confused.

After several days and a few short trips I and son JR2 agreed it needs the doctor so I made an appointment with the dealer for next week in Anchorage. He said his deisel mech. was on vacation but had another as a fill in. Not good situation but what choice do I have.

Well... the next day while driving down the road I wondered what effect the new engine idler would have. To my suprise I found that it was on full. When I turned it off the truck took off like a scared rabbit. Don't know how it got turned on but it definitely slows down the rpm's and throttle response.

I now have tightened all the clamps on the air intake etc. , checked manifold bolts, changed air & fuel filters, installed a new APPS(old TPS)and yes turned OFF the idle control. Still only get max. boost of 18 psi. I believe I will still keep the appointment and have the fuel pressure checked as that is a real possibility of being the culprit.

Question... .

Does anyone know what threads are on the two small ports on the top of the fuel filter and which one to tap to check fuel pressure? (1998. 5, 24v)

Thanks for any info... . Jack

 

[Steve St.Laurent]

They are 1/8" npt . They both can be used to test the pressure, the one towards the front of the truck is after the fuel filter, the one behind is before the fuel filter. Check out this thread on my fuel pressure gauge install - https://www.turbodieselregister.com/forums/showthread.php?s=&threadid=6113&highlight=fuel . You'll want to check the pressures driving down the road not sitting at idle or just revving it up at a stop.



-Steve

 

[JR2]

Steve,

That is stange, Dad (Jack in Alaska) and I tried to screw in an SPA press sensor that is 1/8 NPT and it would not go. I guess the SPA is not 1/8 NPT or Dads fuel filter housing is weird. Depending on how his press tests at the dealer next week, we will probably be installing something like your set up soon and we will figure it out then.

 

[JR2]

Well Jack in Alaska (Dad to me) took his truck to the dealer and the dealer found nothing out of spec. Said the MAP sensor, boost and fuel pressure were all fine. When Dad checked his mpg for the trip he took to get the truck worked on he found that his mileage was up to 19 (from 16-17). The only things that had changed were the fuel/air filters and I cleaned off his IAT sensor. Life is good again.

 

[Todd T]

well Jack I got mine replaced on Monday. I dont know what the mileage is doing yet but i did notice the truck feels stronger in the mid range and seems to runs smoother up through the gears.



my truck had 7PSI at idle and would drop off to 0 at mid throttle, it now has 14 at idle 12 at high idle and 17 on a bump start.

 

[MikeR]

JR2,



I'm wondering if the dirty IAT and MAP sensors are a common cause to degrading mileage. A couple of weeks ago I removed both my IAT and MAP sensors to find them both coated in grungy oil. I cleaned them off,but noticed no difference in instsnt performance. However,it does look like I got another 20 miles out of a tank than I usually get.



-Mike

 

[JR2]

MikeR - I would have to say yes at this point. Cleaning my Dad's IAT only increased his mileage about 2 mpg. We were going to clean the MAP sensor also, but could not find a socket big enough to get it off. Aparently it takes 1 1/4 deep well. He is going to buy a socket and the next time I am around we will clean it too.



Read Marco's post about the IAT sensor and the IAT sensor cleaned thread for more info.

 

[JTBMC]

Hey, Todd was your 7 #'s preasure inlet at idle or out at idle.

Why I ask is I just installed a new pump from Cummins and preasure day 1 was 13 in and 11-12 out and now just 560 miles later on day 2 its 10 in and 7 out and drops to 3 outlet at 70 and 1 at WOT about 100 MPH. The new pump does'nt cycle every time the key is bumped either, something the old pump did do. I called cummins and thier Service Manager said it was in spec, BULL to I aint happy. New pump parts # is 3990105 replacing part # 3938435 as per Cummins. I will reinstall my old pump tomarrow and check preasures, I have 3 gauges 0-15 & 2 0-30 and will test my gauges also.

Thanks, TJ Laffite