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1996 F-350 7.5L gas engine won't accelerate. Idles good. No check engine light
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Brentburek
New User
Nov 6, 2016, 8:46 AM
Post #1 of 2
(2437 views)
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1996 F-350 7.5L gas engine won't accelerate. Idles good. No check engine light
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Truck 223
1996 F-350 Uhaul (dually) 7.5L 460 efi
188,000 miles
This truck has a recently installed rebuilt motor (2weeks ago), new distributor, new belts, hoses, starter, new plugs and wires.
We were driving on the hi-way, under load, up hill.
All of a sudden, Immediate power loss and had to pull over.
Engine temp was normal up to this point.
Engine began to heat up while pulling over.
Got hot but not all the way to red.
Removed catalytic converter
Let it sit and cool.
Starts ok and idles smooth. Obviously loud from no cat. Sluggish in neutral. Put it in drive, barely rolls forward. Reverse same. Nothing, No power. Engine sounds loud as we push on the throttle like it's trying to go, but no power.
No check engine light coming on.
In neutral it's Slow to get engine speed up. Maybe 8-10 seconds pinned, and still not sounding like its at max rpm.
(I don't have an rpm gauge so going off of sound)
We drove it to the nearest town (being pushed from behind by a Prius:) on the downhill we reached speeds of up to 45 mph!
Changed the fuel filter. Made no difference.
Put a fuel pressure test gauge on it. Seemed like a lack of fuel volume. Gauge was in the spec pressure range (of 30psi to 45psi) hitting and resting at @38-39psi but took a long time to get up to it. So we thought low volume.
(although later we realized we may have tightened the pressure gauge too tight on the fuel rail shraeder valve which may have pinched the O-ring inside the fitting and restricted the flow. Maybe.)
We decided to change out the fuel pump which was located in the 40 gallon fuel tank in the rear.
After a day of messing with that we finally got the new pump in !! Yay.
Fired it up--- cold-ish start. Drove it out of the parking lot. thought it was better at first, until half a block later.... It was the same thing. again. No power when accelerating.
It almost seems like it gets worse as the engine warms up. It also seems like it had a little bit less power when you push the throttle more. ..it bogs down. Especially noticeable when driving. This time we hit a max of 11mph
Disconnected fuel lines near the filter. Turned on the ignition and it seems to have good pressure. Turned the engine over and had good visible pressure. Disconnected the return line. Started the engine and it too also had good visible pressure. Emptied out the fuel filter a second time just to see if we could see any debris but everything looked clean.
So I think we can almost rule out the fuel system at the rear. Seems like everything's working ok (as far as I can tell)
--also seems like pressure is ok at the rail. (Other than the couple times we hooked it up, seemed like a long time to build-- so possible low volume) but putting on the gauge a little looser seemed to fix that...We think :)
We popped the distributor cap with the engine running. Looks fine. Nice white/blue sparks all around. Again, engine idles smooth so it doesn't seem like a misfire issue. And I don't think it's the timing because it was working fine before, and the whole distributor assembly doesn't turn so that tells us it didn't move from where it was set at when they put the rebuilt in.
So..
So we thought maybe...
Sensors?
We changed out the MAP sensor and the Throttle Position Sensor, which didn't help anything. Didn't mess with the o2 sensor yet. Or any others. This model apparently doesn't have the MAF (mass airflow sensor)
We put the Catalytic converter back on, as we were getting sick of the obnoxiously loud engine noise.
Still checking the fuel pressure and seems to be ok.
I should mention that at this point, in neutral, the truck is starting to run worse. Seems like a lack of fuel to me. Harder to get it to start. And having to hold the gas pedal down just to keep it going.
After a minute you can let off the pedal and it will idle on its own for a while but will eventually shut off.
Also Shuts off (stalls) under load, especially when you take your foot off the gas.
The problem seems to get progressively worse as it gets warmer, except for on a colder start, where it runs ok for the first few seconds.
We Don't know what to try next...perhaps,
Fuel rail clogged? Seems unlikely.
Injectors? Doesn't seem like it would be one or two injectors because it's not running that rough. And it was fine before
Ignition?
Maybe an o2 sensor
Other sensor or computer
Vacuum lines?
Hmm
I thought initially it could be transmission, but since it seems to act sluggish in neutral as well, it seems unlikely imo.
No engine lights coming on and I believe this one is OBD 1.
We're waiting to get a diagnosis on Monday. Just thought i could try a few things before then. Any suggestions would be greatly appreciated!!
Cheers mates !!
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kev2
Veteran
Nov 6, 2016, 12:57 PM
Post #2 of 2
(2428 views)
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Re: 1996 F-350 7.5L gas engine won't accelerate. Idles good. No check engine light
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1996 should be OBDII - look at radiator support decal OR how many pins in ALDL connector - google OBD2 and OBD1 for diagram to compare.
Always scan for codes even if light is out.
Check the fuel pressure SPEC's I see 39psi as LOW - do a second look.
Some quick looks
>> Engine oil - pressure ok?
>> Coolant - any evidence of head gasket issue? oil in coolant - excess fuel in oil?
>>Remove gas cap - is there a rushing air sound?
>> Intake restriction- air filter back to TB.
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