Fuel Pressure Priming

[thelonerider]

Funny no one mentioned the VSV for Fuel Pressure Up.
http://www.cygnusx1.net/Supra/Library/TSRM/MK3/manual.aspx?Section=FI&Page=116
I also changed out my CSI as well as the temp sensor (higher value) to activate it but I still have do a 2 stage start up. I first let the car turn over for a second then turn the key off. Then when I turn the car over the second time, the car starts up instead of just turning over. If I turn it over the first time and keep it on, the engine will just keep turning without starting... If I try to start the car this way, it seems that the engine is flooded when I retry to start it after a long first attempt. Jumping the B+/FP terminals charges the injectors with pressure but the car still doesn't start on the first attempt.The AFM was changed out a while ago when the original died but this didn't change the starting situation. My next attempt is to check the VSV.
BTW, I am also running a aftermarket FPR (Magen Racing) and the car did this prior to the upgrade and continues to exhibit the same issue. I also will be swapping out the stock pump for a Walbro this summer. I notice that the pressure drops to zero as soon as I turn the key off (not the 25 seconds that the TSRM indicates as the norm.). I am assuming that the old pump is letting the fuel flow back into the tank too quickly and that this also may be the problem.

 

[dbsupra90]

Although I don't have that afpr I'm puzzled why it won't hold rest pressure. I find it difficult to believe it'd be designed with such a basic flaw but as I said I'm not familiar. That'd certainly cause hard hot starts though. If I were you I'd check with aeromotive as to why it doesn't hold pressure and also check the fuel pressure up system to see if it's working.


All that said put the new pump in and see what happens but I'd still resolve the loss of rest pressure if I were you.

it should hold pressure when the car is turned off for atleast a few min. if it doesnt the check valve in the fuel pump is going bad.

on hot starts the vsv should hold vac signal from the fpr.

when replacing the pump that problem should go away.

 

[Dirgle]

Correct, according to the TSRM the fuel system should maintain ~21 psi for at least 5 min after the engine is turned off.

 

[jetjock]

Yes and that's a minimum. It should hang on for much longer. Loss of rest pressure could be a bad pump check valve, leaky injector (including the CSI), a funky FPR, leaky fitting, etc. Just have to hunt it down.

The entire point of keeping the fuel system under pressure is to prevent vapor formation from heat soak during the first 30 minutes or so after shutdown. Since the boiling point of fuel increases with pressure it's a effective method. Same theory behind how the cooling system works.

During a hot start the VSV vents the FPR to atmospheric pressure, thus increasing fuel pressure in the rail. It does that for about 2 minutes and then switches the FPR back to the manifold so the required constant differential between fuel pressure and manifold pressure can be maintained. Without it injected fuel would vary as manifold pressure varied even though fuel pressure remained constant.

Rest pressure is more important than the FPU system because FPU can't do squat if the lines have vapor in them. In fact it becomes a hindrance. Since the FPU system normally raises fuel pressure by throttling return flow it would result in taking longer to purge any vapor. I'm splitting hairs here but you get the idea.

 

[bluepearl]

Yes and that's a minimum. It should hang on for much longer. Loss of rest pressure could be a bad pump check valve, leaky injector (including the CSI), a funky FPR, leaky fitting, etc. Just have to hunt it down.

The entire point of keeping the fuel system under pressure is to prevent vapor formation from heat soak during the first 30 minutes or so after shutdown. Since the boiling point of fuel increases with pressure it's a effective method. Same theory behind how the cooling system works.

During a hot start the VSV vents the FPR to atmospheric pressure, thus increasing fuel pressure in the rail. It does that for about 2 minutes and then switches the FPR back to the manifold so the required constant differential between fuel pressure and manifold pressure can be maintained. Without it injected fuel would vary as manifold pressure varied even though fuel pressure remained constant.

Rest pressure is more important than the FPU system because FPU can't do squat if the lines have vapor in them. In fact it becomes a hindrance. Since the FPU system normally raises fuel pressure by throttling return flow it would result in taking longer to purge any vapor. I'm splitting hairs here but you get the idea.

jetjock, do you mean 2 seconds instead of 2 minutes?

 

[jetjock]

No Sir, I meant about 2 minutes. Primary ECU inputs for determining FPU operation are coolant temp (THW) and the start signal (STA). Coolant temp must be around 100 C or higher. So sayeth the TCCS manual and experience monitoring my VSV seems to bear that out. There's some ambiguity in the documentation regarding whether 7M ECUs also factor in intake air temp (THA) but without running tests I can't say for sure. Even if THA is used I doubt it carries much weight compared to THW.

A simple test of the FPU system (assuming Cletus hasn't removed it) can be done by connecting a vacuum gage to the FPR line during a hot start and time until vacuum appears. Or if you have a fuel pressure gage just watch that. Pressure should drop and respond to manifold vacuum after no longer than a couple of minutes max. Keep in mind the coolant temp requirement mentioned above when doing this. Hope that clears things up.

 

[bluepearl]

No Sir, I meant about 2 minutes. Primary ECU inputs for determining FPU operation are coolant temp (THW) and the start signal (STA). Coolant temp must be around 100 C or higher. So sayeth the TCCS manual and experience monitoring my VSV seems to bear that out. There's some ambiguity in the documentation regarding whether 7M ECUs also factor in intake air temp (THA) but without running tests I can't say for sure. Even if THA is used I doubt it carries much weight compared to THW.

A simple test of the FPU system (assuming Cletus hasn't removed it) can be done by connecting a vacuum gage to the FPR line during a hot start and time until vacuum appears. Or if you have a fuel pressure gage just watch that. Pressure should drop and respond to manifold vacuum after no longer than a couple of minutes max. Keep in mind the coolant temp requirement mentioned above when doing this. Hope that clears things up.

thanks, jj it was the 100c temp that altered my findings. Yes, I do have a cabin pressure gauge.

 

[GrimJack]

Another bit of information to throw in here - I have an Aeromotive AFPR as well, and my fuel pressure doesn't drop when I shut it off. I've never timed it, but it's stayed pressurized for hours at least.

 

[johnathan1]

What kind of fuel pump are you running, GrimJack? And how old is it?

 

[GrimJack]

Still stock pump, the one that came with the car in '89. I have a wally here, but I haven't dropped the tank to install it yet.

 

[thelonerider]

You are right, Dirgle, the TSRM says 5 minutes of pressure after shutoff... Don't know where I got my number from... going to check the shop manual I have as it may be missing an update.
Curious, would the "J" tube have any effect on the shut off pressure with the restricted end? Has someone removed the "J" tube and still measures 5 minutes of pressure?

 

[jetjock]

Anything located after the FPR is part of the return line and not involved in holding pressure ie; the only part of the fuel system that remains pressurized is the supply side and that ends at the FPR.

 

[xanatos]

Another bit of information to throw in here - I have an Aeromotive AFPR as well, and my fuel pressure doesn't drop when I shut it off. I've never timed it, but it's stayed pressurized for hours at least.

Yeah, I'm not losing pressure either when I switch off. I keep pressure in the system for a little over an hour, but after that, I slowly drop back to 0. Now seeing as I never had a gauge on the fuel system prior to that. I always assumed that the fuel pressure system stayed pressurized longer if not constantly until the pressure is relieved or purged from the system.

 

[GrimJack]

Ok, I just went out and checked, I still have ~2 psi and I ran the car last ~7 hours ago.