Engine/Transmission (1998.5 - 2002) What is wrong with my truck?

[VIPERBLUELX]

My truck runs great at part throttle, but if you go WOT and hold it, it starts to nose over. It's actually quicker altogether at 3/4 throttle than at WOT. The TST box has a green fuel light that burns brighter green the more fuel it's adding and I noticed if I got WOT it will flicker and then the green light will go out all together unless I let out to about 3/4 throttle and it's start fueling good again. The truck feels weak at WOT with the TST box turned off also so it's not that. Could it be a TPS sensor or a weak injector pump?

 

[Cummins724]

Sounds like a fuel problem. Possibly the tps, but I'd say that you aren't getting enough fuel on top. Start with the fuel filter, Check for codes, but the filter is the most common. Hopefully you do have a fuel pressure gauge right?

 

[VIPERBLUELX]

Sounds like a fuel problem. Possibly the tps, but I'd say that you aren't getting enough fuel on top. Start with the fuel filter, Check for codes, but the filter is the most common. Hopefully you do have a fuel pressure gauge right?

Fuel filter and lift pump are brand new. Pressure at idle is 13, cruise around 8 and drops to about 4 and holds at WOT.



I have a Vulcan pusher pump kit I've gotta install that should bump the pressure up some.

 

[Cummins724]

I wouldn't run the box until you get that installed... ... or vp44 will be your next step. I had 4psi @ wot with stock pump & vp smoked on my truck.

 

[VIPERBLUELX]

I installed the Vulcan kit today(took a bath in diesel since my tank was full).



Fuel pressure at WOT is not steady at 11psi instead of 4. Truck still drives the same though, feels great at part throttle but at WOT it falls off some and the green(fueling) light on the TST box goes out, I can lift up about 1/2" on the throttle and it starts fueling again. I don't get it.

 

[Cummins724]

Don't just throw parts at it, get it scanned. If you don't have a scan tool, then go to advance auto, auto zone & get it done. The truck should show a code if it is defueling.

 

[Grizly]

Bet you got air in system still , I had this problem too and drove ez for about a week then it cleared all air out and then presure did not drop below 12psi . FYI never let fuel run under 1/4 tank while doing hard take offs or stops air will get in line. .

 

[BK]

I had read a thread on here where one guy found that his intake from the tank was too restrictive. I think he did a FASS system and still had problems. Took the tank apart and found a basic problem, nothing broken. Did a modification, and all was well. You might want to give a search.

 

[VIPERBLUELX]

Don't just throw parts at it, get it scanned. If you don't have a scan tool, then go to advance auto, auto zone & get it done. The truck should show a code if it is defueling.

No check engine light has ever come on, but I'll scan them with my Snap-On diagnostic scanner.



I wasn't throwing parts at it with the Vulcan kit, I think it's a quality upgrade over the stock setup.

 

[VIPERBLUELX]

Bet you got air in system still , I had this problem too and drove ez for about a week then it cleared all air out and then presure did not drop below 12psi . FYI never let fuel run under 1/4 tank while doing hard take offs or stops air will get in line. .

Thanks. The TST box has been on the truck a month now and I never run it below 1/2 tank of fuel.



Also you might have missed my second post, but fuel pressure never drops below 10 at WOT now that I installed the Vulcan pusher pump kit.

 

[jmy]

My truck runs great at part throttle, but if you go WOT and hold it, it starts to nose over. It's actually quicker altogether at 3/4 throttle than at WOT. The TST box has a green fuel light that burns brighter green the more fuel it's adding and I noticed if I got WOT it will flicker and then the green light will go out all together unless I let out to about 3/4 throttle and it's start fueling good again. The truck feels weak at WOT with the TST box turned off also so it's not that. Could it be a TPS sensor or a weak injector pump?

As I recall the TST green light looks at electronic fueling pulses. More pulses = brighter light. If the light flickers or turns off the engine electronics is causing it to de-fuel. The boost signal from the MAP sensor will cause electronic de-fueling.

You may need to get a DRB III scanner on it to look at the fuel system electronic status when the de-fueling occurs.

 

[Cummins724]

No check engine light has ever come on, but I'll scan them with my Snap-On diagnostic scanner.



I wasn't throwing parts at it with the Vulcan kit, I think it's a quality upgrade over the stock setup.

I agree, that was a good upgrade over stock. I meant check for codes before you buy a apps, tps etc. cause I thought you meant you were getting these sensors & gonna just start changing them. At my dealership electrical parts are not returnable, so that is why I said "don't just start throwing parts at it" I was just trying to save you some money.

 

[NoSeeUm]

I had read a thread on here where one guy found that his intake from the tank was too restrictive. I think he did a FASS system and still had problems. Took the tank apart and found a basic problem, nothing broken. Did a modification, and all was well. You might want to give a search.

Just in case you are thinking about another diesel shower.



Vulcan Draw Straw



Glacier Rigid Pick-Up



Jim

 

[VIPERBLUELX]

I scanned the codes, No Fault Codes Present.



I checked the TPS % with the key on and it starts out at 8. 6% and at WOT it's showing 70%.



Should it not be at 100% at WOT?

 

[Fishin Guide]

The TST green fueling light comes on when it sees about 2psi boost. If the light is going out, I would take a look at the MAP sensor first.



Instead of looking at the throttle position, look at boost vs the fueling light. Does it go out above a certain boost point regardless of throttle position? If it does, the MAP sensor most likely has lost the ability to provide a reading above a given pressure.



If that checks out, I would send the box to TST and explain the issue. They can check it out and get it back to you.



Dave

 

[VIPERBLUELX]

The TST green fueling light comes on when it sees about 2psi boost. If the light is going out, I would take a look at the MAP sensor first.



Instead of looking at the throttle position, look at boost vs the fueling light. Does it go out above a certain boost point regardless of throttle position? If it does, the MAP sensor most likely has lost the ability to provide a reading above a given pressure.



If that checks out, I would send the box to TST and explain the issue. They can check it out and get it back to you.



Dave

Thanks, I would have thought the MAP sensor would have turned on the yellow BOOST light instead of the green fuel light, thanks for the tip.

 

[Fishin Guide]

It does. But, the TST is a boost referenced fueling box.



Now, the other thing it does os sense a pulse from the facory harness to the VP. The TST adds duty cycle to this signal. At least that's how TST explained it to me. So, if you had a connection with some resistance, that could be the issue.



And of course, double check all connections.



Dave

 

[Andy Redmond]

How did you tap your "pump wire"? Maybe try one of those pump top covers available from many vendors. You will get a more reliable connection.



I've in encountered your symptoms several times. Most of the time the pump wire connection was the culprit, but the map sensor turned out to be an issue on one of them. .



Good luck

Andy

 

[Fishin Guide]

How did you tap your "pump wire"? Maybe try one of those pump top covers available from many vendors. You will get a more reliable connection.



I've in encountered your symptoms several times. Most of the time the pump wire connection was the culprit, but the map sensor turned out to be an issue on one of them. .



Good luck

Andy

Actually,



my experience with pump taps is this. The aftermarket replacement covers often do NOT produce as good of a connection as soldering the wire.



I personally have had issues with pump covers and a simple soldering job fixed it every time.



Dave

 

[VIPERBLUELX]

It was the TST box, I replaced it with an Edge Comp and the truck runs great.