Thoughts on low RPM hesitation and rough idle?

[Black2010]

Recently I've noticed a couple of things with my 04 (with 29K miles).
- First start after sitting a couple of days takes more cranking than it used to. Almost like it needs to prime the fuel system.
- At idle, it has a slight lope to it. I use rough in the title but it's hard to really describe. It's not rough as in the car shaking but I can feel a slight miss randomly.
- Under light to moderate acceleration the car feels like it's either hesitating or slipping. I was thinking the clutch was starting to go out but in 5th gear at 45 mph, if I give it a good amount of throttle their isn't a slip.

Generally speaking once the RPM's come up the car runs perfect. If I give the car full throttle it spins the tires and runs great. It's just this lower RPM hesitation or slipping that has me some what stumped.

I'm planning on doing some general maintenance to the car now that I finished the complete suspension rebuild/upgrade. I've got some new plugs and will be doing a full fluid flush of the car.

The engine is pretty much stock. Stock pulley with bassani catback. I just added a JLT RAI a couple months ago though.

Does this sound like clutch or something else? I'm open to ideas.

 

[Goose17]

Start easy: how old is the gas? Also get your new plugs properly gapped and installed.

 

[choate]

I've been fighting this too. Only under light throttle and when cold. A data log would tell you everything if you know how to read it or have a tuner that does. If you haven't looked at the plugs and coil packs I'd start there. Not hard to pull except you will have to remove the CAI on the passenger side of the engine for that side which is kind of a pain but not a big deal. Mine has all the plugs and coils out now as a matter of fact. Never done anything like that before but it's very straight forward if you've also never done it

 

[SVTdreamin04]

Vacuum leak, low fuel pressure, old fuel, coil packs and properly gapped fresh plugs is what I would look at. Also make sure the idle air control valve is working properly. Usually the only thing that can be done with them is a light cleaning to make sure they don’t stick.


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[BLOWN9646]

Change the fuel filter as well

 

[Black2010]

I forgot to add that I'm not throwing any codes.

Gas is fresh as I just put 13 gallons in it yesterday and I've driven about 50 miles so far on that tank. I changed the fuel filter when I did the suspension work (over the summer).

I'll put some plugs in the car and will hook my vacuum gauge up to it to make sure it's pulling good vacuum. I'll have to read up on how to test the coil packs though as I've never tested one before.

I'll order an IAC to change as well since they look pretty cheap and I haven't had much luck cleaning them with past cars.

Should I replace the fuel pressure sensor as well? Don't want to just throw parts at the car but it's not exactly a new car so I don't mind doing preventative maintenance things if you think that may be a cause for this as well. Assuming this is the right part - http://www.rockauto.com/en/moreinfo.php?pk=3779376&cc=1425035&jsn=460

 

[whitedevil95]

Might want to clean your MAF.

 

[SVTdreamin04]

If you have a good ohm meter, I can give you the readings off of a new set of coils. I just bought a set. I think it was like 5.8 ohms on the plug itself, but I don’t remember for sure. I doubt it is your coils, but anything is possible.


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[Black2010]

Thanks SVTdreamin04. I have a good ohm reader so that will help. When I change the plugs I'll test them all.

 

[1SL0SVT]

I swear this cold weather must be affecting Cobras all around because my '99 JUST started doing the same thing on cold start up. Does the issue go away after the engine warms up? Because mine sputters on idle and acceleration intermittently until engine gets to operating temp then drives FINE. Guessing its a sensor as well.....

 

[MackDombles]

Ive had egr problems cause this issue. Also i have 2 iac motors and 2 egr valves you are welcome to if you cover shipping for testing. used but in good shape

 

[Black2010]

I swear this cold weather must be affecting Cobras all around because my '99 JUST started doing the same thing on cold start up. Does the issue go away after the engine warms up? Because mine sputters on idle and acceleration intermittently until engine gets to operating temp then drives FINE. Guessing its a sensor as well.....

No, mine I doing it at all engine temps. I really thought it was the clutch but it doesn't do this at all at higher speeds/gears. It's mainly just in 1st and second at lower speeds. I had some things come up so I put the car in storage until I can get a chance to work on it. Right now I've got new plugs, IAC and fuel pressure sensor that I'm going to swap out.

 

[1SL0SVT]

No, mine I doing it at all engine temps. I really thought it was the clutch but it doesn't do this at all at higher speeds/gears. It's mainly just in 1st and second at lower speeds. I had some things come up so I put the car in storage until I can get a chance to work on it. Right now I've got new plugs, IAC and fuel pressure sensor that I'm going to swap out.

Oh ok, yeah i remember seeing a video on YouTube of a black Termi and it was blowing white smoke and idling rough. They changed the FRPS and it was A1 after that.

 

[Mojo88]

Might want to clean your MAF.

Yeah, this was my first thought ^^^^^^^^^^^ spray the heck out of it, with proper cleaner!

I had my car tuned in July and it started running crappy shortly thereafter, mostly with stalling and rough idle. I cleaned the MAF (twice) and after the second thorough MAF douche the car ran terrific. Those damn MAF sensors are incredibly sensitive to contamination or dirt.

Maybe run some fuel injector cleaner in the gas too.

 

[choate]

check COP. that cleaned up my low stumble issues

 

[SuperBreed]

+1 For spark plugs. Was having similar issues, threw in the colder TR7ix's now the car runs like a dream! Hope you figure it out.

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[hotcobra03]

have you done a compression test?

I've learned to do this before anything..

these will run on 7 cyls with a slight miss