[Curtis Creel]

I picked up a wrecked 99 recently. It was a good deal and it turned over and started, ran fine so I had to get it. It's been sitting about a month or 2 while I sourced parts to fix the front end. I start and let it run once a week or so for a little while to keep everything flowing, move it so it doesnt kill grass etc. Last time it ran for about 5 seconds then cut off. Long story short the truck has the platinum FASS on the rail behind the cab and it wasn't running. The relay for it is powered off the battery and the relay coil gets 12v from ECM supply to the stock LP. There's no 12v coming out of the ECM while engine cranking or anything so it sucked the system dry and i played hell priming the system and bleeding the HP side of the system. I got it running jumping the relay, using hand pump to prime the FASS all the way up to the HPP then cracked injector lines loose to bleed the HP side and after about 30 minutes it was running then the dryrotted blue hoses that came with the FASS back to the fuel filler started spewing fuel but luckily i had enough 3/8" hose to fix it.

So main question, I've heard you can bypass the ECM LP driver circuit by supplying 12v off the ignition switch RUN position to the fass relay. Has anyone noticed any downsides to this?

 

[bigfish95971]

The ECM does not run lift pump(or relay in your case) while the starter is actually cranking.

Sounds like you have an air leak in to a fuel line, either a crack, or fitting, commonly the fitting on back of the head in the return line is loose.
Dying after starting can also be a bad Crank shaft sensor, look for tach not showing during cranking or erratic afterward.
Check lift pump pressure when running. If you bump the starter and release key does the lift pump run for several seconds then?
Does the wait to start light come on immediately with the key?
Pull codes with scan tool.

 

[Curtis Creel]

I've verified with a test light under no circumstance does the ecm supply power. It should be a 50/50 on off during cranking then steady after while running or like you said even if I bump the starter. Since supplying a constant hot to the pump with key on it'll run with zero issues. I can drive the truck like normal. I have yet to pull codes but I had one instance while cranking the tach didn't move so I was suspecting crank sensor but I've seen these run fine with tach not working and a bad crank sensor, cel on etc. Also my wait to start light does come on it just usually takes about 5 top 10 seconds after key on for it to illuminate

 

[Stormin08]

Your ecm is toast. The WTS light should flash instantly...even when hot.

Key crank does not supply lift pump power currently ?

You can bypass ecm for luft pump power...but if it is pumping while cranking, you cause other hard start issues


Look into the ecm and alternator AC sine first.

 

[Curtis Creel]

Look into the ecm and alternator AC sine first.

what exactly do you mean here?

 

[Blue Diamond Construction]

what exactly do you mean here?

AC signal i believe. have your alternator tested to make sure it is working properly

 

[Curtis Creel]

I have seen issues where the alternator signal cause rfi up the cross over power wire causing shifting issues with the pcm. I'm going to route my wiring to fix that when I put the front end back together. It's worth a shot. at 306k I'm thinking ecm internal also