Hi everybody. Long time reader, first time writer. In fact I just registered a week ago. I have an 85 4cyl that I bought 2 years ago from a good friend for driving to work. He has taught me a lot but we can't figure this one out. When at operating tempature, it will not idle below 1500 RPM's. Here is what I have done. It has a code 35 (IAC). I replaced the IAC and cleaned the inlets and throttle body and still have a code 35 only when car is at idle for 30 seconds or so. I have replaced the O2 sensor, the throttle position sensor, the thermac temp sensor and replaced every rubber hose I can find that carries air in hopes of eliminating a vacuum leak. I cleaned the EGR but have not replaced it because it performed properly when tested. I have checked the injector and am pretty confident it is not leaking excess fuel into the throttle body, but maybe it still pushing too high a volume of fuel. Speaking of which the fuel pump and filter have been replaced, and of course plugs wires and distributer. The only existing problem I know of, is that I am getting blow by past the pistons. I do not have a head gasket leak (no foam or milkshake in oil) and I have checked the intake manifold for leaks by spraying around the seams (a HPP tip). I know this is a long read, but as you can tell I have done a lot and am now at the end of my limited knowledge. I have learned a lot from you guys here at PFF, can anyone help? Please!

Sounds like you have done a really good job trouble shooting it so don't have a lot of other suggestions.

One idea I might try (not sure about access on a 4 since I don't have one but) is pull the IAC motor and put you finger in the hole to sim. the IAC. (do this cool of course and have someone start it) See if you can regulate the idle with your finger to a normal speed.
Guess what I'm thinking is if you can then it still might be a control/ecm fuction, but if you can't that would tell you you still have vac. leak somewhere and can keep looking.

I didn't see that you tried the CTS. That is also one that can make a big difference in the idle speed and has fixed idle problems of others.

[This message has been edited by Dodgerunner (edited 03-04-2007).]

Did you replace the fuel pressure regulator while you were in there? Or checked the fuel pressure? High idle is usually a vacuum leak though. How does your EGR tube look? Also, did you reset the ECM to clear the IAC code? It will stay until cleared.

You have a ALDL cable and WinALDL software to check the IAC count? '87-'88 with DIS cannot use WinALDL but I "think" your '85 can. Anybody?

Ah, does a 4cyl have an egr tube?

quote Originally posted by Dodgerunner: Ah, does a 4cyl have an egr tube?

lol, no. Dang, and I just replaced mine too. (pulling head out of tailend)

Try disconnecting the vacuum line to the EGR and covering it. Does the idle return to normal speeds? How about the brake booster? If covering over the hose to eather one makes it run right, you've found your vacuum leak.

Thanks for the quick replies. No I did not clear the code, I thought it would automatically. How do I do this? I have not replaced or checked the fuel pressure regulator. According to the Haynes manual it is installed in the fuel meter cover near the injector on the throttle body and is factory set and cannot be adjusted. So I guess the only option is to replace it and see what happens. Is this correct Hudini? Dodgerunner, I'll try the finger thang my next day off. Is the CTS the coolant temp sensor? If so, I have not replaced it. What would the connection be between the CTS and idle RPM's?

quote Originally posted by jimbolaya: Thanks for the quick replies. No I did not clear the code, I thought it would automatically. How do I do this? I have not replaced or checked the fuel pressure regulator. According to the Haynes manual it is installed in the fuel meter cover near the injector on the throttle body and is factory set and cannot be adjusted. So I guess the only option is to replace it and see what happens. Is this correct Hudini? Dodgerunner, I'll try the finger thang my next day off. Is the CTS the coolant temp sensor? If so, I have not replaced it. What would the connection be between the CTS and idle RPM's?

The 4cyl. are low pressure fuel pumps so I wouldn't worry about that. Hudini was thinking it was a V6.
The cts is used by the ECM to control the fuel while running and the idle speed when the engine is cold. That's why when you start it cold it idles high and then the idle speed drops as the engine heats up. If the CTS is bad and the ecm thinks the engine is still cold the idle will stay high.
The dash temp gauge and the ecm cts are to different sensors.
Changing the cts has fix idle speed problems for some others.

[This message has been edited by Dodgerunner (edited 03-04-2007).]

You clear the codes by unplugging the ECM's power for a few minutes. You will then need to drive the car above something like 45 MPH for the IAC to be reset.

I wouldn't worry about the pressure regulator right now. I'd start with the free tests.

CTS is coolent temp sensor, yes. The ECM uses this sensor to determine the engine temp, and to decide when it's warmed up enough that the other sensors are giving acurate information. Until it's warmed up, it completly ignores things like the O2 sensor.
This usually results in a rich mixture, both for easy starting and to prevent dangerously lean running.

I'm still betting vacuum leak at the EGR or similar.

[This message has been edited by Jax184 (edited 03-04-2007).]

Jax184, I did replace the hose in the engine compartment that is associated with the brake booster just because it was there, but I made no connection between the brake booster and a vacuum leak or that it would even be associated with high RPM's. I never checked up front hoses. This has also made me think about another area. The car has cruise control, but it does not work. Could a leaky vacuum canister on the cruise cause enough of a vacuum leak to affect engine RPM's. I did replace all cruise control hoses but not the canister. I assume the hose are hooked properly. I do not have a deck lid for the 85 4cyl, so no schematic. My deck is off a 88 formula.

It certainly would. A brand new hose to a leaky part isn't going to make the car run right. Again, I suggest disconnecting vacuum controlled stuff like the cruise, brakes and EGR and covering the connections to see if that makes the car run properly. Do them one at a time until you find the one that solves the problem.

You guys are awesome!!!!! Plenty of great stuff to look at. I have got to go now, must go to work to pay for more Fiero parts. If there are any other suggestions, please send them and I'll update with progress.

If you have a ohm meter you can also give the cts a quick check using this table as a guide to what the resistance should be.

Check it cold and warmed up a bit.

[This message has been edited by Dodgerunner (edited 03-04-2007).]