My car idles at around 1100-1300 rpm after it warms up. I just replaced the computer today. It did not help.
what else can cause this?

 

In the tech tips section read about adjusting the IAC and the TPS. See if that helps.

 

Has the idle screw on your throttle body been adjusted. On my 85 I noticed a few ways to play with the idle, here they are in random order;
1. throttle screw
2. timing
3. chip (aftermarket prom)
4. TPC
5. IAC
6. intake leak
7. MAF sensor
8. fuel pressure

While all these might not make it a high idle, they will affect the idle

 

mine too it even go up to 1600 sometimes but normally it goes t 1400 rpm. need help to fix

 

Listen very carefully for a high-pitched hiss; a vacuum leak is the most likely culprit. Could be at the throttle body or intake manifold, but I'd bet on manifold vacuum hoses. Fiddle with them all, and if you get a hiss to start/stop/change, and/or your idle fluctuates, replace that piece. In fact, replace any sections that look old--vacuum hose is cheap, and it's one less thing you'll have to worry about. Could also be brake booster related, but you didn't mention braking issues.


If the manifold itself is leaking, it may be more difficult to track down. Try the "cigar trick" if you're not opposed to a few carcinogens, or get a smoke machine intended for leak detection if you are. Fire up your stogie and blow thick clouds across the intake every which-way. If you see smoke being drawn in anywhere besides the throttle plate opening, there's your problem. A smoke machine would be more definitive; it works the other way around to lightly pressurize the intake, and smoke will emerge wherever leaks lurk. Easier to get results, but more $$--your call.


If you can't find a hiss/leak, keep going:


- As mentioned, over-advanced timing can also cause a high idle. Confirm that you're firing plug #1 @ 6 degrees BTDC.


- Excess fuel pressure is probably not at fault...but I wouldn't put a failing regulator outside the realm of possibility. Verify ~36-39 psi with the regulator vacuum hose connected, and ~47-48 when disconnected.


- MAF may be dirty. Remove and clean with the correct MAF solvent; carb cleaner and the like will kill it.


- After that, examine IAC and TPS function. I highly doubt either is the core issue, but the procedure is quick & easy, and essential to restoring proper idle: http://www.digitalcorvettes.com/foru...ad.php?t=58873


Once you ensure that no hoses are leaking, your timing & fuel pressure are correct, your MAF is clean, and the IAC & TPS work as intended...if you are still unable to adjust the minimum idle down to spec following the above link, you almost certainly have an undetected vacuum leak in the manifold or throttle body.


TB is easy to clean & rebuild, so that'd be my next stop. If that doesn't work, check a few less likely suspects before you commit to pulling the intake:


- A failed MAF sensor can cause funky idle behavior, but shouldn't skew it so high on its own--and I'd expect to see trouble codes/check engine, which I assume you would have mentioned.


- An exhaust leak or failing O2 sensor will screw with air/fuel mixture, but shouldn't alter idle speed.


- A bad coolant temp sensor could potentially force the ECM to remain in open loop (which idles a bit higher) without necessarily throwing codes...but again, I highly doubt that alone would cause such a fast idle.


If you get yourself an ALDL cable and a monitoring program like WinALDL, you can verify via laptop that your ECM is entering closed loop as intended, and that these sensors are reporting properly. The cable is cheap, the program, free & easy...but the wealth of diagnostic information you gain access to is priceless. Plus, it eliminates the need to splice into the TPS wiring to check it. I don't know MAF specs off-hand, but O2 should bounce continuously between 0.2 and 0.8 volts. The coolant temp sensor is best checked this way too--your dash display is routed to a different coolant temp sensor than the ECM uses, so it's essentially useless for tracing engine problems.

 

How do you know this? Did you hook up a scanner? Tach is not usually accurate.

Round up the usual suspects first. Plugs are new? Wires not leaking? Cap and rotor? Fuel pressure? Have you cleaned the Throttle Body and that includes taking the IAC and the IAC housing off and cleaning all the passages? Clean fuel filter? Steady at WOT fuel pressure?

 

Get a cheap cigar with the engine off blow smoke in the brake booster line annd others if needed to find the leak.

 

I had a similar problem and it was caused by a vacuum leak around the bottom fuel injector O rings.

 

Not sure but maybe 85 is a bastard year for ECMs so see if the local Autozone has a scanner they can lend you that works. I'd buy a scanner that can read your car and OBD II so you can use it on other cars to defray some of the costs. If you are intent on maintaining cars, a scanner is about as essential as any other tool. The days of throwing parts and guessing what is wrong are over. That is, unless you have more money than common sense. If you do, just remember that you and I are best buddies and you need to float me a loan.

 

Good to check basics. Frozen IAC could be a contributing factor. TB clean & refit may help, if the gasket is leaking. Fuel pressure perhaps, but IMO less likely than other things, like crumbly old vacuum hoses or bad intake seal. Spark items (aside from a loose / over-rotated distributor) & clogged fuel filter would cause rough idle, not high. Maybe EST could be acting up, though...plugging in with a laptop or scan tool would help decide!

 

I'd like to check and see if he is really having the problem before we do the replacement. I take my IAC out every 3 years which is about the time I do injector cleaning and testing. The manifold is already off so why not do something to clean the TB?

With the scanner he can check and see what the RPM really is and if there is an air leak when he tries to set the idle.