Matto'85TA

I have a idle problem with my '89/'85 305 TPI. The engine is a '89 the ECM and harness is the '85. When the engine is cold and/or in open loop the idle is very low and it likes to stall, especially when I put it in gear. When the engine is warm and/or in closed loop it idles fine though i think a bit high, in park at 1000-1100 rpm. I had it in the shop becuase it would die immediatly after starting. They replaced th IAC, they called it a Idle Speed Control Motor or somthing like that though. It looks nothing like my old IAC. It looks bigger and looks like its made of brass. They also have adjusted the idle screw so far out I can see the threads. I tried adjusting it while it was idling in park at 1100 but I could only get it down slightly. The idle was way to loew in drive(500 maybe) and if I went to far the idle would suddenly drop.

What maybe the problem?
Could the ECM be bad?
I don't have any vaccum leaks and I have fuel pressure. When it's warm its fine. I replaced the EGR valve, plugs, wires, rotor, cap, fuel filter. Everything started to happen after I had the intake apart. Everything is in its correct spot.

Help please. I'm out of ideas.

Thanks

Matt

Vader

Matto,

I believe "they" screwed up. The throttle stop screw should almost NEVER be adjusted all the way out. It sounds like you should clean the TB and reset the throttle minimum air position, then check the TPS voltage again.

felo72

If you have an 89 TPI engine, does it have the cold start injector (it's a nineth injector between the runers on the driverside)? If not, that could be most of your problem. From 85 to 88 the TPI's had a cold start injector to throw more gas in until it was warm. 89 and up don't have them anymore. So the 85 ECM and prom could be throwing things off which would explain the idle problem when cold and why you have a high idle when warm. It sounds like your mechanic cured the problem with a band aid. He raised the idle so when the engine was cold it wouldn't stall, but when it's warm it's high.

Matto'85TA

vader-how do I do that, also, what does the TPS have to do with it? I thought the computer ran the engine until closed loop and the sensors didn't do anything then.

No cold start injector. In fact, all my problems started when I tried to put one on. I thought the parts I had weren't matched up and I put the old parts on. The car idled fine before I worked on it. I do have a '86 ECM and I want to upgrade so I can get the cold start program in there. I figured my problems related to the computer. The only thing I need is a Prom and instructiojns on how to convert my harness to the '86 ECM. I looked at the tech article but it didn't make much sense to me. I think I have the repinnning down but I don't get the wire stuff(the splices, removal, and adding).

Thanks

Matt

Vader

Felo has a good point, but actually the cold start valve only works for the first few seconds. After the engine has started, the ECM trims fuel based on MAF, TPS, CTS, and RPM until it achieves closed-loop (good O² feedback).

The ECM trims fuel as described above, so the TPS is crucial to the calculation whether in open- or closed-loop.

The reason the engine would tend to stall is that with a throttle minimum air adjustment that is too low (fully closed throttle plates) the IAC must pass all air at idle. this will require that the IAC is almost fully open, instead of being at its midpoint of travel. Once you enter DRIVE, the need for a bit more idle air cannot be satisfied by the IAC, since it is already fully opened. The ECM doesn't like the feedback, so it will try to open the IAC more, but it cannot. Eventually, the ECM will have to reduce fuel trim to match the IAC/idle speed, so it will stall. (This process all takes about 1½ seconds.)

As for changing over to the '86 ECM, there are fairly good instructions at this site:

http://www.ece.udel.edu/~davis/z28/ecm_swap/

Quoted from the site:

The ECM conversion/upgrade involves
** 10 repinning's in the stock '85 ecm harness connector
** 2 splices's between wires in the '85 ecm harness connector
** 1 wire removal in the '85 ecm harness connector
** 1 wire addition in the '85 ecm harness connector

** A '165 ECM from a Donor '86-'89 TPI Fbody car
** 2 relays (and harness wiring) from a Donor '86-'89 TPI Fbody car
** (Relay Harness plugs can be ordered thru a GM dealer under P/N 12101922 for about $25 each. -- Thanks to Kevin Irving for
this tip..)
** An EPROM configured to your Engine/Trans/etc.

Yes, that's our very own Kevin Irving!

It will probably be worth your effort. Getting an EPROM for the setup will be easy, I can burn a spare stock '86 TPI/Automatic/AC/2.77 axle/WS6 305 EPROM if you run into any real trouble, but I'm sure you can find a better EPROM elsewhere.