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Fuel pressure drop with increased throttle by fierobear
Started on: 06-26-2020 01:32 PM
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Last post by: fierobear on 06-29-2020 08:01 PM
fierobear
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Report this Post06-26-2020 01:32 PM Click Here to See the Profile for fierobearClick Here to Email fierobearSend a Private Message to fierobearEdit/Delete MessageReply w/QuoteDirect Link to This Post
This is on a Formula pushrod 3.4. The engine swap was done years ago, and ran perfectly for years. The car sat for about 8 years, and I'm trying to revive it. I replaced the fuel pump, cleaned the injectors, new battery. Did the factory manual tests on the evaporative canister. Starts, runs and idles fine. After revving the engine, and/or driving it up and down the block, the fuel pressure drops from 34 lbs to less than 20 lbs, and if you keep advancing the throttle it drops more and will stall. I suspect a bad new fuel pump, or some kind of vapor lock. I really need some suggestions. Thanks.
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Report this Post06-26-2020 01:56 PM Click Here to See the Profile for RaydarClick Here to Email RaydarSend a Private Message to RaydarEdit/Delete MessageReply w/QuoteDirect Link to This Post
Check to make sure your tank is not creating a vacuum, that the pump is having to work against. The easiest way is to loosen the gas cap, and see if the problem is still there.
If it goes away, start troubleshooting the vent lines from the tank.
Try unplugging and capping the vacuum line to the FPR. With no vacuum, it should show increased pressure. Or at least no major reduction.
Also try blocking off the return line. (I've seen some "dull" plastic pliers that are used for just that purpose. Clamps the rubber line without pinching or cutting anything.)

[This message has been edited by Raydar (edited 06-26-2020).]

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Blacktree
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Report this Post06-26-2020 02:12 PM Click Here to See the Profile for BlacktreeClick Here to visit Blacktree's HomePageClick Here to Email BlacktreeSend a Private Message to BlacktreeEdit/Delete MessageReply w/QuoteDirect Link to This Post
Regarding the fuel pressure regulator test, unplug the vacuum line from the FPR while the engine is idling. That will trick the FPR into thinking the engine is at wide-open throttle. Fuel pressure should increase by about 8-10 PSI. If it doesn't, the FPR is probably faulty.
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Report this Post06-26-2020 03:17 PM Click Here to See the Profile for fierobearClick Here to Email fierobearSend a Private Message to fierobearEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Raydar:

Check to make sure your tank is not creating a vacuum, that the pump is having to work against. The easiest way is to loosen the gas cap, and see if the problem is still there.
If it goes away, start troubleshooting the vent lines from the tank.
Try unplugging and capping the vacuum line to the FPR. With no vacuum, it should show increased pressure. Or at least no major reduction.
Also try blocking off the return line. (I've seen some "dull" plastic pliers that are used for just that purpose. Clamps the rubber line without pinching or cutting anything.)



I ran some of these tests. I loosened the fuel cap, and there wasn't a change. But the car is on the lift, so it isn't under load. It had fluctuations in fuel pressure, but I wasn't able to replicate the problem of the pressure dropping way off.

Before I take the car off the lift and run it under load, I wanted to talk about the Fuel Vapor Tank, which is located under and forward of the battery. Is there any possible issue that can cause problems with this tank?

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fierobear
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Report this Post06-26-2020 03:19 PM Click Here to See the Profile for fierobearClick Here to Email fierobearSend a Private Message to fierobearEdit/Delete MessageReply w/QuoteDirect Link to This Post

fierobear

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quote
Originally posted by Blacktree:

Regarding the fuel pressure regulator test, unplug the vacuum line from the FPR while the engine is idling. That will trick the FPR into thinking the engine is at wide-open throttle. Fuel pressure should increase by about 8-10 PSI. If it doesn't, the FPR is probably faulty.


