Hard start when cold. No start once warm.

[Mbyoung424]

1987 TBI 305, when cold it'll start after turning over 5-10 times. You can see the oil pressure gauge start increasing and that's when it'll start. Once warm, the gauge doesn't increase and the truck won't start. Putting power to the test lead on the fuel relay starts it up on the 1st or 2nd crank every time. As I understand it, the fuel pressure sending unit by the oil filter supplies the signal to the oil pressure gauge while the oil pressure sensor completes a circuit sending power to the fuel pump from the ECM.

When I got the truck the battery was dead, replaced it, started right up and the next day it was dead again. Turns out the shop that previously worked on it bypassed the oil pressure switch with a paper clip in the connector. I pulled that out, plugged it back onto the oil pressure switch and it started right up, but that's when my issues started.

My first thought was a bad oil pressure switch so I finally replaced it today. Still takes 5-10 cranks to start and then it won't start once warm. I'm going to test the old and new pressure switch in the morning and hopefully I just got unlucky and picked up a bad switch. Problem is, once started it runs fine so I'm at a loss for now. Any other ideas?

 

[Georgeb]

Do you hear the pump start and then stop after a few seconds when you key on but dont crank? If not check the relay.

 

[Mbyoung424]

I previously confirmed that the relay is in good working order and you can still hear the pump when you first turn the key.

 

[Mbyoung424]

I hooked the old oil pressure switch to an air regulator this morning and I get continuity between the two terminals before reaching 5psi. Looks like I went through the hassle of replacing the oil pressure switch for nothing.

Went back and tested the relay again, with about 5Vs the circuit closes. Tested the lines going to the fuel relay and the orange wire is hot all the time while the dark green/white is hot with the ignition on. Everything tests good, but when I plug the relay back in the fuel pump doesn't prime when the engine is warm, but it will prime when the engine is cold. Seems like the relay is bad, but it works when bench tested.

 

[Mbyoung424]

I don't think it's a bad relay, but I have no idea what to try next.

I went ahead and replaced the fuel pump relay and still it only starts when cold.

 

[Georgeb]

Do yoy have powe on the dark green/white with the ignition on when you have the no start condition? How about when cranking? Is there a good ground at the relay when you have the no start condition. I am thinking for some reason somthing isnt telling the ecm to switch the relay on.

 

[Georgeb]

And check your ecm fuse or fuses I think ECM B is the fuse to look for but not sure.

 

[yevgenievich]

Bypass the relay and run a jumper for the pump. If pump does not prime when hot then it or the wiring at fault

 

[89Suburban]

Next time you have a no start condition dump some fuel in the TBI and let us know if it starts then.

 

[Mbyoung424]

With the ignition on the green and white wire has power, but no power when you turn the ignition to start. It is like this when cold or hot. The ground on the relay is good. I agree that it seems like the ECM isnt sending a signal, but the ECM B fuse is good. I checked the fault codes last night and am not getting any.

I jumped the oil psi switch which gives full time power to the pump like it was when I got it. Instead of pulling the fuse every time like I was I pulled the power wire for the spare tank so that I can use the tank selector switch as an on/off switch. Works for now.

 

[Georgeb]

How long does that gr/wt wire have power at key on? Should be 2 seconds. That wire recieves power once the ecm senses a crank signal from the ignition control module. If there is an issue with the signal to the ecm twlling it you are cranking it wont turn the pump on. I have been searching a bit for a more in depth answer on this issue and will post somthing if I find it.

 

[89Suburban]

Something is screaming out to me and it may not mean anything for this particular problem. But a hairline crack in the resin of an electrical component like a ign module or coil will cause the same exact symptoms as far as cold start/hot no start.

 

[yevgenievich]

Something is screaming out to me and it may not mean anything for this particular problem. But a hairline crack in the resin of an electrical component like a ign module or coil will cause the same exact symptoms as far as cold start/hot no start.

that is true, coil is especially capable of the issue. But I though OP stated that it was the fuel pump specifically not turning on when vehicle is hot.

 

[Georgeb]

that is true, coil is especially capable of the issue. But I though OP stated that it was the fuel pump specifically not turning on when vehicle is hot.

He did but doesn't the ecm look for an rpm signal from the ignition to turn the fuel pump on via the green/white lead to the relay?

 

[yevgenievich]

That would come directly from the distributor. ECM will not fire injectors if it sees no rpm signal, but not sure on the fuel pump

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