1990 351 makes no power after 3400rpm...
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1990 351 makes no power after 3400rpm...
<!-- / icon and title --><!-- message -->The 351w in my '90 F-250HD 4x4 5spd falls on it's face at around 3400 rpm, the power drop off isn't gradual either, it just starts making more noise than horsepower. The truck is tuned up full, starts perfect, and drives smooth. No check engine light.
In the last 200 miles it's had a full tune, all new fluids, t-stat, intake temp sensor, coolent temp sensor, and the cat cut out when a new exhaust system was installed. Runs great, no smoke, or noises, just seems low power after a certain RPM point.
In the last 200 miles it's had a full tune, all new fluids, t-stat, intake temp sensor, coolent temp sensor, and the cat cut out when a new exhaust system was installed. Runs great, no smoke, or noises, just seems low power after a certain RPM point.
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Originally Posted by Kwattro
the cat cut out when a new exhaust system was installed.
Post Fiend
Joined: Oct 2003
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From: New Jersey
You're not getting enough fuel, or enough air, to properly support combustion at that RPM.
On your fuel rail, measure your fuel pressure with a mechanical gauge on the fuel rail (schraeder valve). It should be 45psi-ish.
If not, replace the fuel filter first because it's the cheapest thing to fix. Even though your's is new that doesn't mean out of coincidence a chunk of dirt in the gas tank didn't wander into it. If that does't solve it, measure the voltage at the fuel pump to make sure it's getting 13-ish volts. If not, it's a wiring problem, if it is, more than likely the fuel pump is going.
If you have 45psi on the rail go through the air box and hoses, making sure nothing's caught in them.
If neither of the above solves your problem, look at the IAC and measure the vaccum off the intake manifold.
On your fuel rail, measure your fuel pressure with a mechanical gauge on the fuel rail (schraeder valve). It should be 45psi-ish.
If not, replace the fuel filter first because it's the cheapest thing to fix. Even though your's is new that doesn't mean out of coincidence a chunk of dirt in the gas tank didn't wander into it. If that does't solve it, measure the voltage at the fuel pump to make sure it's getting 13-ish volts. If not, it's a wiring problem, if it is, more than likely the fuel pump is going.
If you have 45psi on the rail go through the air box and hoses, making sure nothing's caught in them.
If neither of the above solves your problem, look at the IAC and measure the vaccum off the intake manifold.
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Good input guys. No intake restrictions, new air box fliters, great vaccum, smooth idle and all that. Today I had the fuel pressure tested. The highest reading we could get was 35psi at the rail. Both pumps could acheive that pressure but it took a LONG time to build up after the fuel line had been purged. The front pump being the worst at building pressure. Both pumps are noisey as a son of a gun. We also rechanged the fuel filter to be on the safe side. These pumps are dying 
