3.1L MPFI, has fuel/spark, no start

[rmadursk]

Hey Guys,

1990 Cavalier with 3.1L, MPFI. Fuel pump runs, pressure is 45 psi but it doesn't hold at 45 after the pump shuts off (it bleeds back to 0), plugs are firing, but the motor won't start. This is a brand new rebuilt motor. It sputtered a bit when I first tried to start it but nothing since then.

My question is what would keep the injectors from firing. I listened to them while a friend cranked it over for me and didn't hear any clicking but it was pretty noisy and my hearing isn't that great. I checked the injectors with a NOID light and they are definitely not firing.

Most of the sensors were replaced before I got the car so I am pretty sure that all of the sensors are OK.

Thanks

Ron Madurski

[1meanz]

have you checked where the injector harness plugs in? idk what happened but somehow mine came loose, and the inj's wouldnt fire. just something to check

[Superdave]

Yep, check the injector harness, all the ground connections etc...


with the pressure bleading off like that, it almost sounds like the FPR is bad. it should pump up and stay.

[rmadursk]

more info on no start

I checked the plugs for the injectors and I have good ground and power to the injectors with the key on (at least the 2 that I can see the others are underneath the manifold). Still no firing of the injectors though.

The FPR is supposedly a new unit although that doesn't guarantee that it is a good one.

I swapped ECM units too but I don't know that either of them are good or bad.

What is the resistance supposed to be for the injectors themselves?

Thanks,

Ron

[rmadursk]

Still no injector firing, more information

I checked the wiring harness and did not find any shorts in the system. Both of the Injector Driver Grounds (BD6 and BD7) are OK as well as the Injector Driver lines (BC11 and BC12). There is continuity between the Pnk/Blk and Pnk/Blk wires at the connector which makes sense (I think) since they come together at the fuse block (or somewhere around there). I did not check the Ignition Connector (BA6) since I'm not sure how to check that one.

There is a wire that comes out of the DIS module and connects in with the Pnk/Blk wires but I can't tell from my diagram what that wire color should be. Can I assume that it is also Pnk/Blk? Is that wire carrying a signal that the injector drivers depend on to pulse the injectors open?

Any other hints on what might be the problem?

Thanks,

Ron Madurski

[RailHomie]

I've been messing with a 94 3.4L SFI Camaro engine with a DIS and I know that the computer switches the ground to fire the injectors, not the hot wire. The pnk/blk wire from the injectors should go straight through a fuse to switched 12v. That's how the camaro engine is. I don't know how that compares to your engine though.
Thought I'd throw that in there since no one else replied.

[RailHomie]

Oh, and also, if you have a security system (as the camaro does), if it's not functioning or is locked, it disables the injectors (not the fuel pump or spark, though).
Just giving you some ideas!

[rmadursk]

Getting confused



If the pnk/blk wire through the fuse is showing continuity when everything is unhooked from the circuit (injectors, DIS, ECM) except the fuse block, does that indicate a short?

I'm not an electrical guru but that would seem to be the problem that I am going to have to solve.

No security system installed that I can see.

Thanks,

Ron Madurski

[RailHomie]

If I'm understanding what you're saying, that should be right. The pnk/blk wire should be hot no matter what, as long as the ignition switch is on.
It's sounding like the computer just isn't firing the injectors for some reason. Either something's wrong with the computer, or you have a sensor that's not attached or not working right. OR, if you have a wiring diagram, with a pinout of the computer, look for a wire that goes to a "fuel enable" pin from a security system. Someone on this forum should know for sure if your car has anything like that, but it seems as if all of them have taken a vacation...?
Someone chime in please!

[bszopi]

Maybe this will help...



Rest of the schematics -> http://www.60degreev6.com/gallery/br...=3&userid=9680

[rmadursk]

That diagram is close but not quite the same as what I have. I only have 2 injector driver pins, not 6.

I'll try to scan my diagrams in and post them to show where I am talking about.

Thanks,

Ron

[RailHomie]

yeah, it's probably basically the same, but it's batch fired instead of sequential, so it fires 3 injectors at a time instead of 1 at a time like mine and the one in the diagram.
You've either got a wiring problem, a computer problem, or there's a sensor that is telling the comp. not to let the injectors fire. Someone here would probably know what sensor might do that. I sure don't.
Sorry man,
Pat

[bszopi]

Oh yeah, I just realized I linked you to the 93-95 F-body ECM. Wasn't even thinking that they used SFI when I posted that. I could have sworn we had wiring diagrams on here for the older 7727/7730 ECMs, but I can't find them any more for the life of me. I know there are links to other sites that have the diagrams, so that might be something to do a search for (7730 wiring diagram).