I have a 91' Z28 with a 95' LT1 & 4L60E in it. I been reading about setting the throttle to the minimum air position. Before I do anything though I wanted to make sure I could do this with an LT1? Okay on to my problem. When first start the car the idle is very high then after it warms up it will drop down to around 1500RPM. When I put the car in drive it either stays around 1500RPM or it might go up to 2000RPM. Sometimes even surging up and down. My initial thought was maybe a vacuum leak, but after testing it yesterday the vacuum tester showed a steady pressure between 20 & 22Hg. (17-22Hg is normal operating condition for an LT1). In addition the the high idle in park and in drive the SES light is now coming on. Should I be able to read codes with a paper clip like regular thirdgens? If so how should I go about doing this? Last Question: Do you think these problems would keep my car from performing right at highway speeds? Any help you can offer would be great. Thanks in Advance, Jay

well im not sure if the car has one but it sounds like the IAC (idle air control!!) Do you have all the wiring for the Lt1? if so where is the diag connector? is it early 95 or late 95?..could be obd 2 if late!

Yeah the car has an IAC. All far as the wiring goes I just hooked the LT1 wires to my ALDL connector to make it work. I made sure that I had an OB1 setup because its so much easier to use.

Jay

Sounds like IAC problem

Hey...I actually registered just so I could respond to this thread and hopefully save you some trouble. Sorry about the long post...there's lots of info here.

1) Even if you have a vacuum leak, you'll still often see vacuum in a normal range (18-22 inHG). If the leak is large enough to bring that measurement down, your car will likely not run at all. The best way to check for vacuum leaks is by spraying brake cleaner (carb cleaner rarely works, and starting fluid is just dangerous) on any seam that could potentially develop a leak (intake and TBI gaskets, hoses, fittings, etc), and listen for a change in the idle. When the idle changes (rises or drops), check the same spot again to ensure repeatablity. If it is repeatable, you've found your leak.

2) '89-91 5.7's have a common problem with the intake manifold gasket (at the cylinder head) degrading over time due to the heat in the coolant channel. This will cause a vacuum leak in the front cylinders. This may or may not still be true on a '95 engine, but it *will* cause the excessively high idle and surging that you've been experiencing.

3) Contrary to what everyone is saying (which, to be fair, is an overly common misconception), if the engine is surging, then your IAC is WORKING. The cause would then lie with the IAC control circuitry, which is, more often than not, a direct affected by a vacuum leak. The ECU just doesn't know how to handle the data it's getting from its sensors.

4) the idle speed adjustment screw on the throttle body should NEVER be used to fix a problem. It may be used to make *minor* changes in the idle speed on a modified engine, but the computer should take care of idle speed adjustment for you. Of course, the computer can only do its job properly if the engine is mechanically sound, and the sensors are accurate.

5) If your SES/CEL is on, there should be codes stored. These you can read with a paperclip by shorting Pin B(?) to ground. Google that or call a good repair shop to make sure I got the right pin...it's been a while since I did it that way (I love my scanners).

6) A vacuum leak will lead to an excessively lean condition (that's probably why that SES is on), which can cause knock, preignition, or detonation on the highway....never mind excessive EGT and a melted-down catalytic converter.

7) Of course, this whole volume of information assumes that you replaced the stock ECM with the one for your '95 engine. =)


Jeremy

Jeremy,

Thanks for the reply. I found it to be very useful. It seems that since I have the PCM from the 95' I will just have to get a scanner or buy a cable so I can hook up a laptop.

Jay