I was able to pop the vacuum line off while running, but there wasn't room to cap off the vacuum line. The pressure did go up, but I'm wondering if that could happen due to now having a vacuum leak?
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Report this Post06-26-2020 04:59 PM Click Here to See the Profile for fierobearClick Here to Email fierobearSend a Private Message to fierobearEdit/Delete MessageReply w/QuoteDirect Link to This Post
Started the car, got it warm and run up up and down the block. Same problem, fuel pressure drops way down, drops more the more throttle you give. During the warm up, revving the engine, it was sputtering and backfiring. When running under power, it pings.
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Gall757
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Report this Post06-26-2020 05:53 PM Click Here to See the Profile for Gall757Send a Private Message to Gall757Edit/Delete MessageReply w/QuoteDirect Link to This Post
I think we are all assuming you changed the fuel filter, but you did not include it when listing fixes.
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Report this Post06-26-2020 06:19 PM Click Here to See the Profile for fierobearClick Here to Email fierobearSend a Private Message to fierobearEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Gall757:

I think we are all assuming you changed the fuel filter, but you did not include it when listing fixes.


Yes, sorry, fuel filter was changed when doing the fuel pump.

I used a cheap fuel pump from O’Reilly. Has anyone had issues with new fuel pumps going bad or malfunctioning?
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Report this Post06-26-2020 06:24 PM Click Here to See the Profile for fierofoolClick Here to visit fierofool's HomePageClick Here to Email fierofoolSend a Private Message to fierofoolEdit/Delete MessageReply w/QuoteDirect Link to This Post
Optimally, fuel pressure should be above 40 psi at idle. The stock regulator is set at 43.5 psi and that's what it should provide when the fuel rail is primed without the engine running. After the engine starts, ideally, the pressure shouldn't drop below 38 psi. With it sitting as long as it has, there's probably a bunch of sediment in the tank. We lost a set of injectors after a car had set for about 4 years, and even a fresh fuel filter didn't prevent the superfine particles from passing through and clogging the inlet to some of the injectors.

[This message has been edited by fierofool (edited 06-26-2020).]

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Report this Post06-26-2020 08:35 PM Click Here to See the Profile for wftbClick Here to Email wftbSend a Private Message to wftbEdit/Delete MessageReply w/QuoteDirect Link to This Post
Did you put in a new rubber line that goes from the the pump to the line that runs to the injector rail hardline? they look good untill pressure builds up and then they leak and bleed off pressure.

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fierobear
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Report this Post06-26-2020 09:51 PM Click Here to See the Profile for fierobearClick Here to Email fierobearSend a Private Message to fierobearEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by fierofool:

Optimally, fuel pressure should be above 40 psi at idle. The stock regulator is set at 43.5 psi and that's what it should provide when the fuel rail is primed without the engine running. After the engine starts, ideally, the pressure shouldn't drop below 38 psi. With it sitting as long as it has, there's probably a bunch of sediment in the tank. We lost a set of injectors after a car had set for about 4 years, and even a fresh fuel filter didn't prevent the superfine particles from passing through and clogging the inlet to some of the injectors.



I drained all the old fuel when i changed the pump, and made sure the tank was clean.
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Report this Post06-26-2020 09:53 PM Click Here to See the Profile for fierobearClick Here to Email fierobearSend a Private Message to fierobearEdit/Delete MessageReply w/QuoteDirect Link to This Post

fierobear

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quote
Originally posted by wftb:

Did you put in a new rubber line that goes from the the pump to the line that runs to the injector rail hardline? they look good untill pressure builds up and then they leak and bleed off pressure.



I replaced all of the rubber lines on the back of the tank with new parts, including the two curved hoses that go to the gas fill.
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Report this Post06-27-2020 03:08 AM Click Here to See the Profile for fierogt28Click Here to Email fierogt28Send a Private Message to fierogt28Edit/Delete MessageReply w/QuoteDirect Link to This Post
I think buying a 'cheap' fuel pump wasn't a good idea.

