Author Topic: No Injector pulse at start-up and fuel pump does not shut off after 20-seconds  (Read 230 times)

Offline CreightonC

  • Newbie
  • Posts: 3
  • Newbie here but well seasoned in this hobby
Hello everyone, I have read MANY threads on the issues with there being no injector pulse while cranking but engine will run if primed with fuel through the throttle body BUT I have not read any that talked about the fuel pump not shutting off after 20-seconds (key "on" but not running). We just bought the truck for my 15-year old son from his sister's boyfriend, he said it just started doing that and doesn't know why, here's the info:

1987 1/2-ton 4x4 Silverado with 5.7 TBI
Issue-1: Will only start & run if primed through the throttle body
Issue-2: When the key is turned on the fuel pump runs but does not shut-off

Two of the plug wires have burns on them from the exhaust manifold so it has been missing on both cylinders, because of this I'm guessing it damaged the control module in the distributor but wanted to see if anyone has experienced or diagnosed these combined issues/conditions? I have lots of experience with restorations, engine building, wiring etc but have a really messed up back and would like to whip this problem as quick as possible (as little time bent over the engine or under the dash etc). Thanks is advance for any replies.


Offline VileZambonie

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 17699
So you say no injector pulse, how did you verify no injector pulse? If you add fuel and it runs, that is an indicator of low fuel pressure or the fuel pump relay not energizing.

Run through the applicable diagnostic strategies posted here for TBI

http://forum.73-87chevytrucks.com/smforum/index.php?board=51.0
,                           ___ 
                         /  _ _ _\_
              ⌠ŻŻŻŻŻ'   [☼===☼]
              `()_);-;()_)--o--)_)

Offline CreightonC

  • Newbie
  • Posts: 3
  • Newbie here but well seasoned in this hobby
Thanks for the reply, I said there is no injector pulse because: while cranking the engine there is no fuel coming through the injectors at all but if you prime the throttle body it will start and run and keep running at 30mph or 80mph until the gas tank is empty with no issues at all. I would think that if there was an issue with the relay or pressure it would be evident while driving (which there are no issues after it starts). My assumption that it was the control module was based on what I stated above and the fact that when the key is turned on (but the engine not running) the fuel pump keeps running indefinitely....the control module is responsible for the injector pulse while cranking as well as shutting the pump off if the engine is not running. I have read the diagnostic strategies section that you suggested but did not see any that addressed the pump not shutting off. As I said before....I have a very bad back and want to narrow it down as much as possible before climbing under the hood with diagnostic tools etc.

Offline VileZambonie

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 17699
Unfortunately making assumptions is not going to work. In order to verify injector pulse you need to physically disconnect the injector connector and install a noid light or use a test light to verify B+ and a pulsing ground while cranking. So what you are seeing is no fuel spraying which does not mean there is no injector pulse. As a backup to the system, the oil pressure circuit will supply power to the fuel pump which may be happening in your case. As far as the pump continuing to run beyond two seconds, your truck may be equipped with the hot soak fuel module.

CHART A-5 - FUEL PUMP RELAY CIRCUIT

Circuit Description :
When the ignition switch is turned "ON", the Electronic Control Module (ECM) will turn "ON" the in-tank fuel pump. It will remain "ON" as long as the engine is cranking or running, and the ECM is receiving distributor reference pulses. If there are no reference pulses, the ECM will shut "OFF" the fuel pump within 2 seconds after ignition "ON" or engine stops.

The pump will deliver fuel to the TBI unit where the system pressure is controlled to about 62 to 90 kPa (9 to 13 psi). Excess fuel is then returned to the fuel tank.

A fuel module is used on all 7.4L and some 5.7L engines to correct a hot restart (vapor lock) during a high ambient condition. It is designed to over-ride the ECM 2 second pump operation and will run the fuel pump for 20 seconds at initial ignition "ON".

Test Description : Step numbers refer to step numbers on diagnostic chart.
Turns "ON" the fuel pump if CKT 120 wiring is OK. If the pump runs, it maybe a fuel pump relay circuit problem, which the following steps will locate.
The next two steps check for power and ground circuits to the relay.
Determines if ECM can control the relay.
The oil pressure switch serves as a backup for the fuel pump relay to help prevent a no start situation. If the fuel pump relay was found to be inoperative, the oil pressure switch circuit should also be tested to determine why it did not operate the fuel pump.
Diagnostic Aids :
An inoperative fuel module may be the cause of a hot stall/no start condition. Check for power and ground circuit to the fuel module and a complete circuit to the pump from terminal "A". If OK, and the pump does not run for the specified 20 seconds at initial ignition "ON", replace the Fuel Module.

http://forum.73-87chevytrucks.com/smforum/index.php?topic=37749.msg315615#msg315615
,                           ___ 
                         /  _ _ _\_
              ⌠ŻŻŻŻŻ'   [☼===☼]
              `()_);-;()_)--o--)_)

Offline CreightonC

  • Newbie
  • Posts: 3
  • Newbie here but well seasoned in this hobby
Thanks VileZambonie, those are the systems I already have printed out and ready to diagnose, I was "hoping" for a quick/obvious fix lol. So where is the fuel module located? Is it next to the relay on the firewall? I also had wondered if the oil pressure circuit was bad was the reason the fuel pump doesn't shut off after 20 seconds. I will also test the TPS to see if it is stuck in the wide-open position and causing a "flood-clear" mode. Thanks again and I will update once I resolve.

Offline VileZambonie

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 17699
Here are the best pics I have at the moment
,                           ___ 
                         /  _ _ _\_
              ⌠ŻŻŻŻŻ'   [☼===☼]
              `()_);-;()_)--o--)_)