TBI Fuel Pump Waveform

Juan Technical Support Specialist Lisle, Illinois Posted   Latest  
1989 GMC C1500 Sierra 5.7L (K) 5-spd (MG5)

Performing a pre-trip inspection on a 89 GMC P/U 5.7l TBI. I was checking the current waveform of the fuel pump. It had 5 A peak draw during start up, steady at 3 A. Compared to what I have studied, been told and seen. I suspect a weak/failing pump. I am aware that TBI systems only require 9-13 psi of pressure, so a low amp draw is not very surprising to me, I keep this in consideration due to the low demand of the TBI system. The main issue I have is that I saw no visible commutator segments in the wave form. I was originally testing at the fuse and suspected there was more than 1 load on the circuit. Then jumpered the F/P relay and tested there, same results. Went to tank, same results. I didn't capture the waveform, due to being mobile and my versus battery died. Has any one seen a flat line on a fuel pump wave form with no drive-ability issues. I did not measure fuel pressure or volume either. Im suspecting a dirty commutator gaps, or possible shifted brushes?? I will try my hardest to meet up with the customer this weekend to capture the waveform. 

Thank you, 

Juan …

Marlin Technician
Estacada, Oregon

An OE-style pump cannot draw a steady amperage. Your scope is set up incorrectly or your current adapter is wacky.

0 Ð Bounty Awarded
Randy Curriculum Developer
Raleigh, North Carolina

The low current level doesn't surprise me on a TBI. Let's see what the waveform looks like. Does the vehicle have any symptomatic complaints? I will try to find an old capture and post it for comparison.

+1 Ð Bounty Awarded
Albin Diagnostician
Leavenworth, Washington

I have a terrible time examining a waveform with no waveform present :) That being said, I am also a little on the lazy side, and will use my scan tool to squeeze as much information from a vehicle as possible. In the case of fuel flow and delivery analysis, we are interested in volume & pressure. Given a fuel filter that is capable of flowing the correct amount of fuel, it is a little hard to have the correct pressure to an engine, and have low volume, unless there is a problem with the pressure regulator.

So, use the system to test its self. Most systems on vehicles are engineered for the vehicle. The charging & starting system is made that way, as is the fuel system. The fuel pump is also made that way, and when it starts to degrade, whether the problem is a worn out motor, or a worn out pump, the failure will always rear its ugly head when the demand for fuel is high, such as in a "wide open throttle, maximum engine speed condition".

This is an easy condition to put a vehicle in, unless you are working on high horsepower engines. Use the FRTD (flat fate test drive) and a scan tool. Graph out the oxygen sensor & fuel trim and see what it will say. As long as the oxygen sensor volts are above .9volts at WOT at maximum RPM, there is no need to take a fuel pressure or volume reading, after all, this vehicle is a little hard to do this task.

Scan data graphs just plain rock.

+4 Ð Bounty Awarded
Scott Owner/Technician
Brantford, Ontario

Albin's test is valid, as for a flat line for current, if we think about it's impossible, unless it's brushless. In my experience I used to scope ton's of pump's back in the day , even if they were ugly they still did there job. Now a no start with a bad com bar would show up beauty on the scope. I'd look at the rpm , pattern uniformity and current. We need a capture to go on here. Being it's and 89 it may be time, just for confidence in the vehicle.

I used to know a guy and he'd look at a current wave of a fuel pump and I quote "if this was an airplane I wouldn't fly in it" Many new him as a great instructor. Post a capture please.

+2 Ð Bounty Awarded
Rudy Technician
Montebello, California

Id dump that practice of using FP wave forums for accurate diagnosis.

Its not very practical and can be misleading. It is very much like ohms testing......

-1 Ð Bounty Awarded