Bkannady

Hello,

I have a 04 gt which has a bad misfire at idle. New spark plugs, coils,iac, cleaned maf, checked all vacuum lines, had an induction service, all of the basics. None of it fixed the problem. On top of the misfire, it gets really jerky when engine braking at regular speeds but it runs great while accelerating and cruising. I also get noticeably worse gas mileage than I should. Is it possible I have a bad intake manifold or intake manifold gasket? The engine doesn't drink any coolant and none is leaked out..

dawson1112

Classic indication of failing 02 sensors. Either lack of heated 02 or no switching of the 02

To be sure of what is causing the problem, it would probably be worth the 40 bucks or what ever to take it someplace where they car hook up a techII and scan the live data as the car is idling and driving.

Ive had 2 of these cars with the 4.6l engine and both of them ran like butt until I changed the 02 sensors. Usually it starts off as a random missing at idle with no CEL, gradually increasing to the point where it will run great for a bit, then suddenly start to kick and buck and then the CEL will come on. You may be just starting to see the beginnings of failure. But to know for sure it needs to be scanned.

Another thing on these cars is that can affect how they run is the alternator, low output voltages can play havoc with the PCM causing all kinds of problems.

Where are you in OHIO,, if your close enough to me to bring it Id be happy to look at it and see what I think.

I would expect a cracked intake to run bad on acceleration more than any thing , but its not beyond the realm of possibilities. The only way to know for sure is to have the live data read by a capable scanner.

Now I dont want to scare you here, and this is less likely to be concerned with , but the timing components wear out on these cars, chains stretch, guides break , any slop in the timing chain will create some drivability problems. But almost 100% of the time they rattle like crazy and it almost sounds like engine ticking/knocking.

Bkannady

Well, the car has an o/r h pipe and I have the mil eliminators which from my understanding make them read fine all the time but those are only on the downstream ones of course. The alternator should be pretty new on the car as well. Also, something which has became slightly noticeable the other day and is getting very noticeable as of today, there is a "ticking" noise which sounds like air being sucked in in coordination with the RPM's which is coming from the right side of the motor.
Im located in the Cincinnati area of Ohio

dawson1112

Sounds like you have an exhaust leak ,, that in itself can throw the entire air/fuel mix off.

A leak in the exhaust before the upstream 02 sensor will cause the car to dump exess fuel. It thinks that side is lean. I think if you track down the source of that noise you will find either a blown exhaust manifold gasket, or where the mid pipe connects to Passenger/right hand side manifold just before the upstream 02 sensor there is another gasket that goes there. Its only on the right side, as that side uses a different flat flange than the left. The left side is the old style ball flange and doesnt use a gasket. The gasket is just a thin piece of stainless that goes in there. If someone had it apart to do the O/R H pipe they could have either not gotten it tight or its come loose and needs tighten up,, or they could have left that gasket out all together and its leaking exhaust there.
\ This could explain why its bucking upon deceleration, when you let off the gas the pressure change in the pipe is causing it to suck alot of air at once and really affects the 02 reading.

The down stream sensors are nothing but a way for the PCM to compare exhaust emissions so it can detect any faults in the catalytic converters. The mill eliminators just trick it. Those sensors have nothing to do with air/fuel mix. That is all on the upstreams .

Lol Cinci it a bit far to drive Im clear up close to Toledo.

dawson1112

This is the right hand side mid pipe gasket between the mid pipe and manifold.

https://lmr.com/item/LRS-9450A/96-04...jWvhoC6Hvw_wcB

Z28KLR

^ That gasket is a likely source for a suspected leak. As mentioned, it is just a thin piece of sheet metal but the sealing surface is a compressible lead ring. It is intended to be a one time use only piece, and should be replaced any time the midpipe is disconnected. Many people mistakenly reuse it and experience a leak there.

Bkannady

I honestly never would have figured it was an exhaust leak since I guess I was focused on the intake being cracked and maybe sucking in extra air and the exhaust was done not long ago by a professional, but sure enough when I jacked the car up the top bolt on that flange was loose to where I could move it with my hand... When the guy did the exhaust, he reused the peice of pipe coming immediately off of the manifold. That gasket has never been messed with. I guess that bolt just worked it's way loose over the past 80k miles and 13 years.. Now to replace that small gasket and try to get my hands up in that tight spot.. Thanks for the assistance!

dawson1112

If they reused that section of midpipe they more than likely pulled that out in order to weld to it or cut it. If they used a sawzall to cut that pipe there is a 90% chance your upstream 02 sensors are scrap now. The vibration of the sawzall plays havoc on the internal heaters. I would bet money that if your pull the 02 sensor it rattles when you shake it.

I learned the hard way that you always pull the 02 sensors out before you start cutting on exhaust pipes close to the 02s. Also when welding any where close to them. The upstream 02s on these cars plays a vital roll in how the engine management controls air/fuel. Its a very sensitive system.

If you find you need 02 sensors let me know, I have a brand new set sitting in my tool box at the shop. Still in the box . Id sell em to you for cheap +shipping. They were originally purchased for a my Lincoln Mark VIII but they will fit your mustang. Ford/Lincoln/Mercury all use the same oxygen sensor for about 500 models lol.

Bkannady

So I have since fixed the exhaust leak and have replaced o2 sensors. Mpg has slightly improved but I still have a misfire at idle only... I'm getting pretty frustrated with the car by this point. Any other possibilities?

Z28KLR

I went through the same thing years ago - light intermittent miss at idle. Not present under load or elsewhere, no check engine light. And I went through all of the maintenance items you did as well, including turning off and disconnecting the EGR in case it was flowing excessively.

What finally cured it was closing the gap on the plugs. I went from the spec gap of .054" down to .046"-.048".