Fuel pressure

[B-Co Kid]

1987 351w, holley 570 (about to be replaced

with edelbrock 600) c6 trans, mechanical fuel pump. I've had a fuel pressure problem for some time, and I can't seem to figure it out. My fuel pressure gauge reads abnormally high around 9-10 psi. I was told It could be my liquid filled fuel gauge. Replaced it with a non-liquid filled gauge, and still reads high. I thought it could be the mechanical fuel pump, but it works fine, I think, when I press the throttle, fuel squirts in the bowls of the carb. Its not a performance fuel pump. What causes high fuel pressure with engines using a mechanical fuel pump? And on a side note, anyone considering using Holley's clear sight plugs to adjust the float level, DON'T!! I did, and the friggin plug broke off leaving it threaded in the sight hole, with no way to unthread it. Anyway, high fuel pressure. What causes it and how do I rectify it?

 

[miesk5]

yo,

B

Do you have a fuel pressure regulator installed between pump & carb?

If not, consider adding one.

One cause is a blocked or kinked fuel tank vent pipe

Double D wrote elsewhere awhile ago; "..Once I had a problem like yours, and I kept on chasing this problem for months, massive flooding problems. It turned out the Vent line from the fuel tank was plugged, the tank couldn`t breathe so it was forming air pressure in the tank and forcing the fuel out of the only outlet it had, which was the fuel line going to the carb itself. One day I drove it and it seemed okay, I put $13.00 in gas in, got it in the next day to find it sitting on empty..."

Fill & Vent Tube pics in pre 87 & 87 & up; "...Early trucks use a small corrugated plastic vent tube tucked into the top of the steel filler neck , but it often kinks & blocks fuel from entering the tank easily. Later trucks use a heavy plastic filler hose inside the steel vent neck, which allows MUCH easier filling.The necks are interchangeable, so it's a good upgrade for an old truck, especially if filling is becoming a problem. It also allows the use of a more modern gas cap..."
Source: by Steve @  



http://www.supermotors.net/registry/media/767952



87-96 Filler Neck

 

[Seabronc]

Is it causing some sort of problem?  Are the float bowls over filling?  Is fuel squirting out anywhere? 9 to 10 isn't all that high.  What does the carb manufacturer recommend for fuel pressure? How do you know the gauge is accurate? Just sounds like a good mechanical fuel pump to me.

when you press the throttle, fuel should squirt in the carb.  The float bows are controlled by the float adjustment.

>-

 

[B-Co Kid]

Seabronc, I've been trying to track down a hesitation problem that I've been having when taking off from a stop, there is slight hesitation. Also when driving at normal speed, if I let off the gas, and then get back on the gas there is also a hesitation. In addition, after driving for a while, and I cut the engine and wait a while, it takes a few tries to start back up, like it has flooded. I thought it was the accelerator pump, but it seems to be ok from what I've read on how to check the accelerator pump. I posted a video link on another forum topic that suggested holley street avengers have some inherent problems with the accelerator pump. I'm replacing the holley with an edelbrock, so I'm not concerned with holley issues, I just want to solve the elevated fuel pressure regardless of which carb I use.

Miesk, there is not a fuel pressure regulator installed. The gph rating is fairly low for that pump. However, the vent line you spoke of may be the culprit. The vent tube running from the gas tank to the engine compartment is blocked off. I'm sure it went to some of the discarded emissions stuff, but since I don't have that, I just blocked the end of the tube off. Could that cause the problems I am having with the high fuel pressure??

Miesk, I just re-read your post, and I'm not sure if we are talking about the same vent line. Looks like you are referring to something by the filler neck. The line I was referring to runs from engine bay, along passenger side frame rail, and back to gas tank. Either way, ya'lls help is appreciated!!

 

[miesk5]

yo,

Try opening up that vent tube in engine bay while testing fuel pressure.   I THINK you could roué the vent tube to intake vacuum port.

