siggy_freud

My TPI problems seem to persist. Its an 86 TPI on a 66 gmc. No emissions stuff, no a/c, th-350 trans.

I have cleaned the IAC and throttle body. Reset the idle based on factory procedure. The TPS is set at .54. Timing is at 6. No vacuum leaks that I can detect. At 825 RPM I am pulling about 15 inches of vacuum. Thats where it was before I started having these problems as well.

The problems snuck up on me. I had finished putting on new heads, and had it running very nicely for about 3 weeks and then it started acting up. At first, it would idle nicely, but if you didn't press the throttle slowly, it would pop (sometimes) and cutout like it had suddenly died but then catch itself and run. If you slowly got on the throttle, there would be no problems.

Now, it idles erratically. Not all the time however. Sometimes, in park it will idle nicely (however at a higher idle, 800+ rpm). When I put it in gear, most of the time the idle drops low enough, quick enough that it dies. In drive it idles at 575-600 RPM. At times, this bounces between 400-625 RPM. When it begins to bounce, it almost does so rythmically. Revs, dies down, revs up, dies down. Its about a three second cycle between up and down. If I give it any amount of gas, it holds its RPM's fine. At times, the idle also bounces in park, though not as much, and not as wildly.

I let it idle for about 12 minutes and warm up to operating temperature. It never went into closed loop that my scanner saw. The injector pulse width would vary between 1.7 and 2.1 in park, and in drive, idling, between 2.2 and 4.

I have tried another MAF and it was the same. Could this possibly be a computer fault that is not throwing a code? I've worked with this TPI a lot but this one has my kind of stumped.

Let me know if any other sensor info that might be helpful in fixing this problem. I will list what I have on my mind below.

At idle in park at 825 RPM

MAF Grams per second - 9.
Pulsewidth - 1.7-2.1
Coolant temp - 180
Intake air temp - 63 (its about 40 outside right now)
Oxy Mvolts - 447
Oxy cross count - 2
Rich/lean - lean
Open loop.
IAC steps -145

Let me know if you need anything else lol. I hope someone has an answer. I miss driving it, and at the same time its driving ME nuts lol.

85 500 horse

When was the last time you changed your fuel filter or pump? I have seen Vettes do some very odd things when the fuel filter is clogged. It will run and then die. And then run and then die - If you get my meaning.

It's a cheap investment to try and it sure can't hurt to have a new one in there.

C4Ownr24

i'm going to watch this thread and give it a bump because my vette is doing *exactly* the same thing. especially the rythmic idle deal....

65Z01

With IAC steps at 145 (should be 15-20) the ECM is trying to richen up the idle mixture by closing down idle air. If you cleaned the TB & IAC I would replace the IAC and reseet base idle to 450rpm.

With that coolant temp is should have gone closed loop in minute or so after startup.

What are the BLM cell and INT readings?
Does the O2 sensor value change?
How long was it idling before that scan?
What are your mods?
Where is the O2 sensor located, i.e. how close to the header collector?

If you have an AutoXray scanner use the Monitor mode to capture a 30sec history of sensor data.

EPiC Vette

I had the "rythmic idle" thing. I could also feel it at steady speeds. It would kinda "pulse".

For me, it was the injectors. I had 3 good (0.014 ohms, I believe), 4 questionable (<0.009 ohms), and 1 that was basically a short.

That, and new plugs and wires fixed it.

-Eric

rick lambert

Just curious-for a reason-does it seem to make any difference in RPMs when you put your foot on the brake-and then let off?

siggy_freud

85 500 horse - Its been awhile. I am going to look into that today. Thanks for the idea.

65Z01- I'll try getting a new one and then going through the procedure again. What are BLM and INT? I am not familiar with those. The 02 value did not change at all that I saw. I was monitoring it through the 12 minutes of running. I rescanned a few times and let it monitor so I could see a live feed of the data. The 02 sensor is right in the collector. I'll pull it today and trace wiring to make sure that the computer is still able to get information from it.

Its pretty much a stock TPI. The biggest mod on the Engine are Trickflow heads and 1.6 rockers. The cam specs are unknown but given its powerband and idle i'd say its fairly modest. Never had problems before that I could point to the cam as the cause. Even since these mods, it ran great. Only about 3 weeks after the heads did it suddenly start acting funky.

There doesn't seem to be any relation between the brake, and its idle that I can tell. I tried pulling the vacuum feed from the brake booster and plugging it to make sure the booster wasn't letting vacuum escape. Idled the same.

Thanks for the ideas though. I'll give em a try. Any more ideas are always appreciated lol.

siggy_freud

Do you think gasoline and a match would fix my problem?

MaxLean

??? First of all the IAC has nothing to do with fuel mixture, it is simply a controlled air bypass around the throttle plates. That said, an IAC count if 145 indicates that the ECM ihas opened the IAC to nearly maximum in an attempt to raise the idle speed to where it wants. Is the IAC actually functioning? And as asked above, what are the fuel trims?