QuestionThanks for the info. I'll start digging into the matter with the procedures that you provided. You
asked for the year, make, model, etc. in case there was something in the Off the Wall Fault Book:
It is a 93 full size Blazer, 5.7 Ltr (350). Previous work done on the vehicle includes the previously
mentioned control module replacement (not sure if term is even correct but this is the module
located on the distributor for the 350 chevy) and fuel pump replacement. Other than the usual
brakes, shocks, exhaust, waterpump, radiator and other non-related repairs, the only other thing
that I can recall is the starter being replaced. The engine has always been reliable except when
the aforementioned breakdowns occurred.
A couple of immediate answers to your questions and other notes: The check engine light is not
illuminated but the circuit appears to be fine. Also, we ran the simple diagnostic (something my
son did with the fuse block) for computer codes and nothing unusual came up. We appear to
have good spark but will double check that. Gasoline tank is pretty full and the visible fuel lines
appear to be fine.
Incidentally, I had read somewhere that the oil pressure sensor switch activated by the oil
pressure sensor can go bad, thereby cutting off fuel. There appears to be plenty of oil pressure
but maybe a faulty switch(?)
BTW, I wish we were in New Mexico right now too. We're in Duluth, MN and while it's been
unseasonably warm lately (30s and 40s), we still have plenty of snow and the garage isn't heated!
- Jim-------------------------
Followup To
Question -
We performed the test and you can hear the fuel pump running full time when the jumper is
connected but the vehicle will not start and run. Of course it still starts and runs but cuts out
after a few seconds when the fuel pump relay is plugged back in, as expected.
- Jim
-------------------------
Followup To
Question -
We (my son and I) could not locate a fuel pump test lead anywhere. We
only found a test lead
connected near the coil (which had steady voltage once the ignition
was on). I have tt assume
that there isn't one for the fuel pump on this 5.7 litre Blazer engine.
Any other ideas? If we had
been able to perform the test and the jump would have kept the engine
going, what would that
have indicated (and vice versa)? Any additional feedback is greatly
appreciated.
- Jim
-------------------------
Followup To
Question -
Thanks so much for the immediate reply.
I have heard the same horror stories about remanufactured or
aftermarket fuel pumps going bad
after a short period (a horrible job, especially with the Z-71 plating),
but when the OEM pump
finally failed completely, it wouldn't start at all (although I nursed it
along for several weeks
before replacing several months ago by pounding the gas tank with a
rubber mallet while turning
the key to get it to kick in). The engine does start (and you can hear
the fuel pump kicking in
prior to start) but it cuts off after several seconds, as mentioned. We
can't keep it running by
feathering the throttle; you can see that the fuel jets just cut off. We
did pour gasoline into the
throttle plate area by hand, using a spray bottle to keep it running, but
it only kept running as
long as you could keep feeding it fast enough. Again, no fuel from the
injectors kicked in after
the initial start, even with the manual gas assist. We did check the
wires after replacing the
gasktes and all appear to be in place. I'm pretty sure that we are
getting good spark, at least for
as long as it is running, but will follow your suggestion to check to see
if plug ends are burning
correctly.
We changed the control module a couple of years ago when the vehicle
wouldn't start at all, but I
am not clear as to whether that (or any other module) regulates fuel
flow, and may be bad. I sure
hope it's not the fuel pump, but it may be. I guess I'm not clear as to
whether a pump would
work well enough to deliver fuel at startup but not well enough to keep
it flowing.
Thanks again for your help.
- jim
-------------------------
Followup To
Question -
The engine starts fine but quits after about 5 seconds. The fuel
injectors appear to be flowing freely and the fuel pump was
replaced a year ago, and apears to be working fine. Spark
appears good (new plugs, cap/rotor). Replaced gaskets around
fuel injection system including to manifold. Doesn't appear to
be any vacuum leaks. It seems as if an electronic component is
cutting off fuel flow after starting. Could this be the control
module? Any other ideas? Thanks for your help.
Answer -
You stated:
The fuel injectors appear to be flowing freely and the fuel pump was
replaced a year ago, and
apears to be working fine.
NEVER EVER ASSUME anything. I have seen fuel pumps fail within days,
weeks, months after
being replaced.
Can you keep the engine running if you increase the engine speed ?.
Will the engine start back up immediately after it stops ?.
Did you verify that you have BOTH fuel and ignition ?.
Pull 2 spark plugs and see if the ends appear to be burning correctly.
Did you recheck ALL of your electrical connections AFTER you
reinstalled the manifold as loose
wires are a major problem after the vehicle has been worked on ?.
Although I do NOT recommend this test as it is EXTREMLY DANGEROUS
as you are dealing with
GASOLINE.
