Hey all,

I have a 93' Mustang GT 5.0, original engine block with roughly 83k miles on it, only driven in spring/summers. I've been good about replacing oil; generally more than once every 3k miles. Recently had a full tune up (new plugs, plug cables, distributor, oil/oil filter, air filter, new rear axle fluid, etc.). Shortly before this I had a few things that needed to be fixed for inspection including: driveshaft u-joints, rear brakes, tie rods.

Anyways, I'm making a short story long. It's been running very well since the tuneup and repairs, however i've been driving it longer trips for the past few days (~60 miles a day), and i've been noticing when I start the car in the morning, the idle is slightly rough, and when I give it gas the RPM's will go up a bit, then quickly die down to almost stalling. If i try doing this while in gear, the car bucks alot and makes slight popping out of what i believe to be coming from the exaust end. After about 2 minutes of this, it seems to go away and the car drives just like normal. If i let it idle for 15 minutes it'll work things out, but still is intiially a little rough. It doesn't seem to always be just on a cold engine. If i drive it and stop, shut off the car for maybe 30 minutes, it'll do a similar thing.

I'm not a mechanic, but I was suspecting maybe I got a bad batch of gas because this started happening after i filled a fulltank one night. I've been able to use all this gas except maybe 3-4 gallons, when I put in some Techron concentrate fuel system cleaner and 12 gallons of 89 octane... (I generally run 87, I know I prolly should go higher). After about 40-50 miles of driving with the Techron in, it still has the same symptoms.

What could cause the car to buck when it's initially started for maybe 30sec-1 minute? would Leaky injectors cause this, and then go away? should I give the techron additive more time to work..

Thanks for any advice you guys have, this problem has been pissing me off for days now

 

Does the car have a MAF after the air cleaner.
If so you need to spray some carb cleaner on it. (inside)
It causes startup running peoblems.

 

I say the IAC,injectors,and valves are dirty.
You can clean the IAC and valves by using spray carb cleaner.To do this,remove the hose going into the IAC,place the spray straw from the can of carb cleaner into the hose and replace the hose so that the straw is between the hose and the IAC fitting.Hook the straw to the carb cleaner,start the engine,and let it get to operating temperature.Now the fun part,start spraying.You'll need to keep the engine running by opening the throttle a little and adjust the amount of cleaner going into the engine.This will clean the IAC,upper and lower intake,and best of all,the backs of the intake valves.These get carbon built up on them from years of short drives and can cause just the kinds of problems you are having.
You might want to get a couple of cans of cleaner to do the job.Oh and don't be alarmed about the smoke.
Cleaning the injectors is a longer process that requires you to use cleaner in the fuel over a period of time.Coupled with the longer ~60 mile drives this should work out over time unless there is a piece of dirt actually blocking the injector or the injector is just wearing out.A set of new,stock size,injectors isn't cheap but it will probably go a long way to helping your idle problems as well as giving you better gas mileage.
Going to more expensive higher octane fuel is a waste of money.It doesn't have more cleaners,it doesn't make more power,and it isn't refined any better than the gas coming out of the hose right next to it.
If this doesn't help repost and give us an update and we'll go from there.

 

start simple:

change the pcv valve first
then
clean the IAC and clean the MAF as other posters stated, this should significantly improve idle

 

Checking the engine coolant sensor would be a good idea, too. If the computer thinks the water is warmer than it really is, the resultant lean condition will cause a lot of bucking as well.

 

I agree a stuck IAC is more than likely the cause.try tapping on the IAC with hammer not hard just lightly and see if there is an effect. Cleaning it it not as easy as it sounds replacing is really easy.

 

Cleaning the IAC is very easy actually. Unplug it, unbolt it, then shoot some carb cleaner in there using a pipe cleaner. Bolt it back up and such and be on your way. Always worked for me.