Welcome to the Corvette Forums at the Corvette Action Center!

not holding fuel pressure

Joined
Oct 30, 2001
Messages
2,273
Location
Glen Burnie, MD, USA
Corvette
1986 Bright Red Coupe
This morning I was following the thread about slow starting and checked my fuel pressure (car cranks a number of times before startin). After sitting overnight, I had about 3 PSI. Started it, let it run a bit, read 36 psi (yes, a tad low for 86). Turned the engine off and back to "on" (but not running) and watched the pressure visibly decrease to about 10 psi within minutes. I'm certain that after an hour it would be back to 2 or 3.

O-rings on the injectors? I may as well have the injectors out and cleaned or blueprinted or replaced or something. Anything else to look at?

I think an adjustable pressure regulator might be useful as well.

Yes, I knew I was inheriting someone else's problems when I bought this :(.
 

vigman

Motor head!!!!
Joined
Feb 13, 2001
Messages
3,471
Location
Valencia, CA,USA
Corvette
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
It's a leaker

1st replace regulator
( easier cheaper)

Then observe

I'll bet leaky injetors

Mike
 

vigman

Motor head!!!!
Joined
Feb 13, 2001
Messages
3,471
Location
Valencia, CA,USA
Corvette
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
Try this

Have somebody spin the car over ( after sitting for 20 min or so)

You get by the tail pipes

Do ya small gas on the inital crank & start.


Mike
 

vigman

Motor head!!!!
Joined
Feb 13, 2001
Messages
3,471
Location
Valencia, CA,USA
Corvette
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
The O rings

Are to keep fuel in the rails( top)
and to keep Air out and fuel from leaking
(bottom)

Unless you smell gas under the hood ( a very bad thing)

You have leaky injectors, or a tear in the regulator.

You could do a test to find which one is the leaker.. but by the time you do all the testing
you could have been done with the whole job.

Mike
 
Joined
Oct 30, 2001
Messages
2,273
Location
Glen Burnie, MD, USA
Corvette
1986 Bright Red Coupe
I guess I'm a bit confused about what can leak on an injector. All they are essentially is a pointy tube with a moveable pin stuck down through the end, right? If the pin can't close the gap completely, I guess they could leak that way. That would also completely mess up the spray pattern too, right?
[RICHR]
 

vigman

Motor head!!!!
Joined
Feb 13, 2001
Messages
3,471
Location
Valencia, CA,USA
Corvette
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
Different setups

However
Think ball bearing into a tapered V
As the ball goes up & down it makes a wear pattern
If there is grit in the fuel you get little scratches in the sealing area.

Drip Drip Drip

36 lbs is a fair amount of pressure. And yes it will mess up the spray,
but sometimes thats from excess goop on the head of the injector.

Wait till you pull yours out and look at the spray end of things ( Yeech )

Mike

It's not a perfect world
 

bradfordsvettes

Well-known member
Joined
Mar 28, 2001
Messages
549
Location
Boston, MA area
Corvette
jewel blue 61.cranberry 69 t-top.black rose 92
fuel pressure

the in tank fuel pump also has a check valve i would check this out before chasing injectors the regulator is more likley to leak external into the vacum hose than internal seen more bad pumps than injectors my .02 steve :w
 
Joined
Oct 30, 2001
Messages
2,273
Location
Glen Burnie, MD, USA
Corvette
1986 Bright Red Coupe
Re: fuel pressure

bradfordsvettes said:
the in tank fuel pump also has a check valve i would check this out before chasing injectors the regulator is more likley to leak external into the vacum hose than internal seen more bad pumps than injectors my .02 steve :w

So what's the best way to test the fuel pump check valve? I think this is an in-tank pump? I seem to recall something about taking out the filler neck to get access.
[RICHR]
 

vigman

Motor head!!!!
Joined
Feb 13, 2001
Messages
3,471
Location
Valencia, CA,USA
Corvette
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
Da Pump

Drive the car until ALMOST empty
Lift gas hatch, remove gas lid assy, remove rubber surround around filler neck,
Remove 3( I think ) hoses & 1 electrical connector. 10 (or so) bolts hold the filler neck top to the gas tank top. Remove pump ( which takes a certian finness fo lifting & twisting CAREFULLY).

Replace pump & pickup sock, reassemble.

