jawknee5

Howdy folks. Ill try to make it short with as much info as possible.

So I've got a 1998 mustang GT. Mostly stock- just magnaflow cat back exhaust, 3.73s, some suspension and a KN filter.

Its a really strong motor, even though the engines at around 110k miles. Always well taken care of.

Ive been running into a problem when it warms up. Basically what happens is ill drive it for 20 minutes or more on the freeway, and when i stop at a stop light- the engine stutters and gets a weird lope to it. It never gets worse than that though. Never dies, though it seems like i would like to lol. When it cools down and i restart it, it doesnt do it, until it gets warm again. It happens with the car in neutral with the clutch out and when it's pressed.

I replaced the IAC just to make sure seeing that it probably needed to be replaced anyway. Plugs and wires have maybe 10k or less on them. Motorcraft wires and plugs. O2 sensors have been replaced recently after they were throwing codes and causing problems.

Im wondering if maybe this could be a problem with the coil packs? I read one forum that mentioned once the engine gets hot the spark may not be strong enough to keep up? Any truth to this?

Anyway, im just trying to get an idea of what it could be. Any pointers to possible sensors or anything to look for would be great. Im in the process getting it in shape now that im working again so any suggestions would be great.

Also read that maybe it could be that my cats are old?

jawknee5

Anyone?

jawknee5

Maybe I'll try again...

I've changed a few things but it's still doing it.

So far I've changed my tps, my iac, coil packs, Pcv valve, I've checked spark plugs and made sure they were decent and gapped correctly, I've cleaned my throttle body, sea foamed through intake and adde some in 2 tanks premium fuel. After the seafoamed I rechecked my plugs to make sure they were still okay.

While it does in fact run better as a whole, when it warms up really well it gets this nasty stutter and feels like its misfiring, but doesn't seem to actually be misfiring(?). No codes are being thrown no check engine light. Tested my injectors with an ohm meter and they all were in spec.

Maybe something that has to do with temperature? Maybe maf? I've cleaned it but maybe it's beyond cleaning now? Why would it work before it's nice and warm? Wouldn't it fail at all temps?

Please help, I'm getting desperate...

Lackey'sGT

Seems odd to me I would have thought it would have been your COP's, what brand IAC did you buy. A lot of people have had problems with even brand new off brands. The motorcraft one is defiantly the way to go.

When you changed the cops how were all the clips on them? Did they all clip right in or were any lose, this is a long shot but people have had problems with the connectors that connect to the COP's.

jawknee5

All of my COPs are good, i double checked just to make sure. Nice and tight, locks down well.

The IAC is a BWD. Before i bought it i read around and saw that the brand is decent and reputable enough for me to get it. Seeing as how the motorcraft was 150 i was really hesitant to drop that much on it not knowing if it was the problem. Maybe im an idiot?

I just went out and checked some things out. I let it get up to running temperature and took off the plug for the IAC- No change at all. From what ive read it is supposed to drop in idle pretty far?

Im lost on what to check for next. Its driving me crazy- It seems like it idles fine up until it gets hot then it does what it does. Yesterday i drove about 15 miles on side streets and when i got to my destination it was just stuttering and shaking like crazy. In the meantime while waiting or my girlfriend to arrive i popped my hood and started disconnecting injectors to see if it would mask the problem and single out a cylinder but it didnt lead me to any conclusions...

AHHHHHHHHH. I hate cars.

VTX1800N1

Unplugged, the IAC should cause the engine to die, or nearly die. That will cut off just about all air at idle.

I'm assuming you've checked it again for codes and there are none?

Also check your EGR valve. If it's clogged/sticking, it can cause problems like you are experiencing.

BMWrider

Hey Jawknee5, I also have a 98 Gt (78k orig miles), mostly stock with OEM exhaust. You mentioned your cats might be bad- does it pass an emission test ? You sound like me ...... do your own wrenching, change some parts, spend big bucks, and stiill a prob !
I hate to say to take it to a reputable shop or even a "stealership" but they can diagnosis better with their 'puters hooked up to yours !! Sometimes a sensor can be bad and not throw any codes. Good luck and keep us posted.

jawknee5

I ended up going to pick your part and grabbed a used motor craft iac. It acts completely different. The non motor craft iac, while cold would have my Rpms up to 2k until I drove it and warmed it up.

Did it fix the problem? No. Still doing it.

I don't take it to dealerships anymore. A while back I was having a cooling system problem, excess pressure in my heater hose. They pointed right at head gaskets. Yet it passed a block test and a compression test with flying colors.

I've taken it to a shop for this problem, but it never presents itself when I'm in front of a mechanic.

I'll look into an egr valve, but I'm starting to get frustrated enough to sell it off before it blows up completely. Because if I don't find out what it is , it could go south quickly.

I don't understand, it runs great. Really strong, clean oil, and passes smog fine.

jawknee5

Bumpity bump bump

tadpole1969

check maf clean it if that doesnt solve it buy new one to rule it out