View Full Version : 96 Cobra - Idle issue when AC is on??

06-15-2008, 03:28 AM
Howdy all! :hello:

I just bought my first Ford...a 1996 SVT Cobra :D :fordflag:

I love the car, but having some issues with it and since I'm not familiar with Fords at all (yet!!! :thumbsup: ), I thought I'd request some help.

I noticed that the idle tended to stay up around 2k or so and SLOWLY drop back down to around 750 or so. Since I'm in Vegas (read: it's as hot as hell would be. LOL), I always had the air on since I bought it a week ago. Also, it seems like sometimes when the idle comes down from 2k, it drops like a rock and stalls the motor. Sometimes you can tell the engine "catches" itself and recovers before it stalls.

It finally threw a "check engine" light the other day. I only got to see one of four codes (P0174) because the mechanic went through it too fast, but one of them was something like "Engine running lean (Bank 2)". Oh and it feels like the car tends to hesitate a bit around 3,500 rpm and up. :confused:

I took it back to the dealer and told them the issue (and they noticed it on my test drive as well). They said they'd send it to their mechanic. Well, got it back Thursday and the light/codes were reset, but it still seemed to act the same. GRRRRR! :mad:

So, that night I ended up driving home after the sun went down and did NOT turn the air on. All of a sudden, I noticed the car seemed to act "normal". It would idle around 750 rpm. Wouldn't idle high, wouldn't drop like a rock or anything. It felt great. Even seemed like the "hesitation" at 3,500 rpm and up went away. I was thrilled. :thumbsup: :D :bounce:

Got in it Friday morning and started it up...check engine light is now on again! I kept the air off and it still seemed to drive okay. Maybe a very slight hesitation at about 3500 rpm and up. But then again, it might just be in my head too at this point.

Driving home later, I drove without the air on and it was acting fine. Turned the air on at a light (to test my theory) and the idle went haywire again! Up to 2k on it's own, then dropped like a rock and almost stalled. Then, while driving it, I could turn the air on and off and feel the hesitation either show up or disappear around 3500 rpm and up (according to whether the air was on or off).

I have no idea what to do to fix this (or tell them how to fix it). I'm going to take it back to the dealer on Monday and tell them they have to fix it and reset the codes (again), but if I have an idea or direction to tell them to look into, it would be great...then maybe they could finally fix it and I could enjoy the car! :rotf:

Oh and when I told the dealer I was bringing it back, they said that their mechanic put on new intake seals the other day and thought that would fix it. Something about it being $350.00 worth of work. I told them that was fine and dandy, but it didn't fix the issue. :rolleyes:

When it runs right, it's a darn fun machine to drive...but you definitely can't drive around in Vegas without A/C in the summer! :D :bugeyes: :crazy: :bs:

Any help would be appreciated. Oh, and the car does have 151,000 miles on it...ya, I know it's a lot, but it really does run nice when it's working right. LOL

As an update, I had the local CSK shop check the code today for the Check Engine Light and this time it has only one code set. A P0155 code has come up. To my understanding this is: O2 Sensor Heater Circuit Bank 2 Sensor 1

What's funny is when I took it in the other day, the codes were different (P0174 was one of the four. I didn't get to see the other three) and it indicated the engine running lean in bank 2. So, it seems like it's a problem on that side.

Any help would be truly appreciated!


06-15-2008, 06:55 AM
does the car have a K&N style filter? I think that code is the one for a dirty MAF sensor. This is located in between the filter and the throttle body, it has a little square black box on top of it. Take the filter box out, and remove the MAF sensor from the upper inlet tube. CRC makes a MAF sensor cleaner, that can be found at most auto parts store. Spray the little sensor inside the MAF tube. Put it all back together. It usually takes three normal drives to clear the code. I hope this helps.

06-15-2008, 04:11 PM
Hey there NC,

Thanks for the info. Actually the car is all stock. No K&N A/F on it right now. I'll eventually put one on, but I want to get the car to run right first before I do any modding.

I'm due to take it into the dealer where I bought the car tomorrow. I'll give them another shot at fixing it and see what happens. I just know that if I start replacing items, even if I fix it, I know they won't reimburse me. That's where I'm kind of stuck right now. But I figured if I could get some good info as to what might be the problem, I could give them "suggestions" on what to check to fix it. Unfortunately it's not a Ford dealer that I bought the car from. :(

I'll see what happens tomorrow and if they don't fix it, I'll try cleaning the MAF and see if that helps at all.

