View Full Version : fast idle problem

06-22-2004, 09:58 PM
My 99 Cobra developed a fast idle problem all of a sudden. It idles at around 1200 rpm when it's at complete stop. And the rpm won't drop when I upshift. If I shift at 2500rpm, it will hover there or sometimes it will rise to about 3200rpm. My Cobra has about 67k miles on it. I have replaced the IAC valve and TPS. This did not fix the probelm. I have recently replaced the front o2 sensors. I have inspected the throttle body. It's seems to be shutting all the way. It's not hanging up. I even cleaned the throttle body. Is that my problem? I tried to look for a vacuum leak also. I did not find any. Today, I tried the A/C and no cold air. It was working yesterday. Could it be something in the electrical? Any help is appreciated. Thanks in advance.

My Cobra
06-23-2004, 07:44 PM
Mine does the same thing it does not do it often though. i dunno what the problem is. any ideas guys?

06-23-2004, 10:34 PM
I wonder if this is a common problem on a 99 Cobras.
Any thoughts on this?

My Cobra
06-24-2004, 02:59 AM
i have no idea, makes me mad some times.

06-28-2004, 06:22 AM
There are several things that could be happening here. First start off with reading the codes from the OBDCII port. (most auto parts stroes may do this at no charge FYI autozone ). Excessive idle will be detected and eventually the "check engine or service engine" light will come on. It is always best to start with checking the codes prior to attempting to do guess work. I had a similar issue with a new IAC and almost thought it was defective until I adjusted the throttle stop with a pair of plyers. It could be a dirty throttle shaft or weak throttle return spring. If you have an after market CAI, sometimes they can cause issues with the MAF sensor. When I cleaned my throttle body I removed it completely, along with the intake plenum. Care must be taken when re-assembling the intake plenum. it is best to lube the plastic cladd rubber seal with a light coat of oil.

Another issue could the the PVC valve allowing for air to be sucked in if it is cracked or loose. Also the EGR valve may also cause this if it is stuck open. The codes should indicate which component is at fault (except for the PVC valve). If you do not get any codes pending or reported from the scan, then it becomes guess work from there. Usually if the IAC valve is disconnected, the engine will idle at or above 1200 RPM. I would recheck all electrical connections by unplugging and replugging them. Last thing to do would be to clean off the MAF with electrical parts cleaner in case it is dirty.

06-28-2004, 09:45 PM
Thanks for the great info. I will try and see what happens.
Thanks again for the help.

06-29-2004, 06:28 AM
Oops, I guess mornings are not for me, I stated that EGR may cause fast idle, but mistakenly listed it as a cause which would affect the IAC ad or throttle body by causing build up on the throttle plate and inside the IAC passages. IT may be probable if it was stuck open to supply excess air through the exhaust system but highly unlikely it would cause high idle. More or less it is involved with the throttle plate since you replaced your IAC bypass valve. An easy way to check if the IAC valve is at fault or faulty is to disconnect it while the engine is running at idle. if the engine speed does not change then I would suspect that the IAC solinoid is not functioning or that there is something stuck between the valve seat and the pintle of the IAC. (disconnecting the IAC during Idle will set off the check engine light if left disconnected for two long so I would do this after reading the codes). I would also inspect the travel of the throttle body as well. If there is some grime on the surface of the throttle plate (back side) or on the top of the throttle bore, that may be enough to keep the trottle plate open when it should be closed. Also note that the PCV valve is a two piece assembly and the neck of the PCV may have become loose which will cause idle issues as well. The last thing could be the bellows in the EGR valve assembly, if cracked would cause a vaccum leak but that would only occur when the EGR valve is required to be open since it is operated by a vacumm switch regulator which sits on the back side of the intake plenum (small rectangular box having two vaccum lines attached as well as a wire harness connection). Good luck with finding the fault. I had discovered several issues with my car and finally decieded to purchase a scan tool ($150) which has really helped me find some puzzling problems. I also heard that some programmers can also be used to check codes as well, but that was a bit overkill for my budget.

07-12-2004, 10:21 PM
After trying all of the suggestions, nothing seems to fix this problem.
So, I said what the hell. And I just ordered a throttle body and a gasket.
Replaced the throttle body and what do you know. The problem has been fixed. I disconnceted the neg. batt. cable to reset all the basic setting also.
Now it idles where it's supposed to. I am so happy now :rotf:

07-24-2004, 10:22 AM
That is good that you fixed it. I had much of the same problem after replacing the IAC. I really did not want to go to the expense of replacing the throttle body and since the screws on the TPS were fixed (I think they used epoxy to secure it) I simply bent the throttle stop arm which was the secondary means of adjusting the TPS position. I would have moved the stop set screw but that was fixed with epoxy as well. Bending the arm on the throttle linkage is probably not the best thing to do, however, it did not matter if I broke it since the next step was to replace the TB. I also had a vaccum leak which was very difficult to find. It turns out that the extension tube on the PCV valve (plastic 90 bend) had become loose. Along with a malfunctioning EGR valve which did not throw any codes.

:rotf: I wish I never took organic chemistry in college, either that or stay out of home depot plumbing section, I keep wanting to put down PVC (poly vinyl chloride) instead of PCV (positive crankcase ventilation).

09-11-2004, 08:12 AM
Here is one thing I think someone suggested. I finnaly had this happen to my car too. It turns out that after replacing the coolant this spring of 04, I used a different coolant (the bio-degradable kind) instead of the green. It was not the OAT (organic acid technology) type (yellow or orange). I should have replaced the thermostat but I did not think that was necessary at 26k miles.

After 4000 miles of driving, the motor started running hot 101C to 104C average (214F to 220F) under normal driving conditions. I noticed that the idle would hang in the 2500 rpm range when cruising at that RPM and it would stay there with no throttle.

I replaced the coolant and thermostat (188F) and it is running as it should! No more idle hang and the average temps range from 88c to 94c (190f to 200 f) which is about 20F cooler. The temp basically stays in the 88C to 89C temp when moving and will reach 94C up to 101C when sitting still until the fan kicks in it will drop to 86C and shut off.

I did not think that coolant temperatures would affect idle, but it does to some degree. Just and FYI.