For the 88 Formula, get an AC Delco EP378 which is the stock fuel pump.

You can always connect a mechanical fuel pump gauge to the fuel Schrader valve to find out the fuel PSI pressure.
Should be between 40-46psi.

I'd clue its the fuel pump.

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Patrick
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Report this Post06-27-2020 03:45 AM Click Here to See the Profile for PatrickClick Here to Email PatrickSend a Private Message to PatrickEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by fierobear:

I replaced all of the rubber lines on the back of the tank with new parts, including the two curved hoses that go to the gas fill.


I'm not sure you actually answered the question, as I believe wftb was referring to the short piece of submersible hose in the tank.

 
quote
Originally posted by wftb:

Did you put in a new rubber line that goes from the pump to the line that runs to the injector rail hardline? they look good untill pressure builds up and then they leak and bleed off pressure.

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Report this Post06-27-2020 08:16 AM Click Here to See the Profile for fierofoolClick Here to visit fierofool's HomePageClick Here to Email fierofoolSend a Private Message to fierofoolEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by fierobear:


I drained all the old fuel when i changed the pump, and made sure the tank was clean.


We did, too. Even took a water hose and washed the tank out. It was horrible inside. The water coming out after rinsing was nice and clean.

Installed a new pump and screen and removed the pulsator due to bad seals in it. Not only did we have to replace the injectors, but the new Bosch pump failed, too. MikeMac then rinsed it with a commercial wash.

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Report this Post06-27-2020 01:30 PM Click Here to See the Profile for fierobearClick Here to Email fierobearSend a Private Message to fierobearEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Patrick:

I'm not sure you actually answered the question, as I believe wftb was referring to the short piece of submersible hose in the tank.

[QUOTE]Originally posted by wftb:

Did you put in a new rubber line that goes from the pump to the line that runs to the injector rail hardline? they look good untill pressure builds up and then they leak and bleed off pressure.

[/QUOTE]

There is no rubber hose inside the fuel tank, at least not this one. The pump mounts directly to the pulsator, which mounts directly to the hard metal line.

FYI, this is all on a brand new Fiero Store sending unit setup.
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fierobear
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Report this Post06-27-2020 01:32 PM Click Here to See the Profile for fierobearClick Here to Email fierobearSend a Private Message to fierobearEdit/Delete MessageReply w/QuoteDirect Link to This Post

fierobear

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quote
Originally posted by fierofool:


We did, too. Even took a water hose and washed the tank out. It was horrible inside. The water coming out after rinsing was nice and clean.

Installed a new pump and screen and removed the pulsator due to bad seals in it. Not only did we have to replace the injectors, but the new Bosch pump failed, too. MikeMac then rinsed it with a commercial wash.


I plan to order the AC Delco pump, at worst I'll have one on hand for the next reclamation project. I’ll inspect the tank for contamination and corrosion when i change the pump.
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Report this Post06-27-2020 03:36 PM Click Here to See the Profile for PatrickClick Here to Email PatrickSend a Private Message to PatrickEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by fierobear:

There is no rubber hose inside the fuel tank, at least not this one. The pump mounts directly to the pulsator, which mounts directly to the hard metal line.

FYI, this is all on a brand new Fiero Store sending unit setup.



Ah, you still have a pulsator with the new setup. That's where most Fieros these days now have a short piece of hose installed instead... hopefully submersible hose.

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Report this Post06-27-2020 04:22 PM Click Here to See the Profile for fierobearClick Here to Email fierobearSend a Private Message to fierobearEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Patrick:

Ah, you still have a pulsator with the new setup. That's where most Fieros these days now have a short piece of hose installed instead... hopefully submersible hose.


I figured it might be a year specific thing. 88 is definitely pulsator.
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Report this Post06-27-2020 05:47 PM Click Here to See the Profile for PatrickClick Here to Email PatrickSend a Private Message to PatrickEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by fierobear:

I figured it might be a year specific thing. 88 is definitely pulsator.