 "...The evaporative emission (EVAP) canister is a storage device for fuel vapors that are emitted by the fuel tank in hot soak conditions. When the vehicle is at normal operating conditions the vapors are purged from the EVAP canister. The EVAP canister is controlled by the exhaust gas recirculation/evaporative emission (EGR/EVAP) control solenoid. This solenoid sends vacuum to the EVAP canister purge valve (part of the EVAP canister) which allows the passage of the fuel vapors into the intake manifold where they are mixed with air and burned in the engine. By storing the fuel vapors and purging them into the engine at a later time hydrocarbon emissions are reduced, and fuel efficiency is increased.."
Source: by Ford motorcraftservice.com

 

[B-Co Kid]

I'll give that a try, thanks for your help!

 

[Seabronc]

yo,

Try opening up that vent tube in engine bay while testing fuel pressure.   I THINK you could roué the vent tube to intake vacuum port.

 "...The evaporative emission (EVAP) canister is a storage device for fuel vapors that are emitted by the fuel tank in hot soak conditions. When the vehicle is at normal operating conditions the vapors are purged from the EVAP canister. The EVAP canister is controlled by the exhaust gas recirculation/evaporative emission (EGR/EVAP) control solenoid. This solenoid sends vacuum to the EVAP canister purge valve (part of the EVAP canister) which allows the passage of the fuel vapors into the intake manifold where they are mixed with air and burned in the engine. By storing the fuel vapors and purging them into the engine at a later time hydrocarbon emissions are reduced, and fuel efficiency is increased.."

Source: by Ford motorcraftservice.com

Do not run that tube to a vacuum port!!!!!! Yo will be sucking gas through it.  Put a carbon canister back in the truck and route it to that.  the line is to allow expansion and contraction of the tank due to temperature changes, plus store vapors from the float bowl when the engine is hot and off. 

I don't see how that line could cause hesitation.  You said the accelerator pump seems OK, did you attempt to make the proper adjustment to it?  There are two adjustments.

 

[Seabronc]

Hesitation is usually always a matter of too much gas or too little gas under those conditions.  Other things could be involved but with a new carb I wouldn't immediately suspect them.

 

[Seabronc]

Fuel pressure should have no effect on it, especially at that pressure.  The float valve, if properly adjusted should not allow excess gas into the carb.

 

[Seabronc]

You need to put the proper control back on the carbon canister.  for a carbed engine it is shown in this diagram.  It does eventually get cleared but is never continuously connected to the intake.  Like I said before, connecting that line directly to the vacuum system can cause it to **** gas directly from the tank.  With the proper control it is only sucking vapors at start up.  It is either controlled by a VCV or a computer controlled solenoid.  Again, it has nothing to do with your hesitation when you are moving from a dead stop. 

Holley or whatever, you need to do the installation adjustments and if it is still hesitating then some fine tuning is in line.

>-

 

[miesk5]

yo,

As Seabronc advised the canister needs to be part of the system; I made a Big error in writing " I THINK you could roué the vent tube to intake vacuum port."

Thanks Fred!

 

[B-Co Kid]

Reinstalling the canister is probably easier said than done, as I don't have the canister, VCV, or any other emissions stuff. I got the Edelbrock carb today, so I think I will just install it go from there. I will post results. Thanks again guys!!

 

[Seabronc]

Al,

Not a problem, I didn't want BCo Kid to runoff and do that.  Besides the possibility of sucking gas through the line, with an air tight gas cap,  it will cause the tank to develop a negative pressure after the engine is run for a while and possible fuel starvation.

BCo Kid,

My first suggestion is to get the parts out of another truck and put them back  in.  If you are not going to do that, at least find a way to put a small air filter on the end of that line.  If you plug it, the tank has no way of neutralizing the pressure due to temperature changes or as fuel is drawn out.  Maybe you should also see about placing the vent line outside the engine compartment so fumes don't build up around the engine.  If I had to do that, I'd find some place out of the weather but ventilated.  Also, a possibility would be to at least get a carbon canister from another truck and hook the line to it.

