Dodge RamCharger Central banner
1 - 18 of 18 Posts

·
Registered
Joined
·
946 Posts
Discussion Starter · #1 ·
This morning I checked the fuel pressure before and after the fuel filter to be around 13 psi while running.  When I open up the throttle the pressure would drop to just under 10 psi.  I assumed that I had found the problem (significant loss of power) and replaced the fuel pump.  $100 later I was going for a test drive and it seemed to be back to working order.  When I would floor it it would accelerate.

This evening I figured I'd check again to see if everything was in order.  To the floor the peddle went and after falling on its face for a couple seconds the 90' RC lunged forward then tripped again.

I pulled out the new fuel pressure gauge and noted 14-15 psi idling.  reached up and pressed the peddle to the floor and the pressure jumps around and holds at 10 psi long enough for me to see it and then continues to jump around.

Since a few days ago when it started acting up I have replaced the filter, the fuel pump/screen and still seem to have the same problem.  It seems to be running short of fuel pressure when I open up the throttle.

Why does it seem to be unable to send enough volume/pressurized gas to the throttle body? 

All I can think of, but can't see why would be a plugged fuel line, or power to the pump that comes and goes.  I can't imagine the line being plugged with the filters it has, so what is the path in which electricity moves from the battery to the fuel pump?

Thanks for any help or advice.
Zac
 

·
Registered
Joined
·
946 Posts
Discussion Starter · #3 ·
I will have a look since I dropped the tank again.  Any other ideas?  I did test from the ASD to the pump, there was no significant resistance.  Like .4 ohms.  I'm wondering about the ground also.  I see there are 4 wires going to the tank, a pump positive, signal from fuel level, and a ground right?  Or does the pump and fuel level sending unit have sperate power wires?  I can't find a wiring diagram.

 

·
Registered
Joined
·
946 Posts
Discussion Starter · #4 ·
Just had a thought, Does the pump recieve full time +12V?  If so just as a test, couldn't I take the old (appearently good) pump, set it in a can of gas, run power to it and a hose from it to the fuel inlet on the TB.  Then see if it runs better?  I guess if all that worked it would tell me that the problem lies in the fuel delivery from the tank to the TB.  Or if it does not run properly I could assume that the fuel deliverly from the tank is good.

Is it possible for a bad regulator to send fuel back to the tank all the time, enough that in combination with WOT there is a shortage of pressure to the TB?

 

·
Registered
Joined
·
946 Posts
Discussion Starter · #5 ·
I feel like I'm talking to myself in this thread but here's a little update for anyone reading it.

Checked the hose from the pump to the top of the "Fuel Pump Module".  It was good (checked by plugging outlet with finger and blowing untill my cheeks turned red, I couldn't get any air out).  While I was at it, i blew through the return line from the module down, no appearent blockage.  I then proceded to blow through the supply line from the tank to the filter, and it seemed fine, for blowing through a 7' long hose with about a 5/16" id it wasn't all that hard.

Need to check my ground wire, and while I had the module out of the tank I noted the pin layout, will report back with the results.  Is that ground to the pump switched some place? 
 

·
Premium Member
Joined
·
24,598 Posts
silvermopar1 said:
I did test from the ASD to the pump, there was no significant resistance. Like .4 ohms.
Who said that is not significant? ;)

Voltage drop (on circuit while running) for wires and resistance ONLY for components completely disconnected and/or on the bench (coils, sensors, motors, ect ect)

Ohms readings are not a good way to measure wires. You want to measure voltage with one lead on pump positive terminal and one on battery positive. Over ~.300ish=problem.
 

·
Registered
Joined
·
946 Posts
Discussion Starter · #7 ·
The ground wire consistently had about 3 ohms of resistance.  Would that be enough resistance to cause a problem? 
 

·
Registered
Joined
·
946 Posts
Discussion Starter · #8 ·
I shoulda known it would require the pump running at time of test.  It's going to be difficult to test while running, since when the tank is mounted, there's no room for my arms/hands.
 

·
Premium Member
Joined
·
24,598 Posts
silvermopar1 said:
The ground wire consistently had about 3 ohms of resistance. Would that be enough resistance to cause a problem?
That is definitely not good.

Replace immediately.

Measure ohms of a piece of new wire of that length.

My guess is that ground wire is limiting current to the fuel pump to approx 1/2 of what it needs.
 

·
Registered
Joined
·
946 Posts
Discussion Starter · #10 ·
I will do that in the morning.  Just curious, do you have any idea why it would decide to suddenly start acting up?
 

·
Registered
Joined
·
946 Posts
Discussion Starter · #11 ·
I was talking to my dad a while ago about the problem, he seems to think the pressure regulator mounted on the TB is to blame.  He suggested plugging the return fuel line at the TB.

Before doing so, I want to know a couple things.

Does the pump run all the time?  Pumping the same amount of fuel at idle as WOT?  If not, how does it know the difference?  Does it have some sort of internal sensor that shuts it off at 14psi?  If the pump does run the same at idle as WOT does the regulator and the return line allow it to circulate the fuel when only a small amount of fuel is being used (idling) by the injectors?

