1997/1998 GT with rough idle slight miss

[SmockDoiley]

I've got a GT conv. 4.6 from the 97/98 era, and the customer is complaining of a rough idle. It does seem to have a slight miss. Intake was recently replaced because it was overheated. Also, the vehicle received a complete tune up not too long ago.
Following checks were done and it seems to be passing everything.
Sprayed the intake down and found no leaks.
TPS is good.
IAC checks good-disconnected and car engine started to stall.
Vac. test shows -20 and steady which is right on.
All plugs are firing and all the plugs look coffee and cream.
Sprayed out throttle body with cleaner.
Engine has no codes and passes both self tests.
Fuel pressure is about 32-35 psi which according to my Matco book is right on.
From the backside, the MAF looks clean but I didnt actually look at the thin wire.
Its an automatic with 92k miles. The customer takes good care of the vehicle.
The reason the intake was replaced was because the engine was overheated and the intake cracked or warped.
The vacuum test would have shown a bad valve, or incorrect timing, vacuum leak or plugged exhaust.
At one point I did smell the rotten egg smell but thats just condensation in the exhaust.
The miss seems to be present all through the RPM range, but the roughness is at idle, and more present with a load.

 

[AUTOBAHN]

Are the plug wires original? Wires do go bad with age. Voltage jumping between wires. A quick check is that sometimes you can see the voltage jumping between wires in the dark while running.
Mike

 

[squares98]

I had the same issue until I changed my plug wires@130k miles. Little farts and stumbles, sluggish start up were my symptoms

 

[SmockDoiley]

Nope, brand new plugs and wires.

 

[SmockDoiley]

I need Yount over here, lol. I'm Master certified and this one I cant figure out.

 

[Blackstang3o2]

I had a problem that was close and i could not find it. Eventually I did on the driver side i had a coolant leak right where the t stat nack goes on the intake vary small and slow leak but it was letting coolant get down to the plugs causing a miss. Might not be the problem but its something to check.

 

[SmockDoiley]

One of the other techs had changed the intake about 6 months ago because the engine was overheated. I noticed some build up around the T-stat but I wasnt sure if it was from the previous job or not, that crusty white film. I did a coolant pressure check and it held. What I have noticed is that the Mod motors and those long rubber boots are holding water, and its very humid out here in Corpus Christi, Texas. I found the same deal on a 32 valve 5.4 in a Navigator, the same day and it had a Cyl. 6 misfire. When I removed the almost new plugs they had oily water on them and the threads were wet and steaming, just like the Mustang. I cleaned the plugs on the Mustang and replaced the plugs and the #6 coil on the Navigator. This morning the other tech, Bob, put the plugs back in the Mustang and ran it, and he said its running perfectly. He asked me if I did anything. I guess cleaning them off and getting that moisture out did the trick. I'll be keeping this in the back of my mind now whenever I see a driveability problem on a mod motor. Needless to say, I'm going through lots of Dialectric grease.
So the Nav and the Stang are running well after playing with the plugs.

 

[COramprat]

One of the other techs had changed the intake about 6 months ago because the engine was overheated. I noticed some build up around the T-stat but I wasnt sure if it was from the previous job or not, that crusty white film. I did a coolant pressure check and it held. What I have noticed is that the Mod motors and those long rubber boots are holding water, and its very humid out here in Corpus Christi, Texas. I found the same deal on a 32 valve 5.4 in a Navigator, the same day and it had a Cyl. 6 misfire. When I removed the almost new plugs they had oily water on them and the threads were wet and steaming, just like the Mustang. I cleaned the plugs on the Mustang and replaced the plugs and the #6 coil on the Navigator. This morning the other tech, Bob, put the plugs back in the Mustang and ran it, and he said its running perfectly. He asked me if I did anything. I guess cleaning them off and getting that moisture out did the trick. I'll be keeping this in the back of my mind now whenever I see a driveability problem on a mod motor. Needless to say, I'm going through lots of Dialectric grease.
So the Nav and the Stang are running well after playing with the plugs.

I think you hit it there. I had a similar problem with one wire that had gotten some moisure in the well. I removed the boot and dryed it out but I still had the problem so I took out the plug, dried the threads and the boot again. Solved it. I'm guessing these motors are real finicky about moisture.

 

[5spd GT]

Friends 98 Gt had a bad wire on the #6 hole and he was missing a bit.

Are the plugs gapped correctly? To wide maybe?

 

[Mike97gt]

A lot of times I have run into water/coolant down the plug holes after a intake replacement, even a tiny tiny amount in the plug boots can cause misfires. This is the first thing I would look at..

then if it's not that I would measure the KV and duration and see if you have a weak coil... If that STILL is not it, then do a injector flow test and see if maybe a injector was damaged or plugged up slightly when the intake was done ( I ran into this once, another tech did the job then after he could not fix it it made the rounds around the dealer until I got it)

then if none of that fixes it you may have a valve sealing issue. I ran into this once on a 2004 gt had to replace one head.

 

[SmockDoiley]

The vacuum check would have shown a bad valve, but it was steady as a rock. Like I said, after the plugs were allowed to dry, it cleared right up.