1990 740 Turbo - I have a no start situation. Spark is good. Removed fuel line to injector assembly and I've got no flow when cranked. Fuse is good and relay looks okay (circuit board, solders, contacts look fine) not to mention it was recently replaced 2 months ago. Fuel pump, maybe? I've got an LH Jetronic, so there is only one pump, correct? What's a good way to test the pump? Any advice would be appreciated.

 

Yes you should only have one pump. If you get no fuel then listen for the pump to hum when you turn the key on.
If you don't then look in the back of the car by the spare tire. You should see the wiring for the fuel pump.
I think but not 100% sure the pink wire should be power to the pump. Check for power when you turn the key on.
The relay will turn the pump on for about 20-30 seconds when you turn the key on. Then won't come back on till you crank it.

 

Located the pump and it's actually directly underneath the driver's seat, mounted in a bracket next to the fuel filter. Jumped the relay and did the test. Sure enuff, the pump wasn't coming on. I took a voltage reading and I get 10V when I crank, then t drops down to .02V almost instanty. I'm going to try and bench test the pump, and I'm hoping that't the problem. If not, I've got some electronics and wiring I have to shuffle through. Any testing suggestions?

 

You also have one in the Tank called a Prepump. Usually what happens is the Prepump goes bad and makes the Main one burn up from working to hard.

 

I've seen inner tank pump upgrades that boast greater fuel volume and speed. What kind of performance benefits would that have, if any? Wouldn't you have to also upgrade EVERYTHING in the fuel system from the tank pump to the injectors?

 

Go directly to your pump with 12v. That will at least tell you if it works or not.

JT

 

I bench tested the pump with 12V last week, and sure enough the pump was dead. Replacement part arriving soon. Nice Z, by the way.

 

I wouldn't worry about doing it to the intank pump that is a weak pump it only picks up fuel for the Main one.
Even if you bought a higher main pump it wouldn't work unless you bought an adjustable regulator.
The stock regulator would keep the pressure down.

The stock ones are very good for PSI.

 

Something else regarding the intank pump, or at least in the area of the tank. What would malfunction back there that would cause my fuel guage to not work? I was thinking the float on the fuel sending unit might be stuck. Is this a problem that can be repaired assuming that the guage is okay? I haven't done much research or actually took out the part to look at it yet, but is it something a squirt of WD40 and a good wiggle can fix?

 

Usually the contacts on the sending unit go bad where the float slides on the connection.
You can try cleaning them really good. Then before you install it hook it up and turn on the key and test it and watch the guage and see if it moves.