Engine/Transmission (1994 - 1998) Loss of power

[eberga]

I had a sudden loss of power while driving today. I was running about 65-70 towing an empty trailer along a flat road and when I came to the next hill I was losing mph pretty fast so I down shifted and could only get about 60 in 4th gear with it floored. It felt like i was driving the boss's powersmoke. I checked intercooler hoses and throttle cable, they checked out ok. I dont have any gauges so I cant give any numbers but could it be lift pump, fuel pressure issue? Any suggestions will be appreciated.

 

[eberga]

Will lift pump failure cause loss of power? This morning when I started out the truck was running fine but after running for a while it has low power.

 

[Lightman]

What year is your truck and how many miles, could be the overflow valve, plug fuel filter, or fuel lines.

 

[eberga]

Truck is a 97 with 142,000, I replaced the fuel filter about 5,000 miles ago. How do you check the overflow valve?

 

[moparguy]

Have someone hold the pedal to the floor and check the throttle at the pump to see if there is any unused travel in the linkage. Or you might do what I did and put a stick between the peddle and seat, run the seat forward till the accelerator pedal is on the floor and check for additional travel at the pump.



I noticed I was down on power and smoke, did the test above and found a small bolt backed almost all the way out at the rear of the pump behind and below the fuel shutoff solenoid. Tightened it and everything back to normal.



Good luck, RJR

 

[Lightman]

You need to check your fuel pressure should be 20 to 25 at idle and 26-35 at 2000 rpm. the over flow valve is on the injection pump front top right side of the pump, it holds the steel fuel return line to the pump. if you remove it it has a plug in the top that holds a spring and check ball. the spring should be . 500 long it can be stretched and put back and will work for a while. Be careful when removing the banjo fitting there is a gasket on both sides of the fuel line. Put a rag between the head and injection pump to catch the washer if you drop it, hope this helps

 

[eberga]

On the throttle linkage on the side of the pump there is the main throttle cable and then a small steel cable next to it. It goes down under the battery on the drivers side. When the throttle pedal is full depressed the small steel cable doesnt move. It has about 2" of slack in it when the pedal is pushed down. Does anyone know what the cable is or its function.

 

[eberga]

Checked the overflow valve and the spring was about . 375 or . 400 stretched it back to . 500 and tested it, no change I am starting to suspect the lift pump, how do you check the fuel pressure. Also before at idle when I got on it pretty good it would leave a pretty good black cloud, now I get nothing maybe a slight haze.

 

[Joe G.]

The cable that goes under the battery is cruise control.



The spring in the overflow valve should be a little over 1/2" (. 550). I stretched mine thinking that as soon as it got bad again I would replace it. That was over 100K miles ago.



It will run ok with 18 PSI at idle and 21 PSI at 2000 RPM with no load. If you are seeing something like 15 PSI there is a problem. Mine had 20 PSI at 2000 PRM and no load. That seemed kind of low in spite of it running ok. I stretched the spring. I overdid it a bit so I have higher pressure than I should have, but it's still within range so be careful stretching the spring.

 

[Joe G.]

The lift pumps on these 12 valve engines are pretty sturdy gadgets. If you have some bad diesel the screen in the bottom of the lift pump may be plugged. The cast in nut on the bottom of it is 17 mm. Be careful because you can get a diesel bath taking that loose. The gasget is reusable if you are carefull. If the screen is dirty just clean it and put everything back together.



You can check fuel pressure at the outlet banjo fitting on the fuel filter. A special adapter is needed to fasten the gauge to the fitting. Maybe someone can chime in here with a better way. My pressure gauge is installed permanently.

 

[eberga]

When I first start out after the truck has been sitting for a while then sometimes it runs normal for a mile or two then it is back to no power, other times it is a dog right from the start. Could be the lift pump screen or some kind of blockage that clears after sitting for a while. I will check the lift pump screen tomorrow post the results.

 

[eberga]

I checked the lift pump screen today and it was pretty clean, a little bit of debris on it. I washed it out and put it back together and then checked the overflow valve. I stretched the spring to slightly over . 500 and put it back in. Same problem it will run normally for a while and then after a while it will get really doggy. I guess the next order of business is to check the fuel pressure, where is the best place to get the guage and the necessary adapters? Any more suggestions are appreciated.

 

[illflem]

If it was me I'd replace the overflow valve. The spring isn't the only thing that goes wrong with them, the ball and seat can get hammered and not seal well causing low FP. It's a part that eventually wears out on all engines and must be replaced.



To check fuel pressure:



I drill and tap the top of the injection pump banjo bolt to 1/8'' pipe thread, then install a male x female 1/8'' needle valve with 1/8'' tubing compression fitting in it. Boost gauge or oil pressure tubing pushed though some 1/4'' tubing to protect against cuts and kinks to the gauge. For a gauge you can use a fuel, boost or water pressure gauge, for a 12 valve it must be at least 40 psi. I use rear connected, liquid filled, stainless water pressure gauges and make my own mount. They have no lighting but it's not a gauge you need to look at all the time. They can be mounted low for the same reason. Use no Teflon tape, it doesn't work with diesel. Instead use anti-seize or Permatex #2. Remember you are working on the clean side of the filter, keep everything spotless. Compression fittings with a ferrule that slips over the tubing will leak if overtightened, finger tight with one half turn of a wrench is plenty. The ferrule should be replaced if the fitting is ever removed.



If you can tap your manifold for a pyro tapping the banjo bolt is a piece of cake. Just drill from the inside to keep the hole straight and tap from the outside. You can also connect to the bleed screw on top of your fuel filter with the appropriate adapter to tubing if you don't want to tap. The adaptor is available from TST for $25. After the install start your engine with the needle valve closed then open it slowly till someone in the cab signals you that it's just reading pressure. If you tighten down the packing nut under the valve handle the setting can't move. The valve will control pulsation and buzzing and can also be shut down in the unlikely event that you have a leak. Without a valve to restrict the flow the gauge will pulsate so badly that you won't be able to read it in the short time before it self destructs.



Parts list; 1/8'' needle valve - $2. 50 from the hardware store, plastic oil

pressure tubing kit that comes with the compression fittings - under $10

from an auto parts store. 1/4'' tubing for protective sleeve - less than $2.

Gauge is your choice and may come with tubing. The water pressure gauges I use cost less than $10, bringing the total cost to less than $25 and a couple hours of your time.

I have several gauges that have been in service for over 5 years with zero problems though you may have occasionally close the needle valve a bit more if the gauge starts pulsating.