low rough idle AND hanging RPMs

[mpm]

I feel as if I’m dealing with 3 issues here; possibly related, but I am running out of signs to use for trouble-shooting. The first problem is a low, rough idle that sounds as if my ’04 cobra has a pushrod motor with a loping cam. The second problem, while I originally thought was independent may actually be the computer trying to compensate for the first problem, is a hanging rpm between shifts and at idle after several minutes of driving. Every once in a while the high idle will eventually stop and then settle into a low, rough rumble that creates some noticeable movement in the drivetrain (problem #1 again). And finally, the third issue is a surging idle that I am contributing to the combination problems #1 and #2 until I can find something more solid.

This has been progressive for many months, with it finally getting to a level now that requires I do something to remedy the situation. I changed the spark plugs and received no results. The COPs didn’t show any visible signs of failure. I’ve also unplugged the IAC while at idle, and the engine started to choke out, so that leads me to believe the IAC is working. I’ve inspected the vacuum lines several times to the best as possible, short of pulling them all out and spending hours testing every line.

Could this really all boil down to the car needing a tune? It’s a stock motor with a K&N air filter and an o/r x-pipe. I’ve pulled engine codes and only receive lean readings for banks 1 and 2, which is due to the back O2 sensors being disconnected. Wouldn’t bad COPs trigger an engine code?

I’ve searched multiple threads on here and didn’t find any that addressed all these issues, as well as finding a final resolution. But if you find one, I’m sorry for this post and would love to read that thread.

 

[hotcobra03]

I feel as if I’m dealing with 3 issues here; possibly related, but I am running out of signs to use for trouble-shooting. The first problem is a low, rough idle that sounds as if my ’04 cobra has a pushrod motor with a loping cam. The second problem, while I originally thought was independent may actually be the computer trying to compensate for the first problem, is a hanging rpm between shifts and at idle after several minutes of driving. Every once in a while the high idle will eventually stop and then settle into a low, rough rumble that creates some noticeable movement in the drivetrain (problem #1 again). And finally, the third issue is a surging idle that I am contributing to the combination problems #1 and #2 until I can find something more solid.

This has been progressive for many months, with it finally getting to a level now that requires I do something to remedy the situation. I changed the spark plugs and received no results. The COPs didn’t show any visible signs of failure. I’ve also unplugged the IAC while at idle, and the engine started to choke out, so that leads me to believe the IAC is working. I’ve inspected the vacuum lines several times to the best as possible, short of pulling them all out and spending hours testing every line.

Could this really all boil down to the car needing a tune? It’s a stock motor with a K&N air filter and an o/r x-pipe. I’ve pulled engine codes and only receive lean readings for banks 1 and 2, which is due to the back O2 sensors being disconnected. Wouldn’t bad COPs trigger an engine code?

I’ve searched multiple threads on here and didn’t find any that addressed all these issues, as well as finding a final resolution. But if you find one, I’m sorry for this post and would love to read that thread.

couple of things...see talk section,,at top see stickys...

the rubber hose behind supercharger is a common leak that will cause this issue,,,

more info on your car would help,,,

for better replys,,give code number and or numbers that you get,,they are very important,,,

 

[crazycarlo]

I know its simple but have you tried cleaning the MAF? Some people say the oil on a K&N gets on the maf wire and causes it to run bad. Just a thought.

 

[mpm]

couple of things...see talk section,,at top see stickys...

the rubber hose behind supercharger is a common leak that will cause this issue,,,

more info on your car would help,,,

for better replys,,give code number and or numbers that you get,,they are very important,,,

I checked the FAQ in the sticky's and didn't find anything directly related to my multiple issues.

The code numbers were related to the O2 sensors being unplugged, not affiliated to the ignition.

 

[cobra21987]

Very common leak spot is the vacuum hose underneath the blower

 

[mpm]

Very common leak spot is the vacuum hose underneath the blower

Is this vacuum hose accessible with the blower stalled and the engine in the car? More specifics of which hose you are referring to would be great, because I have checked as many vacuum hoses as I can get my hands on and haven't found even the slightest of wear spots.

