G22-Joe

Hi, this is my first post on this board. I would like to point out that from what I have read and searched on this board, I really like it.

To get to the point.

I just bought a 85 Firebird with the LQ9 2.5L 4 cylinder engine, with 5 speed tranny. Please keep your jokes to a bare minimum. I bought it from the original owner, and it is in decent shape for a 20 year old.

I got it cheap, and so far I get about 32MPG all around driving, and I do 134 miles a day.

I noticed that I seem to stay in the 2K rpm area of the tach, at start-up and at idle even after long periods of drive time. I noticed the engine light would come on from time to time when at a stand still (i.e. waiting for light, so on).

I looked into the trouble codes. and was given a 35 - Idle Speed Control - TPS Sticking or Misadjusted; poor electrical connection, open or short in circuit; defective IAC valve; defective ECM.

I am pretty much a newbie when it comes to stuff, and I have made repairs to other vehicles that I own. I use a manual or shop book that pretty much directs my actions.

So I am curious, is it easy to change those parts out, or would a mechanic be the better option?

Any and all information, tips and tricks will be greatly apprectiated. Thank you.

G22-Joe

Benm109

You don't really see a lot of Iron Duke engines on these boards, but I think we can help you out . Don't replace any parts just yet. It's always better to test things before you just replace them.

Since the code mentions the TPS first, we can check that first. The TPS is located on the TBI unit, on the opposite side of the throttle lever. Here is where the TPS (throttle position sensor) and IAC (idle air control) valve are on a 305 TBI unit:

The TPS is the gray thing and the IAC valve is the gold thing.

To test the TPS, you need a digital multimeter. There are 3 wires connected to the TPS, black, blue, and gray. Strip back the insulation of the 3 wires and set the multimeter to DC volts. Turn the key to KOEO (key on engine off) and put the meter leads on the black wire and the gray wire. You should see 5v. Now put the leads on the black wire and the blue wire. With the throttle closed, you should see somewhere around .5v and with the throttle all the way open, you should see over 4.5v. Slowly open and close the throttle while watching the readings. There should be no voltage spikes, dips, sticking, or anything like that, it should stay linear.

As for the IAC valve, I would suggest removing it, cleaning it thoroughly (don't forget about the IAC passage in the TBI unit itself), and reinstalling it, as well as adjusting minimum idle speed. There are directions on how to do all of this in the "New to TBI" sticky. See if doing those things help at all.


This is just the beginning of testing everything. I know that it may seem like a lot, but it's all fairly easy to do and won't take much time at all. Just simply replacing parts never works well. Especially considering the age of these cars (specifically the wiring).

v8nate92RS

Good write up Bemm, but I disaggree about testing first. With the car being that old, just replace those parts. The parts are dirt cheap and easy to change out. Just take out the torx screws holding the TPS in and switch it out. Just get a wrench and take off the IAC, then screw in the new one. Easy, and you can get the parts at any parts store.

G22-Joe

Thanks for the info guys.

If I swap out the TPS and IAC (after cleaning the chamber for the IAC of course) with new ones, is there any kind of adjustment that I will need to do? I thought I saw a tech bulliten for something mentioning how to set the TPS voltages and such, yet even after a search I could not come up with much.

v8nate92RS

There really isn't any adjusting you can do to those sensors. They are automatically adjusted by the ECM...they either work or they don't.

G22-Joe

Well this weekend, I decided to change out the TPS and IAC valve.

Since I was going to do a full tune-up, I pulled the entire TB off, and cleaned all the carbon build up out of it. I replaced the riser gasket for the TB, replaced the Fuel filter, TPS and IAC valve. Since I had the TB off, I also installed a new valve cover gasket, that was a chore, the original cork one was pretty much "molded" onto what ever it touched. The valve cover gasket came with a new EGR gasket, that I also changed out. I replaced the vaccum lines from the can, since they were pretty much rotted. Replaced the can filter, PVC and breather, as well as a new air filter.

Well I get everything done. Turn on the car, and it blasts at about 2500 RPM, then after a minute or two it drops down to 1000 RPM. It seams to idle very well at that point, and when stepping on the gas it goes very smoothly up to 4000 RPM, and comes smoothly down to 1000 RPM again.

So I take it for a drive. Man does it have some hesitation! It almost feels like I have lost 2 cylinders at times. Shifting from gear to gear, after 1st, it jumps the needle no less than 1000 RPM, and you can feel the engine just give up with every needle jump.

And guess what, I got code 35 again! So now I have a smoother idle, yet after 1st gear I get tons of hesitation, and dropping the brick, just makes it even worse.

Any ideas of what it could be? I believe it could be a fuel pressure issue, yet I do not know if the TPS plays a roll in that or not. I believe there is a way to adjust the pressure, yet I did not do anything to the injector/housing itself, other than remove it from the TB, and install a new fuel filter.

Any and all help is greatly appreciated.

v8nate92RS

Sounds like a vaccum leak...double check around the TBI unit where it meets the manifold and make sure you don't have a leak there. Also, double check to make sure you are not leaking around your EGR and that you have all sensors and hoses hooked back up correctly. Its easy to make small mistakes like forgetting to hook up sensors and hoses, or hooking them up incorrectly. Also, make sure you reset your ECM by unplugging your battery completely. Good Luck!