[Rakkasan21]

Hello I have a 1998 Mustang Cobra. I recently had a brand new engine from put in after the original was blown. Before the old engine was blown I had no problems with it whatsoever. The new engine ran fine for a couple of weeks so I had the ac recharged. The same day the ac was recharged I heard a hissing noise on the highway followed by an immediate noticable loss of power. After the hissing stopped it felt like the power came back. Now it runs fine for about the first 45 min and then it starts sputtering. I press the gas and the rpm will either drop all the way until i release it or it will raise but the car barely increses in speed. It will also shut off after I try to go after stopping at a red light. My father described it as "it feels like when the sparks plugs are missing." Can this be the case? Wouldn't it would feel like that as soon as I started the car? Any ideas of what is wrong? Could the ac being recharged cause some kind of vaccuum leak? I would really appreciate it thanks.

[96GtGoodz]

First what kind of engine is it and what happens when you start it up?

[96BlackSVT]

May sound stupid, but have you let it warm up just setting? Then checking around the engine for loose or missing exhaust bolts? Any clicking noises when you try give it gas? Etc.

I have joined up fairly recent, but have many mustangs and work on all of them. Also, need more details as stated above. Who did the engine swap ? A mech or a buddy? ETC.. It would help a bunch.

[C MY 4D]

Any codes? Post some pics of the engine bay...... I'm thinking maybe vacuum leak with the hissing sound...

[r3dn3ck]

major vacuum leak. wamba predicts PCV hose got a big leaky.

[Rakkasan21]

It's a 4.6 Cobra Engine. A mechanic put the engine in. He used to work for ford. It starts up fine no problem no noises. It drives fine for about 45 minutes and then it will start sputtering and ill have a loss of power. Somtimes it only does this when im at a stop light or a stop sign where it will sputter and move slowly and it will either shut up and ill have trouble starting it back up again or ill press the gas and the rpms will just drop like its going to shut off but as soon as i release the gas the rpm goes back up to normal.

[mdvaldosta]

Originally Posted by r3dn3ck major vacuum leak. wamba predicts PCV hose got a big leaky.

Good chance of that. Definately something to check.

[C MY 4D]

check the hose from the pcv to the intake.... it looks like it isn't all the way on in the pic.

[WilltheThrill]

i dont know what to tell u but i hope u get it worked out soon. that shit sucks. im having trouble with my cobra too. it runs fine for about 8-10 mins. and then it starts acting like its not getting enough electrical power. then eventually it cuts off completely.

[96GtGoodz]

ok well first as stated above check the hose it doesnt seem right secondly test ur plugs wires and coils its hard to diagnose online oh also i dont think so but might as well give it a try check injectors

[96GtGoodz]

where did u get the engine and how long ago was it put in and how many miles this will help

[C MY 4D]

So did it work?

[stanggirl]

The hissing was the Cobra part of him wanting speed! How fast were you going?! Just a joke.

[Rakkasan21]

The Engine came directly from ford brand new with zero miles. It was put in around the end of March and it currently has around 3500 miles on it although the problems started earlier around 1500 miles.
Update: Ok so the clutch went bad so its stuck at my job and havn't had time to try anything yet but as soon as i install the new clutch which should be here Friday I'll try everything suggested and post if anything changed thanks for all the input though I will get back to you guys.

[cobramac]

Originally Posted by 96GtGoodz ok well first as stated above check the hose it doesnt seem right secondly test ur plugs wires and coils its hard to diagnose online oh also i dont think so but might as well give it a try check injectors

this might sound a little strange, but I had a bike that had the same kind of trouble your discribing. here is what the problem was. first my vacuum line was almost all the way off (that was my hissing noise and part of the loss of power) also my petcock was almost in the off position. for those of you who dont know that is the valve that allows fuel to flow from the gas tank to the motor. so you might want to do what 96GTGoodz and check out the fuel injectors and some other fuel stuff like the filter and lines.

[r3dn3ck]

propane and a small vacuum line to aim it is what I use to find vacuum leaks. As the propane gets sucked into the leak it richens the mix back up and smooths out the idle. Once you have the location narrowed down, don't let anything go unchecked. If it's vacuum that's gunna find it.

I can't help but wonder if you just need to disconnect the battery for 10 minutes to clear the adaptive learning. It may have had (past tense) a problem that you didn't notice and the computer would have adapted as much as possible to it. Now that you've got a properly running engine, it may just need a sanity check. Long shot but could happen.

[Rakkasan21]

Clear! *bbzzzzz* revive!
Ok well ive finally been able to try everything suggested and it still has the same prob but now if i take off slow it seems fine but if i really get on the gas it will sputter a bit and take off slowly then the power sort of comes back...could it be a fuel pump prob?

[6banger96]

i would try the fuel filter. i used to have to have that problem with my old car and the filter fixed it.

[B Mad]

Originally Posted by 6banger96 i would try the fuel filter. i used to have to have that problem with my old car and the filter fixed it.

+1 It sounds like a fuel problem, or vacuum

[Trick Tuners]

Do you have the check engine light on? It could easily be the fuel pump on the way out. Does it sound funny at all?