Basically I would like to know more about how/when the Regulator on the TB opens to allow fuel to return to the tank.
 

·
Super Moderator
Joined
·
14,034 Posts
silvermopar1 said:
I was talking to my dad a while ago about the problem, he seems to think the pressure regulator mounted on the TB is to blame. He suggested plugging the return fuel line at the TB.

Before doing so, I want to know a couple things.

Does the pump run all the time? Pumping the same amount of fuel at idle as WOT? If not, how does it know the difference? Does it have some sort of internal sensor that shuts it off at 14psi? If the pump does run the same at idle as WOT does the regulator and the return line allow it to circulate the fuel when only a small amount of fuel is being used (idling) by the injectors?

Basically I would like to know more about how/when the Regulator on the TB opens to allow fuel to return to the tank.
The pump runs for 2 seconds then turns off after you turn the key to the run position. Then with the ASD, it continues to run as long as it is getting a signal from the distributor's Hall-effect sensor (requires the engine to be running).

The pressure is set by the spring in the regulator, calibrated at 15 psi. If there is less than 15 the diaphragm (or piston, etc depending on design) closes the return line until the pressure goes above 15. It then exceeds the spring pressure and lets the fuel go back via the return line.

In this TBI system the pump runs constantly and the pressure is set by simple pressure overflow.

Your father is suggesting a common test - you pinch the return line. If the pump is good, the pressure will get very high. If it stays low, you have a fuel supply/pump/wiring issue.
 

·
Registered
Joined
·
946 Posts
Discussion Starter · #13 ·
I know the system is capable of making 14-15 psi.  It will do so when the vehicle is idling.  However under WOT the pressure drops.  Would you still suggest plugging the return for a test?
 

·
Registered
Joined
·
946 Posts
Discussion Starter · #14 ·
Put it all back together this morning.  Checked that ground wire again and noticed it was not connected on the drivers side fender well.  It checked out to be ok, so I didn't replace the wire. 

Was going to see how the fuel pressure was at idle and WOT before plugging the return line and checking fuel pressure under that condition.  But I found that it held a stead 14-15 psi at idle ad WOT in the drive way.  So I took it for a drive, I gave it WOT proably 10 times and it was fine 7 of those 10.  Went WOT on the highway to accelerate from 40 to 65 mph and it couldn't get over 55 in second, had to shift to drive and back off on the throttle to get it to accelerate. 

So the problem seems to be hit or miss. 
 

·
Super Moderator
Joined
·
14,034 Posts
WOT without a load isn't using much fuel.  Ideally, you would need to rig the fuel pressure gauge so you can see what it is while you are WOT on the road.  This may not be a fuel pressure problem, but you won't know without a proper testing method.
 

·
Registered
Joined
·
946 Posts
Discussion Starter · #16 ·
I was thinking about rigging the fuel pressure guage up so I could watch it going down the highway, but I'm pretty sure it will drop.  Is there a way to test the pressure regulator without a bunch of fancy tools?  I checked the local parts store, it's about $92 for a new one.  Found one online fore $45.  But either way that's too much to just throw at it now. 

I have noticed that pressure does not bleed off much after I shut the engine off.  It held 14 psi for a couple hours today, and I know my guage is not 100% sealed on there.  So it seems the regulator is capable of shutting off.  Could it be sticking?
 

·
Super Moderator
Joined
·
14,034 Posts
silvermopar1 said:
Is there a way to test the pressure regulator without a bunch of fancy tools?
You've already done it. If should hold 14-15 psi while running and stay that way for several minutes to hours after you shut off the pump/truck. If it stuck closed or partially closed it would read higher. If it failed/stuck open pressure would stay too low but go up if you pinched the return line (your finger acting as the regulator in that instance).
 

·
Registered
Joined
·
946 Posts
Discussion Starter · #18 ·
Well I finally got around to doing the test I spoke of earlier.  Rigged up the old (still working) fuel pump into a cut open 1 gallon antifreeze jug filled with gas.  Basically a completely new fuel system supplying fuel to the TB.  Made 3 trips, each time using about half a gallon of gas.  The first trip it seemed to run like a top.  I thought I the problem must be in the fuel delivery.  Second jug of gas I noticed just a couple problems.  Third trip was horrible, I'm 99% sure the problem does not lie between the fuel in the tank and the TB.  I will make a couple more trips tomorrow morning to get a second set of results to confirm this.

In conclusion, it seems the cause for the drop in fuel pressure is not related to the pump, wiring or hoses.  The only thing I can now think of is the pressure regulator, however it would seem to be good.  Since it the system does not lose much/any pressure.  Only a couple psi in an hour and I believe the fittings on my gauge are to blame for that.

Am I missing something here?


Please help me, I feel that I may be slowly losing faith in my mechanical abilities and my RC.  I've spent far more time and money working on it than I have my Pontiac.
 
1 - 18 of 18 Posts
Top