Howdy folks.

Hoping to get a bit of info on my latest truck project. I picked up a 92 3/4 ton Ram with a 360 TBI. I did a general tune-up, as the truck has sat for some time, and changed all the filters/oils, replaced the plugs/wires/cap/rotor/coil, and stripped the throttle body and gave it a good scrubbing.

The truck starts and runs nice, but under load it craps out. Definitely felt like a fuel issue to me. I t'd in a pressure gauge, and have 14-15 psi at an idle which is spec. Revving the engine and watching the gauge, the pressure will start to drop, eventually down to 5-6 psi. When it drops below ~10 psi the engine starts to stumble, 6-7 psi it's at the point of stalling out.

I'd assume if the regulator/pump is working properly, the injectors should be supplied with a constant 14-15 lbs.

The regulator is controlled by vacuum, but there is no vacuum present in the line hooked to it. Hose is good, no cracks/leaks. I'm wondering if it will only pull vacuum at high rpm/load, as the port it's hooked to is mid-level in the throttle body. I'd suspect that to be the case as hooking manifold vacuum to the regulator will drop out the fuel pressure and kill the engine.

I'll most likely replace the pump/regulator as it's nearing 250 000 km, but I'd like to pick up any info the TBI fuel system I can. If anyone's got any info on the regulator function, I'd appreciate it.

**Edit - found out that the regulator is not vacuum controlled, though it's stated in the Haynes Manual. Line hooked to the regulator is simply a drain should the regulator begin to leak fuel internally.

Will swap in a new regulator and try it.

Cheers

Tyler

 

Did you replace the filter sock in the fuel tank? Is this drop in pressure while sitting still or while driving?

 

This sounds identical to the problem I had with my 92' 3.9 V6. The inlet filters in the top of each fuel injector were plugged with crap. Enough gas passed for the truck to idle and run level but not enough when it was under load (like when going up a hill). I removed the injectors, set them up in a board with holes drilled to hold them, sprayed Sea Foam Deep Creep in them and let them sit a while. The sediment was so hard that I had to straighten a paper clip and use it to break the crud up. I sprayed again and let them soak for a few more hours. Next I rigged a 9V transistor radio battery, switch and wires so I could open and close the injectors on my workbench. Applying constant voltage will damage them. One at a time I switched them on for about three seconds and sprayed carboretor cleaner through them from the bottom. The clogged ones will be obvious. The spray should go straight through with little loss in force. I used the little red straw and stuck it right into the discharge end of the injector. This procedure eventually cleaned them. The inlet filters are bright red, so you can look into the top of the injector and see if it is clean. You may need a flashlight and magnifying glass because we are talking small here. Afterwards the truck ran like a dream. If you have a shop in your area equipped to clean injectors you might rather take that route.

Replace the inline filter. It is beside the tank toward the middle of the truck and toward the rear of the tank. Crawl underneath and look up between the drive shaft and the tank and you will see it bolted to a cross member underneath the bed. The fuel pump on this truck does not have a "sock". The inlet filter is a rigid plastic structure about 1.5 inches high and about 4 inches round that fits onto the bottom of the pump housing. You can't access it unless you remove the pump reservoir assembly from the tank. If it is bad your pump will go out soon. Most likely the inline filter is the culprit. It should be replaced occasionally and because it is hidden it may have never been replaced.

 

ol shakey
Look down inside the throttle body when the truck is running. At the base of each injector you should see a fine cone of fuel. The cone should be solid and even with no gaps or streams. The base of the cone should touch the sides of the throttle body throat. You are correct in your assumption about the constant fuel pressure.

granpa49
TBI injectors are built differently than your multi port fuel injectors. They do not have a top screen. They have a large surface area screen that wraps all the way around the injector body and are less prone to screen clogging. Their problems are usually with internal varnish build up and coil failure and both of these problems can be seen by looking into the throttle body.

As far as the in tank filter/sock, I use the term sock because it is old school (like me) and easily differentiates between in tank and in line filters.

 

Fuel filter was replaced first. Injector spray is excellent, good cone pattern and no 'dribbling'. Replaced the regulator and it cured the fuel pressure issue. Truck drives well and won't flatten out under load anymore.

Still has a slight hesitation/lag that feels like a dropout in the throttle position sensor. I'll check it out shortly.

Would this truck have an in-tank filter as well as the inline?

 

Yes it has both an in tank filter as well as an in line filter but if the fuel pressure is OK I wouldn't worry about the in tank filter. The in tank filter is more of a screen than a filter and that is one more reason why I don't like to call it a filter.

 

Yeah, Seal, I should've paid better attention to what he said. The TBI injectors are exempt from the kind of problems I was having. I just saw it was a 92' and jumped right over the details. I'm glad he seems to be on the right track to fixing his problem. BTW the new Carter fuel pump assembly I bought for my truck now has a pleated filter on the bottom instead of just the screen. I thought that was a step in the right direction.