My 93 5.0 Liter was having some over-rich and stumbling problems lately. After a bit of poking around I found out the static fuel pressure was extremely low - 20psi. I adjusted upwards to the 48psi recommendation that the manufacturer of my regulator suggests. I didn't notice this at first because the vacuum regulated pressure was normal. Since adjusting upward, all problems have ceased.
My question is this, how would low static fuel pressure cause an over-rich situation? Shouldn't it, if anything, cause a lean condition?
I don't think it's using oil. I've never noticed, and it only has circa 80k on the motor. But, I'm due for a oil change, so I'll be sure to check carefully.
I checked the plugs today - These are new plugs, about 350 miles old -- and they're starting to get the black goo on them that raised my concern two weeks ago. The fouling is uniform across all spark plugs.
I pulled codes the old fashioned way (CEL) and I think (did I blink or did the light?) I'm getting an IAC code. I pulled the IAC off, and it looks clean. I shot some carb cleaner at it, but I might replace it anyway. As far as I know it's the original.
As it stands now, I haven't had any hesitation problems - probably due to the new plugs - but it backfires and idles a bit high and rough. The backfires are at startup and sound close to the engine. It's a series of quiet bumps rather than pops. At idle it'll subtly backfire every 5 seconds or so, the idle will go to about 1400 and it'll run okay. Then, when it drops back to sub-1000, the backfires will return. If I turn it off then start it up again quickly, the backfires will be much worse for a few seconds, then return to the every 5 seconds rate.
The only thing I've done to the car -- pre hesitation/backfire problem -- was to change from an off-road H-pipe to the cat equipped X-pipe. I put all the EGR/Thermactor stuff back on in the swap. Post problem I put in new spark plugs, cleaned the IAC, O2 sensors, MAF and adjusted the fuel pressure.
Other than the IAC I'm at a loss as to finding a solution.
There is a condition known as wet-fouled which can be caused by oil or an overly rich air/fuel condition, stuck choke, or spark plug heat range too cold.
See #3 on the following page
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I also read somwhere that a bad fuel regulator was leaking fuel into a vacuum line...replacing the regulator cured the sputtering and rough running.
Hmm, are sure you have the correct plugs. Also, make sure the wires go to the correct plugs ! Backfiring at idle should definitely not happen. I wonder if your injectors are leaky?
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