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89, strange hard starting

Jonny-K5

Jonny-K5

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this past month my truck has been acting up. about 2 or 3 times so far, when i go to start my truck, the engine fires and immediatly dies, without idling whatsoever.

the only way it will start is with pressure on the throttle. i gotta hold it there for a few seconds after start up or it will die tho. and once it is idling, its too low (500rpm) for a cold idle.

any ideas? the ecm was just replaced two months ago with a "rebuilt" one.
 
Could possibly be the oil pressure sender going out. After the 2-second prime, the oil pressure sender provides power to the fuel pump relay when there is oil pressure. If the sender going out, it may not be providing the fuel pump power in those situations.

I would hook and ohmmeter up to it and crank the engine. While cranking it should bring the oil pressure up high enough to close the contacts inside.
 
According to the Manual

1) Lose or faulty connections at the dist, coil, or alternator.
2) Bad fuel filter or fuel pump. Check delivery and pressure.
3) Vacuum leak at the gasket surfaces of the TBI unit. Check gaskets and all vac hoses.
 
mouse said:
Could possibly be the oil pressure sender going out. After the 2-second prime, the oil pressure sender provides power to the fuel pump relay when there is oil pressure. If the sender going out, it may not be providing the fuel pump power in those situations.

I would hook and ohmmeter up to it and crank the engine. While cranking it should bring the oil pressure up high enough to close the contacts inside.
Click to expand...

Oil pressure switch is only essential if the pump relay has failed. You'd get NO fire until the engine cranked for a couple seconds if the relay is dead, as it takes around 5PSI to close the pressure switch and run the pump.
 
Kiqman said:
1)
3) Vacuum leak at the gasket surfaces of the TBI unit. Check gaskets and all vac hoses.
Click to expand...

i think this might be the culprit. ive had a rough coldstart idle ever since i installed a tbi spacer. thanks:thumb:
 
dyeager535 said:
Oil pressure switch is only essential if the pump relay has failed. You'd get NO fire until the engine cranked for a couple seconds if the relay is dead, as it takes around 5PSI to close the pressure switch and run the pump.
Click to expand...

I'm suggesting the oil pressure switch is a problem, not the relay.
 
I just went through a similar probelm, and i replaced just about everything i could before i got to the fuel pump.

Turns out it was the fuel pump.

When you get it running does it pulsate when its idleing and when you give it gas does it stumble to get up to speed?

Thats what happend when my pump went out and replacing it was alot easyer and alot less expensive then i thot it would be. Took me about 2 and a half hours and cost me 40 bux with a lifetime warranty.
 
mouse said:
I'm suggesting the oil pressure switch is a problem, not the relay.
Click to expand...

And what I'm telling you and the poster, is that the oil presure switch has nothing to do with starting the truck, UNLESS the relay has failed. If the relay fails, the oil pressure switch powers the fuel pump (at approx 5psi oil presure), and THEN only if the oil pressure switch fails or acts up, could the oil pressure switch affect starting.

The oil pressure switch is a failsafe for the relay, that is all.

Examine the wiring for it if you question my above statement. :)
 
I think you are mssing the point...the engine starts (not the pressure switches responsibility), then dies shortly after (when the pressure switch is needed).

You are saying the same thing I am. (I don't need to look at the wiring diagrams, they are very simple and easily remembered).

Can you understand the point that I am attempting to make?
 
No, I don't think I can. :)

The fuel pump relay keeps the fuel pump operating under all conditions when it's required. Start, run, etc. Maybe I should have been clearer in my last post. "You do not need an oil pressure switch for the vehicle to run or start, *unless* the relay is dead or otherwise not operating properly" If that is what you are saying, we are on the same boat. :)

If the relay is good, you can remove the oil pressure switch and you will not notice any difference in operation.

If the fuel pump relay is dead, the oil pressure switch will allow you to start the engine (albeit with longer cranking than normal) and run normally afterwards as long as oil PSI is over approx 5.

Pressure on the throttle would help in no way (going back to original posters issue) if the fuel pump was not getting power, and I don't see any comments about longer cranking times than normal. Of course, that doesn't mean they aren't present, just that we may not have been told.
 
OK, I'm re-looking the circuit. If the oil pressure sender fails, but the fuel pump relay is still closed, the pump still runs...??? Are you saying B2 provides constant 12v?

"The fuel pump relay keeps the fuel pump operating under all conditions when it's required. Start, run, etc. Maybe I should have been clearer in my last post. "You do not need an oil pressure switch for the vehicle to run or start, *unless* the relay is dead or otherwise not operating properly" If that is what you are saying, we are on the same boat. :)

If the relay is good, you can remove the oil pressure switch and you will not notice any difference in operation."

The diagram that my truck uses is this one pictured below. The computer provides the relay during the initial 2 seconds (when starting), but then drops power, opening the contacts. The only other way to get power to the pump is through the oil pressure switch.

Perhaps we will have to disagree on this one. My understanding (as illustrated below) is that the oil pressure sender will stop power to the fuel pump if the engine dies (low oil pressure). Similarly, a faulty sender would allow the engine to crank, fire, and then die.

