Cold Idle Bouncing / Hunting

[Opticon]

I can't figure this out for the life of me. I've replaced every damn sensor, reset the IAC, and just had the LM flashed for my 42pph injectors. Everything runs great when warm and looks perfect on the OTC scanner but this TII converted Omni GLH can't idle worth a crap on a cold start.

It is smooth as butter for the first 10 seconds after cranking then the A/F meter falls into this cycle of waving slowly from 17:1 lean to 11:1 rich and back again. The idle speed follows the A/F sweep and drops to a stuttering 300rpm before bouncing back up to 2000 repeating again and again until the temp comes up. If I try to drive it before the idle settles it will hang at 2000rpm until I'm almost stopped.

What is wrong with this car?

[black86glhs]

Are you sure the wiring is good to the IAC? Also, the LM terminals making good contact?
When mine did it, the LM was bad.

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[Opticon]

I have suspected the wiring to the IAC before, but there is a notable change when I disconnect it, leading me to believe that it is working properly. But I will ring the whole circuit back to the LM.

I just had Rob mod and flash this LM... I would think a bad LM would have given him trouble. Not sure how I would troubleshoot that problem short of finding another 87 CSX LM and plugging it in (And I'm sure my +20s would throw a stock LM for a loop at idle anyway).

[x.Gen]

I went thru this when I started running +20's and doing my own cal's, it used to annoy me endlessly. you'll always get some wander, but shouldn't be getting to those extremes. if it is only on cold start and fades as engine warms, you're probably going thru what I did. I needed to massage the cold enrichment tables to get it right. this is pretty difficult to do without logging, especially if you are talking about Rob doing it remotely. I haven't looked recently, but I had it pretty tight when finished(13:1 to 15-15.5:1). I didn't put 2 and 2 together until I noticed the RPM's were moving with a/f, and it would lessen as it cycled thru the enrichment tables.

[DevoBuzz]

Sounds like a similar issue I have too - but not as bad. I have the 86/87 GLHS II LM with +20's. The first 15-20 secs it has high idle at cold start like it is supposed to. Then after that it will want to die if I don't give it a lttle gas. When driving in this state it surges a little. After about 2-3 mins it goes away and runs good. The other issue is that as the car warms up even more after driving another 15 mins, idle will be high until coming to a complete stop. The hot idle is about 300-400 rpms higher than cold idle.

[Opticon]

The other issue is that as the car warms up even more after driving another 15 mins, idle will be high until coming to a complete stop. The hot idle is about 300-400 rpms higher than cold idle.

This condition (RPM hanging on decel) comes and goes during my drive. I originally thought it was tied to the temp gauge and the car would cool off enough while cruising to trigger the problem... but it seems to be more random than just temp... Making me think it's a bad wiring connection (like the IAC).

V4L and I just fixed the CV boots so I'll be able to drive and diagnose it this week.

[RoadWarrior222]

If it just started this week, I might go for "time of year".... there's a couple of weeks spring and fall when I get either/both of my cars acting weird at warm up, seems like it's not quite cold enough to get full cold weather enrichment and not warm enough for the "summer gas" to fire up well. Yer kind of at the low end of the range for summer gas, and winter gas ain't out yet... then in the spring, you sometimes get the summer gas early before it's real warm.

[Opticon]

It is not seasonal as this problem has always exsisted on this car. But it does seems to be worse on colder mornings.

I discovered something about the hanging RPM this morning while driving it to work that might be a clue... The engine temp was over 185 (very warmed up) and I was in neutral while rolling. I blipped the throttle and the RPM stuck. The higher I blipped the throttle the higher it stuck. Until I came to a stop then it would drop to 900rpm. This condition came and went during my drive. I tried to link it to the temp gauge or some driving mode (traffic, cruising, etc.) but it seems completely random.

Is there any function of the IAC (throttle follower perhaps) that would do that? If so, what would be making it act like that on and off? If not, would an intermittant IAC connection be kicking the LM in and out of a default table of some kind?

[RoadWarrior222]

Hokay, with those symptoms I'd be hosing out the TB with carb cleaner, checking linkages are butter smooth, testing the TPS and wondering about the SDS.

[Opticon]

The car was just built. The TB/IAC passages and linkage are clean enough to eat out of, and the TPS and SDS are brand new and behaving correctly on the OTC scanner.

... And not you know why I'm asking the forum. lol.

[RoadWarrior222]

Alright, what kind of O2 sensor are you running? Just thinking this miiiight happen if it's expecting a heated sensor (expects it to be hot quicker) and you're using an older style one.

[Opticon]

It's the correct one. 3 or 4 wire IIRC. And no trouble codes.

[RoadWarrior222]

Well if there's no codes you're obviously imagining things and the car is A-okay

But I'd check that heater circuit is working anyway.

---------- Post added at 11:17 AM ---------- Previous post was at 11:14 AM ----------

(oh btw, I'm pretty sure it won't actually set an O2 code until it's been running 20 mins or so, and it's still giving bogus readings)

[ShelGame]

Which direction does it go first? Rich or lean?

It does this all the time when it's warm? Or only when cold, but after starting?

[shelbymonster]

hey i had that problem , when my fuel pump was dying and a hole in the fuel sock and a clogged filter , fixed that and rpm went almost too nice , im running on custom cal and + 40's and the wideband was crazy on idle

[Opticon]

It does the idle bounce only on a dead cold start (after 10 seconds or so). The rpm hanging thing happens randomly cold or hot. I assumed they were connected because they both feel like the result of IAC motor activity... but they could be two seperate issues.

I am not sure which direction the A/F goes first. I'll start it up after work and report in.


I haven't touched the fuel filter and pump in this car since I got it. I assumed that since the WOT A/F holds 10.7 at 18psi of boost with a stock regulator that the fuel setup was more than healthy enough.

[ShelGame]

OK, I'll look at the cal and see if anything seems amiss...

[Opticon]

Which direction does it go first? Rich or lean?

I just checked it. For the first 10 seconds the cold idle is smooth and the A/Fs stay very still around 14.5 (might move slightly but never farther than 13.9 or 15.1). After the 10 seconds the A/F takes a deep swing lean (17) then the idle starts dropping and stuttering... then it swings back to rich (11) as the idle jumps way up.

Then it cycles over and over.

[Opticon]

Coolant temp is around 100 degrees when it finally figures it out and idles well.

[trannybuster]

Only had that issue a few times, when cal wasnt right, or timing was off and when once when my fuel pressure was too low. Maybe those +40s need a smidge more fuel at idle..?