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Discussion Starter #1
Hello all!

My new to me 01 Bullitt has what I feel like a slight rough (lumpy) idle when the engine is warm. When it’s cold it’s fine (higher idle speed too). I can see the idle speed slightly oscillate in the tachometer. It’s not bad but it’s not as smooth as I am used to for a V8. The last modular 2v I drove was a 2011 civilian Crown Vic and it had smooth idle.

the Car has 42k miles FYI. I have no idea of service history And it runs well otherwise. Had some hard start issues earlier but after more driving and some Techron and fresh fuel it seems to have gone away.

taking it into the dealer on Monday for servicing and have this looked at but wanted to make sure I’m not thrown a bunch of parts at if this is normal :)
 

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Given your previous hard start issue, could be a sticky injector. Also could be a coil going bad or fuel pump issue.
When was the fuel filter last changed? How fresh is your gas? What's your fuel pressure and does it hold after shutting off the engine? Good luck.
 

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I had a similar issue just recently before I put mine into winter storage. Rodents had climbed into the engine compartment and chewed into the rubber vacuum line connectors. I had to replace some of the components. Have a look to see if you find any signs of damage to the vacuum lines and connectors. Good luck.
 

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Discussion Starter #4
thanks @bullit4404 im not sure when fuel filter was last changed but am getting that done as part of next weeks servicing. FYI Fuel pump was replaced in 2014 according to carfax

gas is finally fresh, not sure how old the 1/4 tank that came with the car was. I do think Techron + fresh gas and plenty of Italian tune up have smoothed out the starts even as the days are getting way colder.

i don’t have readings of fuel pressure unfortunately but I think it does hold pressure After shut off. It starts good now.

i’ll clean the MAF this weekend and see if that helps.

I read that the Idle control valve could be a problem too but I’ll let the dealer do some diagnosis on Monday before throwing parts at it.

anything else I can clean up ‘ fix on the cheaper Side that might help? I am also going to inspect or just replace the vacuum lines while cleaning the MAF.
 

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you can clean the IAC with some brake cleaner and see if it helps. easy removal and replace...just be careful of not tearing the gasket.
 

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Discussion Starter #7
@bullit4404 I am not sure - no maintenance records.

cleaned the IACV, MAF and throttle body and no improvement to the idle. We’ll see what the dealer says next week.

it’s so weird because the engine runs so well otherwise. Super smooth and powerful. Maybe I just dried out engine mounts?
 

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Discussion Starter #8
Resurrecting this to make sure I’m not being unrealistic. Due to COVID shutting down the service dept the dealer had to return the car and never looked at my rough idle concerns.

I’ve been paying more attention to the rough idle and it actually is not rpm related. Based on reviewing other rough idle threads related to spark, air or fuel delivery I have no other issues with power, hesitation, stumbling, etc. the only one that MIGHT be remotely related is that some warm starts aren’t as perfect - it does not that nice rev up and then come back down to idle but instead kind of turns by the starter and slowly comes to idle speed. But aside from the one time when I just bought it, it has not failed to start.

When I say rough idle I mean that the motor shakes the car at idle including a shaky shift lever. Is this actually normal for a Mustang GT? i have had other V8s in the past so I know they’re not i6 smooth at idle but they don’t shake the car at idle either.

I also searched for engine and transmission mounts and they don’t seem to be a common maintenance/repair item for new edge mustangs but for my old Benzes, motor and trans mounts were almost always collapsed when the cars got old irrespective of mileage and resulted in a similar feeling of shaky vibration at idle. question: anyone replaced their engine/transmission mounts as part of maintenance?
 

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I did mine with the header install. mine were partially collapsed.
 

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Discussion Starter #10
Took the car to the local Chevy dealer to put Bilstein HDs all around and asked them to look over the shakey idle. They couldn’t find anything - no codes and the engine and trans mounts looked good and they told me that it might be normal.

is this the case for those who have had the car since new? the idle speed is steady but the car shakes a bit and the so does the shift lever. aside from the intermittent lazy start when hot theres no other issues, power is good across the rev range.
 

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My money's on a sticky injector or two..... I'd say vacuum hose but lazy start doesn't fit. Someone with a scope/analyzer could diagnose your problem.
 

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How many miles have you driven? Sometimes a code might not appear for 500 miles. IAC could be erratic, cheap part to replace. Cheap scanner will give you Short term fuel trims (STFT) and Long term fuel trims (LTFT) on banks 1 and 2 in live data mode. I did not know what any of that meant until I bought 2001 Bullitt last year. I used a
ThinkOBD2 100 scanner $34 to get basic data: MAF, fuel pressure and fuel trim date etc.
Normal STFT data is + or minus 5% on both banks idle and at 2500rpm. If it is high = 15% or more at idle/ rev idle to 2500rpm. If STFT short term fuel trim goes down to + or minus 5% - you most likely have a vacuum leak.
By the way LTFT is: at idle (-)20-(+)20...... + and minus 5% (2500rpm) neutral.

No code has to be inserted here.
Whenever an IAC component is replaced or cleaned or a service affecting idle is performed, it is recommended that Keep Alive RAM be cleared. Disconnect battery ground for 20 minutes.
 

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I meant to say - Normal STFT data is + or minus 5% on both banks idle and at 2500rpm. If it is high = 15% or more at idle/ rev idle to 2500rpm. If STFT short term fuel trim goes down to + or minus 5% at 2500rpm - you most likely have a vacuum leak. I had to do a smoke machine to find out my EGR valve was open allowing vented air into the intake manifold. (Smoke injected into the intake manifold coming out the EGR valve vent. BAD). I also had an intermittent IAC. (replaced). I had other hose vac leaks. (replaced). The major problem was air entering in at the bank 2 manifold to x_pipe connection. That allowed extra air seen at the o2 1 bank 2 sensor. O2 was signaling a lean condition, for PCM to add fuel. All these issues were interpreted by fuel trims / scanner for clues. I did not start changing parts with out diagnostic indicators. Am not an auto mechanic. Did a lot of reading and you tube videos. The Midas shop here in Ohio had a smoke machine. The mechanic had a personnal $3200 scanner, to verify my reading. They also had a drive on lift. They charged me $89 for the smoke machine and scanner diagnosis day 1. $200 for the exhaust bolts cut off and replace day 2. I replaced the EGR valve myself after they repaired my exhaust and let me go.
 

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Discussion Starter #14
Thanks @McFly ! I do have an OBD2 scanner so I’ll see if I can get the app to log some live data and analyze it against the reference values.

I also replaced the IAC with a new Motorcraft one + a new gasket. I did disconnect ground for 20+ minutes as recommended. The shake at idle is still there once the motor warms up. I think I’ll make it a weekend project to diagnose the reference values as you shared and if nothing else take it to a shop once it’s warmer as it’s easier to reproduce then.
 

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If you have your phone camera handy when you do scan. Consider taking some video of fuel trims: 02 sensors, MAF, fuel pressure etc at idle and when at 2500 rpms. That way you will have the data in front of you as you process and share data with others. It took me a year; of off and on (weekends) racking my brain to get my fuel trims stabilized. My car is a collector piece, so I was in no hurry. Bullitt should idle with no shake what so ever. I have no shake on shifter or other wise. You have a simple little component bad (or vacuum leak) etc most likely. Scan might reveal a hidden code??
 
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