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.
Please help.
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!!
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.
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?
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 that year.
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.
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...
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.
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