Hi, I have a 1998 mustang gt and put a few upgrades on it. I have ported pi heads and cams, 24lb/hr injectors along with a 78mm throttle body and intake and pi intake manifold. I did it all myself but I broke a few things so I took it to a performance shop and had them fix the problems and tune it. I got it back in early January and since then I have been having problems with it bogging when I first get going(not even when I try to launch it) and occasionally loud pops in my exhaust. It sounds like a muffled gunshot in the middle of the right side of my exhaust. I also used to be able to lose traction before I did these upgrades by just stepping on it while going around a turn but can’t anymore(not that I try to do this on public roads). I overall feel like my low end torque is worse off then before I modified it. I don’t know what’s causing it but I have an idea. I noticed that on the right side exhaust tip I can feel a normal amount of exhaust gases coming out but on the left side I feel almost nothing. I can feel the tiniest amount. I think it is due to one of my 6 catalytic converters being clogged. Maybe more idk. Is this creating a lot of back pressure that’s screwing up my performance? Or could the tune have done it, or both? I was thinking of getting long tubes and a whole new header back exhaust system in hopes that it will alleviate all my problems. Anybody with experience with this problem and can share their ideas will be greatly appreciated.

 

Two basic things first: when is the last time you replaced your fuel filter? And not with aftermarket garbage, but with the genuine Motorcraft part? And secondly what’s the status of your coils? Are they Motorcraft or aftermarket replacements? And how many miles are on them? Have you pulled codes yet?

 

I haven’t done any of those things. My coils are stock and so is my fuel filter. Should I replace those two things?

 

Also haven’t pulled any codes and to be honest idk what that means. Like data logging?

 

“Pulling codes” means hooking up a code scanner to the ODB-II port on the car, and scanning for diagnostic trouble codes. You can also take the car to many local auto parts stores and they will let you use their code scanner or even scan for codes for you. Once you get the code or codes they may be able to tell you what they mean or you can do a web search for your year, make, model, and the code and you should get some search results telling you what it means.

So, wait, you’re saying the car has the original coils and fuel filter? Like, never replaced? How many miles are on the car?

 

Well I’m not sure about either. I bought the car for pretty cheap about a year ago and didn’t get a car fax. Changed fluids and stuff but it never had any check engine lights so I didn’t check for any codes. It has 233k on the odo but that is broken and hasn’t worked since I got it so it has a bunch of miles. And no, I never changed either so they could be original or could have been replaced. I guess I should probably do all those things.

 

Here is what I would recommend: take it to your local auto parts store, have them pull codes. I suspect there will be one or more codes related to cylinder misfire. My personal experience in two vehicles with the modular Ford 4.6L V8 is you can have a bad coil or two that only act up under certain conditions, can cause a misfire (significant stumbling, rough idle, muffled backfire), but will not trigger a check engine light. This was true of my 2001 Mustang GT (had the common cracked heater hose nipple on the intake manifold and was leaking coolant down into two spark plug wells), and my 2003 Explorer (simply had a bad coil, and it’s on its second set at 173,000 miles). If you have a misfire code I would consider replacing whatever coil or coils correspond to the misfiring cylinder, or swap coils to see if the problem follows the coil swap. That tells you the coil is faulty. If the coil is faulty, replace it. And you can get whatever brand you want, but I’ll tell you my recommendation is to get the OEM Motorcraft coils.

The fuel filter may be helpful or at the very least good PM.

How are your spark plugs?

 

My spark plugs should be good, I forgot to mention it but I bought new ones when I did Al the other modifications. I’ll go do that now and let you know if it throws some codes. I appreciate your help.

 

Well what do you know. Misfire I’m 4 and 8. Gonna order some new coils right now and I’ll probably just replace all of them to be safe. Again, I really appreciate you’re help and i probably wouldn’t have thought to do any of this if you hadn’t told me so.

 

Well there you go…not running on all 8. That doesn’t necessarily explain why it seems exhaust output is different between the two banks since #4 is on one bank and #8 is on the other…but yes it’s probably time to replace all the coils. If you wanted to verify it’s the coils, like I said, you could clear codes, swap coils, run it again and re-check codes to see if the misfires follow where you moved the coils to. Or you could just shotgun the coils and put new ones in. If you plan on keeping the car for awhile, again, let me recommend just getting the factory Motorcraft coils. They can be pretty pricey from your LAPS. I don’t want to be the ******* that suggests you don’t support your local businesses, but you can always shop online for them. You just have to be careful because online vendors will market coil sets as “equivalent to” the Motorcraft coils, or sell them with Motorcraft spark plugs and the listing will say “Genuine Motorcraft”, but that only applies to the spark plugs. I know the part for my 2001 GT was Motorcraft DG-508. Anyway, keep us posted. Think about the fuel filter too.