Engine 1998 GT - Weird idle and misfire issues

Apr 28, 2018
3
1
13
27
Houston
#1
Hey guys this is my first post here, tell me how I do. I have a 1998 GT that I purchased with 111,000 miles. It has about 114,000 on it now. Has a few mods: CAI, Bassani catted X pipe, 4.10 gears, short throw shifter, and that's about it. Since I've had the car, it has had a slight missfire at idle. When it first starts it is fine, but when it drops down to idle speed it misses every few seconds. No CEL has been thrown since I've had the car. The only other noticeable thing is when I am in a higher gear, such as 4th or 5th, and I let off the throttle (so it is decelerating or engine braking) within a few seconds it feels like fuel is cut to the engine. If I push the clutch in and rev it quick like I am going to rev match, the engine sputters a bit before it revs up. Also, it feels like the car takes longer then it should to crank. Instead of like 2 or 3 cranks it is about 4 or 5. This is all the time, even when warmed up.

Now, all the stuff I have done to fix this:

New plugs and wires: First put in motorcraft copper plugs gapped to factory specs with Ford Racing wires. This did not help. Later I also installed NGK TR6 (One step colder) plugs gapped at .040. This did not change anything.
New coil pack: One of the coil packs appeared to be a newer OEM motorcraft unit. I replaced the older one with the same newer OEM motorcraft coil pack. Did not change.
New IAC: Almost seemed to run worse with a new Duralast IAC.
Cleaned MAF and installed new K&N filter: Didn't change.
New fuel filter: The old filter seemed dirty, so I installed a new one. This did not change anything.
Cleaned throttle body: The throttle body plate and inside of the plenum seemed very black, so I removed the throttle body and cleaned it. This significantly helped the idle, and also helped with the weird deceleration issue.

Now I am at a loss at what to do. I am going to be testing fuel pressure, because I think the fuel pump could be a problem as well. I am also going to put back in the stock heat range spark plugs gapped at factory spec. If anyone has any ideas as to what the issue could be, please respond. I just want to enjoy my car!
 
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Aug 9, 2018
5
0
1
Leonardtown MD
#2
Hey guys this is my first post here, tell me how I do. I have a 1998 GT that I purchased with 111,000 miles. It has about 114,000 on it now. Has a few mods: CAI, Bassani catted X pipe, 4.10 gears, short throw shifter, and that's about it. Since I've had the car, it has had a slight missfire at idle. When it first starts it is fine, but when it drops down to idle speed it misses every few seconds. No CEL has been thrown since I've had the car. The only other noticeable thing is when I am in a higher gear, such as 4th or 5th, and I let off the throttle (so it is decelerating or engine braking) within a few seconds it feels like fuel is cut to the engine. If I push the clutch in and rev it quick like I am going to rev match, the engine sputters a bit before it revs up. Also, it feels like the car takes longer then it should to crank. Instead of like 2 or 3 cranks it is about 4 or 5. This is all the time, even when warmed up.

Now, all the stuff I have done to fix this:

New plugs and wires: First put in motorcraft copper plugs gapped to factory specs with Ford Racing wires. This did not help. Later I also installed NGK TR6 (One step colder) plugs gapped at .040. This did not change anything.
New coil pack: One of the coil packs appeared to be a newer OEM motorcraft unit. I replaced the older one with the same newer OEM motorcraft coil pack. Did not change.
New IAC: Almost seemed to run worse with a new Duralast IAC.
Cleaned MAF and installed new K&N filter: Didn't change.
New fuel filter: The old filter seemed dirty, so I installed a new one. This did not change anything.
Cleaned throttle body: The throttle body plate and inside of the plenum seemed very black, so I removed the throttle body and cleaned it. This significantly helped the idle, and also helped with the weird deceleration issue.

Now I am at a loss at what to do. I am going to be testing fuel pressure, because I think the fuel pump could be a problem as well. I am also going to put back in the stock heat range spark plugs gapped at factory spec. If anyone has any ideas as to what the issue could be, please respond. I just want to enjoy my car!
Every thread I read lead back to the IAC when I had starting problems and an odd idle. I tried the duralast and my 2004 GT idle would hang (high rpm) with hard starts and a rough idle, switched to the newer Motorcraft version and it got better, although, still had a high idle after driving, then picked up the 1785 IAC and it idles perfectly, starts easily, and returns to normal , normally. (Don't forget to check for vacuum leaks first)
 
Apr 28, 2018
3
1
13
27
Houston
#3
Every thread I read lead back to the IAC when I had starting problems and an odd idle. I tried the duralast and my 2004 GT idle would hang (high rpm) with hard starts and a rough idle, switched to the newer Motorcraft version and it got better, although, still had a high idle after driving, then picked up the 1785 IAC and it idles perfectly, starts easily, and returns to normal , normally. (Don't forget to check for vacuum leaks first)
Do you think it would fit on the 98? American Muscle has it listed as 2002-2004. I believe I have also narrowed it down to the IAC. Should'nt have bought the autozone one haha. The bolt pattern appears the same, makes me think it would work.
 