It's just strange to me that it only happens with the AC on. If the MAF was dirty, wouldn't that make the bad idle/hesitation happen whether the AC was on or off? This is my first true "technology" car, so don't mind me if I sound like an idiot. LOL I'm used to working with carburated 60's - early 80's cars. :rotf:

06-15-2008, 05:22 PM
no, the dirty MAF sensor will not make the car run bad. I get that code when mine is due for cleaning, though.

06-15-2008, 06:52 PM
Oh, okay...got it. In reference to the O2 heater code thingy. LOL

I'll wait till I get it back from the dealer and then clean it...probably won't hurt to clean it.

06-16-2008, 06:38 AM
no, it probably wouldn't hurt at all to clean it. Are you planning on doing any mods to the car?

06-16-2008, 07:25 AM
The Service Manual indicates the following possible sources:

Damaged fuel system.
Incorrect cam timing.
Damaged ignition system.
Leaking intake manifold gasket.
Damaged hydraulic valve tappet.
Worn valve stem guide.
Improper valve-to-valve seat contact.
Damaged cylinder head gasket.

Engine idle speed is programmed to intitially idle high (for engine oiling) then drop back to normal idle speed. With the A/C on, the additional load of the A/C compressor is exaggerating the poor idle condition.

The 1996-1998 Cobras have Intake Manifold Runner Controls. There are two intake manifold runners to each cylinder. To promote increased air velocity (for more low rpm torque) one of the runners is closed below 3250 rpm. Above 3250 rpms both runners are open to increase air volume to produce horsepower. With the high mileage of your car the IMRC's probably need to be cleaned to restore proper operation.

06-17-2008, 09:51 PM
NC - Yup, I'm planning on mods as soon as I can. :thumbsup: Probably 3.73s, K&N and not sure what else yet.

Bolt-On - Thanks for the info. The dealer supposedly replaced the intake gaskets a week ago and I wonder if they cleaned the IMRC's? I'm sure they won't tell me, even if I asked. So, I might have to retear it down myself later. Looks like I best buy a service manual for the car! LOL They supposedly fixed it last night.

I picked it up today...same ole' thing. They didn't even touch it. They cleared the O2 sensor heater circuit code and then had the car smogged so that it would pass. I'm sooooo ticked right now! :mad: :mad: :mad:

I don't know what to do now...I'm at an impass. almost seems like I'll have to try and fix it myself and be stuck with the bill. I really don't want to give the car back. :( Either that or I have to drive around without A/C until I can fix it.

I'd much rather be posting about future mods then having to try and find out how to fix this problem. It works fine without the A/C on, but as soon as you flip that A/C switch, it freaks out. Plus, the dealer said the mechanic said the "heater" in my O2 sensor is out/bad (but, of course, he can't remember which side! :mad: :rolleyes: ). That wouldn't cause the "A/C only" issue, would it? Oh, and it threw a code again tonight after driving home...


06-17-2008, 10:19 PM
I highly doubt the dealer cleaned the imrc's. But, you never know.

one good mod, and you wont kick yourself for it, is a short throw shifter. It will give you smoother shifts. MGW is what a lot of the guys in the club run. They are top quality shifters. www.mgwltd.com :thumbsup:

06-17-2008, 10:40 PM
I doubt they cleaned them either. Heck, at this point, I doubt they even replaced the intake gaskets. I just don't think they want to put any money into fixing it because it takes away from their profit. :mad:

I'm thinkin' this weekend I might replace the PCV valve and dump the computer for an hour. Then maybe I'll pray it helps or fixes the situation. If not, looks like I might have to look into replacing the O2 sensors...aren't there like 4 of them in the Cobra? :eek:

Dang that's a nice shifter! :thumbsup: And I thought the one that's in the car has a pretty short throw compared to the GM's I've run. LOL I'd probably throw my fist into the dash the first couple of shifts to third. :rotf:

06-17-2008, 10:44 PM
the shifters are awesome, so smooth. I think they are well worth the money.

not sure, but I think there are 4 O2 sensors.

06-18-2008, 12:43 PM
4 O2 sensors...2 up front, 2 at the rear...

06-18-2008, 01:26 PM
O2 sensors are fairly inexpensive. I'd suggest replacing the two front ones with new and then getting some MIL eliminators for the rear. You'll want those anyway if you start replacing exhaust pieces. The front two are required, the rear two are for information (are the cats working - yes...nothing; no - throw check engine light).