 

[Anabolic]

even if this dones't help but like the member said up there ^....... clean your MAF with the MAF cleaner.. you'll actually get +2mpg too aswell as.. well it just being clean! lol

 

[hotcobra03]

follow pcv valve hose back towards blower,,you will see a tee in the line..after the tee you will see an elbow for hose to back of blower...

this elbow is know to crumble

another bad spot..green hose on egr valve,,harness may drop and green hose melts to egr tube,,,

engine
https://acrobat.com/#d=vef-ZiyODNW-vNu*J3yfXg

 

[kenne-bell321]

I know its simple but have you tried cleaning the MAF? Some people say the oil on a K&N gets on the maf wire and causes it to run bad. Just a thought.

Yea it happend to me on my 93 notch, K&N oil got on my maf wire car ran like a sac of garbage rough idle, low rpms, etc.....

 

[mpm]

I'm going to clean off the MAF when I get to the car in a few days, and then try pushing smoke through the vacuum system to check for leaks. Getting my hands on the backside of the blower is less than fun.

 

[Anabolic]

oh, and also clean your IAC valve....

 

[Anabolic]

Idle air control valve hangs rpms, clean MAF meter with maf cleaner then clean the IAC out iwht MAF cleaner...

 

[mpm]

Incredibly late follow-up, but I did finally clean the IAC and MAF with MAF spray cleaner. The car idled well and drove normal for about an hour...then went back to the same behavior. I have ordered a new IAC and will report back once that has been replaced. My boost/vacuum gauge is reading approx. 20 psi of vacuum at idle, which I believe is correct. So I feel I'm not fighting a vacuum leak.

 

[hotcobra03]

tell more on codes you got,,give the po number..
.
tell us all codes even if you think its nothing

what did you do with rear o2s...im stock and left mine on,,i tried mil eliminators which caused nothing but issues..

 

[mpm]

tell more on codes you got,,give the po number..
.
tell us all codes even if you think its nothing

what did you do with rear o2s...im stock and left mine on,,i tried mil eliminators which caused nothing but issues..

I will have to pull the codes again as I had alot going on that day when I used the scanner and was slightly annoyed with the parts guy who was attempting to give me advice on how not to correct my idle issue.

I have a non-catted x-pipe, so the rear O2 wires are zip-tied to the underbody. The plan all along was to get to a speedshop for a tune, which has been the plan for 2 years now, but life has managed to always get in the way.

 

[mpm]

Replaced the IAC, cleaned the MAF for a second time, and checked my vacuum lines again.....still have the same issue. It doesn't seem to be as annoying as it used to be, but its still idling high after driving for several minutes and then dropping to a low, rough idle once sitting for a few.

I was trying to avoid replacing the TPS, as I'm not too sure if a new stock TPS will need to be calibrated to function properly, but that appears to be the next step.

I notice that turning on the a/c seems to force the issue when the engine is idling normal and driving with appropriate behavior. I don't show any loss of vacuum on my gauge either.

 

[blksvtCOBRA714]

I would check your tps voltage first before replacing anything and try and reset your idle so the car can relearn the idle. I've been having similar issues but they seem to be resolving after I adjusted the tps to .980 volts and reset the idle. My car still needs to learn the idle more but has improved substanially from what it was. Just a thought.

 

[mpm]

I would check your tps voltage first before replacing anything and try and reset your idle so the car can relearn the idle. I've been having similar issues but they seem to be resolving after I adjusted the tps to .980 volts and reset the idle. My car still needs to learn the idle more but has improved substanially from what it was. Just a thought.

Thanks. That was going to be the first step before I replaced the TPS.

 

[hotcobra03]

did you plug o2`s back in?


i have always cleaned my engine and componets with soap and water,,i might be nuts but i got 320k now on my sensors...

i just did the coils at around 300k..

if you have a engine light on you need to fix that issue or you will just be wasting time..

i too read that this needs to be done or that has to..i followed and was in your shoe,

i have lt headers,off road x...all o2s are on as stock...i have no engine light,,but i do get codes for any of the o2s when they go bad...they dont last long...from what i have read going wot is number 1 killer....

regardless of what ive read on rear o2s,,i can prove the rears will cause driveability issues...with just a change of 1 rear o2 sensor,i keep all my parts that i change on my car...for testing and sometimes pics to share..

i suggest putting them back and clear pcm and see what changes you feel..than go from there...this is free


fwiw..the number 1 most changed part on my engine?pcv valve

 

[mpm]

I will dig out the 02 extensions I have packed away and will connect the rear 02 sensors again.