Ok i have been stumped on this for some time now and so have all my buds. Here goes, I have a 1990 Mustang GT that starts hard, idles like it has a lumpy cam, and misses whenever it feels like it. I cant explain the miss to well because it does not do it all the time....but when it does it is usually around 2000 rpms or higher that you really feel it. i took it out the other day after putting on a new fuel pressure regulator and cleaning the EGR valve and now it seems to be at lower rpms but you can feel it want to lose power when you go to higher Rs. Like it sputters a little. Also when it is cold (first starting it) i will feather the gas a little and it pops in the intake. My timing is set at 14 degrees advanced. I have changed all the plugs, wires, rotor cap, rotor button and had the coil tested at auto zone, new o2 sensors as well as a new coolant temp sensor. This has me stumped and i need help!!!!....thanks guys  

Hi. Did you already check(clean or replace) the idle air control?
Also, did you clean the mass air flow sensor? The guy that tunes my car says that he gets about one 5.0 a week that misfires simply due to dirt or oil on the MAF. The problem is fixed by cleaning it off with brake cleaner.
If you've already done these, then sorry I'm stumped with you. . .
                                         - Jenn  

Yeah it has a new IAC as well and i have cleaned the MAF. It sucks so bad i just want to get it back out again! See i scanned it and it gave me code 41 i think which was EGO not switching Bank 1 reading lean. So thats when i replaced to o2. Now my plugs when i pulled them, were dry and white so i am sure that the problem has to be because i am running lean. Thats why i replaced the regulator. it cant be the pump or it would not want to start at all so the only thing left would be the injectors. at least that what i would think. It cant be something catastrophic or it would miss all the time. Now as for the hard start problem i think it is due to having a 70mm throttle body, typhoon upper and lower yet i still have the stock 55mm MAF and stock injectors.  

hmm. . weird. I had problems where my car felt like it would sputter randomly, turned out one of the injectors needed to be cleaned. Recently my car had a huge miss especially on the freeway, and I ended up finding out it had been running way lean because the last shop to work on it had reset fuel press to factory setting, which was too low for the setup.

Steve at Powertrain Dynamics is a engineer/5.0 Mustang tuning whiz. He has helped me on numerous occasions.
He's here in CA, and busy so a little tough to get ahold of,  but if you shoot him an email or call he can probably make some suggestions. www.powertraindynamics.com

Good luck, hope you get it figured out soon, that's annoying.  

The intake and throttle body should not cause a hard start problem. If these are recent additions, you may have a vacuum leak somewhere that's causing the problems. Check to make sure the plenum and EGR spacer gaskets are not torn, and that the upper plenum is not hung up on something like the valve cover and is not sitting flush on the lower.

Check the timing with a different timing light. With 14 degrees and stock heat range plugs, your plugs will probably look white to light brown in color. If your timing is off though because of a bad light or connection, the timing could be a bit off and you could be experiencing misfire/detonation.

Injectors could be the culprit as well, especially if the car has a lot of miles on it. Also, look for the simple things like making sure that all vacuum lines are connected and in good condition (no cracks or holes) and make sure all of the sensors are properly plugged in.

Hope this helps,

Steve  

The throttle body and intake were all on there when i got it and it did not start acting up until about a month ago. The only thing i know is that i took off the throttle body to dremel out the IAC port and when i put it back on i know i overtightened the bolts that go all the way through to the EGR spacer. i know this because i see antifreeze on the gasket. Whether that can can the problem i dont know. The wierd thing is that it does it when ever it feels like it not all the time.  

Ok so i went and bought a new throttle body to EGR plate gasket and put it on right and no leaks but it still did not fix my problem. I did run the code reader on the car though and got a few codes. #91-EGO reading lean, #66 MAF voltage too low/ out of range, #41-EGO not switching always lean, and #31-EGR position sensor below minimum or EGR valve problems. So those are the codes i knew it was running lean but now i dont know what to do about it. injectors are still my number one theory with how it is running. i need oppinions.......  

The typhoon has a bad problem for poorly machined surfaces where the upper meets the lower. Ihave been a ford tech for 12 years and I specialize in mustangs and driveability concerns. If your car has a lean concern. It is a vaccuum leak that is causing your miss. It probably does it worse when its cold right. If you spray brake clean or throttle body cleaner around the mating surfaces around the intake and throttl body area and the idle changes you will find your leak and you can fix it.  

that does make sense but the only thing is i have had the car since october and it had the typhoon on it when i got it and it did not start acting up until about a month ago before then it ran really strong. now it will drive horrible at first then fix itself then horrible again. then i give it a good bit of gas and it has the power, but you can feel where the power drops off and comes back. my throttle response still sucks too. and its not really a hot or cold problem. I think that MAF sensor code might be the big one that is causing it but i am not sure.  

Do you have a scan tool that you ca monitor what is going on with the maf? If not you can use a DVOM/meter to see if the vref, vreturn, and see what the ignition on not running voltage is and then see what the voltage is at idle and see the change as you ramp up the throttle. Also if you haven't ever gone to fordfuelinjectiom.com go there and you can find many things that can help you out. You can also see what color wire are what at the maf so you can test what I explained above. Hope this helps, Rodney  

i had the same problem i canged the cap rotor plugs fuel filter just did about every thing i could think of but then i got the fuel pressure tested and that was the problem u have to have 30 lbs of pressure it will still start but it takes a while to get it goin i would have it checked out  

I put on my buddys MAF sensor and it did wonders all except for a slight miss occasionally when i romp on it a bit but then it goes away. I still have the EGR code so i was curious if anyone knew if a bad EGR position sensor could do this or wether i should go straight to getting a new EGR valve? does this sound like symptoms of a bad EGR? Maybe when i romp on it a little it causes the pintle to stick open or something making it miss. who knows...  

If you are running the original ignition box I would try your buddies to see if it cleans it up.  

I had the same problem with my 93 GT. Turns out the junk cap I bought had aluminum towers instead of brass. My rotor button was shaving aluminum off and letting the spark arc all over inside the cap. It looks like you have pepper inside your cap when the spark starts to get weak.  

I am only saying this cause I had a miss on my 91 and probably cause I am an idiot, but What I had done was put 89 octane gas in mine and I thought I had really torn something up, cause it made it run like crap, I mean it missed, sputtered, was hard to start, and a friend had suggested putting 91 octane in it and it was like magic, it just ran like a new car afterwords.  Not trying to insult you but just my experience