I finally got my freshly rebuilt factory LT4 into it's new home in my 96 SS. When I started it, the idle was very high. I checked for air leaks and didn't find anything. The tach is showing that the idle is at 1500 RPM but it sounded higher than that. When I hooked up the OBD II, it said the idle was at 2300 RPM. And that sounds more accurate. The car has one code and it is for EGR valve system. The factory LT4 didn't even have an EGR valve. Also, the OBD II shows nothing for the TPS. 0%. I used a volt meter and checked the output of the TPS sender and it seems to be working properly. There is 5.03 v coming into the unit and it varies from 0 to 5.03 v as the unit is operated. Right now the LT4 motor is running on the LT1 ECU. I expected issues with that but the motor seems to be running good beside the fast idle, and the throttle response is really awesome. I am afraid I don't know enough to trouble shoot this so any help or info would be welcome. Thanks in advance.

 

If you are sure you don't have a mechanical problem, then try resetting the IAC controller. I'd tell you how to do it but I don't have the manual handy, but you can do a search or maybe someone else will pipe up - it isn't hard to do.

 

OK, I'll look into that. Thanks.

 

Detailed steps are on shoebox site..... Its something like turn key to on posistion, tap the gas a few times and then start it. This is what i did and it always worked.

 

I tried the procedure for resetting the IAC that shoebox has in his trouble shooting guide, with no real result. The only change is that this time the tach was indicating 1300 RPM. I didn't hook up the OBD II analyzer.

 

The LT1 TPS shows 0% at idle. All the idle air should flow through the IAC system. What are the IAC counts at idle? If they are "0" also, it would indicate the IAC valve have been commanded closed in response to the excessive idle speed, and either the IAC motor is not closing down as commanded, or there is another source of air entering the intake manifold.

As far as the TPS voltage, you should see something between 0.20 - 0.90V at idle. The PCM will accept any number in this range and set it as the baseline 0% TPP. When the throttle is opened fully, the max reading should be about 4V higher than the closed throttle reading. If the idle TPS voltage drops below 0.20V it will set a code, and if the TPS voltage exceeds 4.9V at any time it will set a code.

The EGR codes look for the EGR solenoid, and then cycle the EGR valve to see if the EGR system produces the expected change in MAP. Only the solenoid code sets at startup or idle. The flow code diagnostic can only be run under a very narrow set of condtions, generally when the engine is running on the road, at very constant speed at low to moderate RPM. The fact that you happened to install an LT4 engine with no EGR (if I understand you correctly) with an LT1 PCM and LT1 programming would be expected to set those codes.

 

Thanks for the info Fred. I haven't had a chance to do anything with it today. I was going to check the resistance values for the IAC that shoebox provides in his trouble shooting guide. if that tests out OK I guess I need to do a more thorough search for air leaks. One other thing just occurred to me, I had the throttle body apart for cleaning. It was pretty scroungy. Could I have left something out or missed something when I was cleaning it up? I was pretty careful when I was doing it and everything looked good but I am a rookie when it comes to those TBs. A few other questions. Will the OBD II read the TPS values you talk about when the engine isn't running? Also, do you have any idea why there is such a significant difference in the tach reading and the OBD II reading? Again guys, thanks for the help.

 

Did you use a new gasket for the bottom plate on the throttle body? Are you sure you have the right screws in the right positions? It will not tighten correctly with the screws mixed up. I've seen them leak coolant into the intake. Reliable gasket between the intake manifold and TB? Is the EVAP line hooked up to the correct nipple on the side of the TB? The EVAP line goes to a vacuum port. The passenger valve cover line goes to a non-vacuum port in front of the blades. Is the foam seal in the TPS sensor hole?

 

Are you referring to the bolts that hold the TB to the intake or to the small bolts that hold the plate on the side of the TB. I did not have a new gasket between the TB and the plate on the side of the TB. I am pretty sure that all vacume lines are hooked up to the correct ports. I had another running car for a model, plus I took a ton of photos before I took it apart. They all seemed to fit naturally into position. And the foam seal is in the TPS sensor hole but it isn't new. I had 2 to choose from and one was pretty crusty. The other seemed good and that is what I used.

 

I'm talking about the plate on the BOTTOM of the TB, that contains the idle air motor and to which the coolant supply and return connects.

 

OK Got it. Having worked on motorcycles for many years, I am pretty good at getting short screws in short holes and longer screws in longer holes. But at this point I am obviously going to check them again and the gasket.

 

With the engine idling, check the port you have the EVAP purge line connected to. Should have a vacuum. The other port for the valve cover line will not have a vacuum (or only a very tiny vacuum, compared to intake manifold vacuum).

 

Fred, is that the vacuum hose that is bundled with the fuel lines?

 

The charcoal canister purge line is in the bundle with the fuel supply and return lines. It goes to a metal tube that wraps around the front of the engine, connects to a vacuum detector switch near the front of the passenger valve cover, then a rubber hose to the EVAP purge solenoid, then a rubber hose from the purge solenoid to the nipple on the side of the throttle body.

Shoebox has a diagram of the system. But it doesn't really differentiate which of the two nipples on the passenger side of the throttle body the hose connects to. There is no vacuum in the system until the EVAP purge solenoid opens, allowing the vacuum at the throttle body to pull the HC vapor out of the charcoal canister.

http://shbox.com/1/1996_evap.jpg

 

That line on my car is a hard plastic line with some rubber sections. It crosses under the TB. From what I remember, mine is connected to the top port on the actuator. I will compare it against another stock car to make sure. I haven't been able to work on my car for a couple of days. I hope to get to check some of your suggestions today, if I can get free.