Mike
 

69MyWay

Well-known member
Joined
Jul 3, 2001
Messages
4,364
Location
Auburndale, Florida
Corvette
1969 Killer Shark
You can also pull the vacuum line off the regulator while the car is sitting at idle. If you see the pressure go up a tad, that is a good thing. This test will simulate WOT, wide open throttle. If no change in pressure, consider the regulator to be bad. This is not the end all and be all, but it sounds like you already have a pretty good handle on testing pressure.


Go see your G.M. parts dealer for the GOOD stuff when it comes to cleaning the injectors (in the event you conclude the regulator is working). Run several tanks through and run a pressure cleaning on the fuel rail. Injectors are rather durable and if it is just some varnish it ought to clean right up.

The regulator in the tank is more of a check valve that keeps the fuel from backing up the wrong way. It can fail, but don't think it would result in your hard start problem.

You can get a stock replacment regulator for around $25 at the local auto part store. Here is the trick. It will only come as a new diaphram, spring, and maybe screws. Your stock screws that go around the outside of the regulator housing are torx bit with a little tit sticking up in the center. This prevents you from using a regular ordinary torx driver. You will need to get a set of torx that have the hollow tip. Take you time here as they strip easy. The good news is that if all else fails you can use a small pair of vice grips to clamp and unscrew them.

Oh, unfortunately, you do have to pull the upper portion of the intake off. Plan on a full staturday afternoon if you have never done this before. If you are slow and careful you can reuse the old upper plenum gaskets. You can leave the runners in place.

If you buy an aftermarket adjustable regulator it will come with a new housing as it has been modified with a threaded portion and bolt stud that you twist to tighten the spring. Those cost as much as $80.

Good luck.
 
Joined
Oct 30, 2001
Messages
2,273
Location
Glen Burnie, MD, USA
Corvette
1986 Bright Red Coupe
Was planning on a TPIS adjustable regulator anyway; sounds like I should live with the harder starts until I buy it.

Wondering if any of this may be a factor in the high-RPM knocking/pinging I get. Low fuel pressure would certainly cause the engine to run lean and ping. I'm using 93, btw, in an engine that was (according to the owner's manual) designed for 87 - and I'm still getting pinging.

TPIS's AFR is about $90 and they have a higher-flow GM fuel pump for about the same.

Already ran a couple cans of Top Engine Cleaner through the system directly at the throttle body. No help, though the bit that went down through the MAF did a *really* good job of cleaning the screens - until I removed them :)

Thanks...
[RICHR]
 

bradfordsvettes

Well-known member
Joined
Mar 28, 2001
Messages
549
Location
Boston, MA area
Corvette
jewel blue 61.cranberry 69 t-top.black rose 92
i f the check valve in the pump leaks it takes excessive time for it to build pressure then rapid fuel pessure drop after key cycle . to check the injectors and rail components jump the fuel pump to run then use crimping pliers and block the return line then the feed then stop fuel pump. if pressure maintains rail and components. are ok and problem is with pump. does the fuel pressure come right up to spec with the 2 second fuel pump primedoes it drop very rapidly?:crazy :( :w
 

vigman

Motor head!!!!
Joined
Feb 13, 2001
Messages
3,471
Location
Valencia, CA,USA
Corvette
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
Rrubel

Are you throwing any codes....like 44?

Try the unplug the MAF sensor trick.

Drive it and see if it still pings, the MAF was my problem BIG time, not bad enough to throw a code but bad enough to ping under high acceleration.. and the hotter it was the worse it got!

Mike
 

bradfordsvettes

Well-known member
Joined
Mar 28, 2001
Messages
549
Location
Boston, MA area
Corvette
jewel blue 61.cranberry 69 t-top.black rose 92
sounds like you may have more than i problem going on feel free to e-mail me if you want
 

vigman

Motor head!!!!
Joined
Feb 13, 2001
Messages
3,471
Location
Valencia, CA,USA
Corvette
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
Just Do It!

And start doing 1 thing at a time......

Start with the things that are on the maint log
and work your way back, do you have any reciepts from the previous owner?

Pull the pump out.. inspect the sock..If it cruddy then it's a clue to the condition of the pump.

It's hard to advise via the Oujia board, we (at best) can offer suggestions but it's you under the hood.

I do not like pinching off older fuel lines, it could cause premature failure.



