my '93 4.9L started idling rough lately. the SES light has stayed off ever since i replaced the vaccum reserve can, and it also ran noticably smoother, but it still miss fires quite often and idles rough. however one time i accidentaly added alot more of a fuel additive than i was suposed to when i put some gas in it and it ran perfectly, up until i put new gas in it. so im stumped as to why it's running the way it is and not throwing a code. im assuming this is'nt really helping my gas mileage. i just replaced the spark plugs, and the wires only have 33K on them. any ideas?
thanx.

 

you have 33k miles on the plugs and wires, or you JUST replaced them?

 

just did the spark lugs, the wires have 33k.

 

did you check that you put the right spark plug wires on the right spark plugs
you could of switch two or have to backwards so just check the fireing order and if that is not it then you could check the timing.that could be it

 

Sounds like your injectors are dirty. Does the problem go away when you floor it? Have you experienced a slight stumble while cruising around 35 mph under very light throttle?

 

i defenetly got the wires right, i never floor the thing because the last time i did the throttle got stuck, but it doesnt really stumble around 35, it does stumble, however, when i have it in 2nd (manual tranny btw) and i am say, traveling in the turning lane waiting to pull out into traffic, so ive got the rev's at about maybe 2800 (i dont have a tach), going about 25mph in second. under those conditions, the thing jerks and stumbles consistantly.

 

Clean the IAC?

 

Like Pony Rider said, clean the IAC. I cleaned mine and the idle is 100% smoother. Cleaning it help increase my gas milage a little bit too.

 

Sorry, Pony Driver :o

 

but why would the IAC affect the engine at 2800 rpm?

 

ok i cleaned the IAC and now it idles a little smoother but it still startles at like 23-24mph in second and at the same rpms in 1st (didnt try 3rd) iv'e got 2.73 gears in the rear and like i said no tach, but it sound like it's doing it just above 2k rpm

 

Still sounds like dirty injectors. Try this: Buy a couple large bottles of injector cleaner, and when one of your gas tanks becomes almost empty, go to the gas station and put both bottles in the nearly empty tank, and add about 3 or 4 gallons of gas. Drive around for about 30 minutes, and then park it for the night. The next day, go for a test drive, and see if things have improved.

If they have, but not to your satisfaction, you might consider having the injectors professionally cleaned, or replace them.

Post your results.

 

i had a friend who tried that and it ruined his fuel pump, however it was a hyundai but i really dont feel like dropping my gas tank.

 

If doing that damages your fuel pump, it was already dead, but the choice is yours. You can spend more and have them cleaned professionally, and hope that fixes it.

 

does the '93 have O2 sensors? my '97 was doing the same thing... never set off a "check Engine" light... turned out to be the upstream O2 sensor....