Ok so actually this is on an 93 S10 4.3 CPI, however the electrical circuit for the fuel pump is almost the same.
It's my son's truck who doesn't live with me so I haven't had as much time to actually see the problem as I would like to have.
The truck starts hard sometimes. It happens much more often when cold. Once the truck starts the truck will usually start up with no problems after that. No problems with it running, once it is running. He drives it for his job and he drives a lot of miles so once it's running it's quite dependable for 5+ hour trips.
So the fact that it starts hard when cold but is fine once it's running made me think "ah ha - fuel pump relay!" This is easy right? Once the truck is started the fuel pump must be running on the oil pressure switch and that's why it only has hard start problems. Let's hook up the fuel pressure gauge and confirm this is the case.
So with my Harbor Freight Fuel Pressure Gauge (14.95 at Harbor Freight) I see that cold(ish) that the fuel pressure bounces all around during prime. Once the engine is running it holds fairly even. Yep yep - Fuel pump relay! Yep now all we need to do is figure out which relay of those mounted on the firewall is the fuel pump relay. I have looked at the wiring diagram in Haynes and it is pretty much the same as the Fiero's especially in reguards to the fuel pump.
Weird think is the fuel pressure doesn't bounce around nearly as much for prime once the truck is warmed up. OK - well that doesn't fit but that's how these things go sometimes right? idk, but it still really really looks like a fuel pump relay
Well after a bunch of times making the damn thing click and feeling relays and finding some that do click, but they actually click for other ignition related things... None of the relays on the firewall are the fuel pump relay. We can however hear it click. Hunt hunt. Its under the dash, and not accessible because you have to remove the lower black kick panel, and then also the lower dash panel to get to the relay plugs. Someone at GM doesn't like auto repair men but whatever. So finally yep this is it the fuel pump relay. Much smaller than a Fiero one but still it is it, and with the test probe yep it gets power for prime and yeah with it unplugged, the pump doesn't run for prime. Cool cool. Well I'm fairly sure that replacing this relay is it son.
So down to the parts store, and a new relay isn't that much, and lets test after it's installed. Hmm. Well the truck is warm and the problem wasn't really showing up when the truck was warmed up and the pressure flickers a bit sometimes during prime. It also seems to flicker a bit with the engine running so idk. Damn. I'm not so sure we have this thing fixed. Hooked up the test light to the fuel pump power wire and idk, it kinda looks like maybe it flickers during prime, but idlk. We couldn't get it to hard start but well the proof will be found out with driving it for the next week.
Well there is still more to this post so guess what. It didn't fix it. In fact the trouble is slowly getting worse. The hard start was getting worse and worse so he brought it over today and we looked at it again.
Yep the fuel pressure is even worse at flickering around now for prime. Hmm Hmmm Hmmmm
It's not really flickering around once the engine is started. Hmmm Hmm.
Two bad relays that both are iffy in the exact same way? idk. Hooked up a test lamp and looked for flicker. Yeah I think it's flickering but idk if it really is. Hooked up the digital/analog meter (an analog bar across the bottom of the LCD display) That is even worse at pointing to anything. The bar across the bottom is just reacting way too slow and the digital isn't any good at displaying a flicker. Hmm Hmmm Hmmm.
So
Well how about this. Let's hot wire the pump and see. So I aligatored clip wired the pump to the power distribution post (Just like a Fiero, same cover and all) and YES! no pressure flickers at all! OK. But the truck is starting better now also so its back to idk if this is really leading us to the solution or the problem is just hiding like a cockroach when the lights are on. So if the fuel pump is giving problems during prime, and the relay isn't the problem, hmm - well it could be the socket, but with the socket inside it seems unlikely the socket is as corroded as a Fiero's socket might be. Anyways I didn't want to start re-tearing apart the dash on a maybe hunch that might lead us to the problem but might also just be a lession in making assumptions. So anyways I hot wired the pump to his fog light switch for testing. (much easier than tearing apart the bottom pieces of the dash) He will turn on the fog light switch to prime the pump for morning starts to see if that solves the problem. If so then it's time to dig into the dash wiring to see what is really up with the relay circuit not supplying power like it is supposed to. Who knows what we will find. I told him this was NOT the fix but only for testing. I hate it when stuff is rigged to kinda work.
So anyways the question - anyone else have any ideas? Anyone have experence with S10 relays not making supplying a sustaned voltage for prime? Any other ideas? The wiring is exactly like a Fiero's re - either the ecm controlled relay or the oil pressure switch will run the pump.
At the connector at the left front fenderwell is where I measured the voltage of the pump. It is impossible to get to the pump connections without dropping the tank. The fenderwell is the connector closest to the tank on the fuel pump circuit. I can see both the relay supplied voltage and the oil pressure switch supplied voltage there. It is also where i have the temporary hot wire connected into fuel pump circuit.
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http://reddevilriver.com[This message has been edited by phonedawgz (edited 09-30-2011).]
