Odd Fuel Pump Issue

[tlamot01]

Revive for help!

Reviving this thread for help on a very similar issue as the one described.

98 Yukon SLT 4x4, 135000 miles.

Originally, I though my problem had something to do with my "Security" light constantly illuminated. I've had to go through the re-learn procedures several times before reaching a stable security position. However, even during the most frustrating PassLock II issues, the fuel pump would prime each and every time I turned the key. Only lately has the condition below been increasing...
At this point the 3 second prime occurs roughly 10% of the time. The remaining 90% of starts requires hammering the starter for up to 30 seconds before the oil pressure rises enough to activate the secondary circuit. Even when the engine is hot, it sometimes fails to re-start on short stops without an extended crank. I've checked connection from the relay to the VCM and everything looks fine there, so perhaps my VCM is going out?

After the oil pressure switch takes over, the engine runs perfect with absolutely no sluggish performance; telling me that the fuel pump is working fine and the relay remains engaged, so I'm leaning towards the VCM as the culprit.

I'd appreciate any input on how to test this theory or maybe someone out there has resolved this issue by creating their own 3 second primer circuit trigger to replace the one sent from the VCM. I'm no engineer here, but I believe a delay enabled relay would be all I need to make it happen.

The diagram I have of the fuel pump system seems to consist of the mechanical circuit and the VCM circuit tied together. A single relay for the VCM method remains discharged during the initial prime and is activated only when the oil pressure switch completes the ground to the coil.
The only thing I don't understand is the RED wire labeled 490 "Fuel Pump Prime Connector" and what the "Fuel Pump Balance Relay" is.

 

[SunlitComet]

first that is my diagram screen shot.

second it does not apply to your year.

third have you looked at testing your relay continously for bad contacts.

forth ignore the balance relay.

fifth any mode of fuel pum relay control is strictly under pcm control.

sixth that means oil pressure switch has no part in it any more starting 1998.

seventh you could ignore the oil pressure theory now.

eight if you have checked t pcm to relay circuit how do you know that the relay may or may not be recieving it trigger to activate.

ninth have you done continuity checks to check for high resistance or opens on the fuel pump control and power circuits.

tenth have you check the fuel pressure in different instantances of running and not.

and eleven would you like the proper diagram.

 

[tlamot01]

Sunlit, yes, it's been a while since I got that diagram and I now recall it was from you I requested it. Thanks again and sorry for not giving you the right credit.
I think a good starting point would be to have the right diagram in hand. Also, can give me some insight on how when the primer does not function properly that I can still get the truck going after about 20 seconds of hard starter motor labor if the oil pressure switch is no longer a part of the equation.

I'm also skeptical that the fuel pump performance is suffering in any way. The truck has plenty of power even in WOT. There hasn't been any indication that the engine is fuel starved during operation.

 

[tlamot01]

This morning, same problem. My truck just cranked and cranked. I watched the oil pressure gauge very closely this time. As I expected, after the oil pressure bounced around the first two lines for about ten seconds, it jumped to about 1/3 of the gauge and the truck started up just about two seconds after. This is the reason why I still think the oil pressure has something to do with the fuel pump kicking on if the initial prime does not work. The next time I will film this and upload the video for your analysis.

 

[tlamot01]

Based on the diagram I do have and the behavior I'm trying to modify, I think an alternate solution to my problem would be to tie the starter relay to the fuel pump relay. Then as long as the truck is starting, the fuel pump is priming. Let off the starter after it kicks over and the ECU or oil pressure switch or whatever it is that is still working properly will take over. The truck is so old now that fixing it right would take too long and probably cost too much, so workarounds are always a viable option.
Sunlit, I'm still holding out for that alternate diagram. I haven't used the truck in a while so I haven't had time to upload a video.

Any inputs on this would be appreciated.

 

[SunlitComet]

What alternat diagram!

 

[tlamot01]

What alternat diagram!

Sunlit, you stated the above diagram was not for my 98 Yukon and that you'd PM me the correct one - the one that shows the PCM has full control of each state of the fuel pump relay.
Anyway, the alternate method I was considering was to jumper a diode between the control wire for the starter relay and the control wire for the fuel pump. This should energize the fuel pump relay and therefore pressurize the system while the starter is engaged. After the truck starts, the PCM should take over and keep it all running as expected - which is the plan at least...
When I get this done, I'll upload my results. I just don't have the time or energy to invest into hunting down the actual cause of this issue anymore. The truck is just too old and the quirks are piling up. Unfortunately, there is about zero interest in the market for these OBS trucks around here without taking a bath on the sale price.

 

[SunlitComet]

for the 98. fuel pump control is thru relay only and is entirely controlled thru pcm only. can you run a few tests? first is on the tps. looking for voltage out when closed and a smooth increase while opening. or that the egr is not stuck open. i am digging deeper into the system for you about it.

 

[tlamot01]

Thanks for the help SL.
Throttle Position sensor? That seems to be very responsive according to my ScanGuageII. I see immediate change in degrees (which I assume is gradient voltage increases) when I slowly press on the throttle plate.
Do you want me to back-probe the connector with a multimeter? And how do I check if the EGR is stuck open without removing it.

PS. Where is the EGR on this Vortec?

 

[SunlitComet]

nah if the tps seem fine thru scangauge then leave it. for the egr you would have to remove it but catch the gasket cause it might slip right off and fall behind the water pump or crank pulley. it is in front of manifold.