hello everybody,

this is my first time posting so i'll start with a brief introduction. i'm from portland, maine and drive a 1988 volvo 740 turbo wagon. beautiful car with a couple of hopefully small issues to work out.

the biggest issue i'm currently having is that the car is running on only three cylinders. after doing some diagnostics and troubleshooting i've eliminated the ignition system (replaced cap and rotor, plugs, plug wires and tested for spark on each cylinder). my mechanic helped me narrow it down to the fourth fuel injector, and on his advice i bought a used injector and replaced it today, unfortunately to no avail. after i replaced it, started the car and was frustrated the lack of good a new injector did, i checked for voltage to the injector and had none. i suppose i should have checked that first, but i didn't. i then checked the voltage going to the third cylinder and lo and behold..that injector seems to be receiving proper voltage. i even swapped the wires to the third and fourth injectors and simulated the same poor running condition, but on a different cylinder.

so i guess my question is where do i turn next to restore power to this cylinder's injector? are there certain ground connections i can check? is it possibly related to a bad ECU or fuel relay?

very much appreciate the input!

 

Be careful with the wires on that car. I had a '87 and the wires under the hood went bad and the coating on the wires rotted off and allowed bare wires to touch each other and short things out. Hopefully you have newer harnesses.

Other than the plain old BAD WIRES I really don't know.

 

Start looking here and make sure the wires on #4 are getting voltage. Sounds like an open somewhere in the FI harness.

VOLVO 740 1989 - wiring diagrams

 

I'll pile on and say ditto, wiring flaw. The ecu control injector pulse but red blocks batch fire the injectors,i.e., they all fire at the same time so...if 3 fire, nothing wrong w/ the ecu. The grounds for the injectors are soldered to ring connectors under the 2-10mm bolts holding the fuel rail in place. Occasionally they can be problematic. If that's ok, the other 2 places where many issues occur are the harness below the intake where all 4 injector cable merge into one. Heavy handed folks have been known to molest the wiring there. The other spot to examine is the fire wall multi connectors on the passenger side. That's the turbo (say HOT) side of the engine bay and right above the turbo's down pipe so...peel back the sheathing and see what you have. By 88 most of the insulation issues were dwindling but after this much time, even newer cars are having similar issues.
An example of a turbo car's bunk wiring...