x2 with crazy, if you have a multimeter you should be reading 14.6 under load

 

Good theory gents, but the dash tach has no input into the PCM operation.

The tach is run off the alternator, but it is independent of any engine control inputs. They have also been shown to be wildly inaccurate at times.

All the PCM cares about with regards to engine speed is the CKP sensor and that's a 5V reference pulse IIRC.

Fuel is controlled by PCM through reading the APP+CKP+VSS+a whole bunch of other sensors, then the PCM triggers the PMD, which drives the fuel solenoid valve, which sets the pulse width and timing of the event....

 

A while ago I had symptoms suggesting a faulty pump and it wound up being an air leak on the sending unit / tubing connection at the fuel tank. The feed line bends to horizontal right on top of the sending unit and there is no way to find the air leak. Once I took care of that, I had fuel running with the new lift pump from , er, rhymes with Papa... I did not deep 6 my old pump yet since I tested it on the bench and the foolish thing still worked, so I kept it and installed it 6 months later when the "new one" failed. Yeah, I had the new screen on the new sending unit.

This old one finally failed a few months ago... I put my new AC Delco spare in... now I have problems again. This evening the fuel filter was dry. We'll see in the a.m. if a replacement pump is the answer. The existing one clatters rather than purrs. Hasta manana.

 

Often a lift pump will "purr" but not pump, only way to know if a lift is good is to test under a loaded condition Lift Pumps & OPS operation and troubleshooting

 

Thanks Turbine Doc, everyday is a good day for learning something new, and being reminded of the obvious. what, me have dirty sock... no way.

New lift pump (no, not the brand or model I wanted, but that's what they had behind the counter) got put in 2 nights ago (getting better at this, only a 1/4 cup of essence de diesel spilled upon myself). only made it 1/2 block before she sputtered and died... cycled the key a couple times, started her up and backed up into the driveway. sure enough, pump not running when engine is running. Last night learned about jumping the ops sender jack and indeed that makes the LP run... and run... and run. This morning made it 4 times as far, then stumbling and coasting.. into parking lot. opened the filter bleeder and waited ... patiently for the ... fuel... to spill...out the bleeder. Again getting a little sucking sound as I open the bleeder... so fuel starved (and not an air leak).

So Saturday morning I will pull the sock out of the tank... fortunately i have already got the hatchway through the floor of my burb so this'll be easy to do. Also have the new USA made 4 psi pump for 93 and earlier models, new fuel filter, and OPS still on its way. Hoping i have not fried the PDM.

till the weekend when the days are longer

 

Do you get a large vacuum when you remove the fuel filler cap?
A plugged cap vent can cause fuel starvation as well.
Diesel fuel caps are vented and if they have gotten plugged or replaced with a gasser cap bad things happen.
You might want to try running with the cap loose before removing the sending unit?

 

Thanks Racer55 and again thanks to Doc Turbine.

I loosened the fuel cap to no avail. But due to yet more urgent issues at home, I pulled the fuel tank sendiing unit this morning at 5:30 and cleaned the strainer; put it all back together and ran the fuel pump with the blessed cap open. Fuel jumped up about an inch rather than the 2 mm. Still running with OPS jumped. Hope it shows uP tonight .

 

When time permits you may want to remove the strainer and install an inline filter/screen pre LP so it is more easily serviced.
Have the tank cleaned would also be a benefit.

 

Thanks for the suggestion... had been wondering why engineers would put anything in the fuel tank that needs servicing... but yeah why not move it to under the truck myself.
It is now on to more mundane issues like tightening up the steering and replacing the PS pump... finishing a clutch job, and changing some control arm bushings and.. and...and ugh the toilet ... all before the snow flies on Sunday....