If the front 02 sensors are bad....they can cause issues. Also check your IAC valve. It can cause idle issues (my '98 wouldn't idle properly..randomly). It's something you can take off and clean with carb cleaner (it's gets build up and sticks....the carb cleaner will clean the gunk off). I did that to mine.....worked perfectly, idles fine.

http://www.muscularmustangs.com/iac.php will walk you through the unbelievably easy removal/cleaning/install process :)

06-18-2008, 04:15 PM

Ya, I noticed at the local parts store the "front" O2 sensors are about $63.00 a piece for the Bosch ones (the only ones they carry). I have heard/read here that you should replace them as a "pair". So, I'll probably have to do that. I'm getting the feeling the dealer I bought it from is screwing me over and I'll have to pay for all this...so I might as well do it right. The check engine light came back on again yesterday and I'm sure it's probably the O2 sensor heater fault again. Ugggghhh...but if I replace the sensors, then that should clear the code itself, right? (if that's the code it threw)

I have no clue what MIL eliminators are. I'll have to do a search to find out what it is. For right now, it's all stock, so I'll probably have to leave it that way till I get some cash to start doing mods. :thumbsup:

Thanks for the tip on the IAC. :thumbsup: I'll have to check that link out when I get home from work and clean the IAC this weekend. I hope that'll help. If I find out it really does help the idle, especially with the A/C on, then I'll know to replace it. I know it might not be the main cause, but then again, it might.

Again, thanks for all the info. :thumbsup:

06-18-2008, 04:23 PM
Look online, you can get 02 sensors for near 1/2 that price.

If cleaning the IAC works, you don't need to replace it.....I cleaned mine about 3 years ago....still functioning fine.

06-18-2008, 04:26 PM

I actually just printed out all five parts of that link. I might as well go through the entire thing and clean everything. I'll bet it's probably never been cleaned before, so it probably couldn't hurt to clean all that and reset everything. :cool:

However, they suggested to use carb cleaner and a Q-tip to clean the MAF wire...I thought I read somewhere here not to use carb cleaner...so, I'm a bit confused on that part. Any suggestions on that? I don't want to use the wrong cleaner and mess something up! :eek:

06-19-2008, 12:43 AM
Electronics cleaner, MAF cleaner would all be better. Carb Cleaner is a good general all purpose cleaner. It doesn't harm paint but it cuts grease, gunk, oil and dirt like crazy.

06-19-2008, 01:05 AM
Well, then I'll get some carb cleaner to clean the IAC, TB and maybe even the TPS (if needed). Then I'll have to find some MAF or electronics cleaner to clean the MAF sensor/wire. Hopefully doing that and dumping the computer for at least 30 min might help my problem.

Hopefully I won't have a problem disassembling everything. Under the hood actually looks pretty clean. I know that doesn't mean anything, especially when the carbon buildup happens inside, but at least it's nice to look at. LOL :thumbsup:

Thanks for the info!

06-19-2008, 01:57 AM
Okay...I was lookin' into O2 sensors so that I could replace the two front ones...are the front "upstream" or "downstream" sensors? :eek: Before, I've seen them listed as front and back and I need the front ones (the ones that actually do the real reading). But I don't know if that is up or downstream. LOL

Help. :cool: :baby: LOL

06-19-2008, 07:19 AM
just do a google search on something like "02 sensors ford mustang", you should find what you need easily that way.

I'll see if I can remember where I got mine, it's been about 5 years....so I might/might not have any info on where I got them........but I'll see what I can find.

06-19-2008, 07:55 AM
front are upstream.

06-19-2008, 09:34 PM
Thanks a bunch Jeff. :thumbsup:

Hopefully replacing the O2 sensors and cleaning everything will help eliminate the issue. :D

06-20-2008, 09:13 AM
Thanks a bunch Jeff. :thumbsup:

Hopefully replacing the O2 sensors and cleaning everything will help eliminate the issue. :D
That is correct, the front's are "upstream"...advance or your local auto parts store should have them around $50/$60 each...the Bosch replacements seem to work well in case you don't go back to the OEM brand

06-20-2008, 03:37 PM
I'll check into the Bosches then if I can't get the Motorcraft ones. I thought I read here on a post that someone had issues with the Bosch sensors and suggested the stock (Motorcraft, I'm guessing) ones. So, that's why I figured I'd put the Motorcraft ones in. :D

But if people here have run with the Bosch ones without any issues, then I'll just see which I can get a better deal on and go with that. I'm just hoping that once I clean the IAC, MAF sensor, TB, replace the plugs and replace the O2 sensors this weekend that my issue takes care of itself. I'm not lookin' for any performance gains...just to fix the idle issue. :eek:

Thanks for the info. :thumbsup:

06-22-2008, 12:07 AM
Allllrighty folks...

