98 mustang issue

[Texasmustang]

A buddy of mine has a 98 4.6 mustang auto trans. We have been trying to figure out why when it is cold it is fine. But when it warms up it falls on it's face. I mean at half throttle or wot it hesitates and stumbles bad. Let it cool back down and fine til warms up. Any Ideas what it could be. I am thinking the coils.

[shumpertdavid]

likely not the coils, could be a fuel pump getting weak, but it's hard to be certain with that description. That could be a whole list of things. If it hasn't had a tune up lately I'd go with fresh plugs oem motorcraft or autolite's will be fine, good set of plug wires, fuel filter, and clean the mass sensor. See where that get's you and go from there.

[Texasmustang]

All that was just done a few months ago. Along with a tps crank sensor new egr.

[JimD]

CEL on?

[Texasmustang]

It came on once. I forgot what the code was. We changed number 2 plug and it idle fine again. Drove it then got warm and bam on it's face again. It only does it in drive. Checked the vac. Lines. All look good

[Texasmustang]

Ok we have changed plugs,wires,fuel pump,coils, what else should we be looking at or for.

[Texasmustang]

Could it be the idle air control valve.

[davbrucas]

Prior to it reaching normal operating temp the car is in closed loop and the PCM is using feedback mainly from O2/ECT sensors to maintain the AFR. After the car warms up, it goes into open loop and uses the fuel maps set in the tune to maintain AFR...this is a simplified explanation but you get the idea. My guess is the car is unable to maintain a proper AFR in open loop. Start by cleaning/replacing the MAF...word of advice, use MAF cleaner and not carb or TB cleaner.
The car really needs to be datalogged to see what is going on. It can get expensive just guessing and replacing parts.
Best advice, drive that POS off a cliff....prior to going into open loop of course!

[98sohc281]

A buddy of mine has a 98 4.6 mustang auto trans. We have been trying to figure out why when it is cold it is fine. But when it warms up it falls on it's face. I mean at half throttle or wot it hesitates and stumbles bad. Let it cool back down and fine til warms up. Any Ideas what it could be. I am thinking the coils.

I had the same problem with my 98 it ended up being a bad maf. Good luck

[Texasmustang]

That's what I was thinking. I am gonna try to clean his maf and clean the idle air control valve. See which one works. And go from there.we are keeping our fingers crossed it's one or the other. This is getting expensive and old. Glad it isn't my mustang. Lol

[davbrucas]

The IAC helps to control the idle. If it was bad, it would be bad in open and closed loop. But it never hurts to rule things out.

[turbos66coupe]

Prior to it reaching normal operating temp the car is in closed loop and the PCM is using feedback mainly from O2/ECT sensors to maintain the AFR. After the car warms up, it goes into open loop and uses the fuel maps set in the tune to maintain AFR...this is a simplified explanation but you get the idea. My guess is the car is unable to maintain a proper AFR in open loop. Start by cleaning/replacing the MAF...word of advice, use MAF cleaner and not carb or TB cleaner.
The car really needs to be datalogged to see what is going on. It can get expensive just guessing and replacing parts.
Best advice, drive that POS off a cliff....prior to going into open loop of course!

You have this backwards. When cold the car is in open loop using tables. When warm and o2s at temp it goes into closed loop and monitors o2s, iat, maf, etc for proper fuel enrichment. Possible dead o2 sensor. Even if light isn't on you need to pull codes. Check pending, koeo and koer codes.

[davbrucas]

You have this backwards. When cold the car is in open loop using tables. When warm and o2s at temp it goes into closed loop and monitors o2s, iat, maf, etc for proper fuel enrichment. Possible dead o2 sensor. Even if light isn't on you need to pull codes. Check pending, koeo and koer codes.

Yup...not sure how I got that backwards. It was late I guess...but until the O2s get heated the car will be in open loop and doesnt use certain sensors to maintain AFR.

[juiceweezl]

Check the air intake sensor too. If that think is bad, it could just think it's really cold out and dump a ton of fuel. Mine fell out one time and it was running so rich. I replaced the grommet holding it in place and was good to go.

[Texasmustang]

Well now it won't let u do anything but idle. Barely. You can slowly give it gas but if you give it gas fast that's when it does it. Worse at wot. Idk what to look for as mine is a 302 carbed. Never messed with a 4.6. But I am bout to give up. Lol