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Another ECM issue

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my sons 99 model died and would not restart, I have traced the problem back to the ECM which is only sending 8.9 volts to the lift pump. I have disconnected the pump and the 50 pin connector on the ECM checked for short circuit and everything is good. I then cleaned and checked the 50 pin connector reassembled and still 8.9 volts coming out of the ECM.I was wondering if anyone has seen this issue before ?

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  • Owner

Ok... I need to clear up something a bit more when you checking the voltage at the lift pump plug where you cranking on the engine or was that just a starter bump? Reason I ask is the lift pump circuit from the ECM will be PWM (Pulse Width Modulated) during cranking 50% duty cycle and straight 12V on starter bump withthe engine running.As for the lift pump ground its hidden behind the starter.

  • Author

Ok... I need to clear up something a bit more when you checking the voltage at the lift pump plug where you cranking on the engine or was that just a starter bump? Reason I ask is the lift pump circuit from the ECM will be PWM (Pulse Width Modulated) during cranking 50% duty cycle and straight 12V on starter bump withthe engine running. As for the lift pump ground its hidden behind the starter.

Hmmm, I have been checking it lately just cycling the ignition on and off since I have the water/ fuel seperater removed to access the ECM. Initialy we began checking it with the engine cranking.
  • Author

Bump the starter and it will give you 25 seconds of 12 volt signal.

Let me go check it n I will be right back

--- Update to the previous post...

Let me go check it n I will be right back

OK, I had to wait for the "wait to start" light to go off, but it did give give 12v for 25 sec like you said. I have ran strait battery power to the pump and it makes 15-16 psi running. So this leaves me scratching my head again LOL!
  • Owner

So I would put it all back together and then hook up a fuel pressure gauge and take it for a drive and see what happens. The fuel pressure gauge will tell you the story. Now are you running a stock lift pump in the side of the block?

  • Author

So I would put it all back together and then hook up a fuel pressure gauge and take it for a drive and see what happens. The fuel pressure gauge will tell you the story. Now are you running a stock lift pump in the side of the block?

Yes it still has the stock lift pump. Just a little history behind this vehical. Right after he first bought it he had to have the transmission rebuilt at a local shop. About 5000 miles later the transfer case sheared off from the transmission while he was driving. needless to say he came to a violent halt in the middle of the road. after towing it back to our shop I found that the top stud on the case was broken off, and the tranny shop evidently did`nt think it was neccesary. after replacing the overdrive extension housing, and the broken stud we got him going again only to find that the speedometer had quit, and the fuel gauge worked intermittenly for a while then quit all together. I am a little baffled that the PCM has`nt turned the check engine light on, and my OBD 2 is not picking up any DTCs ?
  • Owner

Well as for the lift pump it might be slipping the rotor head or the fuel regulator internally is stuck open. So you have to weigh that into your diagnosis.Double check to make sure no wires and pinched during the transmission work.

  • Author

Well as for the lift pump it might be slipping the rotor head or the fuel regulator internally is stuck open. So you have to weigh that into your diagnosis. Double check to make sure no wires and pinched during the transmission work.

Well I put everything back together and left one of the access plugs loose to bleed the air out of the filter, and the lift pump ran continuously even with the key off. I had to disconnect the battery to get it to shut off
  • Author

Well I put everything back together and left one of the access plugs loose to bleed the air out of the filter, and the lift pump ran continuously even with the key off. I had to disconnect the battery to get it to shut off

The engine cranks but will not start, and once the lift pumps starts its making 12-15psi, but will not shut off without disconnecting the battery? looks like my diagnosis is heading somewhere else now ?

--- Update to the previous post...

The engine cranks but will not start, and once the lift pumps starts its making 12-15psi, but will not shut off without disconnecting the battery? looks like my diagnosis is heading somewhere else now ?

after cranking some more the lift pump is cycling correctly, but still won`t start, and there are no DTCs

I was thinking he might have just ran it out of fuel since his gauge is not working, but I would think it would have tried to hit a lick by now ?

  • Author

So now try the repiming the fuel system...

Ok, just a quick question, Bleeding the high pressure lines, is that at the head? or on the injector pump side?
  • Owner

At the head... Crack 1,3,4 and then crack the engine till fuel spray is seen. Dribbling on the manifold doesn't count... Remember that Bosch VP44 injection pump can produce at minimum of 4,500 PSI at idle and upwards of 20,000 PSI at WOT. But the Bosch VP44 injection pump can't pump air very well so hence the engine won't start.

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    • Author

    At the head... Crack 1,3,4 and then crack the engine till fuel spray is seen. Dribbling on the manifold doesn't count... Remember that Bosch VP44 injection pump can produce at minimum of 4,500 PSI at idle and upwards of 20,000 PSI at WOT. But the Bosch VP44 injection pump can't pump air very well so hence the engine won't start.

    Mike, you have been a great help! and you just earned another premium member. I originally thought he had run it out of fuel, but with the lift pump seeming not to be running I wound up on a wild goose chase. some concern though. it is idleing @ 10psi, and under part throttle "2500" rpm it drops to about 7psi ?
    • Staff

    Mike, you have been a great help! and you just earned another premium member. I originally thought he had run it out of fuel, but with the lift pump seeming not to be running I wound up on a wild goose chase. some concern though. it is idleing @ 10psi, and under part throttle "2500" rpm it drops to about 7psi ?

    Mine did that till I got the Big Line kit from Vulcan Performance. Gets rid of the restrictive banjo bolts and small of fuel lines. Now I'm a steady 14 all the time with no pressure reduction. I forgot to say... try running the truck with the fuel cap off and see if pressures are a little better. That will see if the tank vent is going bad.
    • Owner

    Lift pump is junk... Time to replace it...Minimum pressure is 10 PSISuggested Minimum is 14 PSI for longevity reasons.Maximum (Undocumented) pressure is 20 PSI.Normal pressure fall from idle to WOT at highway speed 2-3 PSIMax Pressure fall from idle to WOT at highway speed is 5 PSIHere is a quick video I did last summer... No comments about sucking a air bubble... :duh:

    • Author

    Thanks guys! is is great talking with people who know WTH they are talking about:cheers:

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    • Like 1

    ...Remember that Bosch VP44 injection pump can produce at minimum of 4,500 PSI at idle and upwards of 20,000 PSI at WOT......

    That is news to me. I thought only the CR's raised rail pressure. Well not really a rail in our case. Just injection pressure the same with duration and timing altered.
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