Jul 12, 2018
21
1
3
Massachusetts
#5
Hi, It can be, and i hope your issue is within your IAC, if you choose to replace it- use the RIGHT MOTORCRAFT part(AND YEAR-98’-2000), don’t want to add to issues. Best bet first- want save some $$ ??(Umm,YES!)
Disconnect the Battery-remove the IAC, likely loaded with Carbon deposits, so clean the ports where the IAC mounts with a Qtip with CRC MAF cleaner(So many uses). Leaves no residue. Also do the same on the IAC with Solenoid up (don’t want cleaner in the Actuator Solenoid area),. Swab out the ports until clean with cleaner on the swab, can do a quick spray inside both ports on Solenoid and where it bolts up). If there was a lot of Carbon in the TB area, it’s likely inside the IAC- can be cleaned out, work just fine. Also Re-clean our the TB to verify its all clean again. Reconnect everything and start the engine- see how it runs. You should be OK with that,if that’s the problem. Next- Cleaned IAC, ran better- replaced- still an issue. Check your Coolant level- if low, can cause Idle surges, etc.. next.. Cleaning out your MAF sensor...
is a good idea to buy a Can of CRC MAF Sensor- excellent cleaner. Battery disconnected for all steps in this text..Gently remove sensor, spray w/CRC MAF cleaner, 2, 3 times, let drip Dry. Gently reinstall- reconnect. Does it have stock (Orange) injectors? If so- no worries, if not- the MAF needs to be sized for the Injectors. This not correct can cause Idle issues, rich mixture (dark colored Plugs- should be a Tan Color).
Always Use a little ‘Never seize’ on cleaned plug Threads during install. Easy to get back out & great Conducive properties for Plug ground. Clean IAC connections, ensure tight & clean. Use CRC MAF or TBody cleaner.
If you did not solve the problem with the IAC Clean/or better it- Try clamping off the Vacuum line to the Brake booster- change idle issue?(Don’t DRIVE it like that). Can be a leaky booster, line or connection.
Have a Scantool? watch O2 Sensors #s moving back and forth (should be changing slightly). If no movement occurs, an 02 Sensor may also be a potential issue. Check Connections, burnt wires-repair, You can remove 02’s and clean them. Don’t ever read O2’s with a Meter, it will damage them.
Aside from Ground wires being solid, Vacuum lines is the next issue, and Plug Wires- arcing. Start the car at night and look for Arc’s between Wires, and anywhere else. Being an OBD-2 Car, most of this can be Id’d with the Check engine light. Any questions, feel free to ask- someone or I will answer, you’re good- we’ve mostly all been there.
 
Apr 28, 2018
3
1
13
27
Houston
#6
Hi, It can be, and i hope your issue is within your IAC, if you choose to replace it- use the RIGHT MOTORCRAFT part(AND YEAR-98’-2000), don’t want to add to issues. Best bet first- want save some $$ ??(Umm,YES!)
Disconnect the Battery-remove the IAC, likely loaded with Carbon deposits, so clean the ports where the IAC mounts with a Qtip with CRC MAF cleaner(So many uses). Leaves no residue. Also do the same on the IAC with Solenoid up (don’t want cleaner in the Actuator Solenoid area),. Swab out the ports until clean with cleaner on the swab, can do a quick spray inside both ports on Solenoid and where it bolts up). If there was a lot of Carbon in the TB area, it’s likely inside the IAC- can be cleaned out, work just fine. Also Re-clean our the TB to verify its all clean again. Reconnect everything and start the engine- see how it runs. You should be OK with that,if that’s the problem. Next- Cleaned IAC, ran better- replaced- still an issue. Check your Coolant level- if low, can cause Idle surges, etc.. next.. Cleaning out your MAF sensor...
is a good idea to buy a Can of CRC MAF Sensor- excellent cleaner. Battery disconnected for all steps in this text..Gently remove sensor, spray w/CRC MAF cleaner, 2, 3 times, let drip Dry. Gently reinstall- reconnect. Does it have stock (Orange) injectors? If so- no worries, if not- the MAF needs to be sized for the Injectors. This not correct can cause Idle issues, rich mixture (dark colored Plugs- should be a Tan Color).
Always Use a little ‘Never seize’ on cleaned plug Threads during install. Easy to get back out & great Conducive properties for Plug ground. Clean IAC connections, ensure tight & clean. Use CRC MAF or TBody cleaner.
If you did not solve the problem with the IAC Clean/or better it- Try clamping off the Vacuum line to the Brake booster- change idle issue?(Don’t DRIVE it like that). Can be a leaky booster, line or connection.
Have a Scantool? watch O2 Sensors #s moving back and forth (should be changing slightly). If no movement occurs, an 02 Sensor may also be a potential issue. Check Connections, burnt wires-repair, You can remove 02’s and clean them. Don’t ever read O2’s with a Meter, it will damage them.
Aside from Ground wires being solid, Vacuum lines is the next issue, and Plug Wires- arcing. Start the car at night and look for Arc’s between Wires, and anywhere else. Being an OBD-2 Car, most of this can be Id’d with the Check engine light. Any questions, feel free to ask- someone or I will answer, you’re good- we’ve mostly all been there.
Thanks for all this! I will be working on my car tomorrow and will be doing everything I can to find the issue. Will update with how it goes.
 

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