Fuel Starvation Issues + Noisy Fuel Pump

422flat6 · 07-30-2016, 02:52 PM

Cross-posted from Early S Registry... This is a 1972 Bosch MFI fuel pump. Question is at the bottom of the post.

The PO of my car had the mixture cranked up super rich. The car ran well but got very poor MPG. Plugs 1-5 were covered with dry black carbon and plug 6 was snow white. I knew something wasn't right with the MFI and set about trying to get it "right". I went through check measure adjust and set everything up correctly. The biggest issue was low fuel pressure at 2.5 psi (!) This must have been causing inadequate fuel delivery to the pump, and the overly rich mixture was compensating for this issue. After cleaning the tank strainer and verifying there was no debris in the bottom of the tank, replacing all fuel lines (supply lines were fossilized and kinked), replacing the fuel filter and replacing the overflow valve I got pressure up to 10 psi and the car ran better. I had a shop with an exhaust gas analyzer adjust the pump mixtures and the car runs great now SOMETIMES with the exception of intermittent stumbling and jerking.

There still seems to be a fuel delivery problem. When the engine gets enough gas it runs great, but it doesn't always get enough gas. The shop recommended relocating the pump to the front since mine is still in the stock 1972 location on the left rear of the torsion tube. I've read all about the benefits of relocating the pump here and on old Pelican threads. I also know Fuel Injection Corp rebuilds these pumps and supposedly does a good job.

While it would be nice to have a freshly rebuilt fuel pump, I don't like just throwing money at a problem. Mine doesn't leak and is capable of 40 psi maximum pressure before the internal bypass valve opens. I have already sent it to Fuel Injection Corp, they bench tested it and sent it back to me saying "it's fine, it makes plenty of pressure, no need to rebuild it". Of course we know pressure isn't the only indicator of a properly function fuel pump. It's also got to deliver enough fuel under all operating conditions.

My pump is super loud and inconsistent sounding. Please listen to it in this Youtube video and tell me if this sounds normal or not. Is this the sound of a weak/dying pump or the sound of cavitation due to the rear-location of the pump? The car is cold in this video, so that should rule out vapor lock due to heat.

https://www.youtube.com/watch?v=V8klTu8myAQ

al lkosmal · 07-30-2016, 03:06 PM

I have an engine test stand and when I am running out of fuel.....i can see the fuel pressure drop and then my fuel pump starts to sound like that....basically from fuel starvation/intermittent flow.

regards,
al

Discseven · Discseven's Garage

Ben... I've replaced a few bad FPs. Some high frequency "rattle" before they go and others have not. Fact that you have had it bench tested and approved opens the door for some other aspect of the fuel system causing pump to rattle. I'm unqualified to make any suggestions in that regard so just tossing up an idea. And like Al, I've run systems dry and as last fuel is pumped... pump rattles.

tobluforu · tobluforu's Garage

Moving the pump will not fix your issues, did you replace the fuel lines from the tank?

gliding_serpent · gliding_serpent's Garage

Clean and possibly reseal tank. Replace fuel line from tank to pump. Ensure fuel pickup in pump is clean. Replace fuel filter when you are at it.

422flat6 · 09:28 AM

I've replaced all fuel hoses, fuel filter and cleaned the strainer. I'm at a loss for what I should do. The only two things I can think to do are have the pump rebuilt or relocate it to shorten the suction line. The tank appears clean looking through the sender hole.

Edit: also, the fuel pressure bounces. It's about 10psi but it's not stable.

422flat6 · 07-31-2016, 08:02 PM

Doh! Figured it out. There was a major kink in the suction hose up front where it was routed up and over the steering rack. It was at the apex of that bend up above the rack where it was really hard to see. This modern Viton lined hose I'm using isn't as flexible as the old stuff. I pulled it out and ran it straight to the hard line beneath the steering rack and all is well again.