Fuel Pump Relay issue...maybe?

[doncaruana]

I have a dumb question about how this all works...
There's an oil pressure switch that will also send power to the fuel pump that is in parallel with the relay. So that begs two questions:
1) Is the fuel pump relay energized the entire time the engine is running or only during startup?
2) If you pull the fuel pump relay once the engine is running, should it stay running (via that oil pressure switch) or stall?

 

[doncaruana]

So...interesting things I did this morning...Yesterday after picking up the truck from a wheel alignment, the starting issue with it's intermittent behavior is back.

First, I made the handy little cheat sheet below and grabbed my test light and went out to the truck. With the ignition off, 86/A1 and 87/B1 behaved as expected. When in the on position and when cranking, 85/B3 did exactly what it was supposed to do - lit up for 2 seconds first then went out, but came on while cranking. Here's where it gets weird...

I started the truck without the relay in at all. I had my wife cranking and I think she let off the ignition too quickly a couple of times tbh (shorter than a normal crank), but once I told her to go ahead and hold the key, it started up. Honestly with relatively no issue. And what's more, 85/B3 was energized the entire time the engine was running. I expected it to shut off, but maybe that was my misunderstanding.

The one thing I didn't actually check is pin 30 to see if I had voltage on it once the truck was running. I also recall hearing something in the engine when the ignition was turned to the on position which I can only assume was fuel getting pushed into the engine? Would that be what I was hearing? But I actually think when I did that, the relay was out already (need to check that again I guess).

Based on my other weird experience of the intermittent fuse with the dashboard lights, I went ahead and swapped in a new fuse for this anyway. Figured it can't hurt. But it's like the more I learn about this, the less makes sense to me. Does it make sense to anyone else??

 

[east302]

The relay pin functions all sound normal to me, so the thing working with the four pin may have been a not-so-nice cosmic coincidence. Once oil pressure is 4-psi, the switch will close to power the pump as a backup if the relay is bad.

Is it still throwing a code?

Does a 97 have Passlock (Security light will come on briefly when ignition is in run)? Might have been for 98 and up, I don’t recall. But when those trip it will start and die within a second or so as the VCM cuts off the injectors.

What is the coolant temperature sensor (ECT) reading when engine is cold and hot? It should be close to ambient when cold and around the thermostat setpoint when hot. If it’s an oddball number, the VCM calls for the wrong injector pulse and can cause a hard start. I doubt that’s your issue but it’s a free check, especially if you can do it right when it’s not starting.

What brand is the distributor cap? Off-brands can cause ignition issues. I had some brand that came with a Summit distributor that randomly caused no-start issues within a month or so of installation. Delco cap fixed it.

 

[doncaruana]

The relay pin functions all sound normal to me, so the thing working with the four pin may have been a not-so-nice cosmic coincidence. Once oil pressure is 4-psi, the switch will close to power the pump as a backup if the relay is bad.

Is it still throwing a code?

Does a 97 have Passlock (Security light will come on briefly when ignition is in run)? Might have been for 98 and up, I don’t recall. But when those trip it will start and die within a second or so as the VCM cuts off the injectors.

What is the coolant temperature sensor (ECT) reading when engine is cold and hot? It should be close to ambient when cold and around the thermostat setpoint when hot. If it’s an oddball number, the VCM calls for the wrong injector pulse and can cause a hard start. I doubt that’s your issue but it’s a free check, especially if you can do it right when it’s not starting.

What brand is the distributor cap? Off-brands can cause ignition issues. I had some brand that came with a Summit distributor that randomly caused no-start issues within a month or so of installation. Delco cap fixed it.

Didn't check codes today, but it threw the P1351 again yesterday. Had to cut short my testing as my daughter drove it to work.

I don't know how (or have equipment?) to do the coolant temperature reading, unless you can do that with scan software (I have DashCommand)

The distributor cap sounds intriguing but I don't know what brand it is and the guy that replaced it sold his business and moved on. This whole ordeal started when it would not fire up for me and I took it to that guy and he said the distributor was really worn and he replaced that, the cap, the ICM, and the ignition coil (it was due).

 

[east302]

I’m pretty sure that DashCommand will read the coolant temperature. If that code is present, then I’d go step by step through that GM procedure for P1351.

The cap is one of those shotgun parts. If you had the same issues before the ignition work, then it’s probably less likely.

 

[doncaruana]

I’m pretty sure that DashCommand will read the coolant temperature. If that code is present, then I’d go step by step through that GM procedure for P1351.

The cap is one of those shotgun parts. If you had the same issues before the ignition work, then it’s probably less likely.

Never had the code before the work on the distributor. I've been through the P1351 stuff though, never got a resolution out of it.

 

[doncaruana]

Brief, still head-scratching, update...
I replaced the fuse for the fuel pump relay circuit and that seems to have reduced the amount of times the issue occurs. I also have adopted a policy of putting the key to the on position then waiting a second for the fuel pump to energize, and then cranking. I've asked my daughter to do the same and she said she forgot and tripped it again. Not sure if that's coincidence. The fuse replace was before the strategy and it seems as if both help...maybe?

As far as the P1351 code, that only gets thrown when I end up with a long crank and it doesn't start. Otherwise it's fine. So it seems like it's definitely a symptom and not the disease. Today I had a slightly long crank but it started and ran fine. It was also 15 degrees out, so I'm not too worried about that.

The fuse I used was the spare under the hood. I ordered some replacements from RockAuto and I think I may do a few more given the age.

BTW - is there a better place to get these fuses? They are super cheap through them but they do kinda get ya on shipping. Still cheaper than Amazon or a parts store from what I've seen. And I'm only getting littelfuse or buss...

 

[exp500]

Cycling Key to ON 2-3 times before start ian't too bad of a penalty. Usually happens when fuel pump is on it's way out.
Have you considered changing Oil Pressure switch? And have you tested firewall and engine grounds to Battery?

 

[doncaruana]

Fuel pump is only about 5 years old. Haven't looked at the grounds or the oil pressure switch. I was thinking it's probably not the oil pressure switch since I can start it without the fuel pump relay in - is that a valid thought?

 

[exp500]

Long cranking time seems to set the P1351 for you. Sets on voltage Hi to ICM.(4.9 I think) Check firewall ground with a meter. Non GM coils or ICM Known to cause this. A bad ground could do it too.
Oil press switch- The long cranking time again. If you are not getting full voltage or amps to the fuel pump it could be a possibility. Have you ever done a fuel pressure test?
Have you removed the fuse relay box from fender and looked for corrosion or damage? Unplugged the computer and looked for corrosion or water?
There are many tests you can do to narrow down and eliminate possible problems for free. The manual has a very good troubleshooting section, along with the wiring diagrams to teach and show you how.