I have a 1991 GMC pickup truck with 350 engine. It has a throttle
body fuel injection system. It's been running poorly or most of the
time, not at all. I first replaced the fuel filter, and that made a
difference but soon it did not run or ran poorly again. I tried a
spare (used) fuel pump I had laying around, and no improvement. I
bought a brand new fuel pump, and it ran very well and I thought the
problem was solved. Several days later I tried to start it and it
would not start. I ran the battery dead and had to charge the batt.
The next day it started but sounded like it was running on 3
cylinders. Barely had enough power to get up a slight hill on my
driveway. I revved the piss out of it, and it suddenly smoothed out
and ran fine. I used the truck several times after that with no
problems. Yesterday I tried to start it and it would not start. I
ran the batt dead, charged it, and tried again. I poured a little gas
down the throttle body and it started right up, but only ran a few
seconds. I did this again, and a 3rd time. Same results.
It's just not getting gas at all.
I have new fuel pump (can hear it running)
I changed the filter not long ago
No lines are kinked or leaking
YET, NO FUEL to the intake.
What could be wrong?
I have to say that I have always had carburated engines and do not
know much about fuel injection.
PS (I am wondering if this is possible, so I am going to mention it.
When I took out the second fuel pump (the used one), I noticed the
boot filter (sock) had a tear in it, so it was not likely doing it's
job. Could enough crud have gotten sucked up to clog the (NEW) filter
again? I plan to change the filter first (again), but because the
problem is intermittant, I dont thing that's it.
Bad fuel pressure regulator on the injector assembly.
Injectors not firing.
Injectors plugged with crud.
Could be. But I'm betting it's a problem with the injectors themselves.
To check if the injector pulse it there go rent a set of noid lights and
see if the pulse is there.
Also grab a pressure gauge and see what the pressure is at the line.
Fuel injection isn't that difficult once you understand how it works.
It's a LOT easier than working on a Q-Jet!!
Noid lights? What's that? What do they do?
I wish I could find a website that actually explains how FI works. I
never had all these problems with carburetors. In the year and a half
I have owned this truck, it's only run properly for about 3 months.
Last winter It didn't run all winter causing me to be snowed in for
several weeks because my car cant handle the snow like a 4WD. Yet, my
old carburated chevy Impala car starts immediately even in 30below
temps. I sure am not sold on FI. And I wonder how many thousands of
dollars I have to spend to get all the equipment to test a FI engine?
On a carb engine I only needed a timing light and a pair of plyers.
They are a small specific test light that plugs into the harness in
place of the fuel injector. They will flash as the injector pulse comes
from the ECM.
EFI works real simple actually. Here is a quick simplified version.
Fuel from the tank goes through the pump up to the regulator and then to
the injector(s). Fuel that is in excess of the regulator pressure gets
returned to the tank through the return line.
At the engine end the injector(s) receive a control pulse from the ECM.
This control pulse tells the injector how long to stay open (it is
actually more of a string of pulses but they are VERY fast)
These pulses are timed by the ECM by taking a signal from a crank
position sensor and a cam position sensor. Those tell the ECM exactly
where the crankshaft and pistons are during operation.
The ECM calculates how long to pulse the injectors and when to shut them
off using feedback from the above two sensors, the Throttle Position
Sensor and the oxygen sensors. It also uses intake air temperature and
air flow into the engine.
The system sounds like a mess BUT the nice thing is that most of the
parts are not hard to actually test. Many can be tested with a simple
multi-tester and a service manual. About the only part that cannot be
easily tested is the ECM, BUT they are more or less self testing for
The injectors themselves are just small solenoids with a fuel control
valve on the end. As they pulse they spray fuel out of VERY small holes
that can plug up.
The pressure regulator is a simple spring type that keep a constant
pressure for the injectors, if it is failing and the pressure isn't
correct the engine will run like crap.
From the sound of it I would say you have more than one problem. When
EFI is operation correctly it will start much better than any carb.
Tool wise you don't have to own many of the special tools needed. 99% of
them can be "rented" from places like Advance Auto or Auto Zone. For the
average DIYer that is a better way to go because you won't be using them
daily and having a $200.00 dollar tool setting there collecting
rust/dust is a PIA.
One tester that is handy to have is an Actron Sensor Tester. It is an
older unit that allows many tests on parts without having to buy the
bigger tools. Mine was something like 30 bucks 20 years ago.
The only item that you really should own are the service manuals for the
vehicle. They make ALL of the testing easier and have a lot of
flowcharts and tests spelled out for you.
For instance the Throttle Position Sensor:
Sounds like a very difficult item to test. BUT in reality it is nothing
more than a variable resistor just like a volume control on your stereo!
Testing involves pulling the connector and putting a meter across a
couple contacts, measuring the total resistance and then moving the
throttle while watching the meter for SMOOTH motion with no drop-outs.
If it meets the specs it's OK.
Intake air temp sensor is a simple thermistor. You pull it. then measure
it and compare the readings against the chart in the book. If the chart
says you should see 1.2 ohms at 32 degrees, 2.4 at 50 and 3.6 at 90 just
measure at those temps and if it matches it's OK. (not actual values)
The MAP is the same thing but controlled by engine vacuum. It can be
tested using a hand vacuum pump and a tester.
One item that can throw off a LOT of the sensors is a dead battery or a
bad ground/positive line. The computer looks at the return voltages on
most sensors as compared to a ground or 5 volt standard. If the engine
isn't grounded well you get bad data. Same thing if the battery isn't
charged. The ECM gets bad data and you end up with a mess.
On Sun, 13 Jun 2010 18:14:33 -0500, firstname.lastname@example.org wrote:
Earlier you said "used"... Is it really new, or used? Hearing it
running is not significant WRT it being any good. It simply means that
it is getting (some) power.
Did you check fuel pressure, and is it up to specs? If no, why not?
And did you change it again? What condition was the old filter in when
you changed it? Is it possible that there is sufficient contamination
in the tank that the new filter is also now bad?
That you can see, at least.
Well, why? Again, check fuel pressure to the TBI. Sufficient by the
book? If yes, then the TBI or related components are what to look at.
If no, trace back along the fuel path.
Just guessing, maybe low voltage to the fuel pump (have you measured
the voltage ACROSS the pump? Don't measure to ground, as that won't
tell you if you have a bad ground point. Possibly that 'new' used fuel
pump is also defective.
Yes possible. Was there any 'junk' on what was left of the sock?
(1991 GMC pickup 350 TBI engine)
...intermittent good/poor/not running...
My 1992 Buick with a 350 TBI had the same symptoms.
The problem turned out to be an intermittently failing
ignition (TFI) module. On my engine it was a little
package mounted inside the distributor.
Check your alternator. The battery should read above 12.65 volts when
vehicle not running, then 13/14 volts after starting. That is the alternator
kicking in and charging the battery.
If the alternator is not working, the battery will run down. If the battery
is run down, electric things like a fuel pump will not have enough
electricity to work right...
How to use a multimeter...
Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here.
All logos and trade names are the property of their respective owners.