Fuel Question

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dave s

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I'm still battling an occasional no start issue. I haven't put this 94 5.0 truck on the road yet and am still tinkering with things but this no start is puzzling to me. So far I've replaced the fuel pump, Delphi unit, fuel pump relay, rebuilt the tbi and installed the cfm technologies fitting and fuel pressure gauge to monitor that. Also fuel filter twice. Just to make sure. Also replaced tps and set it according to instructions I got on here using my meter. had to elongate the mounting holes to get it correct. Anyways, it all worked fine. Haven't put ten miles on it since November. Around March it wouldn't start. No spark, replaced module, good to go. Then the other day I go up, no start. I was busy and tried it again a couple days later, no start again. Yesterday morning I was going to do the tests on coil and module as per instructions I got on here and it started right up. Anyways I shut it down and it seems like it's sluggish trying to get going again when you try to start it. Cranks fast but doesn't fire right away. I am curious about the fuel pressure sequence. I turn on key, fuel pressure goes up to 10 1/2 or so then drops to zero. I didn't have anyone to crank it to check but what does fuel pressure do when you start to crank? It should go back up, am I correct? Can someone explain the fuel pressure sequence from key on to crank to start? The original problem was the tps sent a signal the motor was flooded and cut fuel pressure. I could give it a shot of starting fluid and it would run just fine. I did wiggle all the wires under the hood prior to trying to start it yesterday so that made me wonder if I have a wire bad somewhere. Right now, this dang truck is so unpredictable that I'm afraid to put it on the road. My son in law got stranded six hours out in Idaho once with this no start, then got it towed and ignition system replaced. He went out hunting the next weekend again and same dang issue! Lol. Anyways if anyone has the fuel pressure sequence I'd be grateful for that info.
 

PlayingWithTBI

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Anyways if anyone has the fuel pressure sequence I'd be grateful for that info.
I'll try -
When you turn the key on the fuel pump will "prime" for 2 seconds then shut off. TBIs don't hold pressure like Vortecs do so, it'll bleed down almost immediately.

When you start cranking the ECM sees ignition pulses and starts giving it fuel. If the fuel pump relay doesn't work, the oil pressure switch on the back of the engine will turn on the fuel pump.

Some trucks came with a Hot Fuel Module that will turn on the fuel pump (when engine is hot) for ~20 seconds.

Here's a schematic for an 88. It should be close to what yours is.
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You s/b able to find yours here. HTH
https://www.gmt400.com/threads/88-95-service-manuals.43575/
 

kenh

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When you start cranking the ECM sees ignition pulses and starts giving it fuel. If the fuel pump relay doesn't work, the oil pressure switch on the back of the engine will turn on the fuel pump.

I'll be checking my fuel pump relay. (90 C1500 with 5.7) The engine always starts but it has to crank for "a few seconds" before it fires. Like it has to build some oil pressure first????? Been like this since I bought it so thought it was normal.

Thanks for that heads up!

Ken
 

dave s

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Reviving an old thread. Over a year ago I finally bought both ICM and new AC Delco coil and installed. Seemed to correct my no start problem. Still haven't licensed the truck but I start it a couple times a week in the nice months. Sometimes daily and use it around the farm.

Wouldn't start again the other day. No spark. I had saved to favorites the nice GM tech troubleshooting steps someone had provided me the link to and ran through them all and it all pointed to bad ICM, again, number 3.

While checking the pickup coil, I was getting a reading of 904, steady, but wiggling the wires it did seem like it changed a few times though it could have been the leads just not maintaining good contact as I was holding both in place with my thumb.

So I put it at No 1 and lined up the timing marks and yanked out the distributor. Bearing seemed tight with no wobble so I was just going to replace the p/u coil but after looking online, I bought a complete distributor.

Put new one in and hooked everything up trying to gauge from my reference marks where everything had been. Unplugged brown wire (sticker on rad support said it was on firewall but google said inside under glove box), which it was, nice easy access.

Truck started right up, it was 2 degrees atdc so I adjusted it to zero and snugged things back up. I may tinker with that a bit at a later date. Anyways plugged brown wire back in and went for a ride. Man, all I can say is Wow! Either something was really wrong with that distributor or the timing was off this whole time. (Shame on me for never checking it before) Because it runs so much better. Not smoother or anything but will pull a hill now and accelerates so much better. More like I remember it when new.


I just figured it was lazy due to it's age and mileage and I hadn't driven it for 14 years. So it's running great, I'm still not sure what took out the new module but I'm hoping a new distributor will solve the continued issue I was having with ICM's dying.

I'm also keeping the original distributor and going to put a new p/u coil in and get that new AC Delco ICM checked or replaced and have a spare distributor on hand. That had to be the easiest distributor R&R I ever did.

Lastly I can't thank everyone enough for all the information provided on this site because it only took me a couple minutes to login here and find my favorites where I had saved that troubleshooting info that helped me go through the steps to figure stuff out.
 

Schurkey

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TBI distributors have a known problem with the magnet integrated into the pickup coil assembly. Cracked magnets are epidemic.

Failed magnet = erratic signal to the ignition module. Could be easily mis-diagnosed as a failed module.

 

dave s

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TBI distributors have a known problem with the magnet integrated into the pickup coil assembly. Cracked magnets are epidemic.

Failed magnet = erratic signal to the ignition module. Could be easily mis-diagnosed as a failed module.

I will pull that thing apart later this weekend. I was away all week for work, just got home. I'll let you know.
 
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