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I bought this 86 in the summer, and my garage I park in is heated, 65F or above.
Month or so ago I happened to move it outside while doing some other stuff on a chilly day
and when I went to bring it back in it fired up as normal and had a lower than normal idle and
didn't take throttle very well. I thought huh, odd. Next day in the shop it was normal again.
And by normal I mean 11-1200 cold idle, comes down pretty quick as it warms up, etc.
I had occasion today to park it outside again when it was 50F or so for four or five hours
and it did the same thing. I plugged my handy dandy OTC 4K scantool into it to look at some
live data and it was showing invalid rpm, no request for proper idle RPM, no change in airflow via
the AFM regardless of RPM, proper ecu temp and MAT, and the IAC pegged.
Photo below is at idle, indicated 800rpm on the cluster which was likely correct.
This all occurred inside 4 minutes or so but I let it run a moment, keyed off and restarted a couple times and the last time it seemed to wake up and realize it was a car and stated acting normal.
Like so:
I can't think of any failure mode that would make the ECU report the wrong engine RPM, no AFM flow and not even try to actuate the IAC other than an ECU failure.
There was no CEL. I know these are supposed to have a limp home mode but while not great it ran better than I expected one to in limp mode.
It is text book normal when it hasn't been sitting in the cold. My next step is to park it outside when it's cold with a space heater in the passenger floor
and if it does not act up I'll assume it's the ECU (or prom I guess?).
That's 980 rpm, and that is after it woke up and started working. At about 120f coolant temp yes it's normal, least for this motor. It starts when dead cold about 1150-1200rpm and idles down as the coolant temp rises.
It was walking the IAC steps down to hit the target when I took the photo. Acted perfectly normal after it started working.
What about ref pulses ? Wouldn't the distributor be suspect here ? Coil, pickup, ICM, or connectors ? These items (mostly wound coils) seem to be temperature affected right.
Ref pulses affect most everything fuel and timing oriented. Fuel pump on affects MAF power.
Maybe inspect / clean G104 engine ground. Probably not it, but a maintenance item that should be done at 38 years old.
Are there any stored codes ?
If the reference pulse was gone I wouldn't think it would run, and it runs okay, not right but nothing crazy bad. I don't know how good the limp-mode is on these things?
And I'd expect it to store a code and it did not, no CEL.
The pickup is NOS GM and the HEI module is a new AC Delco, coil's new too.
My experience with the HEI module is when they die it's a no-start but I dunno.
Pretty sure I've touched all the grounds.
No, not missing of course not, but hindered somehow. It would be nice to see KOEO data.
backprobe a 5vdc source and measure it when the fault is occuring.
.
I can't imagine how the car could be firing the injectors correctly but not report RPM or AFM flow and have no idle control other than a temp dependent ECU failure.
Bad solder joints aren't uncommon stuff this age. We're basically driving around with Commodore 64 age and level of tech running our engines.
KOEO looks just like that first photo. I didn't have the TPS voltage on the screen at the time but I did scroll past it and it was normal closed throttle.
I might stick it outside again one day this week or I might just ignore it for now and get into my trans swap and worry about it later.
Seems fine long as it's parked in the nice cozy shop.
Did replace the ECU with a newer revision and it seems to have fixed the odd results on the scan tool
And the idle was acting normally after a drive and a couple on/off's.
However, be this coincidence or not, about the 4th drive after replacing it the idle went weird again.
Differently weird though, not just when it was cold. The IAC valve was sticking and causing all sorts of
confusion. Once I swapped THAT out, it went back to pretty much normal.
The idle system would be a lot less drama filled if it had a position sensor in it instead of just trusting
that it goes to it's home position.
The point that I was trying to make earlier was that the scan data examples that were shown did not really appear to be unusual (except for the desired idle rpm being divided by factor of 10). I don't know why your scan tool reports the desired rpm in this way but I will write it off as a tool issue.
The reported behavior did seem to suggest a sticking and/or out of sync IAC which can be difficult to determine without commanding idle speed sweeps with an emulator or fancy tuning.
Note: I've increased the max IAC counts to 200 here, but as you can see it doesn't have much if any effect above 145 counts, but still enough airflow to support an extra 1,000 rpm or so.
The odd thing was that the reported current rpm was wrong, and the air flow meter reading was static regardless of rpm while the idle issue
was occurring.
I can't say if it was a coincidental tool issue (**** did indeed happen sometimes back even when these were new) but it hasn't done it since.
It was only doing that while it was failing to fast idle and everything worked perfectly normal when it was above 50F or so.
No tool issue with the replacement ECU so far and the IAC did crap out but it was a few days of normal behavior later and in a different
manner fwiw. Could well have been a weird coincidence.
When the IAC did fail a few days later it was temp independent and it was failing to return to it's home position some of the time so the ecu was trying to control idle but didn't
know what it's position actually was. The previous failure was just flat not opening along with the weird scan tool readings.
If it weren't for the wonky scantool data I'd have just called it an IAC failure to start with.
Back a million years ago when this type of ECU (pre-obd2) was still in regular use and the cars weren't that old we replaced a crap TON of them.
They did fail, usually but not always a no-start though.
We had a huge diag computer that had interface harnesses that would plug inline between the ECU and the vehicle and eavesdrop on
everything going and coming that was really good at finding errant behavior. I don't remember who made the thing but it was worth
it's weight in gold for cutting down diag time at an indie shop. When I left the guy was looking at getting a dyno to use with it.
I guess everything is just data out of the ECU these days.
86 ECU failure below 55f ambient
