okay. so i've been driving this 93 f-150 for 8 years. you'd think i know more about trucks by now. it has the 4.9L I-6 and a 5-speed manual tranny. almost 129k miles.

so about two weeks ago, i notice some lower gas mileage, a little rougher than normal idle, and exhaust that smells like the engine is burning rich. so i start checking stuff out. plugs, wires, distributor; they're in great shape (less than 10 mos. old and firing great). go to the autozone and get the codes read. oxygen and coolant temp sensors bad are listed as malfunctioning so i replaced those.

trucks runs great, check engine light (which had previously been coming on and off sporatically for a few mos.) lights up. so i go back and get tested again. this time i'm getting the 172 and 173 codes (obd1) for burning lean and burning rich (respectively). so now the ecm is telling me that i've conflicting problems.

so the autozone guy tells me to reset the ecm by unhooking both leads from the battery and bridging them so the circuit is completed and all the capacitors are drained. so this i do. wait about 45 mins and rehook everything up. the autozone guy tells me that it should run rough for the next little bit. which it does....very roughly in fact. it idles smooth now after it's been ran awhile. startup can still be rough. checked the codes both last night and again tonight and i get the 173, i.e., burn is rich. the 173 is in the continuous code section and the in the self-test i get a 522 (transmission is not in neutral or all accesories are not turned off, but they both are).

should i be examining timing? could fuel injectors be a problem? what about the ecm itself? or could there be any of a couple thousand of things that are going wrong on it? any help would be great.

 

Normally the O2 sensor would take care of the engine running rich. The ECM may be confused, but yes it could be a host of things; including injectors. Try a simple fuel system cleaning, a real one not in a bottle. That may help. If it's running really really rough with the ECM defaulted then I'd look to clogged/dirty or faulty injector.

 

what is your idea of a good fuel-system cleaning? those not in a bottle that is. at this point it smells like it's burning mighty rich. on another board (identical post) i had the suggestion to examine the fuel pressure regulator. an advance auto parts guy also mentioned that i should actually take out the fuel injectors as well and see how the truck runs (one at a time that is).

 

so to make matters more odd, i've got this problem. i had ruled out the spark plug/distributor cap/plug wires earlier because they were so new. so now i'm trying to figure out what's going on so i try everything.

i pull plugs and wires. wires have low resistance (less than 5 Ohms). but the plugs are fouled. very black and sooty. when i plug up the timing gun, the light blink is intermittent. i would expect it to be fairly regular. but it isn't. how do i test my timing gun? is there a chance that it's shorting out? i mean obviously this is not a good thing. i figure the only other way to test it is to use it on someone else's vehicle.

thanks

 

Get some facts before you start tearing stuff apart and throw parts at it. Do a fuel pressure test, and do a fuel pressure leakdown test. This will tell you if your fuel injectors are leaking. If they are, clean and/or replace.
Next check the MAF, be sure it is clean and working. Check the AIR DUCT from the MAF to the Throttle Body to be Sure it has no cracks or leaks.
If fuel and air is good, then go to the Ignition and PMC.

 

this is where i don't know any better than to do some of this. i also, in alot of cases don't have the tools to do it. timing is off by the way.

 

Just a thought here but by any chance you have a vacuum gauge?

I'm kinda curious what kind of vacuum it has, you might have an intake gasket blown out, cause if I'm not mistaking a 93 doesn't have a MAF sensor it has a MAP sensor which will cause it to be rich if it doesn't get the proper amout of vacuum.