Fuel pressure drop with increased throttle (Page 1/5)
fierobear JUN 26, 01:32 PM
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.
Raydar JUN 26, 01:56 PM
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).]

Blacktree JUN 26, 02:12 PM
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.
fierobear JUN 26, 03:17 PM

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?
fierobear JUN 26, 03:19 PM

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?
fierobear JUN 26, 04:59 PM
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.
Gall757 JUN 26, 05:53 PM
I think we are all assuming you changed the fuel filter, but you did not include it when listing fixes.
fierobear JUN 26, 06:19 PM

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?
fierofool JUN 26, 06:24 PM
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).]

wftb JUN 26, 08:35 PM
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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