Just thought I'd give an update to the situation. First off, I'd like to thank everyone for their helpful advice. :thumbsup: Totally cool!

Okay, here is what I fixed/cleaned/replaced today on the Cobra:

New plugs (couldn't afford the wires yet...workin' on it)
New air filter (The one in it was NASTY! I can't believe the car even ran!)
New PCV valve
New O2 sensors (both upstreams/fronts)
Cleaned the entire intake air hose
Cleaned the IAC
Cleaned the TB
Cleaned the MAF (ya, it was dirty...couldn't even see one side of it shining)
Cleaned the PCV tube to the intake
Checked the voltage on the TPS (right at 0.99 volts)
Dumped the EPROM/computer for a few hours

Soooo, after all that fun stuff :cool: :D I finally got to restart the car and take it for a 30 minute test drive. :bounce:

When I started it...no check engine light. :thumbsup: That was the first good sign because it was on for a few days prior to me taking the neg battery cable off to work on the car.

Next, the idle (without the A/C on) ended up settling at 700 rpm and works fine. No stumbling when in neutral and the car comes down to idle. Also, no hesitation through the rpm range. :thumbsup:

Now it was time to switch the A/C on. :eek: Ended up switching it on and the idle didn't change from the "non A/C" idle. I hit the gas in neutral and the rpms went up and came back down to about 1200 and then settled around 800. AWESOME! :bounce: :bounce: :D Before it would stick at 2k and then drop like a rock and stall the car. So far, so good. I then rode it easy for a bit and stopping normally at lights and it only stumbled once (on the first stop, but I'm guessing the computer was "relearning"). After that, no stumbling...it'll come down to about 1200 rpm, then settle at about 750-800 rpm. Workin' great! Plus, no more hesitation through the rpm range! :bounce: :bounce:

I'm soooo happy! So, I'm not sure which thing (or which combo of things) helped the car, but something did!

Anyway, so it's runnin' like a champ now and I couldn't be more thrilled!

Thanks again to everyone for your help. I do have a couple of other issues I need to work on, but I'll just create a separate thread for that. :thumbsup:

06-22-2008, 07:20 AM
great to hear you have the problem solved. It sounds like the car just needed some TLC, which you are giving it. I am sure everything was the root of all evil. Now, you can truly enjoy the car:thumbsup:

07-15-2008, 05:40 AM
The issue with the idle dropping quickly from 2K to down below 1k might actually be normal operation, though the car obviously shouldn't stall during normal idle operation. During normal idle operation, the engine computer automatically kicks-in the ECC (Emissions Control Circuit) to minimize emissions at idle. As the ECC kicks-in, the idle speed will normally drop quickly from 2k down to ~900 RPM, as the car goes through the normal idle cycle. However, for future reference, if the car stalls when the idle drops below 1k, you might have a problem with the ECC, which includes the IAC, the EGR, the PCV, the two tuned vacuum switches mounted on brackets at the front of the engine, the air pump, and the high-temp hoses that connect to the air pump and tuned vacuum switches.

Speaking from experience, I suggest that before you go chasing rabbits through the ECC circuit, you might first quickly inspect the high-temp hoses leading from the air pump to the "T-junction" that feeds the pair of tuned vacuum switches. When my car was stalling at idle, it turned-out that one of the high-temp ECC hoses had collapsed like a bad artery. It had collapsed at the dog-leg near the air pump.

Obviously, as noted in earlier posts, there are a number of things that can cause the car to stall at idle, but if one of the high-temp hoses connected to the air pump and the tuned vacuum switches collapses, that will also cause the car to stall at idle. Thus, it is a good idea to quickly inspect these high-temp hoses before getting into extended, time-consuming troubleshooting.

cheers, :cheers:
--Professor Chaos

07-22-2008, 05:06 PM

Thanks for the info...if it starts to happen again, those will be the first things I check next time. :thumbsup:

But so far, so good....all is still working well with the A/C on or off. :thumbsup: :bounce: