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Thanks a bunch Fitz - for offering to loan me your ECU (mine is a 544). I’ll check the local yard first and see if I can pick one up. Whereabouts are you by the way ? I live in the bay-area of California.
I too had a bad engine-harness problem until I replaced it with a good one from the yard a couple of years ago. Around the time I cleaned the IAC, Throttle-Body, Injectors etc, I noticed that the wiring from the engine-harness-connector that was going into the car on the passenger side (to the ECU etc) was starting to degrade as well. I stripped all the outer black-insulation from the harness and insulated each wire all the way to the firewall. Also checked and make sure none of the wires were shorting each other.
Regaring the possible culprit:
1. Bad or damaged ECU: I’m trying to exclude all the other components first– so if all else fails this will probably be the only thing left. Hope not though..
2. Failing temp sensor: I checked this at the ECU connector end – so as to make sure there were no breaks/shorts along the way from the sensor. On a cold engine read about 2400 ohms, and on a hot one around 300.
3. Shorted IAC/TPS wires in the original wiring harness: Again, checked both the TPS as well as the IAC connections at the ECU connector end so as to factor in break/shorts along the way. Everything showed up fine, with no shorts along the way. Checked TPS open/close as well as IAC resistance.
4. Improperly adjusted throttle body: I too wanted to know how much of the high-idle was solely attributed to the butterfly-valve – so I removed the IAC and bridged the hoses coming to it with a solid pipe – so no air was coming in via the IAC line. I then fully closed the idle-bypass screw. Car barely started, and idled at around 300 and died very soon. To allow it to run, I had to loosen the idle-bypass screw almost all the way. Achieved an idle of around 600. So I don’t think this is a factor, considering the idle I am seeing is between 1000 to almost 2000 rpm. When I was doing the TB cleaning and had the idle-stop screw and TPS fully-removed, I noticed that the butterfly-valve didn’t completely seal shut. There was a very tiny sliver of light that made it’s way through along the sides. I could have filed the 2 spots on the butterfly-valve that were stopping it from completely sealing shut – but decided instead to compensate for this very-slightly open position by not tightening the idle-stop screw that extra 90 degrees when setting it.
5. Damaged IAC Valve: I checked the IAC by applying ground to close and then open terminals, and both worked fine. I even blew through the hoses in both positions to verify that it was closing & opening correctly. I was beginning to supect that maybe the commutator was worn in the areas that correspond to the partially-open position only (which is probably the position it spends most of it’s life in) – which was causing it to behave erratically when idling – but allow it to pass the fully-open and close tests just fine. Since I already had the IAC off and the IAC line to the manifold already blocked-off, I decided to test it with the engine idling to see how the ECU controlled the valve. Before starting the engine I manually closed the IAC. On startup, the IAC shutter-valve opened about 1/2 way. I touched the shutter-edge with a pen to see how strong the motor was – and it was pretty firm, and you could feel the motor-buzz. Over the next 30 seconds the shutter slowly went to about 7/8 closed, and then crawled back to the 3/4 closed position. Of course this had no effect on the idle-speed because it was offline and not controlling the intake-air. I then opened the throttle expecting the IAC to completely close, but it had no effect which was surprising.
On opening the IAC I didn’t find anything visibly wrong with the commutator. It was not gleaming– but not dirty or worn either. This was surprising given that the motor is not completely isolated from the valve-area of the IAC. Considering the valve-end gets so dirty you would expect some of that dirt to make it’s way down to the commutator. Was surprised it was designed this way though…
SO – Will clean the commutator & brushes, oil the shaft and install it back in tonight. I’m not too optimistic it will fix the problem, but let’s see. The idling with the IAC connected had actually got much worse this morning – it now idles at around 1800, and if you open and then close the throttle with a jerk, it drops to 700 and then immediately jumps back to 1800. Not good considering I have a ZFHP22 tranny attached… :(
One question: How would the engine behave smog-wise without the IAC attached, and with the 2 hoses blocked ? That’s how I’m currently driving it, and apart from a low idle of about 700, it seems to run fine. Anyone have an insight on how this would affect a smog-check on a dyno ?
Thanks again for all your inputs, and will let you know how things go.
Noel
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