Good luck with the new carb,

>-

 

[B-Co Kid]

So, I installed the edelbrock, and took it for a spin. The hesitation problem is gone! I've always been a holley guy, but I must say the throttle response is excellent on the edelbrock. The idle speed needs to be adjusted, and I will fine tune the air/fuel using vacuum gauge, other than that I'm satisfied with the carb; however, the friggin fuel pressure still reads around 9 psi. Should I look to replace the fuel pump, or install a pressure regulator??

So, I installed the edelbrock, and took it for a spin. The hesitation problem is gone! I've always been a holley guy, but I must say the throttle response is excellent on the edelbrock. The idle speed needs to be adjusted, and I will fine tune the air/fuel using vacuum gauge, other than that I'm satisfied with the carb; however, the friggin fuel pressure still reads around 9 psi. The fuel tank vent tube is unblocked and routed out of engine bay. Should I look to replace the fuel pump, or install a pressure regulator??

 

[Seabronc]

There is nothing wrong with 9PSI.

 

[nelbur]

So, I installed the edelbrock, and took it for a spin. The hesitation problem is gone! I've always been a holley guy, but I must say the throttle response is excellent on the edelbrock. The idle speed needs to be adjusted, and I will fine tune the air/fuel using vacuum gauge, other than that I'm satisfied with the carb; however, the friggin fuel pressure still reads around 9 psi. The fuel tank vent tube is unblocked and routed out of engine bay. Should I look to replace the fuel pump, or install a pressure regulator??

9 psi is not alarming.    The pressure from a manual fuel pump is caused by the strength of the return spring in the fuel pump.   The arm driven by the engine is used to "****" the pump and the pumping is done by the return spring.   If you have unblocked the vent, and have no electric pump in the line, that is the intended pressure.   If you have no flooding problem I would think the pressure is not a problem.

 

[B-Co Kid]

Ok, thanks to everyone for all of your help. I'll have a chance to take the B-Co out for a cruise this weekend, so I'll see how everything holds up. Thanks again!!

 

[miesk5]

yo B,

If the FP really bothers you; see what Edelbrock advises;

#1400 EDELBROCK CARBURETORS PERFORMER / THUNDER SERIES AVS 3110.2 Kb
http://www.edelbrock.com/automotive_new/misc/tech_center/install/1000/Performer_Thunder_Carburetors_130815.pdf

"Do not use more than 6.5 psi fuel pressure.  Excessive fuel pressure may cause flooding.  At IDLE, the fuel pressure must not exceed 6.5 psi.  If the vehicle has an adjustable fuel-pressure regulator, it is highly recommended to set it to 5.5 psi.  With most fuel pumps, the minimum fuel pressure is encountered at high rpm and wide-open-throttle.  Fuel pressure should not drop below 2.0 psi.  If it does, a fuel pump with more capacity may be required.  Note that some later model vehicles (the 5.0L Ford is one example) have mechanical pumps that will give more than 6.0 psi at idle.  The vehicle will perform well, but may be prone to stalls on quick turns and stops with the clutch disengaged.  If this problem occurs, check the fuel pressure.  If it is more than 6.0 psi at IDLE, it should be reduced through the use of a regulator, such as Edelbrock #1727 and #8190, or by creating a restricted by-pass bleed to the fuel return line.  Edelbrock Street Fuel Pumps are highly recommended for all Edelbrock Performer and Thunder Series AVS carburetors."

or

#1400 Performer Series Smog Legal Carburetor 119.9 Kb
http://www.edelbrock.com/automotive_new/misc/tech_center/install/1000/1400.pdf

" Fuel pressure regulator #8190 (If stock fuel pressure is greater than 6.5 psi"
 

 

[Seabronc]

That's all well and good to fix a weakness in Edelbrock carbs.  I've run with 8-9 PSI for years and that is no problem with a properly adjusted Holley float in the fuel reservoir.   Apparently the fuel reservoir  system in an edelbrock is a little on the weak side and can allow  overfilling of the reservoir.  That is the only way having 9 PSI can cause the problem described in the Edelbrock notice could happen.  The same is for Holleys if the float system isn't working or set properly.