Mike
 
Joined
Oct 30, 2001
Messages
2,273
Location
Glen Burnie, MD, USA
Corvette
1986 Bright Red Coupe
Ok, not in any order:

1. Unplugged the MAF the other day and drove about 20 miles at various speeds. Still pinging.

2. No codes being thrown (except when I disconnect the MAF :)

3. Pressure does NOT rise above 3-4 PSI until I start the car. Then it rises to about 35-36 and stays there until I shut the car off (it drops a bit when revving the engine; haven't been able to test it at WOT under load because of local weather conditions). After I shut the car off, it visibly drops to about 10 within seconds.

4. I have some receipts from the previous owner, but they're mostly for tuneups and tires and the like. I think he also did the head gasket, and there's definitely non-OEM plug wires (but they look pretty thick; may be a set of performance ones). Have new plugs (Bosch +4's) , will be replacing them tomorrow.

5. One of my mechanic friends suggested the blocking of the return line, but said to disconnect it and plug it rather than crimp it. May try that.

When I had the car inspected prior to purchase, I was warned about the long crank time - that particular mechanic knows Vettes well, but is somewhat expensive and also very busy. And I want to do some of this work myself anyway.

I do realize it's hard to diagnose issues long distance (my dad and his computer problems come to mind... "Son, my modem disconnects for no reason. What's wrong?") and I appreciate the time spent and suggestions offered.

My goal is to get the car into peak shape by late spring for a May or June track day so I do have a couple months.

[RICHR]
 

vigman

Motor head!!!!
Joined
Feb 13, 2001
Messages
3,471
Location
Valencia, CA,USA
Corvette
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
Still pinging

How many miles on the car?

Is your TPS sensor set to .54 volts @idle

Do you have "wiggle"in the throttle linkage, grab the bit the cable hooks to and see if you have any verticle play..could be an air leak there.

How's the VAC at idle? 18 inches?? You might have a VAC leak. Do the AC or heater vents change under hard acceleration? check the connection at the cannon ball. Does cruise work?

When was the last time the fuel filter was changed?

How's the EGR?

More to chew on

Mike
 
Joined
Oct 30, 2001
Messages
2,273
Location
Glen Burnie, MD, USA
Corvette
1986 Bright Red Coupe
Re: Still pinging

vigman said:
How many miles on the car?

132000 and change.

Is your TPS sensor set to .54 volts @idle

I am having difficulty getting a measurement from the TPS. Can't feed the leads of my VOM into the harness.

Do you have "wiggle"in the throttle linkage, grab the bit the cable hooks to and see if you have any verticle play..could be an air leak there.

Seems solid.

How's the VAC at idle? 18 inches?? You might have a VAC leak. Do the AC or heater vents change under hard acceleration? check the connection at the cannon ball. Does cruise work?

Cruise works, no vent change, vacuum from the intake manifold is 20 inches. I did find a cracked vacuum line (think it was cracked before I started playing; it's totally broken now :( ) between the intake manifold and some T-connector by the firewall on the driver's side.

When was the last time the fuel filter was changed?

Today... along with plugs. And whoever designed the rear cylinder on the passenger side was a total sadist.

How's the EGR?


I can't FIND my EGR valve and solenoid. According to my Haynes, it should be pretty much under the distributor (86 L98). I have nothing there except a weird rusty-metal pipe leading from the top of the block under the intake down to an exhaust pipe, with a sensor in it. See attached picture. Warning - BIG picture, to show detail.



Side note - after replacing the fuel filter, the start seems better. Started on the first crank a couple times while out running errands; we'll see how it starts tomorrow.

Also - more food... was out testing the knock and found that it's not always related to RPM. In second, if I stomp on it at 2k RPM, I get power and then massive knocking. If I gently accelerate to ~4500 RPM, I get lighter knocking.

Also, after replacing the fuel filter, my fuel pump is a LOT noisier.

[RICHR]
 

bradfordsvettes

Well-known member
Joined
Mar 28, 2001
Messages
549
Location
Boston, MA area
Corvette
jewel blue 61.cranberry 69 t-top.black rose 92
still think it is a fuel pump problem do your tests and narrow it down i have seen it more than once at the shop not only corvettes all gm cars steve best of luck
 

Corvette Forums

Not a member of the Corvette Action Center?  Join now!  It's free!

Help support the Corvette Action Center!

Win Both Corvettes!

Win Both Corvettes and Get Extra Bonus Tickets Now!

Supporting Vendors

Dealers:

MacMulkin Chevrolet - The Second Largest Corvette Dealer in the Country!

Parts/Accessories:

Dead Center Foundation

Vetteskins

Advertise with the Corvette Action Center!

Partners

Top Bottom