I will have a look at the ECM schematic for B2.

747ecm2.jpg
 
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Your manual diagram for the fuel pump appears to be for an S10. However, ALL gm stuff is wired up like this that vintage. (minus the hot fuel thing, it wasn't used on cars, I know that much)

http://brochures.slosh.com/wiring/87%20Chassis%201.pdf That's supposedly '87. Dunno about that hot fuel thing, I didn't think it was used in all TBI truck apps, could be wrong, I saw it in every year of those diagrams up to '91.

In that diagram, just backtrace the fuel pump and wiring going from it. Mentally erase the oil pressure switch and all wiring up to the splices...yep, still path to fuel pump. Same path actually, but the switch gets you wired AROUND the relay. It's not separate from the circuit at all, but not necessary because the relay does the same thing.

B1 is fused 12V, (power "in") GRY is power "out" to the tan/white fuel pump wire.

Here's another diagram, this is GM: http://s25.photobucket.com/albums/c59/daleearnhardt01/1990%20GM%20V%20Series%20Wiring/?action=view&current=111-1183_IMG.jpg

Same thing, 440-1.0 ORN feeds power to both the relay and the switch via splice 104, and tan/white from BOTH the relay and switch at splice 103 feed power to the pump.

Without the switch, you still have 12V going into the relay, and 12V going out to the fuel pump. Imagine wiring that circuit up with JUST an oil pressure switch. You'd simply feed 12V to the switch, and once it closes, 12V goes to the pump. Ignoring all the splices, that IS how it's wired up. It's a backup for the relay, nothing else.
 
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Your diagrams definitely show the oil pressure switch in parallel with the relay, not series. I have to say using your technical data that this appears correct. BTW, after looking at several vehicles in junkyards, etc, I've only seen one with the hot fuel handling relay.

On my 1989 suburban, the fuel pump circuit has the oil pressure sender wired as the diagram I posted. There is no tan/white wire (or splice 103), and the gray wire (which isn't on the second diagram of yours) is also missing.

I have many of these schematics and diagrams on my computer for reference, but prefer to use the diagrams since they show what I need and not all of the optional wiring differences. I grabbed the one I posted because it was the first one I came upon. I will post a 1988 schematic (trimmed for easier viewing), that shows the oil sender in series with the relay to power the pump.
 
We'll see how this looks...

A little small but I don't want to try posting the whole thing. There appears to be more than one right answer to the riddle of the fuel pump circuit.

Dorian, you have proven your case, but I believe the oil pressure sender is designed to kill the engine in case of low oil pressure. In the schematics you reference, this does not appear to be the function of the sender, and, as you speculate, the sender looks as a backup to the relay...does not make sense to me in an engineering standpoint or from manufacturing as this would be redundant and cost more $$ to produce.

fuel.jpg
 
I have no idea what that third component is (right after the oil pressure switch, maybe a connector?) but again, pnk/blk feeds the tan/wht pump wire...the splice is *after* the oil pressure switch closes, with the relay output. ORG wire feeding the oil pressure switch is the same one feeding the relay. Remove either one of those wires to the oil pressure switch, relay still gets power and you still have a path to the fuel pump.

In other words, the oil pressure switch can not interrupt the flow of power from the relay to the pump.
 
Dorian is right. The oil pressure switch does not directly control or interupt power to the fuel pump. The fuel pump operation is totally controled by the ECM. If the ECM sees no pressure at the oil pressure switch it shuts off the fuel pump relay drivers after a few seconds. If you look at the wireing diagram it shows the pressure switch picks up 12 V off the fuel pump power wire. 12 V is routed thru the pressure switch then back to the ECM.Not to the fuel pump! If the ECM sees no oil pressure it shuts off the driver to the coil that activates the relay. The relay is a NO (Normally open) relay. It opens and the fuel pump shuts off.
That is why the pump only runs for a few seconds and shuts off when you turn on the key.
The circut is energized when the ign is turned on.
The relay closes and The pump comes on.
If the engine is not started, the ECM sees no oil pressure and shuts off the pump.
 
Thunder said:
Dorian is right. The oil pressure switch does not directly control or interupt power to the fuel pump. The fuel pump operation is totally controled by the ECM. If the ECM sees no pressure at the oil pressure switch it shuts off the fuel pump relay drivers after a few seconds. If you look at the wireing diagram it shows the pressure switch picks up 12 V off the fuel pump power wire. 12 V is routed thru the pressure switch then back to the ECM.Not to the fuel pump! If the ECM sees no oil pressure it shuts off the driver to the coil that activates the relay. The relay is a NO (Normally open) relay. It opens and the fuel pump shuts off.
That is why the pump only runs for a few seconds and shuts off when you turn on the key.
The circut is energized when the ign is turned on.
The relay closes and The pump comes on.
If the engine is not started, the ECM sees no oil pressure and shuts off the pump.
Click to expand...

Thunder, which pins doe the ECM sense oil pressure? I will research this using the ECM schematic and brush off my transister reading capabilities. I never recall the ECM having this capability.
 
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