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2003 F-150 Harley Davidson with Pullies, Diablo Tune, and ported stock supercharger.
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Discussion Starter · #1 · (Edited)
So I have a 2003 f150, ported supercharger, pullies, tune. Truck ran awesome for a while on around 15-16 lbs of boost. Ever since about 2 months ago, it started not letting me go about 3-4,000 RPMs. The engine started stuttering like crazy. The boost gauge twitches like crazy, but it does seem accurate since the engine is not running right. At idle and all the way up to 3,000 it runs great.
Through testing, we found out that if I rev it at idle really quick up to almost red line, it revs up fine because it is not under a load. However, on its way back down, it stutters really bad. It seems like it is missing like crazy and just does not want to run. It shakes like crazy and barely still runs.
I started off thinking it was a vacuum issue, the main line that goes around the back of the engine to the bottom of the intake. I took off the whole supercharger and saw that that line was fine. While the supercharger was off, I checked pretty much all other vacuum lines on the engine. Engine has only about 40,000 miles at most. Everything on the engine is stock other than what is it the beginning and it still has EGR and all original vacuum lines. While the supercharger was off, found out the whole intake and intercooler was full of oil. We cleaned the intercooler and intake as well as installed a catch can on passenger side. Also note that my boost gauge indicates 19 inches of vacuum.
Next we thought it was ignition. Removed and replaced spark plugs with NGKs. Did not solve the issue. My neighbor has the same truck that runs great so I borrowed his COPs to see if that was the issue and it was not. Even the power balance test said that cylinders 1,2,3 and 4 had negative indicators.
Just today, I checked fuel pressure thinking that was the issue. It was not. At idle, about 30 lbs. While revving it up, got it up to about 60 lbs.
I even tried three different tunes. The custom tune i had ran great until recently like I said. I don't think will go bad just outta no where. I put the stock tune back on to see if that was the issue, and it wasn't. Even tried the can 93 pulley tune that came with the Diablo predator and that did not change anything in terms of the stutter.
I just don't know what else it could be. Any help would be greatly appreciated. I know this is a lot but thanks.
 

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Registered
2003 F-150 Harley Davidson with Pullies, Diablo Tune, and ported stock supercharger.
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2 Posts
Discussion Starter · #2 ·
Well it turned out to be the catalytic converters in the exhaust. Unbolted the exhaust from the headers and drove it and it had all the power like it should. Its a little bit of a pain to unbolt the exhaust and to hang it. Burnt a little bit of the protectant underneath but everything works fine. Put in some high flow cats and the trucks back to a 100%. Old cats were blocked pretty bad. On the passenger side, the front #1 cat's platinum inside was completely dismantled and blocking a lot of air.
 
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