Actually, it's a fuel pump specific thing. The earlier years originally had pulsators installed as well. I forget the name of the style of pump from the factory, but a pulsator was required to reduce/eliminate pulses in the fuel pressure. The newer style fuel pumps don't produce these same pulses in fuel pressure, therefore pulsators have mostly become redundant. That's what I've read here over the years anyway.
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Report this Post06-27-2020 06:31 PM Click Here to See the Profile for fierosoundClick Here to visit fierosound's HomePageClick Here to Email fierosoundSend a Private Message to fierosoundEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Patrick:

Actually, it's a fuel pump specific thing. The earlier years originally had pulsators installed as well. I forget the name of the style of pump from the factory, but a pulsator was required to reduce/eliminate pulses in the fuel pressure. The newer style fuel pumps don't produce these same pulses in fuel pressure, therefore pulsators have mostly become redundant. That's what I've read here over the years anyway.



They changed from a roller vane design to a gerotor design.
https://www.onallcylinders....t-fuel-pump-designs/




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[This message has been edited by fierosound (edited 06-27-2020).]

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Report this Post06-27-2020 07:06 PM Click Here to See the Profile for fierobearClick Here to Email fierobearSend a Private Message to fierobearEdit/Delete MessageReply w/QuoteDirect Link to This Post
Great info guys, thanks.
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Report this Post06-28-2020 04:48 PM Click Here to See the Profile for fierobearClick Here to Email fierobearSend a Private Message to fierobearEdit/Delete MessageReply w/QuoteDirect Link to This Post
Matt Meyer dropped by, and he noticed something that i missed. The vacuum line running from the evaporatorative canister to the throttle body wasn’t connected to the boot on the throttle body. He said that will keep the canister from opening to the gas tank, and will cause a vapor lock. I haven’t connected it yet and done a test run. Probably get on that tomorrow.

Sometimes, its the little stuff you miss...
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Report this Post06-28-2020 08:03 PM Click Here to See the Profile for PatrickClick Here to Email PatrickSend a Private Message to PatrickEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by fierobear:

He said that will keep the canister from opening to the gas tank, and will cause a vapor lock.


I think Raydar's suggestion of temporarily loosening the gas cap while the engine was running would've eliminated that potential issue, but we'll see!
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Report this Post06-29-2020 01:51 AM Click Here to See the Profile for fierobearClick Here to Email fierobearSend a Private Message to fierobearEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Patrick:

I think Raydar's suggestion of temporarily loosening the gas cap while the engine was running would've eliminated that potential issue, but we'll see!


Good point! I did mention that to Matt. Ill run it past him.
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Report this Post06-29-2020 03:02 PM Click Here to See the Profile for fierobearClick Here to Email fierobearSend a Private Message to fierobearEdit/Delete MessageReply w/QuoteDirect Link to This Post
Here is the problem...the vacuum line not working causes a vapor lock situation in the fuel lines, not the tank. That’s why loosening the gas cap didn’t make a difference. I have the Rodney Dickman stainless steel lines, and the bend was off just enough to keep the line from doing its job under vacuum.
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Report this Post06-29-2020 07:19 PM Click Here to See the Profile for PatrickClick Here to Email PatrickSend a Private Message to PatrickEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by fierobear:

...the vacuum line not working causes a vapor lock situation in the fuel lines, not the tank.


Interesting. So it's okay now?
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Report this Post06-29-2020 08:01 PM Click Here to See the Profile for fierobearClick Here to Email fierobearSend a Private Message to fierobearEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Patrick:

Interesting. So it's okay now?


Well, no.i put it all back together. Now, instead of it dropping out and sputtering randomly, it has a regular surge and drop pattern under hard acceleration. I didn’t have the fuel pressure gauge on it. Ill rerun that test tomorrow.
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