If you introduce a small amount of gas into the throttle plate area just
as the engine is about to
stop will it run for a few more seconds and or ATTEMPT to run ?.
let me know.
autohelp
Answer -
You obviously know where the fuel pump relay is located. If you look
near the relay usually taped
to the electrical harness is a single wire female style electrical
connector that is NOT connected
to anything.
This wire is your FUEL PUMP TEST LEAD and if you insert a standard 12
volt test light and or
meter into this connector you will see that you will have battery voltage
for about TWO SECONDS
as soon as you turn the ignition switch to the ON position.
Once you locate this TEST LEAD, You can by using a 10 amp fused
jumper wire from the battery
attach full time battery voltage to this TEST LEAD and this will cause
the fuel pump to run at all
times as long as you have battery voltage connected to this TEST LEAD.
You can start and run the vehicle with this test lead attached to the
battery.
Perform this test and let me know.
autohelp
We (my son and I) could not locate a fuel pump test lead anywhere. We
only found a test lead
connected near the coil (which had steady voltage once the ignition
was on). I have tt assume
that there isn't one for the fuel pump on this 5.7 litre Blazer engine.
Any other ideas? If we had
been able to perform the test and the jump would have kept the engine
going, what would that
have indicated (and vice versa)? Any additional feedback is greatly
appreciated.
- Jim
Answer -
There is a test lead there somewhere.
OK, Then let's do it the hard way.
You know where the fuel pump relay is. using your test light identify the feed wire that goes back
to the fuel pump. You can do this by probling the fuel pump wire at the fuel pump relay and
when you turn ON the ignition switch as I stated earlier, the test light will only illuminate for 2
seconds and then go out.
Once you have identified the feed wire going back to the fuel pump, DISCONNECT the fuel pump
relay from the electrical plug and using the previous style 10 amp fused jumper, apply battery
voltage to th electrical connector terminal wire that feeds the fuel pum and the fuel pump should
run all the time. DOES IT ?.
If it DOES and you can physically hear it running, see if the vehicle will start and run.
let me know.
autohelp
Answer -
OK, Then your next test will be to test the fuel pump PRESSURE and VOLUME. I ahve seen similar
problems where when the fuel level gets under half a tank the rubber hose that connects to the
ful pump and the fuel pickup tube can become loose and or suck air. Make sure that you have
atleast 3/4's of a tank of gas as this will put the fuel level ABOVE the rubber hose.
Did you check to be sure that you did not accidently squash one of the rubber fuel lines when
you replaced the pump cause if you did it usually takes quite a while for the rubber hose to get
soft enough so that it collapses at the point of where it is pinched.
OK, if your fuel PRESSURE and VOLUME are ok, Then you must have an ignition problem.
Let's back up a little and tell you how the fuel pump circuit operates.
When you initially turn ON the ignition, the fuel pump relay is energized via the vehicles
computer for ONLY 2 seconds and then shuts off. This 2 second run pressurizes the fuel system
in order to easily start the engine.
Once the engine is running the fuel pump relay is NO LONGER UTILIZED in supplying battery
voltage to the fuel pump as the fuel pump feed is redirected and fed to the fuel pump directly
from the vehicles computer.
This is why I had you install the jumper to keep the fuel pump running ALL THE TIME to see if the
computer was NOT supplying battery voltage to the fuel pump at all times when the engine was
running.
OK, Now your problem has got to be ignition related and you MUST be losing ignition to the
spark plugs and or getting WEAK ignition to the spark plugs. Pull some plugs and see if they are
WET and or CARBON FOULED.
Usually LOSS of ignition and or weak ignition is caused by:
1. the crankshaft sensor and or wiring from the crank sensor to the ignition module.
2. An intermittent fault in the ignition module.
3. a faulty coil pack (if applicable to your vehicle).
Are you SURE you do NOT have an exhaust restriction ?. Did you perform an exhaust BACK
PRESSURE test ?.
Has the CHECK ENGINE light on the dash ever illuminated ?.
Does the CHECK ENGINE light circuit operate ?.
Try unplugging the vacuum line that goes to the EGR valve as possibly the EGR controller may be
pulling the EGR fully open due to a fault in the controller.
If you can spray fuel into the throttle body when the engine tries to stall out and keep it
somewhat trying to run, Your problem is most likely fuel related.
I wish I had access to this vehicle here in New Mexico as I know I would have your answer with 30
minutes.
Give me the YEAR, MAKE, Model and ENGINE SIZE as well as any previous work done on the
vehicle of your vehicle and let me look in my OFF THE WALL FAULT BOOK that I keep records in
when I run across unusual problems. But I doubt your problem is off the wall.
autohelp
AnswerOK let me know.
As to the oil pressure switch being the cause, Since we already fed the fuel pump direct battery voltage and the pump ran all the time and the engine would still not start this eliminated the oil pressure switch as a possible cause because we bypassed it via the direct battery feed.
You can check it just to satisfy your own curiosity.
autohelp
