"If it weren't for bad luck- I'd have no luck at all"

[brcidd]

Wasn't that a great song on the old Hee-Haw show--- My '94 Astro Van engine went 318,000 miles two yrs ago and finally spun some rod bearings- wife was driving in 2nd gear (by mistake) at 70 mph pulling trailer- (no tach on Astros)- anyway- I felt really good- found a $500 87k mile engine from a '95 Astro- had it in, in less than a week's time- ran great- pulled a stock trailer better than ever- then 15,000 miles later- last month- for no apparent reason, this engine was seizing- again pulling a stock trailer- on the way to a Nativity scene with the appropriate animals on board- anyway- got it home on a flatbed- terrible rod knock etc-- pulled the oil pan off last weekend- and lo and behold the oil pick-up tube and screen assembly is laying in the bottom of the oil pan- the "press fit" was not there- it slips in and out- somebody must of been asleep at the oil pump machining station- what a bummer!!

What further peeves me off - is that the way GM wires their fuel pumps- the oil pressure switch is in parallel with the fuel pump relay, not meerly strictly through the oil sensor switch. My 77 Vega had a fuel pump relay activated by the starter circuit- that turned off when oil sensor switch took over-- If I was loosing oil pressure, the fuel pump would cut out-- NOT any more- I suppose with the advent of fuel injection- it is completely necessary to prime the fuel pump before cranking....So the big question is-- why does the fuel pump circuit even go through the oil sending unit switch- if it is always in parallel to the ecm driven fuel pump relay? The service manual says it is to "serve as a back-up" to the fuel pump relay- what kind of logic is this? If they want a backup- install two relays- the oil pressure switch does nothing to protect the engine- the gage circuit grounds through the sending unit and did flicker the graphical display on the dash- but I have replaced 2 or 3 sending units on this van for the exact same reason- irratic gage readings....I'd wire my FP through the sending switch- but I don't want the extended crank time it takes to wait on oil pressure to make the switch- (I have a remote starter)- I did this before on one of my in-law's Astro van- took an extra 2 or 3 seconds of cranking.......

Oh well- I found another 4.3L engine out of a 95 S-10- with only 57,000 miles on it- $700- brought it home- and am transferring all that needs to be- including oil pan- I had to transfer the pick-up tube as well- so this time it will be altered to be a "welded fit" I am going to wire a flashing light off of another oil pressure switch to "double-up" the oil pressure monitoring. I lost a perfectly good engine the old way. I do like buying used engines- heck I got every sensor now as a spare-- and even another CMPFI spider assembly- that a $300 part- so let's see if my luck improves..............

Note:- I originallyposted this over in the A/C forum "off topic chat" - but thought I'd share this here....

[old_master]

GM has "experimented" with the double, or back-up circuit to the fuel pump several times. I service a fleet of Astro's and Safari's and I can tell you for certain, some of them shut the pump off when oil pressure gets below 7psi, and some don't. Of three vans I thought were identical, two of them stall when you unplug the OPSU. If the PCM is programmed to shut off power to the fuel pump relay after the engine is running, then you have protection from the oil pressure sending unit. If the PCM does NOT shut off the relay after the engine is running, your engine is vulnerable to the very problem you experienced.

There is a way to shut off power from the PCM to the fuel pump relay after the engine is running by installing a standard 5 terminal relay. By doing this, fuel pump power will only be supplied by the oil pressure sending unit after the engine is running. The fuel pump will be controlled by the PCM for priming purposes only. GM's thought process behind the double circuit is to increase available amperage to the fuel pump. By using two wires of the same gauge, the amperage flow capacity will increase, reducing resistance. The draw back to only using one wire to supply the fuel pump is that resistance is increased and the connections may get hot and melt the connectors. Most auto parts stores sell the relay for under $10.00 USD. If you're interested, PM me.

[brcidd]

Just to add to this thread- I have some new insight from an S-10 I worked on over the weekend.....

So another buddy needs his '98 S-10 Fuel Pump replaced- (tired of kicking the tank to get the pump to run) - So he tows it to my house- I jumper the test lead- kick the tank 5 times- pump comes on- pressure builds to 65psi- start the truck- pull it into my barn- Just for fun, I pull the jumper off- and pull out the FP relay- the fuel pressure starts dropping- the truck stalls- I am perplexed- figure the GM method of wiring (per the current wiring diagram) says this is a parallel circuit- FP should run off oil pressure sending unit switch- So- after I replace the FP (lifted the bed off the truck- 8 bolts super easy with a full tank of gas) I jumper the two big wires at the oil pressure sending unit- and truck runs without the relay now- as designed- so I figure a bad sending unit- I go by a new sending unit- counter person brings me one with a single terminal- I say no way that is for oil pressure only- mine has the wiring for FP as well- I need the three terminal unit- she looks up a '99 S-10- it has a different part number- brings me the sending unit- it has three terminals- I install it (tough to do- have to pull the distributor) and I find the OEM sending unit has only ONE terminal (duh- the parts girl was right). Since I have the wiring for the three terminal unit- I install the three pronged unit and everything runs as designed- pull the sending unit plug- truck runs, replace sending unit plug, pull the relay- truck runs.

Now the big question- Why was the OEM and replacement sending unit designed for oil pressure only? when the truck is wired for the FP parallel circuit (and the service manual shows it as well)- can certainally become confusing- So any of you out there that say the circuit is not in parallel- check your sending unit- for a single terminal!!! GM must have saved some money that year with the single terminal sending units.....Just my learning experiences...............