New fuel pump results in boost gain

Night Wing

Registered User
Please excuse the length of this posting but perhaps it can help some others. About two weeks ago, my 89 5 speed SC (that was running great) literally overnight developed what I term a "sputter" (felt like the ignition system breaking down under load) as soon as I went into boost at any rpm level effectively eliminating any power.

Beginning with the most common causes sited on this board, I replaced the plugs with the recommended Motorcraft type and gap, also wires, and fuel filter. No change at all, still sputtering. Even though vacuum was normal, I checked for vac or boost leaks and I couldn't find any. I had recently put new cam and crank sensors in so didn't consider them as likely culprits. My fuel pressure was normal, however since the fuel pump is one of the only original parts still on the car with about 200,000 miles on it and on borrowed time anyway, I thought best to change it. Still the sputter remained. Hearing that DIS modules seem to go out frequently in some cases, I replaced it with a new one with still no change. In spite of hearing that coil packs rarely go out, my friend had a spare one lying around and with nothing to loose we swapped it with mine. Bingo, the sputter was gone and in fact the motor seemed to rev much smoother.

Ever since installing many bolt ons listed on my member page (including a 10% overdrive SC pulley) and blowing the head gaskets after overheating it the first day I drove it, my motor was completely torn down, rebuilt, including having the heads redone with new stock size valves and some porting and polishing, my boost level dropped from 15 psi (before the head gaskets blew) to a reduced max of 11 psi (compared to 12 psi when stock). In spite of carefully going over all IC and blower sealing points, I was unable to find any leaks. In spite of lowered boost, the car ran faster than ever. I just began to think that the head work allowed better breathing and enhanced flow that translated into lowered boost pressure (restriction) readings.

After eliminating the sputtering problem by installing the new coil pack and replacing my old fuel pump with a new Walbrough 255 LPH forced induction unit (still have the original 30 lb/hr fuel injectors that will be upgraded to 42 lb/hr) I took the car out for the first real test drive last night. I immediately noticed that the car felt much smoother and very strong. The real surprise was when I hit it hard and noticed the boost gauge now maxing out at about 14 psi instead of the previous 11 psi. Instead of just chirping the tires when hitting second gear hard, I can feel the tires actually break loose for an instant. It is really pulling now. Apparently, I was fuel starved at high boost levels and didn't realize it. The fuel pump is the last thing I would have thought about causing lowered boost and wonder if anyone else has encountered this? It is hard to believe I picked up about 3 lbs of boost by installing a new higher flow fuel pump. This makes me wonder if I'll gain anymore when upgrading the injectors.

Bill Sturm aka Night Wing
 
I bought my first 89 XR7 ..do to fuel starvation ...start run die ..start run die ....noticed very little fuel pressure on the fuel rail...disconnected the fuel filter ..had lots of gas pressure from the pump ...so it was a glogged filter ...put a new one on and drove it home ...not looking back ...they had spent alot trying to figure out what was wrong with it ...dave
 
Dave,

That was the funny thing about this. I didn't have any problem what so ever with the engine killing or starting, and it even ran great until the "sputtering" at boost began two weeks ago. It still started fine and ran without ever killing even with the "sputter" problem that turned out to be the coil pack. Actually, changing the fuel filter was the very first thing I did with no improvement in the "sputter". Apparently the fuel volume produced by my original pump was enough to give good performance but inadequate to feed it at near max boost levels after having the heads rebuilt with some porting and polishing done. I didn't think it was enough to effect flow that much but it was the only variable changed before the difference showed up (boost immediately dropping from 15 psi to 11 psi). Any other theories from some of the "SC Mentors" on here would be greatly appreciated.

Thanks,
Bill Sturm aka Night Wing
 
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